The effect of acoustic deterrent devices on bottlenose dolphin depredation in the Spanish mackerel gillnet fishery

Abstract

Bottlenose dolphins (Tursiops truncatus) reduce catches of Spanish mackerel (Scomberomorus maculatus) when they interact with coastal gillnets off Hatteras, North Carolina. Acoustic alarms, or pingers, are one means of potentially reducing these adverse interactions. We measured the effect of Save Wave alarms on the incidence of dolphin depredation and target fish catches. The study had two goals: (1) to quantify the effect of alarms on target species catch and (2) to quantify the effect of alarms on dolphin depredation. Fieldwork was conducted off Hatteras during June and August 2004. During this period, we attached alarms to 118 Spanish mackerel gillnet sets (63 active and 55 control). All catches were standardized for effort by incorporating measures of net length and soak duration. We collected data on total catch-per-unit effort (CPUE), Spanish mackerel CPUE, counts of fish discards, and evidence of depredation. There was no difference in total CPUE (p = 0.25) or Spanish mackerel CPUE (p = 0.94) between active and control sets. Net position (inshore, middle, offshore) also had no effect on total CPUE (p = 0.48) or Spanish mackerel CPUE (p = 0.35). We observed several dolphins interacting with nets with active alarms, but did not observe a sufficient amount of dolphin depredation to quantify their response to the Save Wave alarms. These results suggest that Save Wave alarms do not affect fish catches. Fieldwork for this project will continue during the summer of 2005 to determine whether or not the alarms reduce dolphin depredation.