Effects of Land Use Change on Juvenile Fishes, Blue Crab, and Brown Shrimp Abundance in the Estuarine Nursery Habitats of North Carolina

Abstract

The U.S. coastal region is home to more than half of the American population. Also, the coastal counties' population is growing much faster than that of inland counties. With a high density and an increasing population, there are rapid changes in land-use and land-cover (LULC) types, characterized mostly by the increase in areal coverage of anthropogenic land uses (agriculture and developed), while "natural/unaltered" land uses (forest and wetland) are in decline. The human population growth and land-use and land-cover changes caused by humans generate nutrients and pollutants to coastal waters, which can affect water quality and aquatic life. Trends in the land-use and land-cover changes that might impact fish and invertebrate species abundance at 71 selected estuarine stations sampled by the North Carolina Division of Marine Fisheries (NCDMF) juvenile sampling program (Program 120) were analyzed. Land use categories of interest were forest, wetland, agriculture, and developed areas. The selected fish and invertebrate species were: Atlantic croaker (Micropogonias undulatus), Atlantic menhaden (Brevoortia tyrannus), pinfish (Lagodon rhomboides), southern flounder (Paralichthys lethostigma), spot (Leiostomus xanthurus), blue crab (Callinectes sapidus), and brown shrimp (Farfantepenaeus aztectus). Geographic information system (GIS) data, remotely sensed data and statistical techniques were used to quantify the LULC type changes between 1980 and 2000 within the immediate coastal watersheds of North Carolina. Forest has been the most affected, losing about 30.1% of its total area to the increase (~24.1%) of agriculture area. The wetland and developed land use varied depending on location, but their overall changes were small when the whole study area was considered. The long-term trends in abundance of juveniles of selected fish and invertebrate species indicated declines at certain sampling stations, and increases at others. In order to determine whether land use changes were correlated with changes in the selected species, and also to find which other factors might influence changes in their abundance, I analyzed seven predictor variables [(1) percent land use change within local catchments centered on the NCDMF sampling sites, (2) number of pollution point sources in large USDA Natural Resources Soil Conservation watersheds, (3) number of people in US Census tracts within watersheds, (4) water temperature, (5) water salinity, (6) station depth, and (7) distance to inlet (minimum distance by water to an ocean inlet) for each NCDMF juvenile fish and invertebrate trawl sampling program station] in a classification and regression tree statistical analysis to predict normalized change in trawl catch for the selected species in NCDMF Program 120 data between 1980 and 2004. Land use changes were found to be influential to the number of blue crab, southern founder and Atlantic croaker, and declines were observed at 47 stations when land use changes were greater than 13% (blue crab), at 30 stations when land use changes were greater than 21% (southern flounder), and 6 stations when land use changes were greater than 53% (Atlantic croaker). Water salinity was found to be more important than land use change for southern flounder catch, and increased catch was observed at stations with salinity < 14 ppt. No significant changes due to land use could be associated with changes in Program 120 trawl catch of brown shrimp, Atlantic menhaden, pinfish or spot. There was a long-term increase in pinfish in the Program 120 data. Pinfish increased most at stations where bottom temperature was < 25 C. Atlantic menhaden declined at stations where bottom temperature was < 24 [degrees]C. Brown shrimp abundance was increased at stations where the distance to inlet was > 21 km and highest when salinity was > 14 ppt. Spot showed an increase in abundance when distance to inlet was greater than 42 km and the human population was > 883 people/census tract in the year 2000. Land use change impacts were observed in the classification and regression tree analysis for blue crab at 66% of the NCDMF stations, 42% of stations for southern flounder, and 8% of stations for Atlantic croaker. These three species were ranked first, second, and fourth in commercial value in North Carolina fisheries, and were the only species of those selected for this study that were considered to be overfished in 2000 by the NCDMF. Thus, land use change had the greatest impact on species that were targets of intensive commercial fishing and had low adult spawning stock. This result suggests an interaction between commercial fisheries harvest and land development for agriculture along the coast. Recruitment of these species may have been low because of reduced spawning stock due to commercial harvests, and this reduced recruitment was most noticeable where land use changes were high. Species that were not intensively harvested (pinfish) or had stable adult stock sizes as determined by the NCDMF (brown shrimp, Atlantic menhaden, and spot) were able to produce many recruits, and this high recruitment may have allowed colonization of areas with marginal habitat due to land use changes. Few stations showed declines in abundance of the juvenile stages of these latter species, suggesting that land use change was not a significant factor between 1980 and 2004. However, post-recruitment mortality (survivorship after June throughout the summer and fall) was not monitored by NCDMF and should be studied in the future. The results of this study serve as an early warning to coastal managers regarding the potential impact of coastal land use changes. The abundance and growth of these valuable fishes and invertebrates in North Carolina estuarine nursery habitats could be reduced in the future, given the national trends in coastal development and fishery harvests.