Study indicates currents slowed by fish farms could negatively impact local environment

A newly released, 11-year study into the impact of a fish farm in Port Mouton Bay could change how environmental assessments are done for new aquaculture sites, and may cause pen size and location to be re-considered for existing sites.

In 1995 a fish farm was approved for Port Mouton Bay, a small community with a big bay on Nova Scotia’s South Shore (Atlantic Coast). After the farm went into operation residents noticed changes in their water and shore, lobstermen found a decline in catch. The residents formed an organization, “The Friends of Port Mouton Bay,” and enlisted the help of researchers and ocean scientists to study their situation. The research team were aided by 30 fishermen working on 15 boats. 

Inka Milewski, a marine biologist and research associate at Dalhousie University in Halifax, and lead author, says the study was conducted in the last two weeks of May for 11 years, between 2007 and 2017. Port Mouton Bay is an area that has been studied by scientists since the 1940s and with generations of lobstermen working the waters, had a quantifiable history. 

“The weeks chosen are a period when the lobsters are known to migrate in. The fishermen in each of the fishing regions provided us with data on their landings of market and berry lobsters on a confidential basis,” Milewski said. “We standardized the data by effort, so we converted the data to catch per unit of effort or counts per thousand trap hauls. And we used this information to calculate the numbers and catch per unit effort (CPU). And we also had some bottom temperature data in one of the fishing regions, which is important because temperature has an important role in the catchability of lobster.”

The data was collected in the bay over a period of 11 years, between 2007 and 2017, and showed that whether the fish farm was in operation had an impact on the catch lobstermen were pulling in. 

“In that 11-year period we had two feed and two fallow periods, meaning periods when the fish farm was in production and periods when the fish farm wasn’t in production,” Milewski said. “The pattern is what is important. During periods when the fish farm was in production the catch across the regions dropped an average of 42 percent for market lobsters. Berried lobster counts dropped by 56 percent.”

The research team also considered other factors for this discrepancy, to ensure they weren’t missing anything. 

“We looked at temperature, and found that during feed and fallow periods the temperature was not significantly different. So we know that things like molting, bait, traps, effort, wind, all of these things affect the catchability of lobster. And we eliminated pretty much all of those,” Milewski said. 

Lobster’s molting wasn’t a factor, as fishermen catch them before they molt. The fishermen were also using the same type of bait, and the same type of traps. Temperature was standardized, and found to not impact the lobster during fallow periods. During periods of time when the aquaculture facility was in production, it seemed to override any kind of temperature affect, according to the study. 

“We saw that more significantly with the berried lobsters. We think the reason for that is these berried lobsters are carrying eggs, which they are trying to protect and are far more cued to small temperature changes,” Milewski said. “But the aquaculture effect, added on to the temperature effect, made the counts even lower.”

After accounting for temperature, bait type, trap type and more, the only thing left that could affect the catch, according to Milewski, is factors related directly to the fish farm located in the bay. 

“We think it’s water quality changes, specifically odor plumes and fecal or particulate matter produced by the open net pens,” she said. “Odors play a significant role in the behavior and ecology of lobsters. They provide chemical cues to locate food, find mates, select habitant and detect predators and environmental stressors. And we know that lobsters are particularly sensitive to dissolved sulfides, ammonia and low oxygen conditions. Berry lobsters are more sensitive to these odors and temperatures.”

The fish farm had a license for 400,000 fish. It originally held salmon, then went fallow from 2009-2012, was sold, and returned with rainbow trout. It was been fallow from 2015-2017. 

During productive times for the farm, local residents and fisherman noticed increases in algae on the beach, eel grass and traps. 

“We did an earlier study, published in April, looking at the waste discharge from this fish farm and its dispersion potential, meaning how far do the currents take these nutrients,” Milewski said.  “We had some current meter data and found there is really not a very strong or strong enough current in the vicinity of the fish farm to really dilute and disperse the waste coming from this particular fish farm to counteract those waste discharges. We concluded there is insufficient current speed to mitigate the impacts that could potentially occur because of the volume of waste discharges from the fish farm.”

This led the researchers to pinpoint a problem with current environmental assessments, which doesn’t take into account the influence the fish pen will have on the current. Current assessments are done without the fish pen in place, and don’t factor in the direct influence the pen and the fish inside will have. 

“They often put in current meters and get a reading and think that’s okay. But what the science and the literature shows is that when you suspend these net pens in the water and fill them with fish, there’s a baffling effect that basically slows down the current speed,” Milewski said. “Depending on the height of the net pens in relation to the depth of the water the ability to erode all of that fecal waste and uneaten food at the bottom is diminished if that ratio of net height to water depth is about .5. In Port Mouton the net pen height and water depth is so small, like .8, the ratio is large that there is a huge baffling effect that can get a 40-60 percent reduction in the current.”

Milewski believes a more robust environmental impact assessment process includes the baffling effect of net pens would eliminate unsuitable sites in advance.


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