Inferring shark population trends from generalized linear mixed models of pelagic longline catch and effort data

We estimate recent (1992–2005) trends in relative abundance for Northwest Atlantic oceanic and large coastal sharks, using generalized linear mixed models to standardize catch rates of eight species groups as recorded by U.S. pelagic longline fishery observers. Models suggest precipitous (76%) declines in hammerhead (Sphyrna species) and large coastal (dusky, night, and silky shark, genus Carcharhinus) species, and moderate declines (53%) in blue and oceanic whitetip sharks over this period. In contrast, mako and thresher sharks appear to have stabilized, and the tiger shark population appears to be increasing. A comparison of nominal shark catch rates from this fleet's observer and logbook data (to evaluate the veracity of trends previously estimated from the latter) showed a high degree of concordance for each species group, both in individual sub-areas and overall. Models of these two datasets for the common time period (1992–2000) show that compared to the observer data the logbook data indicate greater declines for some species, but lesser declines for others. Signs of recovery for some shark species are encouraging, but must also be set in the context of the significant declines that occurred in previous decades.