The quantification of perceptual and behavioural variation in commercial fishing logbooks
Harris, Lei-Wun Emily
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Reliable data are vital to the successful management of a fishery. Commercial fishing logbooks are a valuable source of data, yet may be inaccurate due to data collection methodology or fishing strategy. As a result, these data are currently under-used in research and in management. Potential biases in the logbook data need to be identified and quantified so that these data can be analysed with greater confidence. I examined the records of catches of cod, haddock, and pollock in commercial logbooks from offshore bottom trawlers on the Scotian Shelf for inaccuracies due to both the perception of a haul and the effect of the presence of an observer. Typically, a captain underestimated the catch and frequently did not report small catches of all three species, but particularly cod and haddock. The probability that a captain reported these smaller quantities of cod and haddock could be predicted by the amount of that species and the amount of the other two species in the catch. A comparison of observed and unobserved trips on the same vessels revealed that captains tended to fish at different positions or depths when monitored. In addition, hauls containing cod were recorded more frequently but hauls containing pollock were less commonly recorded on trips that were not observed. The resultant total landings from unobserved trips were more valuable. I was unable to distinguish between targeting and hi h-grading as the mechanism to increase the proportion of desired species in the catch.