Monte Carlo simulation modelling of purse seine catches of silky and oceanic whitetip sharks

This paper applies Monte Carlo simulation modelling to investigate the impacts of purse seine fisheries on silky and oceanic whitetip sharks under different scenarios of fishing effort on free schools and FAD- associated schools. Two scenarios were explored: the redistribution of effort on FADs to free schools; and, redistribution of free school sets to FAD sets. Probability distributions for school association and species specific shark presence were generated through standardisation of purse seine observer data. It was not possible to fit robust statistical models to non-zero catches. Therefore values were based upon the mean and standard deviation of non-zero catches identified by observers. Redistribution of effort from FADs to free schools resulted in substantial reductions in estimated catches of silky shark (by 83%) and oceanic whitetip shark (by 57%) compared to the ‘status quo’. There was large uncertainty in total catch estimates due to low confidence in assumed estimates of non-zero shark catches.