Junk food in marine ecosystems

Overfishing and changes in climate risk putting marine birds and mammals on a “junk food" diet, shows a new Stockholm Resilience Centre study.

This kind of diet has originally been suggested as a potential explanation for the dramatic decline of Stellers sea lions in the Gulf of Alaska. The new study illustrates that “junk-food" can be of importance for marine ecosystems around the world.

As overfishing has led to the extensive removal of predatory fish, energy rich forage fish lower down the food chain has increased in numbers. An expanded commercial fishing on forage fish, coupled with changes in climate, can influence the fish stocks dynamics, both in terms of abundance and quality, in turn affecting marine predators.

A switch from high energy- to low energy fish has been called switching to a “junk food" diet.

Quantity and quality of food affects population growth
Because fish-eating seabirds and mammals are depending on high densities of energy-rich prey, even small changes in diet quality can have a negative impact on population parameters .

- Under adverse environmental conditions, scarcity of food and reduced food quality, can result in reduced breeding performance in top predators, says Henrik Österblom, one of the authors behind the Oikos-published article Junk-food in marine ecosystems.

Österblom is a joint theme leader for Governance and ecosystem management of coastal and marine systems and researcher at the Baltic Nest institute. 

Decreased abundance and reduced energy content of seabird prey in the North Sea led for instance to a massive breeding failure in 2004. 

- Seabirds, seals and sea lions are central place foragers during the breeding season. During this period they are constrained to forage within a certain distance of the breeding site and are consequently limited to resources within that local area. Reduction in the number and quality of food can therefore lead to prey depletion or interference competition, especially around large colonies, he says.

Can predators adapt to these changes?
So what is the minimum prey abundance and quality required for good predator performance? According to the Resilience centre study, this differs between species and their sensitivity to such changes. 

- For instance, the seabird Common Guillemot is potentially very vulnerable to reductions in prey quality. This species only carries one fish at the time from the sea back to their chick on the cliff. Much of the time and energy consumed is spent when travelling between the foraging area and the colony, which is particularly expensive for a bird with an exceptionally high wing loading and low load carrying capacity. Carrying a single fish of low energy content simply does not pay, Österblom says. 

The “junk-food" study has highlighted the large ecological importance of forage fish for sustaining the resilience of seabird and mammal population worldwide.  

- We already knew that changes in forage fish numbers were important. This “junk-food" aspect, that predator respond to changes also in prey quality, adds a level of complexity in the challenge of managing ecosystem sustainability, Österblom says.