Is bycatch a big problem for small fish? Assessing and addressing the impacts of tropical shrimp trawling on small fish species.

My research, focused on the industrial shrimp fishery in southern Gulf of California, aimed to determine whether tropical shrimp trawling is a problem for viability of incidentally captured small fish species. My first objective was to use life history information to evaluate possible impacts on four small fish species from their incidental capture. I applied length based indicators and qualitative criteria to information on captured sizes, reproduction, and distributions across the study site. My results suggested potential for overfishing silver stardrum and bigscale goatfish, largely because most sampled individuals were immature. Silver stardrum may be particularly affected because its occurrence and density declined during the trawling season whereas goatfish apparently recruited to the study area. Most sampled sandperch were mature, suggesting greater resilience to trawling. In contrast, sampled lumptail searobin, although mature, had not yet spawned, indicating potential for adverse fishing effects. Because human behaviour affects the success of fisheries management, my second objective was to shed light on the social dimensions of tropical shrimp fisheries management. My interviews with industrial trawl fishers suggested that proper enforcement and reliable governance are essential for a sustainable fishery. If enforcement were strong, then most fishers would support trawl free areas. The effort data I gathered point to areas where protection might be socially acceptable. My third objective considered the biological appropriateness of trawl closures for small fishes. The divergent distributions of bigscale goatfish and silver stardrum, just two of many small species in bycatch, implied that trawl restrictions would have to cover many depths and latitudes. Further, although my matrix model was of limited use for assessing population status of silver stardrum, it clearly indicated that precautionary management should focus on increasing survival of younger fish. This could be achieved with trawl closures where smaller fish live. While the approaches I used identified small fishes that might be vulnerable to trawling, they are too data intensive to be viable for the hundreds of such species in bycatch, and too inconclusive to confirm impact. It may be necessary to apply precautionary methods such as trawl closures to avoid potential effects of indiscriminate trawling.

Foster, S.J. (2010). Is bycatch a big problem for small fish? Assessing and addressing the impacts of tropical shrimp trawling on small fish species. Doctoral dissertation, University of British Columbia, Canada.