Using distribution patterns of small fishes to assess small fish by-catch in tropical shrimp trawl fisheries

Ecologically sound fisheries management and improving future food security require that small fish by-catch in tropical shrimp trawl fisheries is maintained at, or reduced to, sustainable levels; restricting trawling in particular places, or at particular times, has been suggested as a means for achieving this goal. The purpose of our research was to compare patterns in occurrence, density and body size across depth, latitude and time for four small fish taxa caught as by-catch in the southern Gulf of California shrimp trawl fishery: Diplectrum spp., Prionotus stephanophrys, Pseudupeneus grandisquamis and Stellifer illecebrosus. We then used these results to explore the potential for trawler impacts on these taxa, and the possible placement and timing of fishing restrictions to mitigate potential impacts. Our results confirmed, however, the difficulties of regulating such fisheries for multiple by-catch species, in that their distribution patterns varied in a way that precludes a 'one size fits all' solution. The four taxa analysed - only four of the hundreds obtained as by-catch in this fishery - exhibited distribution patterns at odds with one another. Observed intra- and inter-taxon variations in the relative importance of different spatial and temporal variables in determining occurrence, density and size argues that several permanent trawl closures covering a range of depths and latitudes, and not temporal ones, might be required to mitigate potential trawl impacts on these fishes. Our results also suggested a higher potential for impact on S.illecebrosus than the other taxa: occurrence and density of the former declined, whereas occurrence or density of the others increased across the study area as the fishing season progressed.