Shifts in Open-ocean Fish Communities Coinciding with the Commencement of Commercial Fishing

We identify changes in the pelagic fish community of the tropical Pacific Ocean by comparing recent data collected by observers on longline fishing vessels with data from a 1950s scientific survey when industrial fishing commenced. A major shift in size composition and indices of species abundance and community biomass accompanied the start of fishing. The largest and most abundant predators, such as sharks and large tunas, suffered the greatest declines in abundance (21% on average). They also showed striking reductions in mean body mass. For example, the mean mass of blue shark (Prionace glauca) was 52 kg in the 1950s compared to 22 kg in the 1990s. The estimated abundance of this species was 13% of that in the 1950s. Overall, the biomass of large predators fell by a factor of 10 between the periods. By contrast, several small and formerly rare species increased in abundance, e.g., pelagic stingray (Dasyatis violacea). However, the increases in small species did not balance the reductions in the biomass of large predators. Of three possible explanations (fishing, environmental variation, and sampling bias), available evidence indicates fishing to be the most likely cause for the observed patterns.