Coming close on the heels of last week’s post about estimated worldwide seabird bycatch in gillnets, an experiment that basically did itself. Paul Regular, William Montevecchi and several colleagues have a paper out in Biological Letters examining the effect of the closure of two major Canadian fisheries on murre populations. In 1992, cod and salmon gillnetting was effectively closed down in Atlantic Canada. This resulted in the removal of tens of thousands of gillnets from the foraging waters of seabirds like murres and razorbills, which are heavily impacted by bycatch in these nets. What followed was thus a kind of de facto experiment on the population impact of removing bycatch mortality for these birds. The study authors looked at impacts not only on birds likely to be entangled in gillnets, but also surface feeding seabirds like gulls which are comparatively unlikely to suffer that fate.
Using both bycatch data and breeding colony census information, the paper tracks the populations of the various species over the decades since the fisheries closures. Data was sufficiently robust to look closely at two species: Common Murres and Herring Gulls. The researchers found a dual and approximately opposite effect on the two species. Murre numbers climbed substantially once pressure from gillnets was removed, but Herring Gull numbers showed a decline. This is likely due to the withdrawal of offal and other fisheries waste when the gill nets were hauled up and stowed away in 1992. Opportunistic feeders, gulls feed heavily on discarded bait and other material tossed overboard by fishermen. As those anthropogenic food sources become less available, gull survival declines. The same phenomenon seems to have followed the closure of many open landfills after the 1970s. The declines in breeding gull populations over the past 30 years seem to reflect a “correction” of sorts–a return to baseline numbers in the absence of man-made buffets. Simultaneously, more specialized feeders like murres have benefited not only from closure of some fisheries, but also from stricter laws governing the discharge of petroleum products by ships off the coasts of the U.S. and Canada.
The closure of the cod and salmon gillnet fisheries in Canada afforded this opportunity to study the effects of bycatch pressure on seabirds, and it appears to be substantial. Fisheries don’t generally remain closed forever though, and if and when the salmon and cod fisheries off the Canadian Maritimes do reopen, the authors of the study point to potential changes in methods to continue to protect murres and other diving birds. One such possibility is cod pots, which operate similar to lobster traps, sitting on the ocean bottom with bait inside. Fish swim in and can’t swim out. This type of gear is highly unlikely to entrap seabirds, though I have known lobstermen who reported finding Common Eiders in their traps from time to time, so never say never.