An Ocean Warming: Atlantic Cod and Northern Shrimp Search for Colder Water

Perhaps no other New England species has felt the effects of the Gulf of Maine’s rapid warming like the fabled Gulf of Maine cod. An iconic species since its discovery centuries ago off of New England’s coast, cod profoundly impacted the way our region developed, and has shaped our coastal economies.

Today, however, the state of the species looks much different, with cod stocks in the Gulf of Maine hovering around three percent of what scientists say are sustainable levels. In an attempt to quell the overfishing of cod, restrictions on fishing quotas have been enacted. However, instead of seeing the cod stocks rebound, the numbers have continued to plummet.

There are two reasons for this: First, continued fishing pressure hasn’t allowed for enough of an opportunity for cod populations to recover. The other reason? You guessed it: drastic warming due to climate change.

Cod’s home is no longer the comfort zone

Climate change has led to warming ecosystems all over the world, but the Gulf of Maine is experiencing this warming trend faster than most. Cod, like every species, has a range of temperatures at which they can live comfortably, which in turn makes a range of geographical areas suitable for them. Historically, the Gulf of Maine has been at the southern boundary of their range. But, with Gulf temperatures rising due to climate change, cod are now starting to move north in search of cooler temperatures.

Because of this shift, restrictions on fishing quotas, even if effective on their own, have failed – as they didn’t account for what would happen due to warming waters.

As scientists and fishery managers have begun digesting this information and its implications, they are increasingly calling for something called “ecosystem-based management,” a management principle that considers the environmental factors in play in a given ecosystem. With widespread use, this broader understanding of what’s happening, and why, could provide a brighter outlook for the future of Gulf of Maine cod.

Northern shrimp headed further north

Another Gulf of Maine species, northern shrimp (pandalus borealis), face a similar situation. This small crustacean is an integral part of the Gulf of Maine food web. It’s unique because it is hermaphroditic, meaning the shrimp first mature as males at around two and a half years of age, and then about a year later develop into females. Northern shrimp feed on plankton and benthic invertebrates, and are then prey for several important species of fish, including cod, redfish, and hake.

For decades, the northern shrimp fishery thrived, until a few years ago, when stocks showed massive declines and low levels of shrimp reaching the age when they’re large enough to be fished. Particularly warm water temperatures during what is known as the “ocean heat wave” of 2012 caused stocks in the Gulf of Maine to drop even more dramatically. Although heavy fishing pressure is partly to blame for the driving the stocks down, scientists also point to the shockingly low levels of fishable shrimp. As the northern shrimp’s lifecycle is highly dependent on temperature fluctuations – with colder temperatures producing higher levels of fishable shrimp – the decline can be linked to warmer waters.

Northern shrimp, like cod, have a narrow range of temperatures in which they can thrive and, similarly, the Gulf of Maine is at the southern end of their range.

Long-term fishery closure a warning sign

Since 2013, the northern shrimp fishing seasons have been closed entirely. With ocean temperatures predicted to continue their warming trend and the species’ already vulnerable status, it looks increasingly bleak for the fishery.

Little by little, we are losing cod and shrimp to colder waters, and others soon may follow. It’s clear the Gulf of Maine and its composition is changing. That’s why plans to mitigate climate change must pay close attention to oceans and the warning signs they’re giving. And in the meantime, fishery managers must do everything within their power to ensure that management strategies incorporate the best climate science possible.