Fish Size and Exposure to Air: Potential Effects on Behavioral Impairment and Mortality Rates in Discarded Sablefish

—Fisheries models often assume that discarded undersized fish and target species will survive and contribute to future recruitment and yield. If smaller fish are more susceptible to capture stressors than larger fish, then the assumption that smaller discards would contribute to recruitment may not be true. We tested the hypothesis that small sablefish Anoplopoma fimbria show more behavioral impairment and mortality than large fish when exposed to air (10–60 min) at various temperatures (10–188C). Sablefish captured by trawl, longline, or trap are commonly exposed to these conditions during warmer seasons when brought up on deck and sorted. Two size-classes of fish (small: 32–49 cm total length [TL]; large: 50–67 cm TL) were used in the experiments. Behavior was measured as upright orientation and startle responses to visual and mechanical stimuli 1, 2, 3, and 24 h after air exposure; mortality was measured through 7 d after air exposure. Small fish mortality increased as air time increased and was at higher levels than in large fish. Only 10 min of air exposure caused behavioral impairment in small and large fish, which could lead to increased predation on discarded fish. At 24 h after air exposure, normal behavior had not generally resumed and small fish had more behavioral impairment than large fish. Size-related mortality and its causes in discarded fish, with smaller fish showing greater mortality than larger fish, is an important factor to consider in models of fisheries recruitment and yield. Fisheries management often relies on minimum size rules for harvest, with the assumption that discarding of undersized fish will contribute to future recruitment and yield (Halliday and Pinhorn 2002). Target species may also be subjected to ‘‘highgrading’’ in which smaller fish are discarded for economic reasons. If mortality rates of discarded smaller fish are high, the benefits in future growth or reproductive potential are thwarted (Pinhorn and Halliday 2001). While size-related mortality has not been investigated in sablefish Anoplopoma fimbria, smaller fish of other species have been observed to have higher discard mortality rates than larger fish under field conditions (Neilson et al. 1989; Richards et al. 1995; Milliken et al. 1999; Parker et al. 2003) and laboratory conditions (Davis and Olla 2002). Size-related behavioral impairment in discards, in which normal behavior is decreased or completely inhibited, has not been studied in any detail and may result in * Corresponding author: [email protected] Received February 12, 2003; accepted July 28, 2003 predation becoming a significant source of delayed