Vertical habitat utilization by large pelagic animals: a quantitative framework and numerical method for use with pop-up satellite tag data

A quantitative framework and numerical methodology were developed to characterize vertical habitat utilization by large pelagic animals and to estimate the probability of their capture by certain types of fishing gear. Described are the steps involved to build 'vertical habitat envelopes' from data recovered from an electronically tagged blue marlin (Makaira nigricans) as well as from a longline fishing gear experiment employing temperature-depth recording devices. The resulting vertical habitat envelopes, which integrate depth and temperature preferences of tagged fish, are conducive for comparative studies of animal behavior and for calculation (and visualization) of degrees of overlap - be it among individuals, species or fishing gear. Results of a computer simulation evaluation indicated our numerical procedure to be reliable for estimating vertical habitat use from data summaries. The approach appears to have utility for examining pelagic longline fishing impacts on both target and non-target species and could point to ways of reducing bycatch via modification of fishing strategy or gear configuration.