A team of co-authors led by Ann-Marie Osterback, a marine ecologist at the University of California-Santa Cruz, suspects that garbage and fishery discards might play an underrated part in the proble of decreasing steelhead trout.. The hypothesis is that local food wastes inadvertently subsidize Western gulls in the Monterrey Bay area, and these gulls in turn prey on the juvenile steelhead trout.
The dramatic decline in steelhead numbers would normally mean that fish-eating birds around Monterey Bay would have to move down the food chain to survive. That’s what’s happened to Brandt’s cormorants and marbled murrelets, and their populations have declined as a result.
But according to Osterback, the number of gulls have doubled or quadrupled in different parts of the bay just since the 1980s — thanks to a steady diet of landfill garbage and fishery discards. Osterback and her co-authors found that each individual gull now eats less steelhead than in the past, but the combination of a greatly increased gull population and a severely reduced run of steelhead trout adds up to a dramatic rise in predation pressure. She estimates that the gulls may eat up to 30 percent of juvenile steelhead en route to the sea.
Ryan Carle, Program Manager at Año Nuevo Island Seabird Research and Conservation, provides more context:
I would like to clarify that, although it is true that the population of Western Gulls has indeed risen greatly since the 1970’s and 1980’s at Año Nuevo, it is misleading to assume that all of this growth was fueled by human food subsidy, or even that the current population is necessarily greater than historical populations (i.e. 1800s or earlier). The Western Gull population at Año Nuevo grew from around 100-200 nests in the 70’s and peaked at around 1200 nests in 2005- a lot of growth. But the 1970’s numbers were at probably the lowest ebb of population, after around 100 years of disturbance and direct persecution during the mid 1800’s- mid 1900’s when a manned lightkeeper’s station operated on Año Nuevo Island. There are no data from Año Nuevo pre-1970’s, but at the Farallon Islands its well documented that the gull population was drastically reduced during the 1800’s and early 1900’s and only recovered when the islands were protected. Año Nuevo was protected in the 1960’s and the gull population has recovered since. This may have more to do with the growth we’ve seen than the trash. During the 1980’s-1990’s increase in gull population at Año Nuevo and other nearshore colonies, there was a major decrease at the offshore Farallones, so the overall popualation in central California may have remained stable.
What’s more, the Western Gull population has been annually declining at Año Nuevo since its peak in 2005. There were only 643 nests in 2015 compared with 1234 nests in 2005. Many factors could be contributing to this decline, most probably lowered availability of natural prey such as krill and forage fishes. Similar declines are being seen at the Farallones, indicating a larger scale pattern than just local landfill management.
Finally, there are several published studies showing that Western Gull reproduction is actually worse, not better, for individuals that eat a lot of trash (see multiple papers from Alcatraz Island by Pierrotti and Annett). This implies that contrary to intuition, trash may actually not help subsidize population growth in this species after all. Trash may buffer adult survival in years of poor natural food availability but this has not been thoroughly studied.
Also, the assertion that Brandt’s Cormorants have declined due to reduced steelhead numbers is not entirely accurate. Though these cormorants do eat salmon sometimes, they are more influenced by species like anchovy and juvenile rockfishes. Brandt’s Cormorant population trajectory is somewhat similar to the gulls’–they increased from 2 nests in 1992 to over 2,000 nests in the mid-2000’s: a thousand-fold increase! Similar to the gulls, their populations dropped at the Farallones as the nearshore populations grew.
The narrative that the Western Gull population has unnaturally exploded, causing them to be an unnaturally large threat to the steelhead, is more nuanced than presented here.
shared via: e360.yale.edu/feature/unnatural_balance_how_food_waste_impacts_worlds_wildlife/2945/
article: Richard Conniff and comment by Ryan Carle
photo: Public domain