Atlantic cod harvest: Decline mirrors rising sea temperature
A study in tomorrow's Science1 finds a strong correlation between warming ocean water near New England, and declining harvests of Atlantic cod.
The rapid drop in catches in the Gulf began in the early 1980s. Since then, cod-fishing quotas were slashed to allow cod -- and the cod-fishing industry -- to recover.
It didn’t work.
The new study places the blame on rapid warming in the Gulf of Maine, where the sea surface temperature was rising an average of 0.03 ° C annually between 1992 and 2013.
That's three times faster than the global average.
In hot water
"Correlation is not causation," as the maxim says. So how strong is the connection between warming water and cod decline? Fishing effort, a key factor in cod abundance, has been dramatically reduced as regulators -- over industry opposition -- have tightened limits on harvests and fishing time.
How might a 1°C warming harm cod? One link may occur during "recruitment," when new individuals enter a population.
"For many years, we have been seeing poor numbers of young fish in surveys," says study contributor Lisa Kerr, a fish biologist at the Gulf of Maine Research Institute in Portland, Me.
The transition from larva to juvenile fish is highly sensitive, she says, "so it's not uncommon to see a relationship" to environmental factors. The warming water could, for example, influence the number, activity and behavior of predators or prey.
"A lot of what we describe is most likely hypothetical; its hard to say we know the exact mechanism," Kerr concedes. Since it's impossible to experimentally change conditions in the ocean, computer models are used to evaluate the effects of variables like fishing and water temperature support the link between warming and declining cod.
Who's gonna fish?
If global warming accounts for the failure of cod to recover in the Gulf of Maine, what should we do?
Since fishing boats "have done a very good job of staying within the quotas," Kerr says, it may help to adopt the "ecosystem approach to fisheries management," which inserts environment factors to management decisions.
Although that approach is supposedly common, a 2015 study2 found that only 2 percent of global fish stock regulations are accounting for what's called "ecosystem drivers."
At present, the cod industry in New England and Eastern Canada is watching as boats rust in harbors and the owners and crews look for work in places with few alternatives.
Will the industry ever return to the sea and produce a meaningful catch? A lot depends on climate change, Kerr says. "One of our biggest recommendations is that we need to revise our expectations for cod productivity. Current expectations are based on history, but the capacity may not be there to reach past abundances because we are moving into a warmer climate."
"It's heartbreaking to see what this has done to the industry in our region," Kerr says, "how many boats are tied to the docks, facing really hard times because of this shift in climate, and we have no control over that."
– David J. Tenenbaum
Kevin Barrett, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer
- Slow Adaptation in the Face of Rapid Warming Leads to the Collapse of Atlantic Cod in the Gulf of Maine, by A.J. Pershing et al, Science, 30 October 2015. ↩
- Ecosystem processes are rarely included in tactical fisheries management, Mette Skern-Mauritzen et al, Fish and Fisheries, DOI: 10.1111/faf.12111 ↩
- Grim results of overfishing in Massachusetts. ↩
- Maine lobster booming as cod retreats to deeper waters. ↩
- Gulf Stream ring water intrudes onto continental shelf like 'Pinocchio's nose' ↩
- The enigma behind America’s freak, 20-year lobster boom. ↩
- Cape Cod fishermen raising concerns over high herring catches. ↩