GILLNET CALIBRATION FOR SPINY DOGFISH ABUNDANCE ASSESSMENT

Abstract

Spiny dogfish (Squalus acanthias) is one of the most abundant species in the Northwest Atlantic however; the stocks collapsed in the late 1990s and were declared overfished April 3, 1998 by the Atlantic States Marine Fisheries Commission. Spiny dogfish have recently been declared recovered well ahead of the 15 to 20 year recovery time estimated by Atlantic States Marine Fisheries Commission, resulting in the methods of the Northeast Fishery Science Center's trawl survey to be questioned. Gillnets are widely used for fishing operations and assessments. This project was undertaken to determine the feasibility and estimate parameters for using gillnets to estimate abundances. The goals of this study were to determine the percentage of spiny dogfish vertically distributed above the fishing height of the NEFSC calibrated trawl survey and determine the catchability of spiny dogfish in a gillnet. The catchability of spiny dogfish in a gillnet was determined by multi-pass depletion sampling inside a corral net. The maximum efficiency was 8.6% with a 95% confidence interval of 5.8% to 11.3% during a 10 minute soak time. The minimum catch efficiency was 1.6% with a 95% confidence interval of 0.9% to 2.2%. Recalculated population estimates from a previous study using gillnets produced estimates between 3.87 x 10⁶ and 9.09 x 10⁶ metric tons of mature females, depending on the method of estimating catch efficiency used for coastal North Carolina waters. Catch efficiency was not affected by dogfish being tagged; tagged dogfish are captured at the same rate as untagged. A vertically elongated gillnet was used to estimate that the NEFSC is missing a minimum of 21.8 % (SD = 25.1%, Variance = 6.3) of the dogfish vertically suspended in the water column depending on water depth. The findings of this study indicate that a large number of spiny dogfish may not be adequately sampled by the NEFSC calibrated trawl survey and supplemental sampling methods should be considered.