A different sort of fish sandwich

The seas’ most sought-after fish are swimming between a rock and a hard place: the fisherman’s net and an encroaching mass of suffocating water.

A recent study has uncovered a new dose of bad news for ocean fish and the fishing industry. Areas of the deep ocean with little dissolved oxygen, called dead zones, are expanding and, thus, shrinking many fishes’ watery homes.

One driving force behind the predicament is none other than that pesky climate problem.

“Climate change is actually working in tandem with overexploitation of the animals to push these populations into a real dangerous place in terms of population collapse,” said Eric Prince, a fisheries biologist with the National Oceanic and Atmospheric Administration’s Southeast Fisheries Science Center and co-author of the study.

For example, Prince and his colleagues calculated that the Atlantic blue marlin, an economically valuable fish that was a focus of their study, has lost about 15 percent of its habitat from expanding dead zones since 1960. Dwindling habitat threatens not only the lives of fishes, but also the sustainability of the already ailing fishing industry.

Breathing room

Like their above-water brethren, fish need oxygen, which is dissolved in the water. Big, predatory fish, such as the blue marlin, need more dissolved oxygen than most, because they require lots of energy to grow and survive. Without sufficient oxygen, they’ll suffocate.

The level of oxygen in the water thus partly delineates fish habitat boundaries. Dead zones often draw these borders.

As climate change causes open ocean dead zones to balloon, fish habitat deflates.

Diagram modified from one originally published in Deep Sea Research Part I: Oceanographic Research Papers, Vol 57, Issue 4, Lothar Stramma, Sunke Schmidtko, Lisa A. Levin, & Gregory C. Johnson. Ocean oxygen minima expansions and their biological impacts, 587-595, Copyright Elsevier (2010).

Technically known as oxygen minimum zones, dead zones are actually a natural occurrence. Found at depths of between 200 and 1000 meters, they are caused partly by seawater circulation and partly by the decomposition of organic matter, namely deceased sea critters that sink from surface waters.

As aerobic bacteria nosh on the organic matter, they use up the oxygen in the water. Eventually, hypoxia happens—the water becomes so depleted of oxygen that many creatures can’t survive.

Since deep-sea dead zones are insulated from the ocean’s surface, where the water borrows oxygen from the atmosphere, they can only reload with oxygen if currents make a long-distance delivery, according to Sunke Schmidtko, an oceanographer at the University of East Anglia, the other co-author of the study.

Deep-sea dead zones are different from their coastal cousins like the one in the Gulf of Mexico. Coastal dead zones form due to a buildup of agricultural fertilizer that rivers, such as the Mississippi, collect and then flush out to sea, causing abnormal blooms of plant life.

De-fizzing the ocean

But climate change is turning what Mother Nature does normally into a big problem. As the air is getting hotter, so is the water, and warmer water can hold less oxygen than colder water.

This is similar to what happens to a soft drink on a hot day. After sitting in the heat and sun, the fizz fizzles, and you are left with a flat, carbon dioxide-depleted beverage.

Also, warmer surface waters are less likely to sink to the ocean’s lower layers, because warm water is lighter than the colder water below, Schmidtko explained. In other words, as the oxygen-rich surface layers heat up, they could have a harder time delivering oxygen to the deeper ocean.

Schmidtko clarified that oceanographers are still trying to determine how exactly climate change is affecting the ocean, but with their knowledge of how water works, these represent their current speculations.

The rock below

With less oxygen to go around, oxygen minimum zones are swelling and intruding on many fishes’ living zones.

For example, marlins often dive deep to feed, sometimes as far down as 800 meters. However, in the eastern Atlantic’s growing dead zone, which is already one of the largest in the world, Prince found that marlins can’t dive as deep as their west-side counterparts.

“They need to go where the food is and where they can breathe,” he said.

With less breathing room below, the floor of their habitat rises, and they are pinned to the surface layers. With nowhere to go but up, marlins become squished into tighter, testier quarters with other predatory fish and their prey. They also find it harder to dodge a waiting fishing hook or net.

“Concentrating them makes it much easier for overexploitation by [humans],” said Prince.

The increasing concentration of animals at the top could also lead to a boost in the amount of sinking organic matter, which would further worsen the oxygen shortage below.

Softening the hard place above

As a prized catch, Atlantic blue marlins are already victims of overharvesting. In fact, their populations have dropped 60-64 percent over the past three fish generations (14-18 years).

But the growing dead zones can actually fool scientists and fishermen into thinking fish populations are doing just fine, since more fish are squeezed into a smaller area. Thus, to ensure the dead zone-fishing vise does not become their demise, Prince said scientists must more carefully monitor fish populations, as well as the expansion of the dead zones.

While fish stock assessments are starting to incorporate this information, Prince warned the pace needs to quicken.

And if the Earth is to continue warming, as most scientists predict, Schmidtko added that humans should chill out on fishing.

After all, we will never be capable of “ventilating the ocean,” he said.