11. Population Estimates of Spiny Dogfish Aggregations Overwintering South of Cape Hatteras, North Carolina, Using an Area Density Method
Roger A. Rulifson and Tina M. Moore
Abstract.—Little information is available about the coastal distribution of spiny dogfish Squalus acanthias south of Cape Hatteras, North Carolina, and whether these fish are an extension of the population that overwinters in continental shelf waters off the North Carolina Outer Banks north of Cape Hatteras, or a separate population that remains south of Cape Hatteras. A coastal roaming survey was conducted in February and March 1999 from south of Cape Hatteras to the South Carolina state line to estimate the number of dogfish in coastal waters. Fish aggregations were located by sonar, and a commercial-grade sink gill net of seven different mesh sizes was deployed in waters to assess whether the aggregations were dogfish. Six large dogfish aggregations were located in shallow (10–16 m) coastal waters of Raleigh Bay, Onslow Bay, and Long Bay, covering an estimated surface area of about 66,922 ha. Two additional sets marked by sonar were not dogfish aggregations. No dogfish were caught in exploratory deepwater sets (46–55 m). Using a sensitivity analysis, total population size of all aggregations was estimated at 1.102 to 2.223 million individuals or 2.470 to 4.984 million kg. The sex ratio was 27.1:1 females to males. Aggregations were located near the bottom at a temperature range of 10.4°C to 15.7°C. Temperatures varied little vertically through the water column; laterally temperatures varied by less than 1°C for five of six aggregations. The largest aggregation, in Raleigh Bay, was exposed to the greatest spatial variability in temperature (3.6°C across 15,135 ha). This is perhaps a result of its proximity to the Gulf Stream at this time of year. We believe that dogfish south of Cape Hatteras during the winter are a small portion, probably less than 1%, of the U.S. migratory stock.