Behind the Catch

(Over)Fisheries: The Ripple Effects of Overharvesting

Northern shrimp remain at low biomass levels, a result of increased ocean temperatures in the Gulf of Maine. Photo credit: NOAA

This is the fourth and final blog in our series, Behind the Catch, in which we explore New England’s waters and fisheries from a scientific perspective. By examining the region’s fundamental ecology, we can better discuss and tackle marine conservation issues.

Through a series of complex relationships, ocean species are all connected. We humans are part of the marine food chain, too, but our unprecedented overfishing has caused drastic, unforeseen changes to ripple through entire ecosystems—from the surface swimmers to the bottom feeders.

This ecological phenomenon, referred to as a trophic cascade, can alter fish stocks, biodiversity, and predator-prey dynamics. As the global overharvest of fisheries continues, ocean ecosystems are more vulnerable to these changes than ever before.

Messing With Sharks

The classic example of a trophic cascade comes when humans eliminate an apex predator, such as sharks. When sharks are removed from ecosystems, the species that they would normally feed on—rays, for example—grow unchecked. In turn, the rays predate more heavily on mollusks until their populations are depleted.

This trophic cascade illustrates an accepted principle in ecology: the elimination of a top predator can set off a chain reaction that cascades down to lower species on the food chain.

A Shrimp Saga

Of course, trophic dynamics are rarely so cut-and-dry. In fact, some researchers have critiqued the shark case study above for its simplicity and for ignoring other variables like pollution. While the jury is still out on that critique, it’s true that when cascades occur, many factors can play a role. Consider a more nuanced example from the Gulf of Maine: northern shrimp.

Historically, northern shrimp has been an intermittently productive fishery in New England. But when cod stocks fell in the early 1990s, northern shrimp landings experienced a marked increase, likely for a combination of two reasons: (1) there were fewer groundfish around to eat them, so the population grew, and/or (2) fishermen targeted, and therefore landed, northern shrimp more frequently to make up for their cod losses.

This rise in landings, however, did not last. From 2010 to 2013, the fishery’s seasons were cut short because landings surpassed recommended caps every year. At the same time, sea surface warming in the Gulf of Maine may have inhibited the population’s reproductive capacity. Consequently, a moratorium on northern shrimp has been in place since 2014. Generally, overfishing caused the stock to collapse, but environmental factors have prevented recovery.

This shrimp saga begins to demonstrate just how complicated trophic cascades can be. While northern shrimp’s population ecology remains poorly understood, it is clear that they are intertwined with a number of commercially valuable species. For example, northern shrimp consume plankton and small bivalves, and northern shrimp themselves are food for redfish and hake. As warming continues and shrimp stocks stay low, only time will tell if these other species begin to feel the ripple effects.

Implications for Fisheries Management

Perhaps the most important lesson for fisheries management is to think holistically. Overfishing one species never just affects one species. Overharvesting impacts an intricate collection of animals interacting in a dynamic environment. The overall effects of a single overharvest are difficult if not impossible to know and often larger than the sum of their parts.

To effectively sustain populations, we need to stop looking at stocks as isolated entities and instead manage them as one part of a larger ecosystem—particularly in the face of climate change and a rapidly warming and acidifying Gulf of Maine.

Admittedly, this is not a simple task. As we have learned in New England, there are key legal considerations that need to be addressed (the Magnuson-Stevens Act requires overfishing levels to be determined on a stock-by-stock basis). But we know that the current set-up is not sustainable: New England has more overfished stocks than any other region in the U.S., and failing stocks regularly lead to headlines like, “Does fishing have a future in New England?”

A changing ocean calls for a change in management. We need holistic fisheries management and innovative stock assessments that account for climate change. And let’s not forget that humans are a very important part of the marine ecosystem. Our fisheries, our culture, our economy, and our laws impact what happens beneath the waves—from the surface swimmers to the bottom feeders.

Clayton Starr is a guest writer for Talking Fish. He is a recent graduate from Bowdoin College where he earned a bachelor’s degree in Environmental Studies and English with concentrations in Marine Science and Creative Writing. Clayton hopes to communicate science to spread understanding and, ultimately, to inspire climate action.


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