Posts Tagged ‘fish resilence’

Losing Apex Predators

Monday, February 11th, 2013

We’re losing or have already lost the apex predators from most of our ecosystems. This has been going on for a long time – remember saber toothed tigers? – so it’s obviously not news that we are a particularly difficult species to co-exist with.

Over the past few decades global capture fisheries have added most of the large fish species of any commercial value to the list of missing apex predators. Among those that are still with us, an unexpected response has occurred.

In a comparison of 37 commercially fished stocks, the majority matured earlier and at a smaller size. The effect is clearest in heavily fished populations.

The size of first spawners of Arctic cod has declined, as it has in many other heavily fished species (nature.org).

The size of first spawners of Arctic cod has declined, as it has in many other heavily fished species (nature.org).

Is this a genetic change, an evolutionary shift towards smaller size-at-age due to the selective harvesting of the oldest, largest and fast-growing individuals? If it is, it is a dramatic change, and will be difficult to reverse.

It could as well be a response to climate change, with physiological declines in growth rates occurring due to increasing sea temperatures and decreasing oxygen in warmer oceans.

At the same time, we wonder why fish that we have overfished don’t recover when we stop harvesting them. With their huge reproductive potential, fish surely should be resilient, and recover quickly. Famously, though, the cod of the northwest Atlantic have not recovered from their collapse.

The famous graph of exploitation of cod in the northwest Atlantic, leading up the moratorium in Canada in 1992. (Wikipedia.org)

The famous graph of exploitation of cod in the northwest Atlantic, leading up the moratorium in Canada in 1992. (Wikipedia.org)

Why not? What stops or delays recovery? And what have we actually learned about the impact of the damage we have done to marine ecosystems?

In fact we have learned quite a lot. We have learned that the responses of an ecosystem to the loss of apex predators are likely to be complex and convoluted, and often unpredictable. Shifts occur within the community of species, involving changes in mortality rates, growth rates, competitive interactions, and prey-predator relationships. (Two fine reviews worth reading were published in Science: Estes et al, July 15, 2011; and Garcia et al, March 2, 2012)

Pandalus borealis, the northern shrimp, became abundant after the collapse of cod, and is in part responsible for the lack of cod recovery. It is also the sweetest shrimp you would ever want to eat. (biology.com)

Pandalus borealis, the northern shrimp, became abundant after the collapse of cod, and is in part responsible for the lack of cod recovery. It is also the sweetest shrimp you would ever want to eat. (biology.com)

We have also learned that sufficiently perturbed ecosystems break abruptly into alternative stable states that are usually of lower trophic status and of far less commercial value. Coral reefs have become algal covered rubble. Jellyfish have replaced fish as top consumers.

We have learned that trophic degradation is an inevitable outcome of eliminating or radically reducing apex predators.

And we have learned that there are limits to resilience.

Iconic cod are showing signs of recovery in the northwest Atlantic - not enough to lift the moratorium, but enough to suggest hope lives (ctv.news0

Iconic cod are showing signs of recovery in the northwest Atlantic – not enough to lift the moratorium, but enough to suggest hope lives (ctv.news0

Out of these fisheries disasters has comes some decent advice. For instance, fishing pressure should be spread over more species and sizes, probably netting more fish, but reducing the risk of wiping out a species or restructuring the community. Biomass drops but not biodiversity,
a more ecosystem-based approach.

But also we are aware that the only truly reasonable response is to try to restore the apex predators. If we don’t, biodiversity will decline, trophic degradation will continue, ecosystem phase shifts will occur, and the current global mass extinction will just continue. The world becomes ever more diminished.

Cod captured by trawler in 1949 were often huge. Nevermore. (heritage.nf.ca)

Cod captured by trawler in 1949 were often huge. Nevermore. (heritage.nf.ca)

Does it help to understand the reasons for a catastrophe, if there seems to be little chance of preventing or recovering from it?

The answer must be yes. If recovery from the catastrophe is even remotely possible, we can encourage it. And we can use our knowledge to mitigate the impact of other catastrophes-in-waiting.