There have been numerous wake-up calls about the effects of climate change on marine life. As ocean waters heat up, they are bleaching corals. Growing levels of carbon dioxide are acidifying seawater, which is degrading the shells and skeletons of sea organisms. The rising temperatures are prompting fish to migrate to colder waters, even causing them to shrink.
Now climate change is starting to affect their sense of smell, a phenomenon that will worsen in the coming years if global warming continues unabated, according to new research. A sense of smell is indispensable to fish. They use it to find food, detect imminent danger and elude predators, to find safe environments and spawning areas, even to recognize one another.
To lose it could threaten their very survival. If this happens, it also would mean big trouble for the fishing industry, tourism and, most importantly, global nutrition, since many of the world’s people — including its poorest — depend on fish for food.
Fishermen in Vietnam. PHOTO: Pexels
“Future levels of carbon dioxide can have large negative effects on the sense of smell of fish, which can affect fish population numbers and entire ecosystems,” said Cosima Porteus, a researcher at the University of Exeter and author of the study, which appears in the journal Nature Climate Change.
“This can be prevented, but we must reduce carbon emissions now before it’s too late.”
Carbon dioxide combines with seawater to produce carbonic acid, which makes the water more acidic. Since the Industrial Revolution, oceanic CO2 has risen by 43 percent and is projected to be two and a half times current levels by the end of this century, according to the scientists.
Experts believe that about half of anthropogenic carbon dioxide — that is, emissions produced by human activities, such as the burning of fossils fuels — has over time ended up in the oceans, lowering the pH of seawater, and making it more acidic.
The sea bass used in the study. PHOTO: Cosima Porteus/Nature Climate Change
They found that sea bass exposed to the more acidic conditions swam less and were less likely to react when encountering the smell of a predator, offered to them in the form of very dilute monkfish bile. Also, they were more likely to “freeze,” a sign of anxiety, she said.
“I found the longer they were in high CO2, the worse they fared,” she said. The scientists also measured the ability of the fish to detect certain odors in different levels of acidity by recording their nervous system activity. “I recorded the olfactory — smell — nerve response by measuring the electrical activity of the nerve to these different odorants in the water that flowed over the nose of the fish in both normal and high CO2 seawater,” Porteus said.
“The odorants tested were those that would be involved in finding food — amino acids — and in recognizing fish of the same or other species, including bile acids, bile, intestinal fluid, etc., at different concentrations, and at levels they would encounter in the wild,” she added.
The researchers found that seawater acidified with levels of carbon dioxide that are expected by the end of the century — if global warming continues — reduced the sense of smell of sea bass by half, compared with today’s levels.
“Their ability to detect and respond to some odors associated with food and threatening situations was more strongly affected than for other odors,” Porteus said. “We think this is explained by acidified water affecting how odorant molecules bind to olfactory receptors in the fish’s nose, reducing how well they can distinguish these important stimuli.”
They did not compare the impact of today’s ocean acidity levels with those of pre-industrial times, although they plan further research to do so. “It is possible that sea bass are already being affected by a rise in oceanic pH,” she said.
Fresh catch at the fish market. PHOTO: Pixabay
The researchers also studied the impact of high levels of CO2 and acidity on genes expressed in the nose and brain of sea bass and found them altered — but not in a good way. Rather than adjust, things deteriorated, Porteus said.
“The gene expression experiment was conducted to see if these fish were able to compensate for their loss of sense of smell over a short period of time, not generations,” she explained. “Animals have some ability to respond to a stressful condition by making more proteins or different proteins that work better under different conditions.”
Researchers can determine this by looking at what genes change or are different between animals exposed to different conditions, normal and high CO2, for example, according to Porteus.
“One way to smell something better is to have more receptors detecting these smells in order to increase the chance that particular smell will be detected, and therefore increase the expression of these receptors,” she said. “Another way is [for them] to make a slightly different receptor that works better under lower pH. However, we did not find any evidence this was the case.”
Instead, they found the fish were making fewer such receptors, making it more difficult for them to detect smells, she said.
“There was a decrease in ‘active’ genes, indicating that these cells were less excitable, therefore responding even less to smells in the environment,” she said. “This means that these fish had a reduced sense of smell and instead of compensating for this problem, the changes in their cells were making the problem worse. This matched our observations of their behavior.”
Sea bass. PHOTO: Pixabay
The team chose to study European sea bass because they are an economically important species, both for food consumption and for sport fishing, Porteus said.
Nevertheless, “we think the ability to smell odors is similar in most, if not all, fish species, so what we have found for sea bass will almost certainly apply to all fish species, and maybe invertebrates too, such as crabs, lobsters etc.,” she said. “So all the commercially important species are likely to be affected in a similar way, such as salmon, cod, plaice, turbot, haddock etc.”
This is important because 20 percent of the protein consumed by 3 billion people comes from seafood, and about 50 percent of this comes from fish caught from the wild, according to Porteus. “Therefore, increases in carbon dioxide in the ocean have the potential to affect all fish species, including those that many people rely on for food and livelihood,” she said.
Marlene Cimons writes forNexus Media, a syndicated newswire covering climate, energy, policy, art and culture.
[Note: This is the third blog in a series about the WCS-led marine megafauna survey, which is gathering data on whales, sea turtles, sharks, and other marine species inhabiting the coastal waters of Bangladesh. Data from the effort will identify biologically important locations for future consideration as marine protected areas.]
The WCS marine megafauna survey currently underway along the coast of Bangladesh involves two vessels. The larger vessel follows a transect line surveying for whales, dolphins, and porpoises (collectively known as cetaceans). Researchers on the smaller vessel focus on active fishing vessels by collecting data related to their catches and learning from fishers about the nature and scale of their fishing effort.
Recording various gear specifications contributes to our understanding of coastal artisanal fishing practices. Photo credit: WCS Bangladesh.
The team aboard the second, smaller survey boat, F. B. Jobeda, has the task of assessing the catches and bycatches of several different types of fishing gear operating in the coastal waters of Bangladesh. While the targeted finfish are of considerable interest, we are especially keen to investigate other species such as dolphins, porpoises, turtles, sharks and rays, as well as sea snakes—some of which are accidentally taken as bycatch.
On day two our team was preparing for the day’s work when we spotted a long liner—locally known as the Boiral—a fishing vessel that uses lines with thousands of baited hooks. By the time we reached the boat and established contact, the fishermen were already pulling in their lines.
A bigeye houndshark was identfied for the first time in our waters. Photo credit: WCS Bangladesh.
As a researcher, investigating this haul was fascinating, but on a personal level it was deeply disturbing. Among the 1150 hooks we examined there was a shark caught on almost every one. We managed to identify four different species, including an immature tiger shark.
The catch also included several individuals of a relatively small shark species that the local fishermen call “Gule Kamot” and which we didn’t immediately recognize. These beautiful fish have large sparkling eyes and a stout snout. Upon looking at them more, we identified them as big-eye hound sharks, a new species we had yet to record in Bangladesh.
The baited hook and line fisheries target large finfish but catch large numbers of sharks. Photo credit: WCS Bangladesh.
One of the females had 19 pups in her belly, and several others were clearly immature. We learned that these sharks have a very low market value and are used only as bait or simply discarded. Also, we discovered that some of the longliners are targeting endangered hammerhead sharks, due to their high market value.
This experience has strengthened my resolve to help find a balance between protecting of our country’s large and diverse marine megafauna and ensuring that fisheries are sustainable. Fortunately, WCS is partnering with the Government of Bangladesh to establish a network of marine protected areas that will promote sustainable fisheries while conserving threatened species such as hammerhead sharks.
This beautiful honeycomb whipray was caught in a drifting gillnet along the southern coast of Bangladesh. Photo credit: WCS Bangladesh.
The information collected by survey participants, as well as other data collected by a WCS-led citizen science network at fish landing sites throughout coastal Bangladesh, will also help inform future management actions and international shark and ray conservation efforts.
——————————————— Shanta Shamsunnaharis the Marine Protected Area Program Coordinator for WCS’s Bangladesh Program and a research participant in the survey.
Humpback whale lunge-feeding on menhaden in New York waters. Photo: Artie Raslich/Gotham Whale
One commercial fishing company is cruising off the shores of New York, taking whales’ food.
One year ago we reported that small, herring-like fish called menhaden, a major food source for whales, dolphins and large fishes, were making a comeback in New York waters. But then, last year, fisheries managers decided to increase the menhaden catch limit by 8 percent, or 216,000 metric tons of fish—allowing hundreds of millions more menhaden to be caught off the U.S. East Coast each year.
Why? One fishing company won out over whales, dolphins, fish and thousands of recreational fishers and whale watchers. As Safina Center Fellow Paul Greenberg has prolifically discussed and has covered in his new book, The Omega Principle, menhaden and other small, oil-rich fish are caught by major corporations in huge quantities, only to be ground up and turned into fertilizers, aquaculture feed, human health supplements and pet food. This creates a situation where there are fewer fish for marine animals to eat, and where fish caught are not used to directly feed humans. It is one of the least efficient ways to use fish.
Since the catch limit was increased, menhaden fishers in waters off New York have become especially active, pulling up enormous quantities of fish, reports Paul Sieswerda, executive director at Gotham Whale, a New York City whale research and advocacy organization. And Sieswerda says that one company—Omega Protein Corporation—is pulling up extra-large quantities of these important fish.
One of Omega Protein’s ships catching menhaden just outside New York waters where humpback whales and other marine animals feed. Photo: Gotham Whale
We recently spoke to Sieswerda to learn more about the current situation in New York’s waters and what needs to be done now to protect these important fish from exploitation by companies like Omega Protein.
Safina Center: What are menhaden, and why is menhaden conservation a focus of your organization?
Paul Sieswerda: Menhaden, called “bunker” in New York (and by many other local names along the U.S. seaboard from Maine to Texas) is a bone- and oil-filled herring-like fish, 10 to 12 inches in length. It has also been called “the most important fish in the sea,” by H. Bruce Franklin, the author of a book by the same name. Virtually all predators in the sea, except humans, eat menhaden directly. The whales of New York City feed directly on these fish and are the reason for the comeback of marine mammals to our area.
SC: Why is menhaden so important for the health of whales? Do any other marine animals rely on menhaden for survival?
PS: Whales come to northern latitudes to feed all summer before returning south to warmer waters to mate and give birth. They must acquire all their fat reserves while feeding here in the north because they do not feed during their winters in the tropics. The menhaden are at the bottom of the food pyramid feeding on algae and zooplankton. They provide food for sport fish like sharks, bluefish, striped bass, seabirds and anything with a mouth big enough to take them at each stage of their growth.
SC: Why do people catch menhaden?
PS: There is a local fishery for bait. Fishermen jig them individually for single hook sport fishing. Local fishers net them as bait for their traps and to ship to Maine for the lobster fishery. And the Omega Protein Corporation nets them on an industrial scale for the “reduction fishery” where they are turned into animal feed and additives for a myriad of products. This is a single company industry, operating out of Reedville, Virginia, which was recently bought by the Cooke Seafood, a Canadian aquaculture and seafood conglomerate.
SC: What threats does the Omega fleet pose in terms of conserving menhaden?
PS: The Omega Fleet uses a very efficient method of fishing. The fleet consists of 10 large “factory” ships close to 200 feet long, some of which can transport two boats of about 40 feet each, filled with nets. These boats ply out the nets around the menhaden school, guided by spotter planes that also are used to find the menhaden. The purse seine net surrounds the entire school, closing off the bottom giving the fish no escape. The nets are then brought next to the larger ships and vacuumed into the holds until they are filled and returned to Virginia.
Rinse and repeat…. This method cleans the local area of bunker much like clear-cut loggers reap the forests of the northwest. There is nothing left for grazing whales.
SC: What actions does Gotham Whale seek to take to alleviate the threats posed by the Omega fleet?
PS: Gotham Whale is trying to raise public awareness because the legislative, regulation, and policy approach is not working. The limit of the total allowable catch by industrial fishers is, in our opinion, too high. The State of New York, which prohibits menhaden fishing without a permit, only extends three miles off the coast, which is barely a stone’s skip compared to the enormous area where fishing is allowed. While we are supporting the efforts to improve these restrictions, the fish could be gone before any action takes place. We hope to influence the company itself, through public relations or investor direction.
SC: What can readers do to help or take action?
PS: Gotham Whale is developing an action plan to support better regulation and public awareness, to try to influence this and other companies. Please stay tuned for updates by following Gotham Whale on our website, Facebook, Twitter and Instagram. Help us fight the fight!
Another Humpback whale lunge-feeding on menhaden in New York waters. Photo: Artie Raslich/Gotham Whale