Yesterday, the Times and Scientific American both covered new research that suggests warming oceans and a fishing ban have led to a recovery in Atlantic cod off the coast of Canada.

But a little further south, the story is less positive. Another study, published today in Science, finds that warming waters in the Gulf of Maine, off the US Atlantic coast, are contributing to the population decline in the heavily-fished region.

So, how is warming helping one population of cod while hindering another? Carbon Brief takes a look at the research.

Population collapse

Alongside bluefin tuna, Atlantic cod has become a poster child for the perils of overfishing.

Populations of this once-plentiful fish have collapsed in the wake of increasing demand and technological advances in trawling. Atlantic cod is now classed as vulnerable on the International Union for Conservation of Nature’s Red List of Threatened Species.

In the Gulf of Maine, the focus of the new Science study, Atlantic cod was once so abundant that Cape Cod was named in its honour. But numbers have plummeted 90% since 1982, leaving stocks at just 3% of what is needed to sustain a healthy population.

Yet despite increasingly strict quotas on commercial fishing – culminating in an effective ban in 2014 – fish populations have not rebounded as expected. The new study suggests that rapidly-warming water in the Gulf are holding back the recovery.

‘Remarkable’ warming

Using satellite data, the researchers found that sea surface temperatures in the Gulf have risen by an average of 0.03C a year between 1982 and 2013. This is three times the global ocean average, says lead author Dr Andrew Pershing, chief scientific officer at the Gulf of Maine Research Institute.

From 2004, the rate of warming increased to a “remarkable” 0.23C per year, the paper says, meaning the Gulf has warmed more quickly than 99.9% of the world’s oceans. You can see how the oceans have warmed in the map below, with the Gulf of Maine picked out in the black box.

The rapid warming is down to both natural and human-caused factors, Pershing tells Carbon Brief:

The long-term warming trend is most likely due to climate change. The rapid warming between 2004-2013 is part of a natural cycle adding to the background warming.

Such warm conditions aren’t ideal for cod, Pershing says, with females tending to produce fewer surviving young, and fewer juvenile fish making it to adulthood.

The lower survival rate is likely to be down to several factors, the paper says, such as availability of food in the Gulf and higher temperatures pushing the cod into cooler, deeper waters where they are more likely to be eaten by predators.

But the adverse impacts of high temperatures hasn’t been factored in when fishing quotas have been set over the past decade, the paper says. The result being that even staying within quotas has harmed cod populations, says Pershing:

Essentially, the impact of the warming was so rapid that our management system could not keep pace.

Canadian contrast

Meanwhile, in the Canadian waters off the coast of Newfoundland and Labrador, the Atlantic cod population is making a bit of a comeback, says a study in the Canadian Journal of Fisheries and Aquatic Sciences.

After collapsing in the 1990s, the population now appears to be rebounding “from tens of thousands of tonnes to more than 200,000 tonnes within the last decade,” the paper says.

A 20-year moratorium on commercial fishing and the return of plentiful capelin to eat are in the cods’ favour, but warmer ocean conditions could be a factor, too, the study finds.

So, why are higher temperatures having a contrasting impact on cod? The answer lies in the fact that the Gulf of Maine is just about as southerly as Atlantic cod will venture, says Pershing:

Because Newfoundland is cold, warming makes their ecosystem more favourable to cod. Our work shows that warming in the Gulf of Maine, one of the warmest places where cod are found, will reduce the productivity of the cod stock.

So, while warming is pushing temperatures in the Gulf of Maine beyond the limits that cod are used to coping with, it’s pushing Newfoundland “squarely into the comfort zone,” Pershing told Scientific American.

This means the chances of similar rebound in cod population in the Gulf could be hampered by further temperature rise, the paper says:

[H]ow quickly this fishery rebuilds now depends arguably as much on temperature as it does on fishing.

Dr Jon Hare from the National Oceanic and Atmospheric Administration (NOAA) Fisheries Service, who wasn’t involved in either study, says the Gulf of Maine paper highlights the challenge of managing fish stocks with so many different factors at play. He tells Carbon Brief:

The main take-home point for me is that warming in the Gulf of Maine poses real-world, tangible challenges to fisheries in the Northeast US. We are going to have to work hard and in partnership to address these challenges.

Pershing, A. J. et al. (2015) Slow Adaptation in the Face of Rapid Warming Leads to the Collapse of Atlantic Cod in the Gulf of Maine, Science, doi:10.1126/science.aac9819