Posts Tagged ‘cod recovery’

Culling Seals

Wednesday, April 24th, 2013
Cape Fur Seals breed on the coast of Namibia, where the annual 'hunt' removes about 90,000 seals, most of them pups (africareview.com)

Cape Fur Seals breed on the coast of Namibia, where the annual ‘hunt’ removes about 90,000 seals, most of them pups (africareview.com)

The Canadian government has approved a cull of 70,000 Grey Seals over the next four years to protect cod (guardian.co.uk)

The Canadian government has approved a cull of 70,000 grey seals over the next four years to protect cod (guardian.co.uk)

Seals of course eat fish. As opportunistic feeders, they’ll eat any fish they can catch.

When those fish decline in numbers or fail to recover when fishing is curtailed, a natural response has been to blame the seals.

Culling seals to protect fish populations of interest to human fishermen has been going on for more than a hundred years – California Sea Lions, Ringed and Grey Seals in the Baltic Sea, Harbor Seals in both BC and along US East Coast, Grey Seals on the US East Coast, Iceland, Norway, and the UK, and currently Cape Fur Seals in Namibia. In all cases seal populations experienced huge declines.

Culling, as opposed to harvesting, refers to killing the seals without intent to market them in some way. Though the market for seal parts is now close to non-existent, Canada considers its annual Harp Seal hunt to be harvesting, not culling.

Grey Seals on Sable Island: are they really responsible for failure of cod to recover? (truenorthimages.com)

Grey seals on Sable Island: are they really responsible for the failure of cod to recover? (truenorthimages.com)

But have the culls resulted in increased fish stocks for human fishermen? Oddly enough, nobody actually knows! There simply are no data, no experiments, nothing to indicate whether culling is effective or not. If anything, almost all of the examples suggest that no particular impact occurred on the target fish species.

Is there then any evidence we can point to indicating an effect of culling? This is of current concern, for Canada has approved a cull of Grey Seals to encourage the recovery of cod, and the Baltic States are considering a cull for the same reasons.

Fishermen want the cull, politicians are sympathetic, and marine scientists are unanimous in opposing culling. This is a familiar stand-off. What’s needed is evidence.

And now there is some, and it illustrates just how complicated ecosystem dynamics are.

Sable Island is an arc of sand on the Scotian Shelf, where Grey Seals breed in large numbers, in a region where cod were once abundant (oceantrack'org)

Sable Island is an arc of sand on the Scotian Shelf, where Grey Seals breed in large numbers, in a region where cod were once abundant (oceantrack’org)

It involves cod. Following the moratorium on fishing North Atlantic Cod following the collapse of stocks in the early 1990s, everyone assumed the stocks would recover. They didn’t. But Grey Seal numbers exploded to around 400,000, particularly those breeding on Sable Island on the Scotian Shelf, near one of the past major cod stocks. That seems to indicate that seals have suppressed cod recovery, and therefore culling ought to help.

475 ships have wrecked on Sable Island since the 17th Century. Feral horses are the only permanent residents (getimage.php)

475 ships have wrecked on Sable Island since the 17th Century. Feral horses are the only permanent residents (getimage.php)

Instead the story goes like this. When the Scotian Shelf populations of cod and haddock, both large bottom predators, crashed from overfishing in the early 90s, the result was a major restructuring of the food web, a ‘regime change’ of the sort we have now learned to expect to occur. With the loss of the cod and haddock, planktivorous fish like herring, capelin and sand lance, as well as macro-invertebrates like Northern Shrimp and Snow Crabs became abundant instead – hugely so in some cases, and they have supported alternative fisheries. A new and stable balance of species seemed to have developed, with cod and haddock unrecovered. Grey Seals numbers increased greatly during this time.

Herring exploded in numbers, but have now crashed (fisherycrisis.com)

Herring exploded in numbers, but have now crashed (fisherycrisis.com)

But this was not in fact a stable system. The biomass of fish in the system increased to an estimated 10 million tons, where carrying capacity is estimated to be less than half of that amount. The fish ran out of food – the zooplankton abundance crashed, the herring and capelin starved, and their populations crashed.

And then, with the herring and capelin gone as predators on cod and haddock larvae, cod and haddock have begun to recover. Particularly haddock. A return to the earlier food web appears to be underway, though how far it gets is unknown, for of course so much else is also involved, such as the impact of climate change, pollution, and continued fishing.

Cod show signs of some recovery (fisherycrisis.com)

Cod show signs of some recovery (fisherycrisis.com)

Haddock recovery is greater (fisherycrisis.com)

Haddock recovery is greater (fisherycrisis.com)

The really good news is that ‘regime changes’ can reverse back to what previously existed. And in this case, in this ecosystem, the evidence indicates that Grey Seals, though obviously eating fish, are not responsible for preventing the recovery of the cod over the past two decades.

Since culling seals probably has no impact on the recovery of overfished populations, decisions to cull them anyway are then political, disregard science, and are so unfortunate.

Grey Seals, Sable Island, waiting to be culled (theglobeandmail.com)

Grey Seals, Sable Island, waiting to be culled (theglobeandmail.com)

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.