There are few animals as iconic to an area as the cod is to the northeastern United States. For centuries, cod was a staple industry in New England. A giant wooden cod even hangs in the Massachusetts State House. But cod is also an ecologically important species – a top predator that kept other species of fish in check. However, in the early 1990s, overfishing led to a collapse of cod populations. Now, what fish are left may soon leave for cooler waters.

According to a new study published in the journal Global Change Biology, climate change is causing many fish to migrate to new territory. Cod, for example, are leaving New England waters for cooler water further north. This will not only impact U.S. fisheries, it will affect the cod’s prey. For instance, fish like herring and sand lance, common prey species for cod, are not as sensitive to temperature and so are not moving north along with their traditional predator.

Rebecca Selden, a postdoctoral researcher at Rutgers University and lead author of the paper, says that this will cause some ecological shuffling. Free from the pressures of predation, the populations of these fish may explode. But warmer waters may also allow other predators to take the cod’s place. Spiny dogfish, another major predator off the east of the U.S., prefers warmer waters, and their populations will likely expand to take cod’s place.

Species interactions, even between a predator and prey, are complex. They are determined by size, metabolism, reproduction rate and competition. So while Selden notes that spiny dogfish will likely be able to ameliorate some of the effects that come from the loss of cod, climate change is altering how species interact all over the world, and not all of them can rely on such ecological buffers. The impact on people and coastal economies may be severe as well – Massachusetts probably won’t hang up a giant spiny dogfish in its state house anytime soon.

Oceans Deeply spoke with Selden about her research and the impact that these changes could have on ecosystems and economies. The interview has been lightly edited for length and clarity.

Oceans Deeply: One of the things that you mentioned in the paper is that a lot of research on how climate change will impact species and fish focuses on single species, but you really chose to look at the interactions between species. Why?

Rebecca Selden: So, species interactions is one of the major ways that we think that we might be surprised by the way climate change is affecting ecosystems because a lot of our previous sort of studies have been on the single species perspective, and we know that species interactions are really important in determining where we find species and how they perform and where they are, through competition and these predator/prey interactions.

And so taking that perspective would allow us to anticipate where warming might be affecting these interactions and have impacts that are different than what we were expecting if we just looked at the species alone.

Oceans Deeply: One of the reasons that these interactions are not studied more closely is that they’re really complicated. So, when you’re talking about things that are as complex and multifaceted as animal behavior and species interactions, how confident are you even in the conclusions you were able to draw?

Selden: I guess we were confident in these results applied to the potential for interaction, because we’re putting these animals in the same space and we know that they interact because we have some diet studies that back up the fact that these are really important interactions between the species.

But there are things, like behavior, that could mitigate some of these results. Say, we’re putting more spiny dogfish in this region and it’s overlapping more and more with these prey species, it’s possible that prey do have defense behaviors that would minimize their actual interaction. I think it’s true that the potential compensation that we’re seeing by spiny dogfish and some of these other warm water predators may be mitigated. But I think that, in terms of the loss for cod, if you don’t have cod in the same space as this prey it’s pretty hard to imagine that it would interact with those prey, so I think our estimates of the loss of cod in the system are conservative.

Oceans Deeply: One of the things you talk about in the paper is that cod will generally probably move north to find colder waters, whereas the spiny dogfish are going to expand their region because they are much more okay with warmer waters. What effect is that going to have on cod besides them changing their prey? Will they actually have less prey as they move north? Do you know how that will impact their population?

Selden: That’s a good question. I think cod is quite a generalist. And so, as it moves into more Canadian waters there are other suitable prey available, but that these key interactions that they had in the past are likely to be disrupted.

I think cod’s pretty resourceful, but that’s probably not going to be a major constraint on its success in its new area. However, there are some other studies that have shown that where we find cod is very strongly driven by the abundance of sand lance, and so the availability of prey in its new range may very strongly drive where we find cod as well.

Oceans Deeply: How much can you say about how climate change and how these interactions will impact the fishing industry or public policy around these fish?

Selden: There’s a lot of restoration efforts in the Gulf of Maine to actually try to restore [commercial fish] including cod. There’s been some efforts to get rid of dams to allow the [growth of] alewife populations that could actually serve as prey for cod.

It does make it more difficult to recover cod in these areas where you’re competing against warming waters. So, no matter what you might do to benefit cod from the bottom up, it may have this more global constraint on its ability to recovery in these areas.

There is evidence that black sea bass are actually moving north, and it’s a coastal predator as well but it actually eats lobster. So there’s more things to think about for fisheries that are becoming more and more reliant on lobster.

Oceans Deeply: It sounds like you went into this knowing that you were going to be looking at how climate change and these interactions were also going to be tied to policy and industry.

Selden: Yeah, I mean, one of the other projects that I’m working on right now is trying to understand how fishing communities are adapting to these changes in fishing distributions. We see a lot of variety in the capacity for these communities to adapt to some shifting, to a shifting species.

There are some fishing communities that can follow their target species as it moves further and further north, while others stay very loyal to a particular fishing ground. And so in those other communities they perhaps can adapt by shifting their target species, but then there are lots of questions about whether there’s a new market for any new species or whether there’s a price difference. They might not have the same profits with this new target catch, and then there may be other constraints.

So there’s a lot of really interesting consequences for fishing communities, and it’s definitely something I’m very interested in working on moving forward.

An earlier version of the story incorrectly stated that white hake was a common prey species of cod. In fact, white hake is not a common prey species for cod.