Influence of illumination and temperature upon flatfish reactivity and herding behavior: Potential implications for trawl capture efficiency

For both survey and commercial trawls, selection of flatfish species and sizes is strongly influenced by fish behavioral reactions to the ground-gear. Herding begins with initial disturbance, where flatfish are flushed from the sediment by ground-gear. Yet this initial behavioral reaction is poorly understood, especially relative to effects of environmental parameters such as light and temperature. The goal of this study was to examine disturbance and herding behavior in English sole, northern rock sole and Pacific halibut using a simulated trawl footrope, in both light and darkness at 9 °C, and additionally, for rock sole, at 2 °C. In the light, all species exhibited a disturbance response consistent with the initiation of herding; directed swimming in close proximity to the substrate. This response was diminished in darkness, with fish hopping or slowly rising off the bottom, such that the footrope quickly passed beneath them (precluding herding). Low temperature also influenced rock sole behavior, increasing the likelihood that fish would simply hop off the bottom, rather than initiate herding. These results help lay a behavioral framework for understanding such phenomenon as diel and seasonal variations in flatfish catches and fish size composition, and why diel patterns for survey and commercial trawls often differ. Further, difference in behavior between rock sole and Pacific halibut suggest possible avenues of reducing halibut bycatch at night.