Abstract
Many marine mammal predators, particularly pinnipeds, have increased in abundance in recent decades, generating new challenges for balancing human uses with recovery goals via ecosystem-based management. We used a spatio-temporal bioenergetics model of the Northeast Pacific Ocean to quantify how predation by three species of pinnipeds and killer whales (Orcinus orca) on Chinook salmon (Oncorhynchus tshawytscha) has changed since the 1970s along the west coast of North America, and compare these estimates to salmon fisheries. We find that from 1975 to 2015, biomass of Chinook salmon consumed by pinnipeds and killer whales increased from 6,100 to 15,200 metric tons (from 5 to 31.5 million individual salmon). Though there is variation across the regions in our model, overall, killer whales consume the largest biomass of Chinook salmon, but harbor seals (Phoca vitulina) consume the largest number of individuals. The decrease in adult Chinook salmon harvest from 1975–2015 was 16,400 to 9,600 metric tons. Thus, Chinook salmon removals (harvest + consumption) increased in the past 40 years despite catch reductions by fisheries, due to consumption by recovering pinnipeds and endangered killer whales. Long-term management strategies for Chinook salmon will need to consider potential conflicts between rebounding predators or endangered predators and prey.
Highlights
Marine mammal population recoveries are a conservation success story in many parts of the world[1,2]
Marine mammal populations never threatened with extinction benefited from protection, with some populations recovering to high abundance levels
Within the International Whaling Commission (IWC), some argue that rebounding baleen whale populations are responsible for reductions in commercially fished prey populations and certain whale species should be culled, whereas others argue that natural fluctuations in targeted fish populations and fisheries management are responsible for declines in yield[8]
Summary
Marine mammal population recoveries are a conservation success story in many parts of the world[1,2]. Marine mammal populations never threatened with extinction benefited from protection, with some populations recovering to high abundance levels (e.g. harbor seals Phoca vitulina richardii;[4,5]). Examples of potential impacts of higher trophic level consumers on other species in the food web include: reduced recovery of forage fish such as Pacific herring (Clupea pallasii)[7], increased competition between marine mammal species that share the same prey, such as pinnipeds and killer whales (Orcinus orca) in the Northeast Pacific[6], and lastly, increased direct competition between marine mammal populations and fisheries. Like other generalist predators, quantifying the impact of these pinnipeds on prey species can be challenging because pinnipeds may consume fish at a variety of ages. A second challenge in quantifying the impact of these pinnipeds is that their diets vary in space and time, as predators alter their foraging to exploit local concentrations of prey
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