It was a mystery that puzzled researchers at the University of Maine’s Center for Cooperative Aquaculture Research (CCAR) in Franklin, Maine: What caused hundreds of Atlantic cod in a research project there in 2004 to develop severe cataracts at higher rates than expected?
Based on extensive studies carried out in the last five years, UMaine Ph.D. student Kevin Neves and CCAR Director Nick Brown have determined that cod living at high densities were exposed to higher levels of carbon dioxide, causing them to grow cataracts and eventually go blind. Fish that cannot see to locate their food cannot grow.
“Any animal produces carbon dioxide when it respires, and the water systems that we have here are all recirculating,” Neves says. “Unlike humans, who breathe out carbon dioxide and it dissipates into atmosphere, (in water) carbon dioxide has a tendency to dissolve. Back in 2004, we didn’t have an accurate test for dissolved carbon dioxide in seawater, so we used a formula based on temperature, pH and alkalinity to get an approximate value. We found out that the carbon dioxide could have been 30 parts per million when the average seawater has less than one part per million.”
Although Neves says the blind cod are perfectly safe to eat, there are aquaculture implications for fish with cataracts. If a cod with cataracts were to survive long enough to make it to market, it would have lower value.
“The major goal of the fish raised here in the net pens is for them to be sold in the live markets in New York, Boston and Philadelphia,” he says. “There’s a lot of money at stake. These fish were almost jet black instead of the silvery brown color you’d expect from a cod. They had bright white eyes. Some of them were decent size but most were smaller. They just didn’t look like something you would buy in a market.”
In fact, the impact of carbon dioxide in water could reach beyond aquaculture in the deep future, when carbon dioxide levels are predicted to rise and could affect many more species.
In 2004, Brown was working on a grant with local salmon farmers to determine the feasibility of using existing salmon farms to also grow Atlantic cod. The cod used in the research were initially raised at CCAR and were then to be transferred to net pens at salmon farms. However, due to unforeseen delays, the cod stayed at CCAR for longer than anticipated, and at high densities. By the time the fish were moved, it was discovered that more than 90 percent of the cod had developed cataracts.
The cod did poorly, experiencing high mortality and low growth. Within another year, 98 percent of the cod were blind. The salmon were unaffected.
The researchers took tissue, blood and other samples from the blind cod, and ruled out viruses, toxins and pathology. Diet may have been a factor, the researchers theorized, but all of the food the cod had been fed was long gone. Genetics could have been an issue, but there are multiple males and females, and the chance that every single one would have been blind is very unlikely.
Brown later read a Danish Technical University study of carbon dioxide and Atlantic cod, which anecdotally noted that as researchers increased the levels of carbon dioxide, they noticed more cataracts. This, Brown reasoned, could explain the 2004 incident.
Brown approached Neves, who arrived at UMaine in 2008 for graduate school, about pursuing the research. Neves considers himself a fish nutritionist and didn’t know a lot about the fish eye or carbon dioxide, but agreed to take on the mystery.
Based on a conversation with one of Brown’s previous colleagues, Grethe Rosenlund at the Aquaculture Research Centre in Norway, which is run by major aquaculture feed producer Skretting, the researchers decided to investigate whether an amino acid that helps salmon maintain proper eye development would have an effect on cod. The amino acid, called histidine, is thought to buffer the eye from changes in the environment, which is critical for salmon that move from freshwater to saltwater and are susceptible to a type of cataract.
Skretting formulated three feeds – one that was histidine-deficient, one that had the adequate amount of the amino acid, and another that had extra histidine. The researchers and CCAR systems manager Christian Cox also set up three treatments at CCAR – one with carbon dioxide levels of 20 parts per million, which is considered a safe level, and two other tanks at lower levels. It wasn’t an easy feat of engineering, Neves says. The amount of carbon dioxide delivered to each treatment was controlled using automatic valves connected to carbon dioxide sensors, so if carbon dioxide levels rose too high, the valves closed and when the levels dropped too low, the valves opened.
Carbon dioxide was physically removed from the water bypassing it through one of two degassing towers, which helped to precisely control the levels of carbon dioxide. There were an industry-standard 150 fish per tank, and the fish were raised on the food with the different levels of histidine until they weighed 100 grams.
Ultimately, the different diets didn’t matter. Within one month, cataracts were already starting to show up in the high-carbon dioxide tank. Six months later, the researchers saw 96 percent cataracts in the high-carbon dioxide tank, regardless of histidine levels in the feed. Even in the tanks with lower levels of oxygen, up to 40 percent of the fish had cataracts after six months. Neves, along with CCAR staff, also sampled 600 cod that had been moved to the net pens when they were much smaller and had therefore spent less time in the tanks, and found just 1.1 percent had cataracts.
The fish in the high-carbon dioxide tank were also smaller than average and nearly jet black instead of the brown silvery color of cod.
Although the fish were smaller than average, they were likely able to survive because the tanks were a sort of artificial space. The tanks had a bottom, which meant the cod could find food that had fallen to the floor of the tank.
Neves and Brown also considered the possibility that something else was causing the cataracts, and sent out hundreds of blood and tissue samples for analysis. So far, nothing has come back as positive. They also worked with Clive Devoy, the laboratory supervisor at UMaine’s Sawyer Environmental Chemistry Research Lab, to track changes in the level of calcium in the eye; Dawna Beane, a histologist with the UMaine Animal Health Lab, to track physical changes in the eye; and Brian Perkins, a research assistant professor in UMaine’s Department of Food Science and Human Nutrition, to do amino acid analysis.
Neves, who has Type I diabetes (diabetes raises the risk for cataract development), also measured blood glucose because fish do not have a good method of maintaining their blood sugar levels. He found that the healthiest fish had the highest blood sugar levels.
There is no way to tell how many cod in the open ocean have cataracts, Neves says, because those fish would never survive an environment in which they could not see their food. However, considering the models that project increases in ocean acidification, which is the result of increased carbon dioxide in the atmosphere, Neves believes there is a possibility in the future that cod and other fish could be affected.
“Right now the levels in the ocean and atmosphere are at equilibrium, but as more carbon dioxide is pumped into the atmosphere from greenhouse gas emissions, eventually you’re going to shift the carbon dioxide into the water, which will lower the pH of the water and increase the carbon dioxide levels,” he says. “The levels they’re predicting, say, 100-200 years down the road, are those levels we would see at our low carbon dioxide treatments. It’s already been shown to affect invertebrate animals such as corals and some clam species.”
Although those outcomes are likely hundreds of years away, Neves hopes his research will heighten awareness of the need for water quality parameters, and the effects of those parameters on aquaculture and local cod farmers.
“Water quality parameters are clearly species-dependent, so carbon dioxide levels of 20 parts per million could be OK for very hardy fish such as tilapia or catfish,” says Neves. “But to extrapolate that to a very wide range of species is not going to work, and hopefully this can help people be more productive in terms of Atlantic cod aquaculture. Right now, if 10 percent of the cod have cataracts, that’s 10 percent the farmers have lost right there.”
Contact: Jessica Bloch, (207) 581-3777 or firstname.lastname@example.org