ABSTRACT
Kenneth J. Brennan. AGE, GROWTH AND MORTALITY OF LANE SNAPPER,
LUTJANUS SYNAGRIS, FROM THE EAST COAST OF FLORIDA. (Under the
direction of Roger A. Rulifson) Department of Biology, December, 2004.
Lane snapper, Lutjanus synagris, otoliths were collected from headboat and
commercial fisheries landings and from fishery independent sampling along the east
coast of Florida from 1997 to 2003 (n = 1414). Specimens ranging in size from 25 mm to
547 millimeters total length (mm TL) were measured and assigned ages. Ninety - eight
percent of sectioned otoliths could be aged. Fishery-independent samples were used to
clarify formation of the first annulus and to complement the fishery dependent data set
for other analyses. Marginal increment analysis established that rings formed annually,
primarily in June. The oldest fish encountered was 12 years and 406 mm TL. The east
coast of Florida was separated into north and south regions with the dividing line at Ft.
Pierce. The range in age and size for back-calculated total lengths were by regions, ages
2-10 years for north Florida were 153-437 mm TL, while south Florida fish for ages 1-12
years were back-calculated to 131-397 mm TL. The von Bertalanffy growth equation for
north Florida was U = 443.9 (1-e -0'30(' + o.82^^ and L, = 311.4 (1-e -o63(( + o.6i)^
Florida. The length and weight relationship was determined using additional headboat
data from 1998-2003 (n = 5837). The relationship was significantly different between
regions: W = 9.50 x 10'^ TL (R^ = 0.93, n = 2939) for north Florida, and W = 6.94 x
10“^ TL (R^ = 0.81, n = 2898) for south Florida, where W = total weight (grams).
Also, lane snapper from north Florida were typically larger at age and reached asymptotic
length slower than fish from south Florida.
AGE, GROWTH AND MORTALITY OF LANE SNAPPER, LUTJANUS SYNAGRIS,
FROM THE EAST COAST OF FLORIDA
A Thesis
Presented to
the Faculty of the Department of Biology
East Carolina University
In Partial Fulfillment
of the Requirements for the Degree
Master of Science in Biology
by
Kenneth J. Brennan
December 2004
AGE, GROWTH AND MORTALITY OF LANE SNAPPER, LUTJANUS SYNAGRIS,
FROM THE EAST COAST OF FLORIDA
By
Kenneth J. Brennan
APPROVED BY:
DIRECTOR OF THESIS lA >
Ro^r A. Rulifson, Ph.E^.
COMMITTEE MEMBER
Gerhard W. Kalmus, Ph.D.
COMMITTEE MEMBER
Charles S. Manooch, III, Ph.D.
COMMITTEE MEMBER.
D^glas S. Vaughan, Ph.M
CHAIR OF THE DEPARTMENT OF BIOLOGY
Ronald J. Newton, Ph.D.
INTERIM DEAN OF THE GRADUATE SCHOOL |
(/
Paul Tschetter, Ph.D.
ACKNOWLEDGEMENTS
I would like to express my thanks to my thesis committee members for their
unwavering support, guidance, and patience. I would like to thank Dr. Roger Rulifson
for his encouragement and help through my graduate work and for keeping me on course
toward the completion of this thesis. I would also like to thank Dr. Charles Manooch for
believing in me from the start and for helping me every step of the way. My thanks to
Dr. Gerhard Kalmus for steering me through my academic requirements and for his
sincere understanding of the many issues an older student faces. I would like to thank Dr.
Doug Vaughan for his patience and willingness to share his statistical expertise at any
give time.
Also, I thank Jennifer Potts for her patience, help, and advice. She was a guiding
light and instrumental in analyzing the data and organizing the results.
The National Marine Fisheries Service made it possible for me to work and go to
school, and for this I am most appreciative. A special thanks to members of the Reef
Fish Team and staff at the NOAA Beaufort Laboratory; without their support this would
not have been possible.
Finally, above all I thank my wife, Vicki, she has been supportive in every way
and has worked equally hard in order for this to happen. And to my children,
Jonnell and Kenny, you were my inspiration.
TABLE OF CONTENTS
LIST OF TABLES iv
LIST OF FIGURES v
INTRODUCTION I
MATERIALS AND METHODS 9
RESULTS 19
General Considerations 19
Age Analysis 19
Age Validation 20
Fish Length - Otolith Radius Relationship 20
Weight - Length Relationship 24
Growth 27
Lee’s Phenomenon 40
Von Bertalanffy Growth Parameters 40
Age - Length Key 43
Mortality 46
DISCUSSION 50
LITERATURE CITED 56
LIST OF TABLES
1. Number of lane snapper otolith samples by fishery and region 8
2. Mean observed total length (mm) of lane snapper at age from the east coast of
Florida by regions 28
3. Observed total length (mm) of lane snapper aged by sectioned otolith for
Manooch and Mason (1984) from southeast Florida 29
4. Back-calculated total lengths (mm) of lane snapper aged by sectioned otoliths 35
5. Back-calculated total lengths (mm) of lane snapper aged by sectioned otoliths
adjusted for BPH - north Florida 37
6. Back-calculated total lengths (mm) of lane snapper aged by sectioned otoliths
adjusted for BPH - south Florida 38
7. Age - length key for lane snapper from south Florida 45
8. Estimates of natural mortality (M) for lane snapper from the east coast of Florida,
by region 48
LIST OF FIGURES
1. Commercial and headboat landings for lane snapper on Florida’s east coast
1986 -2003 6
2. Sagittal otolith and transverse plane for sectioning 10
3. Image Pro photo of sectioned lane snapper otolith and measurements 11
4. Geographic scope of study region, with bouy locations 17
5. Marginal increment analysis for lane snapper from the east coast of Florida 21
6. Frequency distribution of measurements from focus to each annulus, ages 1 -6,
for lane snapper from the east coast of Florida 22
7. Total length - otolith radius relationship for lane snapper from the east coast of
Florida 23
8. Weight - length relationship for lane snapper from the east coast of Florida 25
9. Total length - weight relationship for lane snapper, east coast of Florida (1998 -
2003), by region 26
10. Mean observed total length (mm) at age, present study vs. Manooch and Mason
(1984) 30
11. Total length at age for lane snapper, by region, north and south Florida 31
12. Mean observed total length (mm) at age by region, east coast of Florida 33
13. Total length (mm) at age of lane snapper by fishery from the east coast of
Florida 34
14. Back-calculated (to last annulus) total lengths (mm) of lane snapper, this
study vs. Manooch and Mason (1984) 36
15. Mean back-calculated size at age for lane snapper, by region corrected for
body proportional hypothesis (BPH) 39
16. Lee’s phenomenon comparison between regions, north and south Florida 41
17. Theoretical growth of the present study compared to Manooch and Mason
(1984) 42
18. Theoretical growth of lane snapper for the east coast of Florida, by region
(present study) 44
19. Age frequency distribution of lane snapper from the east coast of Florida, by
region 47
INTRODUCTION
The complex of snapper and grouper species from the offshore waters of the
South Atlantic and northern Gulf of Mexico supports numerous commercial and
recreational fisheries. One important member of this complex is the lane snapper,
Lutjanus synagris. The South Atlantic Fishery Management Council, one of eight
regional fishery management councils in the United States, is charged with developing a
fishery management plan for the lane snapper and other reef fish species from North
Carolina to Key West, Florida. In the case of lane snapper, the last thorough and
published examination of the age and growth was completed 20 years ago (Manooch and
Mason, 1984). In this study I attempted to produce a current validated age and growth
analysis of the lane snapper from the east coast of Florida to be used to update stock
assessments.
The lane snapper, Lutjanus synagris, is a tropical marine fish found in the western
Atlantic from northern Florida through southeastern Brazil (Manooch and Mason, 1984),
and including the Gulf ofMexico. A member of the family Lutjanidae, the species is
important recreationally and commercially throughout its range. Adults are common in
Florida waters but rare farther north. However, juveniles and larvae occur at least as far
north as North Carolina (Adams, 1976; Ahrenholz, 2000; Tzeng, 2003) but do not
contribute to fisheries. Lane snapper occur in a variety of habitats including coral reefs,
hard bottom limestone outcroppings, shallow water seagrass beds, and turbid mangrove-
bordered estuaries. Moderate sized (to 3 kg), these fish are distinguished from other
snappers by seven to eight yellow lines diagonal to the lateral line and a diffused black
2
spot located below the soft portion of the dorsal fin. Diet consists of a variety of small
fishes and crustaceans along with worms, gastropods and cephalopods (Claro and
Reshetnikov, 1981; Franks and Vanderkooy, 2000).
Spawning patterns of lane snapper vary by geographical location. Erdman (1976)
reported that lane snapper have a protracted spawning period with seasonal peaks. In the
western Atlantic spawning is primarily in the summer months (Luiz Barbiéri, personal
communication, Florida Fish and Wildlife Conservation Commission, St. Petersburg,
Florida). In Bermuda the spawning season is late May to early September, with a peak
between June and August (Luckhurst and Dean, 2000). In the Caribbean, spawning may
be more protracted. For example, in Cuba (Claro, 1994) and in Trinidad (Manickchand-
Dass, 1987) lane snapper spawn from about March through September. Spawning
variability may coincide with warmer water in the southern parts of its range allowing
mature fish to spawn earlier, more often, and possibly even year round.
Age to maturity is early in life but may vary with location. Male lane snapper
may become sexually mature at age 1, but females may not mature until age 2 (Claro and
Reshetnikov, 1981; Manickchand-Dass, 1987; Luckhurst and Dean, 2000). Nevertheless,
Munro and Thompson (1983) reported that both sexes matured during their first year of
life in Jamaica.
Since juvenile lane snapper are uncommon in North Carolina waters and adults
are rare, the source of lane snapper larvae known to ingress through North Carolina inlets
is uncertain (Adams, 1976; Tzeng, 2003). Perhaps the progeny of spawning aggregations
off the east coast of Florida are transported north by the Gulf Stream currents. Other
3
dispersal mechanisms such as the North Brazil Current may contribute significantly to the
dispersal of larvae throughout the eastern Caribbean (Fratantoni and Glickson, 2003)
while the Florida Current and associated gyres may supply southwest Florida, the Straits
of Florida, and possibly southeast Florida (Lee, 1994).
Numerous age and growth studies on lane snapper conducted throughout the
South Atlantic, Gulf ofMexico, and Caribbean Sea indicate they are relatively short
lived, rarely reaching the age of 19 years, compared to most reef fish species, and reach a
maximum length of 50 cm TL. Most specimens landed in fisheries are <10 years old and
<25 centimeters total length (cm TL) (Alegría and Menezes, 1970; Cruz, 1978; Claro
and Reshetnikov, 1981; Manooch and Mason, 1984; Manickchand-Dass, 1987; Torres
and Chavez, 1987; Acosta and Appeldoom, 1992; Johnson and Collins, 1995; and
Luckhurst and Dean, 2000). Growth is rapid and by age-1 lane snapper reach about 19
cm TL in south Florida (Manooch and Mason, 1984), 19-23 cm TL in Trinidad
(Manickchand-Dass, 1987), and 23 cm FL in Bermuda (Luckhurst and Dean, 2000).
Growth during subsequent years is slower. Generally males grow slightly faster than
females and are larger at age than females (Luckhurst and Dean, 2000; Manickchand-
Dass, 1987).
Lane snapper have been aged using whole otoliths (Manickchand-Dass, 1987) and
sectioned otoliths (Manooch and Mason, 1984). Annulus formation is similar to other
lujanids with an opaque ring forming annually on the otoliths of lane snapper (Manooch
and Mason, 1984). Marginal increment analyses suggest that annulus formation in lane
snapper occurs April to June in Bermuda, (Luckhurst and Dean, 2000), April to
4
September in the northern Gulf of Mexico (Johnson and Collins, 1995), May to August in
Trinidad (Manickchand-Dass, 1987), and May and June in Cuba (Claro and Reshetnikov,
1981). The maximum age of lane snapper varies geographically, with younger and
smaller fish typically found in the southern distribution. Maximum estimated age ranges
from 7 years in Trinidad (Manickchand-Dass, 1987), age 10 in south Florida ( Manooch
and Mason, 1984) and Cuba (Claro and Reshetnikov, 1981) age 17 in the northern Gulf
of Mexico (Johnson and Collins 1995), and to age 19 in Bermuda (Luckhurst and Dean,
2000).
The older fish recorded in Bermuda and northern Gulf of Mexico exceed values
from the Caribbean and south Florida by a considerable margin, and may be related to
colder water in the winter months that may produce slower growth and longer- lived
fishes (Pauly, 1980). It is hypothesized that exploitation of older fish may not be as great
in these areas because older age classes are still represented. However, the northern long-
lived, slow growing populations may be as susceptible to overfishing as the faster
growing southern populations.
Almost all U.S. landings of lane snapper come from Florida, where lane snapper
is of minor importance commercially but moderately important to recreational anglers.
Commercial and recreational fishermen in Florida catch lane snapper using a variety of
gear including fish traps, beach seines, trawls, and hook-and-line. Commercially, adult
lane snapper (>30 cm TL and weighing 2 kg) are caught in deep offshore waters
(>30 m) with other lutjanids such as mutton snapper (Lutjanus analis), gray snapper ( L.
griseus), and red snapper (L. campechanus) ( Manooch and Mason, 1984). Recreational
5
fishermen catch lane snapper from headboats and private boats using hook-and-line
(Huntsman, 1976). Smaller lane snapper typically are caught inshore by anglers fishing
from piers, jetties, bridges, and shore (Manooch and Mason, 1984).
In 1983 the South Atlantic Fishery Management Council (SAFMC) developed a
fishery management plan (FMP) for the snapper-grouper complex of the U.S. South
Atlantic. Twelve amendments to the Snapper-Grouper FMP have restricted commercial
gear and created recreational bag limits. The FMP implemented an 8-inch minimum (203
mm TL) size linait and a daily bag limit of 10 fish per person for lane snapper. Similar to
other members of the family Lutjanidae, lane snapper landings have experienced
significant decline in the southeastern United States over the past 20 years (Figure 1).
The primary objective of my study was to produce a validated age and growth
analysis of the lane snapper from the east coast of Florida to be used to update stock
assessments. A secondary objective was to determine if there is a difference in growth
rates and estimates ofmortality of lane snapper between regions of Florida’s east coast.
In this study I will test the following hypotheses;
1) Ho: The opaque zone on the otolith is annular.
2) Hq: There is no change in age and growth for the same geographic area due to fishery
management regulations imposed in 1983.
3) Hq: There is no difference in age frequency, or size at age, or growth parameters,
between regions for Florida’s east coast.
4) Hq: There is no difference in the instantaneous rate of total mortality (Z) and the
instantaneous rate of natural mortality (M) between regions for Florida’s east coast.
50000
? %
40000
(k ) 30000landings 20000Total 10000
0 I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I
CD 00 o CM CD 00 O CM
CO 00 CD CD CD CD CD o O
O) CD CD CD CD CD CD o O
1“ 1“ 1“ T— 1“ 1“ 1“ CM CM
Year
Figure 1. Commercial and headboat landings for lane snapper from Florida’
east coast 1986 - 2003. (Commercial = Florida landings data,
Headboat = South Atlantic Headboat Survey).
MATERIALS AND METHODS
Otolith samples (n = 1303) were collected at dockside from Jacksonville, Florida,
to the Florida Keys from 1998 to 2003 by port agents of the National Marine Fisheries
Service (NMFS) South Atlantic Headboat Survey (recreational fishery) and the NMFS
Trip Interview Program (commercial fishery) (Table 1). Additionally, 111 juvenile fish
(25 - 196 mm TL) samples were collected using a fishery independent survey in Florida
Bay during 1997 and 1998. All fish were measured to the nearest mm for total length
(TL) or fork length (FL), weighed to the nearest 0.01 kg, and sexed when possible.
Sagittal otoliths were removed by lifting the operculum, exposing the otic bulla,
and using a wood chisel to shave away bone from the fluid-filled cavity to expose the
otolith inside. This method was used to minimize disfigurement of fish destined for
market or angler photographs (Matheson, 1981). Forceps were used to extract the otolith
from the cavity with care not to break the sagitta or push it deeper into the cranium.
Otoliths were stored dry in a coin envelope labeled with site of capture and pertinent
morphological measurements.
Otoliths were processed at the National Oceanic and Atmospheric
Administration (NOAA) Center for Coastal Fisheries and Habitat Research located in
Beaufort, North Carolina, according to standard methodologies outlined by Matheson
(1981). Otoliths collected from fishery dependent sampling were mounted using methods
applicable for age determination of adult fish. Whole otoliths were mounted transversely
dorsoventral) using Crystalbond*™, a thermo plastic cement, to adhere each earbone to
8
Table 1. Number of lane snapper otolith samples by fishery and region.
(north Florida = Georgia/Florida border to Fort Pierce, south Florida = Fort
Pierce to Key West, Rorida Bay = 1 mile north of Islamorada).
Fishery
Region Independent Commercial Headboat Total
North Florida — 135 144 279
South Florida — 934 90 1024
Florida Bay 111 — — 111
Totals 111 1069 234 1414
9
a tab (1 X 1 inch) of thin cardboard. The tab was then aligned and mounted on a Buehler
Isomet, model 11-1180, low speed saw equipped with a diamond gritted wafering saw
blade. Three serial sections approximately 0.25 mm wide were cut from the otolith: one
section containing the core of the otolith and the others immediately adjacent to the eore
(Figure 2). Sections were permanently mounted to glass microscope slides using
Crystalbond'*", and labeled for examination.
Prepared slides were placed on a black background and viewed under a
dissecting microscope at 18.8x magnification using reflected light. A video camera and
monitor were connected to the microscope to facilitate viewing and analysis. Immersion
oil was applied to each slide to increase clarity of otolith sections. Under reflected light,
otolith sections have alternating opaque white rings and dark translucent rings. Opaque
rings are hypothesized to be annuli (Manooch and Mason, 1984) and each was counted as
one year’s growth. Using Image Pro software, measurements (mm) were recorded in
along a lateral plane on the dorsal lobe of the otolith section from the focus to each ring
(annulus) and from the focus to the otolith edge (radius). The distance between the last
annulus and the otolith edge (marginal increment) also was recorded to validate the
annual periodicity of opaque ring deposition. Data were entered into Excel computer
software for analysis (Figure 3).
Sagittal otoliths of juvenile lane snapper (< 203 mm TL) from Florida Bay were
analyzed to determine time of first annulus formation and to complement fishery
dependent samples for the basic relationships of fish length (TL) and otolith radius (OR),
and fish length and fish weight (W). Otoliths were mounted for micro-structural analysis
10
Anterior Posterior
Figure 2. Sagittal otolith and transverse plane for sectioning.
Figure 3. Image Pro photo of sectioned lane snapper otolith and measurements:
A. = Annular measurements; R. = Otolith radius; M.I. = Marginal increment.
12
using methods described by Secor et al., (1992). Each otolith was embedded in resin,
sectioned, and mounted to a glass slide. Each section was then polished until the otolith
core was visible and a thin section (10 microns) was achieved. The otoliths were
examined at 18.8x magnification using the same microscope and software as used for
ototliths removed from larger fish. Measurements were recorded from the focus to the
radius and, if present, to the first annulus.
Using fish ages 2, 3, and 4 the month of annulus formation was determined by
calculating the mean monthly marginal increment values by age and for all ages
combined. These means were plotted against the month of capture, with the minimum
values indicating the month and season of annulus formation.
An otolith radius-fish length relationship is used to generate back-calculated
size at age for separate and pooled regions. First, I examined the relationship using
linear regression:
TL = a -i-b (OR),
where TL = fish total length (mm),
OR = otolith radius (p), and
a and b are the intercept and slope of the regression, respectively.
Next, the linearized In-ln regression:
TL = aOR*’,
corrected for transformation bias with Vi MSE (Mean Square Error), was analyzed to
determine the best fit (Beauchamp and Olson, 1973).
Back-calculated total lengths were derived using the log transformed regression
13
model of the observed total length (TL) on otolith radius (OR) with the body
proportionality hypothesis (BPH) method to account for individual fish growth
(Francis, 1990). The following equation represents this method:
TLi = ( ( a + b * Si )/(a + b * OR))*TL,
where TLi = fish total length at annulus i,
a = intercept from the TL - OR regression,
b = slope from the TL - OR regression.
Si = measurement to the ith annuli, and
OR = otolith radius.
Back-calculated size at age data were used to test for size selective mortality,
otherwise known as Lee’s phenomenon. This phenomenon exists when back-calculated
lengths at age are smaller for younger ages when using ageing structures from older fish
in the sample (Ricker, 1975). This would imply that faster growing individuals are
recruiting to a fishery sooner, or that sampling gear is preferentially selecting faster
growing fish at a young age. To determine if this trend was present, the distance from the
focus to the first and second annuli on age were regressed on observed fish age. If the
slope is significantly different from zero, size selective mortality exists. And if the slope
is negative, then older fish have smaller distance measurements when young, (e.g., 1-2
years old). The size selective nature of fishing, especially when managing with minimum
size limits, causes bias in apparent mean size at younger ages. This larger fraction of
young fish, when used in fitting a growth model (e.g., von Bertalanffy), can biased model
parameters (Goodyear, 1996). Young-of-year fish from Florida Bay were included in
14
analyses estimating von Bertalanffy (1938) growth equations for north and south Florida
to partially overcome this problem.
Mean back-calculated sizes at age were fit to the von Bertalanffy (1938) growth
model and theoretical growth parameters and sizes at age were estimated. Size at age
was estimated using the following equation:
L, = L„ (1- exp(-K(/- to)),
where Lt = mm TL at age t,
Loo = the theoretical asymptotic length,
K = the growth coefficient, and
to = theoretical age when fish length is equal to 0.
Theoretical lengths at age were derived for back-calculated lengths at age calculated from
all annuli measurements as well as from the those using only the last annulus. Vaughan
and Burton (1994) recommend using only length at last annulus in von Bertalanffy
(1938) fit to avoid violating the assumption of independence among measured lengths.
The back-calculated lengths from this study were compared to Manooch and Mason’s
(1984) study to determine if any significant change has occurred in growth due to
fishery management regulations imposed in 1983.
Age-length keys (Ricker, 1975) were developed for north and south Florida by
grouping aged fish in 25- mm length intervals for all ages. Additionally, the percentage
of fish, by age group, were calculated to compare age frequency distributions by region.
Initially, only samples collected for this study with whole weights reported were
analyzed to determine the length-weight relationship (n = 185). Due to this relatively
15
small sample size in this study, an additional analysis was completed to strengthen this
relationship by using additional lengths and weights for lane snapper obtained from the
headboat survey from 1998 to 2003 for the east coast of Florida (n = 5839). The weight -
length relationship was determined by using a linearized (In-ln) regression. The
equation:
/n(W) = a +b *ln (TL)),
was transformed to W = aL*’, adjusting for the transformation bias with V2 MSB from
linearized regression (Beauchamp and Olson, 1973), where W = whole weight in (g)
and L = total length (mm).
Various life history approaches for estimating natural mortality (M) were
explored. Two methods, Pauly (1980) and Hoenig (1983) are commonly used in stock
assessments (Vaughan et.al., 1992; Manooch et. al., 1998; Potts, 2000). Ralston’s (1987)
equation was developed with data obtained from 19 populations of snapper and grouper
stocks. A method by Alverson and Carney (1975) also has been used in stock
assessments to estimate M.
I calculated M using the following equation from Pauly (1980):
logioM = 0.0066 - 0.279 logioL» + 0.6543 logioK + 0.4634 logioT,
where = the asymptotic length,
K = the Brody growth coefficient, and
T = the mean annual seawater temperature (°C).
Sea surface temperature readings were derived from buoys operated by the NOAA’s
National Oceanographic Data Center during 2003 (Figure 4). Monthly averages provided
16
mean annual temperatures for north Florida and south Florida.
Hoenig’s (1983) method derives M using the following equation;
Ln M =1.46 -1.01 In tmax)
where tmax = the maximum age in an unexploited population.
In actual practice this equation is usually In Z, but if the data is from an unexploited
V
population the Z = M. If the population is exploited than Z > M and this estimate would
be the upper limit on M.
Ralston’s (1987) method derives estimates of M from the equation;
M = 0.0189+ 2.06*K,
where K = the Brody growth coefficient.
Alverson and Carney (1975) equation forM is,
0.38 * tmax = 1/K * ln{M + 3*K)/M,
where tmax = maximum age of the fish, and
K = the Brody growth coefficient.
Natural mortality estimates for the present study were compared to Manooch and
Mason (1984) using the same method, Pauly (1980), as well as the estimates derived
from all the equations.
The estimates for instantaneous rate of total mortality were preliminary values
based on the age frequency data from this study and did not include more detailed catch
data from all fisheries that land lane snapper. Total mortality (Z) was estimated by
regressing the log of the age frequency on age for fully recruited ages. Modal age or
age + 1 is used to determine fully recruited ages. The absolute value of the slope of the
North Florida
Cape Canaveral
South Florida
Fowey Rocks
Dry Tortugas Florida Bay
Figure 4. Geographic scope of study by region, with buoy location^ • ) for sea surface
temperatures for 2003 (Source: Burton, 2001).
18
descending right limb is Z (Beverton and Holt, 1957). These estimates were compared
by region and for all data combined.
RESULTS
General Considerations
Otolith samples collected from the headboat and commercial fisheries from 1990
to 2003 ( n = 1604 ) were processed for age determination and analysis. Earlier years,
1990 to 1997, were inconsistent in sampling effort by area and fishery, so consequently
the sample set was reduced to those collected from 1998 to 2003 (n = 1414) (Table 1).
Fishery-independent samples collected from Florida Bay were included for age validation
of 0-1 year old fish, which otherwise would not have been available with a size limit
(8-in TL; 203 mm) imposed on the recreational and commercial fisheries.
Most fish lengths were recorded in total length (TL); fork lengths (FL) were
converted to total length using the equation TL = -2.6252 + 1.0891 FL, R = 0.999
(Manooch and Mason, 1984).
Age Analysis
Lane snapper otolith sections showed a clear concentric pattern of alternating
translucent bands and opaque rings, which could be examined and used to age the fish.
Young-of-year fish from Florida Bay improved overall understanding of first annulus
formation. When present, false annuli occurred primarily between the first and second
annuli. Ninety-eight percent (1396 of 1414) of the sectioned otoliths were assigned ages.
Fractures and lack of clarity were the reasons for otoltihs not being aged.
20
Age Validation
Marginal increment analysis of all ages and ages 2-4 years old were calculated
and plotted for all fisheries and regions combined to determine month of annulus
formation (Figure 5). Opaque zones were revealed as annular and formed in late spring,
primarily in June. Validation using frequency distribution of measurements from the
focus to each annulus showed consistency in the range of modes for ages 1-6 (Figure 6).
The overlap of measurements between annuli in this distribution increased with age,
while the distance between the rings decreased with the slowing of somatic growth in
older fish. This was also reported by Manooch and Mason (1984) and subsequently
influenced their decision to exclude 9 and 10 year old fish from further aging procedures
because the distances were not discernible. In this study the use of Image Pro software
facilitated measuring these distances with confidence, allowing older fish to be included
in back calculations.
Fish Length - Otolith Radius Relationship
The total length and otolith radius relationship was analyzed using In-ln
transformed linear regression. The data fit this regression better than the linear regression
(Figure 7). The resulting equation for all data combined was
TL = 71.99 X OR '(n = 1396, R^ = 0.87).
Fish length-otolith radius relationships were calculated for each region. Fishery-
independent samples (small fish ages 0-1 year old) were included because regulations
excluded lane snapper smaller than eight inches (203 mm TL). Incorporation of these
21
A.
(iMncrm|eamragjeinna)tl
Figure 5. Marginal increment analysis for lane snapper from the east of
Florida. A. All data combined (n = 1353); B. Ages 2-4 (n = 937).
22
fPfrPrefPreqrqecruqcrueceunenetnctncytcyy
Figure 6. Frequency distribution of measurements from focus to each annulus
ages 1-6, for lane snapper from the east coast of Florida.
23
Otolith radius (fi)
Figure 7. Total length - otolith radius relationship for lane snapper
from the east coast of Florida.
24
small fish improved the fit of the regression:
TL = 65.37 X OR ‘ (n = 379, = 0.95) for north Florida, and
TL = 71.81 X OR * (n= 1119, R^ = 0.91) for south Florida.
Weight - Length Relationship
The relationship between weight and length increased exponentially (Figure 8).
The low sample size (n = 185 ) resulted from most fish being eviscerated at sea. Only
185 of the 1303 fish sampled were landed whole. The equation that best fit these data was
W = 3.27 X 10 TL , R ^ = 0.98, n = 185.
This In - In transformed regression was corrected for transformation bias with Vi MSB.
The relationship is similar to that reported by Manooch and Mason (1984), and was
expressed
W = 1.02 X 10'^ TL = 0.96, n = 101.
To strengthen the length - weight relationship when comparing regions, an increased
sample size was used by pooling all lane snapper samples with recorded weights and
corresponding lengths from the headboat survey on the east coast of Florida from 1998-
2003 (n = 5837). The length - weight regression was tested statistically using ANCOVA
and showed a significant difference in the intercept (t = 2.09, p = 0.0365), whereas the
slope (t = -1.63, p = 0.1031) was not significantly different between north Florida and
south Florida. Both regions increased exponentially with the north being slightly heavier
at length then the south region (Figure 9). These relationships were described by the
following equations:
25
(Wgwrheaoigmlhets)
Figure 8. Weight - length relationship for lane snapper from the east coast of
Florida (present study).
26
Whole
Figure 9. Total length - weight relationship for lane snapper, east coast of
Florida (1998 - 2003), by region.
27
W = 9.50 X 10'^ TL (R^ = 0.93, n = 2939) for north Florida,
W = 6.94 X 10'^ TL (R^ = 0.81, n = 2898) for south Florida, and
W = 6.79 X 10'^ TL (R^ = 0.89, n = 5837) for data combined.
Growth
Both oldest and largest fish were caught by the commercial fishery. The oldest
fish, a 12 year old female (406 mm TL), was caught in south Florida; while 10 years was
the oldest age recorded from north Florida. The largest fish captured was a 8 year old
female measuring 547 mm TL landed in north Florida. Observed mean total lengths
showed wide ranges in lengths at age for all ages (Table 2), a phenomenon also reported
by Manooch and Mason (1984) (Table 3). The protracted spawning season for lane
snapper, March to September (Luiz Barbiéri, personal communication, Florida Fish and
Wildlife Conservation Commission, St. Petersburg, Florida) may contribute to the range
in lengths related to each year class. When comparing size at age of my study to the
Manooch and Mason (1984) study from 20 years ago, the present study showed larger
mean lengths at age for 2-7 year old fish and similar lengths in both studies for ages 8-10
(Figure 10). Ages 0 and 1 were constrained by sample size in the earlier study due to the
absence of young-of-year fish (fishery-independent samples).
Comparing regions revealed overall total length (TL) at age was greater for north
Florida than for south Florida (Figure 11). This difference was tested statistically using
ANOVA (F = 399.37; P< .0001) resulting in a highly significant difference between
regions. Ages 2-3 showed only a slight difference in mean length 255 and 292 mm for
Table 2. Mean observed total length (mm) of lane snapper at age from the east coast of Florida by regions.
north Florida south Florida All Areas Combined
Age n Mean TL (mm) S.E. Range n Mean TL (mm) S.E. Range n Mean TL (mm) S.E. Range
0 42 89 35 25-135 42 89 35 25-135
1 9 237 14 217-263 73 151 41 97-263
2 9 255 20 216-295 154 253 20 204-329 163 253 20 204-329
3 51 292 37 233-394 412 281 27 212-372 463 282 28 212-394
4 82 335 33 261-417 229 290 30 226-422 311 302 37 226-422
5 80 355 40 266-472 109 312 37 248-425 189 330 44 248-472
6 31 381 45 295-460 65 322 47 242-429 96 341 53 242-460
7 11 420 59 333-501 21 314 55 242-492 32 350 75 242-501
8 8 422 70 338-547 7 321 31 282-363 15 375 75 282-547
9 3 479 42 430-505 4 315 31 280-350 7 385 94 280-505
10 2 459 13 450-468 2 386 41 357-415 4 423 49 357-468
1 1
12 1 406 406-406 1 406 406-406
Total 277 1055 1396
to
00
29
Table 3. Observed total length (mm) of lane snapper aged by sectioned otolihs for Manooch
and Mason (1984) from southeast Florida (regions and fisheries combined).
Total length (mm)
Age Number Mean length Standard deviation Range
0 1 168.0 — —
1 2 193.5 6.4 189-198
2 29 219.4 12.0 195-250
3 33 243.2 21.3 208-309
4 69 273.7 32.7 205-357
5 69 296.1 40.9 237-397
6 57 305.1 50.7 242-416
7 35 345.7 57.2 286-457
8 18 375.8 63.7 283-495
9 5 428.0 55.7 335-474
10 2 461.0 72.1 410-512
Total 320
30
Figure 10. Mean observed total length (mm) at age, present study vs. Manooch
and Mason (1984).
31
]600500 qOI 400£O) 3000) à« 200 - Oo o Florida BayA south Rorida100 - o north Roridaj I j
0 1 2 3 4 5 6 7 8 9 10 11 12 13
Age (years)
Figure 11. Total length at age for lane snapper, by region, north and
south Florida (present study).
32
the north, 253 and 281 mm for the south, respectively. Ages 3-10 diverged, with north
Florida showing a significantly larger size at age for this range (Figure 12).
Observed size at age for lane snapper was also compared by fisheries to determine
if differences were present between fishery-independent, headboat and commercial
growth characteristics (Figure 13). The overlap for all ages suggests very little difference
occurs, and for this reason subsequent analyses combined data across fisheries.
Mean back-calculated sizes at age for the present study and for Manooch and
Mason (1984) are shown in Table 4. Because Manooch and Mason’s (1984) results were
uncorrected for BPH (body proportional hypothesis), the method described by Francis
(1990), back-calculated means from the present study were also calculated without BPH
adjustment for this comparison only (Figure 14). Overall, the data and plots are similar
with only a slight difference in size at age.
The preferred back-calculated lengths at age were derived from the method
described by Francis (1990) for the body proportional hypothesis (BPH). This method
uses the regression of the length on age, and accounts for variation among individual fish
from the value predicted by the regression, by assuming constant proportionality between
observed and predicted back-calculated length. Florida Bay data were combined with
north Florida and south Florida separately as was the case with the regression model for
TL on OR. Mean back-calculated sizes at age for both regions were consistent with other
Growth characteristics examined in this study. The mean back-calculated lengths at age
for north Florida was greater for ages 3-10 (Table 5), while ages 1-2 were slightly higher
for south Florida (Table 6). This noticeable separation can be clearly seen in Figure 15.
33
600
500
E 400 A
E
o> 300
«
S
p 200
100
”1 1 > 1 r n 1 1 r 1 1
1 23456789 10 1 1 12
Age (years)
Figure 12. Mean observed total length (mm) at age by region, east coast of
Florida (present study).
34
600 n
E
500 - QL
A
400 - ?
1
"b
O Fishery Independent
S Commercial
A Headboat
0 2 4 6 8 10 12 14
Age (years)
Figure 13. Total length (mm) at age of lane snapper by fishery from the east coast of
of Florida, all data combined (present study).
Table 4. Back-calculated total lengths (mm) of lane snapper aged by sectioned otoliths.
A. Back-calculated total lengths (mm) of lane snapper aged by sectioned otoliths, no BPH used - Florida all data combined (present study).
Mean back-calculated total length at time of annulus formation
Observed age N 1 2 3 4 5 6 7 8 9 10 11 12
1 72 135
2 163 162 229
3 462 159 221 264
4 310 155 214 257 288
5 189 155 211 254 288 315
6 96 156 209 250 283 310 333
7 32 170 224 261 289 314 337 357
8 15 156 212 251 280 303 326 350 371
9 7 149 208 243 271 295 317 338 359 382
10 4 146 202 245 276 302 326 350 371 392 412
11 —
12 1 150 227 252 282 303 322 347 365 392 410 424 444
Number of calculations 1352 1280 1117 655 345 156 60 28 13 6 2 1
Weighted means 157 218 259 287 313 332 352 368 387 411 426 444
Increment 157 61 41 28 26 19 20 16 19 24 15 18
B, Back-calculated total lengths (mm) of lane snapper aged by sectioned otoliths, no BPH used - Manooch and Mason (1984).
Mean back-calculated total length at time of annulus formation
Observed age N 1 2 3 4 5 6 7 8 9 10
1 2 160
2 27 148 205
3 26 139 201 235
4 54 133 196 237 264
5 57 132 192 231 259 283
6 43 132 193 228 257 282 302
7 26 131 192 230 261 289 312 331
8 16 134 197 235 266 286 318 340 359
9 4 129 196 233 264 294 325 356 385 412
10 3 127 189 232 267 302 335 363 389 409 426
Number of calculations 258 256 229 203 149 92 49 23 7 3
Weighted means 135 196 233 261 285 310 338 367 411 426
Increment 135 61 37 28 24 25 28 30 43 15
36
Age (years)
Figure 14. Back-calculated (to last annulus) total lengths (mm) of lane snapper, this
study vs. Manooch and Mason (1984), not adjusted for BPH.
Table 5. Back-calculated total lengths (mm) of lane snapper aged by sectioned otoliths adjusted for BPH - north Florida.
Mean back - calculated total length at time of annulus formation
Observed Age N 1 2 3 4 5 6 7 8 9 W
2 9 153 225
3 51 157 222 270
4 82 155 220 274 316
5 80 148 204 256 302 338
6 31 149 205 253 297 336 365
7 11 159 216 265 301 339 373 402
8 8 149 202 245 281 312 344 379 408
9 3 146 205 248 285 318 355 389 426 461
10 2 127 190 234 272 301 325 353 379 409 437
Number of calculations 277 277 268 217 135 55 24 13 5 2
Weighted means 152 213 264 305 335 361 389 408 440 437
Increment 152 61 51 41 30 26 28 19 32 -3
U)
Table 6. Back-calculated total lengths (mm) of lane snapper aged by sectioned otoliths adjusted for BPH - south Florida.
Mean back - calculated total length at time of annulus formation
Observed Age N 1 2 3 4 5 6 7 8 9 10 11 12
1 72 131
2 154 161 227
3 411 159 220 262
4 228 152 209 250 277
5 109 153 209 248 277 300
6 65 150 201 239 268 292 313
7 21 154 202 233 255 274 292 307
8 7 141 197 232 254 270 285 300 315
9 4 128 181 210 233 251 264 278 291 306
10 2 146 194 238 263 286 310 331 349 364 378
11
12 1 137 205 228 254 272 289 312 328 351 367 379 397
Number of calculations 1074 1002 848 437 209 100 35 14 7 3 1 1
Weighted means 154 216 254 274 293 304 304 314 329 374 379 397
Increment 154 62 38 20 19 11 0 10 15 45 5 18
U)
00
39
O 1 1 1 1 1 1 1 \ 1 1 1 1
1 2 3 4 5 6 7 8 9 10 11 12
Age (years)
Figure 15. Mean back-calculated size at age for lane snapper, by region
present study corrected for body proportional hypothesis (BPH).
40
Lee ’s Phenomenon
Smaller size at a given age for fish captured at an older age was found, suggesting
size-selective mortality was present in both north Rorida and south Florida. Linear
regression of the measurements from the focus to the first annulus (Al), and to the
second annulus (A2) was used to test if the slope was significantly different from zero
(Figure 16). In north Rorida for Al, the difference was only slightly significant (n = 276,
p = 0.0977) and insignificant for south Florida (n = 1014, p = 0.8663). Comparing
regions by the same method for A2 resulted in a highly significant difference in the slope
of the regression for both regions. North Florida at the p<0.1 level was p = 0.0010, while
for south Florida at the same level of significance it was p = 0.0015. The slope of the
regression where it was significantly different was negative; i.e., decreasing size at ages
lor 2 with increasing age of fish.
Von Bertalanffy Growth Parameters
Theoretical growth parameters were compared for the present study to Manooch
and Mason (1984) and by regions in Florida. Both comparisons used the von Bertalanffy
(1938) growth equation, however when comparing studies, the back-calculated lengths at
age of Manooch and Mason (1984) did not adjust for BPH (Francis, 1990). Therefore,
data from the present study used back-calculated lengths without the BPH correction and
all measurements to annuli strictly or the purpose of comparison with Manooch and
Mason (1984) and should not be used in future analyses. The results showed theoretical
growth is greater from years 1-9 for the present study and similar in older fish (Figurel7).
41
A.
3.0 1
north Florida
south Florida
2.5 -
distance 1.5 i —I 1 1 1 1 1 I0 2 4 6 8 10 12 141i Age (years)3.0 n( 2.5 -2d'i^stan)ce) 2.0 - —north FloridaRing —B— south Florida
1.5 - -1 1 1 1 1 1 I
0 2 4 6 8 10 12 14
Age (years)
Figure 16. Lee's phenomenon comparison between regions, north Florida and south
Florida. A. Age 1 ring mesurements. B. Age 2 ring measurements.
42
Age (years)
Figure 17. Theoretical growth of lane snapper, this study vs. Manooch
and Mason (1984).
43
The following von Bertalanffy (1938) growth equations were obtained:
Lt = 501 (1-e Manooch and Mason (1984), and
Lt = 516 (1-e Present study (without BPH correction).
Because evidence of size-selective mortality for both regions was observed, only
back-calculations to the most recent annuli (Vaughan and Burton, 1994) were used for
estimating growth parameters of the von Bertalanffy (1938) growth equation. Results
from the analysis between regions showed a significant difference (Figure 18). The von
Bertalanffy (1938) growth equations using the corrected back-calculated lengths at age to
the last annulus were
Lt = 381.4 (1-e for all data combined;
L, = 443.9 (l-e ” ^®^‘^°^^) for north Florida, and
L, = 311.4 (1-e ^ “ ^'^) for south Florida.
These equations represent my best estimates of the theoretical growth curves for east
coast lane snapper. Significant difference was determined comparing the 95%
confidence intervals for north vs. south. North Florida had a range of 415 - 472 for Lq
for the 95% confidence interval, while south Florida had a range of 306 - 317 for Lg.
Since the confidence intervals are disjoint (do not overlap), these theoretical growth
curves can be considered significantly different.
Age - Length Key
Age - length keys were developed by grouping aged fish by total length in 25-
mm length intervals by age class, for north and south Florida. The percentage of fish by
age group was calculated for each length interval (Table 7). Age- length keys can be
44
Figure 18. Theoretical growth of lane snapper for the east coast of Florida,
by region (present study).
Table 7. Age - length key for lane snapper from south Florida (A.) and north Florida (B.). Total fish by age class (percent).
Age Yyears)
1 23456789 10 11 12
TL (mm)
A.
200 1 (6.7) 11 (73.3) 3 (20.0)
225 9 (8.9) 54 (53.5) 30 (29.7) 4 (4.0) 1 (1.0) 1 (1-0) 2 (2.0)
250 2 (0.6) 78 (24.4) 148 (46.3) 75 (23.4) 11 (3.4) 6 (1.9)
275 8 (2.5) 148(47.1) 88 (28.0) 40 (12.7) 18 (5.7) 9 (2.9) 2 (0.6) 1 (0.3)
300 2 (1.4) 55 (39.6) 35 (25.2) 23 (16.5) 14 (10.1) 5 (3.6) 3 (2.2) 1 (0.7) 1 (0.7)
325 2 (3.2) 18 (29.0) 16 (25.8) 16 (25.8) 7 (11.3) 2 (3.2) 1 (1.6)
350 10 (24.4) 7 17.1) 10 (24.4) 10 (24.4) 2 (4.9) 1 (2.4) 1 (2.4)
375 2 (20.0) 5 (50.0) 2 (20.0) 1 (10.0)
400 2 (15.4) 2 (15.4) 6 (46.2) 1 (7.7) 1 (7.7) 1 (7.7)
425 1 (50.0) 1 (50.0)
450
475 1 (100)
500
B.
200 1 (100)
225 1 (10.0) 9 (90.0)
250 6 (31.6) 9 (47.4) 3 (15.8) 1 (5.3)
275 1 (3.1) 17 (53.1) 9 (28.1) 4 (12.5) 1 (3.1)
300 8 (17.4) 22 (47.8) 14 (30.4) 2 (4.3)
325 4 (7.0) 26 (45.6) 21 (36.8) 4 (7.0) 1 (1.8) 1 (1.8)
350 2 (4.9) 10 (24.4) 15 (36.6) 9 (22.0) 2 (4.9) 3 (7.3)
375 2 (6.9) 8 (27.6) 14 (48.3) 4 (13.8) 1 (3.4)
400 4 (25.0) 7 (43.8) 3 (18.8) 2 (12.5)
425 3 (25.0) 6 (50.0) 1 (8.3) 1 (8.3) 1 (8.3)
450 1 12.5) 2 (25.0) 1 (12.5) 2 (25.0) 2 (25.0)
475 2 (100)
500 1 (33.3) 2 (66.7)
525
550 1 (100)
46
used to develop catch at age matrices derived from length frequency data, assuming age
samples were randomly sampled from fisheries. Age frequencies distributions from
north and south Florida were compared to determine when lane snapper recruit to the
fishery (Figure 19). Fish in south Florida were caught primarily at age 3 years old, while
in north Florida the trend shows the majority of fish are 4-5 years old.
Mortality
Natural mortality (M) was estimated and compared by region using various
equations (Table 8). The method used by Pauly (1980), which includes La and K, along
with mean annual seawater temperature, estimated M at 0.71 for north Florida and 0.69
for south Florida. Hoenig’s (1983) equation, which derives M using maximum age (t^ax),
yielded estimates of 0.42 and 0.35 for north and south Florida respectively. Ralston’s
(1987) method used K (Brody growth coefficient), and estimated M at 0.62 for north
Florida and 1.32 for south Florida. Finally, M was estimated using the method of
Alverson and Carney (1975), which uses K and the maximum age of the fish. The
estimates ofM for this equation were 0.42 and 0.11 for north and south Florida,
respectively.
My estimate ofM = 0.63, based on all data combined, was higher then Manooch
and Mason’s (1984) estimate ofM = 0.40, using the same method by Pauly (1980).
Estimates calculated from the equation by Hoenig (1983) compared more closely to the
previous study.
Estimates of total mortality (Z) were obtained by regressing the natural log of the
47
Age (years)
Figure 19. Age frequency distribution of lane snapper for the east
coast of Florida, by region (present study).
48
Table 8. Estimates of natural mortality (M) for lane snapper from the east coast of
Florida, by region.
Method North Florida South Florida Variables
Pauly (1980) 0.71 0.69 Loo, K, Mean seawater temperature
Hoenig(1983) 0.42 0.35 Max age
Ralston (1987) 0.62 1.32 K
A1verson and Carney (1975) 0.42 0.11 K and Max age
49
age frequency on age for fully-recruited fish from north and south Florida separately, and
all data combined. Lane snapper are fully recruited to the fishery at age 5 for north
Florida and age 4 for south Florida based on the age-length keys for each region.
Estimates for north Florida were lower (Z = 4.42) than south Florida (Z = 5.47); and the
estimate for all data combined was slightly higher than south Florida (Z = 5.65). These
estimates are preliminary results using the data in this study and do not include the
various components of a more comprehensive estimate for total mortality.
DISCUSSION
Lane snapper otoliths exhibit opaque zones which I have validated as annuli on
sagittal otolith sections. Annuli are deposited in late spring, primarily in June, when
otolith growth is slowest and the mean marginal increment distance is minimal. This is in
agreement with recent findings for lane snapper from southeast Florida (Acosta, personal
communication, Florida Fish and Wildlife Conservation Commission, Marathon,
Florida). For comparison. Gray snapper, Lutjanus griseus, from the same geographic
range, lay down an annular mark during the same time of year (Burton, 2001). Biological
and environmental factors such as temperature, food availability, maturity and other
causes may affect time of annulus formation. These factors may also create checks or
false annuli, adding uncertainty in age determination. Nevertheless, sectioned otoliths
remain the preferred structure for assigning ages to many species of fish.
Beamish and McFarlane (1983) strongly recommend validation for all ages in any
age determination study. Young-of-year (0-1) are not readily available for most age and
growth studies where samples are obtained from fisheries, making validation of the first
annuli very difficult. However, my study validated first annulus formation using fishery-
independent samples from Florida Bay (n = 111). Marked-recapture and rearing
juveniles are methods that can also accomplish validation of the first annuli. Mark-
recapture methods have associated problems from handling that may affect growth and
survival. Validation using controlled rearing has been attempted most recently on red
porgy Pagrus pagrus, and black sea bass Centropristis striata (James Morris, personal
communication. National Marine Fisheries Service, Beaufort, North Carolina). Although
51
this method has merit, the growth rate of most fish under confined conditions is difficult
to relate to the natural population (Manooch, 1987).
The relationship between weight and length was consistent with other studies that
showed weight increased exponentially with increased length (Grimes 1978; and Garcia
et al., 2003). Comparing the weight -length relationship by regions showed a similar
increase based on a large sample size for Florida’s east coast. Lane snapper from north
Florida were found to be slightly heavier, which may be related to genetics, metabolic
rate, primary food production, water temperature, or other environmental differences.
Mean observed and back-calculated sizes at age for 2-7 year old lane snapper
were somewhat larger for my study compared to findings from 20 years ago (Manooch
and Mason 1984). Mean back-calculated lengths from Manooch and Mason (1984) for
ages 1-5 were 135, 196, 233, 261, and 285 mm TL, respectively, were smaller than my
study for the same ages 157, 218, 259, 287, and 313 mm TL. Parameter estimates of K
and Loo from the two studies using the same method were similar. My study had a
slightly greater theoretical asymptotic length (L» = 516) than Manooch and Mason
(1984), Loo = 501, and the Brody growth coefficient (K) was somewhat lower for my
study, K = 0.10, in relation to Manooch and Mason (1984) who reported K = 0.13.
Collectively the results of this comparison based on the same geographic area and ageing
method would suggest that size and bag limits implemented in 1983 may have had the
desired effect for the east coast of Florida. These regulations are intended to increase
yields and allow smaller fish the opportunity to attain larger size at age over their life
span. Bag limits may have had a positive effect as well with under size fish being
52
released, and thus theoretically able to contribute to the stock over time.
Conversely, separating Florida into two regions led to the observation that
management success was not as apparent as when the entire east coast of Florida was
examined. Differences by region were significant for size at age, and other growth
characteristics, such as L» and K. Noteworthy is that this study is the first to report these
latitudinal differences for lane snapper. It is similar to other studies, however, in that fish
from north Florida are typically larger than are those from south Florida (Manooch and
Matheson, 1981; Burton, 2001; Potts and Manooch, 2001).
Back-calculated lengths at age were greater for north Florida using constant
proportionality of observed fish length to predicted fish length (BPH). I choose to use the
body proportional hypothesis (BPH) method which is more widely used rather then the
scale (otolith) proportional hypothesis (SPH), also described by Francis (1990). The SPH
method assumes constant proportionality of otolith radius (OR) to predicted OR (Francis,
1990) for back-calculations.
The lack of older fish from both regions may have influenced these findings,
most notable, theoretical growth. The K (Brody growth coefficient) values for north and
south Florida are 0.30 and 0.63, respectively. The south Florida K value is higher then
previously reported for lane snapper, with published ranges of K from 0.126 - 0.530
(Claro and Reshetnikov, 1981; Manooch and Mason, 1984; Manickchand-Dass, 1987;
Acosta and Appeldoom, 1992; Claro et. al., 2001). The differences in values are
attributed to the method used to derive the von Bertalanffy equation, this is evident when
comparing values from the Manooch and Mason (1984) study to the present study. The
53
von Bertalanffy equation resulted in K values considerably lower, 0.13 for Manooch and
Mason (1984) and 0.10 for the present study. In general, growth estimates for other
lutjanids show high variability throughout the south Atlantic, Gulf of Mexico, and
Caribbean with published ranges for K of 0.078 - 0.70 (Pauly, 1980; Manooch, 1987;
Claro et. al., 2001). Advances in methodology, i.e. BPH, and equipment for reading
otoliths, which were used in this study may also account for variation between
contemporary studies and earlier studies.
Another explanation for the differences in growth estimates in this study is that
fishing pressure is greater in south Florida. Studies by Manooch and Matheson (1981),
and Burton (2001) show higher estimates of fishing mortality (F) on gray snapper for all
fisheries for south Florida compared to north Florida. This was attributed to the fact that
population density is greater on the southeast coast of Florida as opposed to northeast
Florida, and access to the fishing grounds is much easier (Burton, 2001). In the
Caribbean, Claro (1981) reported the oldest lane snapper from the Golfo de Batabano,
Cuba to be 6 years old and attributes the lack of older fish to extensive fishing pressure.
Although older fish (> 6 years old) were reported in my study, the frequency of
occurrence was low, making up only 11 % (n = 156) of the total fish aged (n = 1396). It
is this author’s opinion, that fishing pressure in Florida is the primary reason older fish
are not better represented in this study and population, since fishing gear, primarily hook
and line, does not exclude or reduce capture of larger (older) fish.
Compounding this issue is the possibility that younger and faster growing fish
from the south appear to be recruiting to the fishery sooner than the north. This would
54
result in size selective mortality. Furthermore, to the extent that growth is a heritable trait,
harvesting the faster growers could result in a shift in the stock toward slow-growing
individuals (Goodyear, 1996). Although Lee’s phenomenon was present in north Florida,
asymptotic size was considerably higher (443.9) compared to south Florida where L» is
much lower (311.4). Lane snapper from south Florida exhibited rapid growth from 0-2
years old, but reached asymptotic length earlier then fish from north Florida. Asymptotic
length was still lower in the recent study by Acosta (personal communication, Florida
Fish and Wildlife Conservation Commission, Marathon, Florida) which reported
Leo = 268.4 for lane snapper in the Florida Keys. Pauly (1980) contends that asymptotic
lengths (Lod) of fish from cooler regions are larger and the growth coefficient (K) is lower
than those of fish from warmer habitats. This supports my findings and the differences
between regions using growth estimates for north and south Florida. These distinct
differences between regions could be characterized as separate stocks within this
population.
Mortality estimates for this study were made with aged samples under the
assumption that sampling methods were unbiased. If this is not the case, this may lead to
bias in my results. Another factor that must be considered is the obvious differences
between regions of Florida. For this reason, it is my recommendation that mortality
estimates be developed from a more complete consideration of fisheries landings, length
frequency sampling, and application of my age-length keys (Table 7). Estimates of total
mortality that are estimated in a stock assessment for this species, must include the
landings data for all fisheries, generally by year, area and gear type. Area-specific
55
age - length keys are combined with length frequency to calculate the percentage of fish
at age for each fishery and year. This proportion at age multipled by catch in number
would give catch in numbers at age for each fishery and year. Total catch added across
fisheries for a given year would provide total catch in numbers at age for all fisheries,
otherwise known as a catch matrix. Catch curves based on cohorts can be used to derive
instantaneous total mortality (Z), or more complex virtual population analysis (VPA).
In conclusion, this is the most current and comprehensive study on lane snapper
for the east coast of Florida since 1984. I have demonstrated regional differences in
growth characteristics in Florida that must be considered in future stock assessments and
management alternatives. Although the population of lane snapper on the east coast
appears healthy, the evidence presented in this study suggests that the more heavily
fished regions may require a review of management policies in those fisheries. Increased
size limits may be appropriate to address size selectivity in these areas. The minimum
size of 8 inches TL (203 mm TL), should be increased to 10 inches TL (254 mm TL) in
south Florida to reduce the problem of smaller, faster growing fish entering the fishery.
Since lane snapper are recruiting to the fishery at an older age and larger size presently in
north Florida, this increase may not be necessary in that region. Tagging and genetic
studies should be completed before any final decision. Finally, analyses should also
include comprehensive mortality estimates and VPAs for each region, with management
recommendations based on these results. In the future, managers should consider a
regional approach to management issues in Florida, and other areas with increased
demands on stocks.
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