Science Links Ocean Warming to Gulf of Maine Cod Fishery Collapse
The Gulf of Maine has been hit with a double whammy – declining Atlantic cod stocks and ocean warming – and a new study published Thursday in Science definitively links the two. The paper is already receiving headlines (The Boston Globe; Associated Press) and has major implications for how we manage our fisheries.
In their paper, Gulf of Maine Research Institute (GMRI) scientists comment that “the Gulf of Maine cod stock has been chronically overfished…Despite [progressively stronger management] efforts, including a 73% cut in quotas in 2013, spawning stock biomass continued to decline.”
From 1982-2013, Gulf of Maine sea surface temperatures rose three times faster than the global mean rate, and between 2004 and 2013, the Gulf of Maine warmed faster than 99.9% of the world’s ocean. Over this time period, the scientists report, “The Gulf of Maine experienced decadal warming that few marine ecosystems have encountered.”
The researchers fit current Gulf of Maine cod population assessments with stock-recruit models that did and did not account for temperature effects. The models suggested that higher temperatures resulted in increased mortality of juvenile cod and age-4 cod (those reaching maturity), increased metabolic costs, and below average weight-at age. In other words, in the face of climate change, Atlantic cod has a “lower probability of survival.”
GMRI uses these conclusions to explain why Gulf of Maine cod stocks continue to decline even when fishermen do not exceed their quotas:
“During warm years, fewer fish are available for the fishery. Not accounting for this effect leads to quotas that are too high. The resulting fishing mortality rate was thus above the intended levels, contributing to overfishing even though catches were within prescribed limits.”
Essentially, management that did not account for temperature trends, accompanied with socioeconomic adjustments in the fishery, led to the downfall of cod. “Ignoring the influence of temperature produces recruitment estimates that are on average 100% and up to 360% higher than if temperature is included,” the study reports. “The failure to consider temperature impacts on Gulf of Maine cod recruitment created unrealistic expectations of how large this stock can be and how quickly it can rebuild.”
The study does outline three possible rebuilding scenarios under different temperatures (cool, warm, and hot), and Atlantic cod might not be completely doomed. All three scenarios, however, assume that fishing mortality is zero.
Ultimately, the scientists say, “how quickly this fishery rebuilds no depends arguably as much on temperature as it does on fishing.”