More squid, less fish: North Pacific seabirds alter their prey preferences

Over the last 125 years, and particularly after an uptick in industrial fishing since 1950, North Pacific seabirds - typically fish consumers - have shifted their prey preferences, a new study reports; they are eating lower on the food chain, consuming more squid. The seabirds' changing position within the food web, or trophic position, provides important insights into the intricate changes occurring in ocean ecosystems more broadly, changes that may have extensive environmental or economic impacts. Seabirds are opportunistic predators whose diets shift in accordance with the availability of different kinds of prey. Because ocean fish are among their prey, studying seabirds' feeding behaviors provides a window into marine ecosystem changes. Previous studies of marine ecosystem status have relied on fishery-dependent data, or scientific surveys at sea. However, these data are subject to strong biases that can lead to somewhat misleading conclusions. To address this, Tyler O. Gagne and colleagues analyzed the isotopic composition of seabird tissues to obtain a fisheries-independent metric of the diet of eight species of central North Pacific seabirds from 1891 to 2016. The method is particularly powerful for distinguishing between vertebrates such as fishes versus invertebrates such as squid. Species of predatory seabirds declined in their trophic position throughout the time series, with the rate of decline doubling after 1950 when industrial fishing grew more intense. Squid consumption increased markedly while consumption of four families of fishes correspondingly declined. The authors attribute these massive shifts in ecosystem structure to commercial fishing, climate change, and seabird adaptation to their changing environment.

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