What happens when endangered whales eat endangered fish

What happens when the endangered marine mammals are busy scoffing endangered fish?

That's exactly the conundrum that could soon be facing fisheries managers and conservationists in the Salish Sea in the northeast Pacific, the home of southern resident killer whales and their favourite food the Chinook salmon, both 'at-risk' species.

For the first time, scientists have worked out accurately how many salmon these resident killer whales eat, and hence how much conflict there is likely to be as plans go ahead to restore the depleted populations of both predators and prey.

Publishing in the journal PLos ONE, the big international research team, including from Oceans Initiative, set up a series of computer models linking age, length, weight, sex, and energy consumption of killer whales based on information from captured and captive animals - they didn't catch any killer whales themselves, but used existing information from whaling records.

These models were then used to calculate how many fish are eaten on an annual basis by the 87 killer whales that currently live in the Salish Sea.

And it turns out they already eat a lot of fish - it's estimated they eat 12-23% of the salmon from the Fraser River each year, compared to 10-40% natural mortality for the same fish. And the killer whale consumption could go up by 75% if efforts to boost killer whale populations are successful: the target is a 2% increase every year for the next 28 years.

Obviously, that means there will need to be a lot more salmon available for them to eat.

How to resolve the conflict between endangered predator and endangered prey?

Short term measures could include reducing the number of salmon caught by people in fisheries, while steps to boost the productivity of salmon take time to kick in - efforts like taking out dams that block the salmon migration routes, restoring their spawning habitats, and controlling diseases spreading from salmon farms.

One solution proposed is that the killer whales are officially allocated their share of the Chinook salmon under the Pacific Salmon Treaty.

This study highlights the importance of setting aside old-fashioned, single-species approach to managing fisheries and protecting species. Instead we need to look at the whole ecosystems, and consider the interactions between different species, in this case killer whales, salmon, and people.

Find out more:

Williams et al (2011) Competing conservation objectives for predators and prey: estimating killer whale prey requirements for chinook salmon.
PloS One.