Blue marlin mean length: simulated response to increasing fishing mortality

Mean length of caught fish often declines noticeably with increased fishing mortality and this trend is regarded as an indicator of excessive fishing mortality. The most recent stock assessment results for Atlantic blue marlins and Atlantic white marlins indicate that these stocks are heavily overfished. However, available data do not suggest a strong trend of decreasing lengths of marlins in the catch. The lack of such a trend has been used to argue that the assessment results are in error. This study uses population simulation to characterize the expected response of the size distribution of blue marlin to the vector of fishing mortality and F-MSY estimated for the base case in the most recent stock assessment, and contrasts the results with a similar analysis for Atlantic swordfish. The results indicate that blue marlin should not be expected to exhibit strong trends in mean length with respect to fishing mortality within the range of fishing mortality estimated in the most recent stock assessment, and that the trends in the observed size composition are consistent with the findings of the stock assessment. The findings indicate that declining mean length is not a necessary outcome of excessive fishing mortality in all species.