Using data from five different marine ecosystems, researchers have tested the hypothesis of predator‐pit dynamics for forage fish. By examining the consumption of fish by seabirds and the effect of such predation on fish population dynamics, they found that seabird-induced mortality of forage fish varies with fish abundance.

Data from five countries and three continents

Seabirds are widely distributed and abundant forage fish predators on all continental shelf ecosystems around the world, and they can have significant local impacts on forage fish species often also targeted by commercial fisheries. An international group of researchers has examined to what degree natural mortality rates of forage fish may be influenced by seabirds. Compiling data on seabird numbers, diets, energetic needs and prey energy content and abundance, they investigated top‐down processes exerted by seabirds on forage fish stocks in five contrasting marine ecosystems off Norway, South Africa, Peru, Sweden and Scotland.

Recommended threshold for prey biomass

Results from the study showed that the proportion of a fish stock consumed by seabirds was generally low, but increased sharply at low levels of prey abundance. Predation by seabirds became a source of important additional pressure on prey stocks when prey biomass decreased below 15–18% of its maximum recorded value. For the sake of the forage fish themselves, the authors therefore suggest that this threshold should not be exceeded to avoid extra cautious management of fisheries. With regard to food requirements for seabirds, an earlier study demonstrated that seabird breeding success was heavily impacted already when prey stocks fell below 33% of their long-term maximum biomass.

Appeal to fisheries management

Despite exceptionally high rates of predation on some occasions, prey entrapment due to seabirds alone was not found in any of the five ecosystems investigated. The study clarifies predator-prey functional relationships between forage fish and seabirds and underlines the importance for fisheries management to account for ecosystem constraints when setting catch limits in periods of low forage fish biomass.

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Contact person: Tycho Anker-Nilssen, NINA

Puffins with herring larvae. Photo © Tycho Anker-Nilssen
Two adult puffins on their way to the chick with a nice load of herring. Photo © Tycho Anker-Nilssen.