Food and feeding habits of the spiny dogfish Squalus acanthias overwintering off the coast of North Carolina and the effects on the marine community

Abstract

Spiny dogfish (Squalus acanthias) are small, highly migratory sharks that occur in large numbers in North Carolina nearshore waters between November and March. This species has long been considered a pest by commercial fishermen, and is suspected of being a major source of predation mortality for economically-valuable species. The goals of this thesis research were 1.) to assess the efficiency of a non-lethal method for collecting stomach contents from dogfish, 2.) to determine if ontogeny, sex, and habitat selection influence the dogfish diet, 3.) to identify important prey species for dogfish overwintering off of North Carolina, and 4.) to describe the predatory and competitive interactions between spiny dogfish and another piscivore (striped bass, Moronoe saxatilis) during the overwintering period. To accomplish this, 399 spiny dogfish were captured in North Carolina waters during research bottom trawl surveys in February and March, 2010. Size and sex data were recorded for each dogfish, as well as depth, salinity, and temperature data at each station. Stomach contents were sampled by either dissection or stomach tube gastric lavage, in which an acrylic tube was inserted through the esophagus and flushed with water. Prey items were identified to the lowest possible taxa and quantified in terms of number, weight, and frequency of occurrence, then importance was determined by calculating the Index of Relative Importance for each taxa. Consumption during the sampling period was estimated using estimates of daily ration from previous studies. Stomach tube lavage proved to be efficient, and tube diameters within 10-20 mm of the shark’s mouth width were nearly 100% efficient. Spiny dogfish showed significant differences in habitat selection by sex and size: females occupied significantly shallower, less saline, and cooler water than males, and dogfish began utilizing shallower depths between 600-650 mm total length (TL). The dogfish diet also shifted from an invertebrate-based to a teleost fish-based diet between 650-700 mm TL. Atlantic menhaden, (Brevoortia tyrannus) and bay anchovy (Anchoa mitchili) were the most important prey for dogfish sampled in this survey, though menhaden only dominated the diet in February. Dogfish predation may account for 14.08-3.56% of menhaden landings. Spiny dogfish and striped bass showed high spatial and potential dietary overlap, and dogfish consumed the equivalent of less than 0.91% of the striped bass stock biomass. Dogfish and striped bass are potential intraguild predators, but this relationship does not appear to affect the abundance and distribution of either species.