Bycatch-susceptible diving birds suffer, while surface-feeders thrive

Fishing gear causes the deaths of many non-target species (“bycatch”) each year. While conservationists assume that these mortality rates lead to decreases of entire populations of impacted animals, data deficiencies have made it difficult to study this directly—until now, that is.

By taking advantage of a United Nations moratorium on high seas driftnet fishing, a group of Canadian conservationists has been able to assess the effects of gillnets on north Atlantic seabird populations. The research team obtained data on fishing effort both before and after the ban, which was initiated in 1992. This allowed them to calculate fishing effort throughout their study area over the past twenty years. This information was then related to census population data collected for both diving seabirds (common murres, razorbills, Atlantic puffins, northern gannets) and surface-feeding seabirds (herring gulls, great black-backed gulls, and black-legged kittiwakes) nesting in nearby seabird ecological reserves.

Unsurprisingly, gillnet fishing activity was found to decrease sharply after the moratorium was initiated. Simultaneously, populations of diving seabirds increased, while populations of surface-feeding seabirds decreased. Diving birds are particularly susceptible to bycatch, so removal of the gillnets likely led to increased population size by reducing annual mortality rates. Surface-feeding birds, on the other hand, take advantage of discards and offal produced by fishing efforts; elimination of these treats has previously been associated with reduced breeding rates and probably drove the population decreases observed here.

The authors believe their study may be the first ever to support the idea that bycatch affects not only individuals, but entire populations. These findings may be useful in promoting future moratoria and other conservation efforts aiming to reduce bycatch.

Further reading:

Regular, P. et al. 2013. Canadian fishery closures provide a large-scale test of the impact of gillnet bycatch on seabird populations. Biology Letters 9(4): 2013088 (online advance publication).