A simulation study of krill fishing by an individual Japanese trawler

A model is set up for the operation (which includes both searching and fishing) of a Japanese krill trawler over a half-month period, and the output is compared to statistics from a sample of data from the commercial fishery. Twenty-four candidate "CPUE" biomass indices are considered, and their performance investigated in response to six different ways in which the overall krill biomass in a 600 n. mile square oceanic sector might decline by 50%. In most cases there is essentially no response of the index, or a response rather smaller in relative magnitude than the biomass decline. Catch statistics collected at present (centred primarily on catch per fishing time) are of low utility in detecting biomass declines. Indices incorporating search time information may give improved performances, but would probably be reliable only for detecting changes in within-concentration krill distribution parameters. Other approaches (e.g. research vessel surveys) need to be considered to monitor changes in the number, distribution and density of krill concentrations. Priority for further analyses should be given to examination of existing scientific krill survey data to improve the krill distribution model underlying this analysis (this might perhaps be appropriately effected by constituting and organising a meeting of a small joint CCAMLR-BIOMASS Working Group). This is because of a major discrepancy between the typical length of Japanese tows (generally through a single swarm) which is much larger than the typical size of krill swarms recorded in scientific surveys; this discrepancy needs to be resolved before greater confidence will warrant being placed in the results from a simulation model of the fishery.