Behavioral impairment after escape from trawl codends may not be limited to fragile fish species

摘要

Field studies indicate great variability between fish species in their susceptibility to direct mortality resulting from stress incurred during entrainment and escape through trawl codends. Moreover, stressors that do not directly kill fish may still cause indirect mortality, such as behavioral impairment leading to predation. However, it is unknown whether resistance to direct mortality also imparts resistance to behavioral impairment. Juvenile sablefish Anoplopoma fimbria are more resistant to direct mortality resulting from physical damage and stress than are walleye pollock Theragra chalcogramma. We measured juvenile sablefish resistance to behavioral impairment resulting from simulated trawl passage and compared results to those for walleye pollock, a species already studied. Juvenile sablefish (18-27 cm) were subjected to three levels of simulated trawl/escape stressors in the laboratory: (1) control: no stressors; (2) swim: forced swimming for 90 min at 0.33 m s(-1) in a towed net, followed by escape through 8 cm square mesh; and (3) swim/pin: forced swimming for 60 min, then pinning against net meshes for 30 min, followed by 3 min crowding prior to escape. Subsequently, we examined sablefish behavior in the presence of a threatening but non-lethal predator (38-49 cm sablefish). In a second experiment, equal numbers of trawl-stressed and control fish were mixed and exposed to predation by a lingcod Ophiodon elongatus (62-87 cm). The first experiment demonstrated that sablefish suffered the same behavioral impairments as walleye Pollock: stressed fish swam slower, shoaled less cohesively and allowed the predator to approach closer than did controls. In the second experiment all three levels of trawl stress caused fish to be consumed in greater numbers (by lingcod) than control fish, again, like walleye pollock. Therefore, although differing in susceptibility to potentially lethal stressors, both species exhibited similar impairments in response to sub-lethal stressors. This suggests that for numerous fish species, behaviorally impaired individuals escaping codends may be consumed by predators, contributing to unobserved mortality