A long-awaited multiyear study of the impact of cage aquaculture on freshwater lakes gives some relief and hope to a controversial Great Lakes industry, but isn’t a slam-dunk endorsement of raising fish in open pens.
The study, “Impacts of freshwater aquaculture on fish communities: A whole‐ecosystem experimental approach,” was published in the February 2019 issue of Freshwater Biology. The ambitious investigation deliberately overloaded one small lake, to produce results that could guide aquaculture industry operations, or conceivably bring them to an end. The main concern has been the impact of nutrient loading, from fish and fish-food waste, on the greater ecosystem.
Already, concerns about possible negative impacts on the natural environment have slowed industry growth, and there has been a lot riding on this study’s findings. Ontario produces about two-thirds of the farmed rainbow trout in Canada, averaging about 4,500 tonnes annually since 1996, but not all farms are open-water cages. Existing open-water cage operations are in Parry Sound and the North Channel of Georgian Bay. For all Ontario aquaculture operations, see Ontario Seafood Farmers.
Ten thousand rainbow trout were raised annually in aquaculture cages from 2003 to 2007 in a “whole lake” experiment, using Lake 375 in Canada’s IISD Experimental Lakes Area, a region of 58 small lakes and watersheds in northwestern Ontario. (The study almost didn’t survive the federal government’s decision in 2012 to close down the ELA, which had been run by the Department of Fisheries and Oceans. The ELA was rescued in 2014 and is now run by the International Institute for Sustainable Development.)
Researchers employed other ELA lakes as controls, and used and gathered data for the aquaculture lake for eight to ten years before the experiment and ten years after it was over. The result was a comprehensive assessment of how an open-cage freshwater aquaculture operation affects other species, something that has not been extensively studied.
Overall, the study can be considered good news for the aquaculture industry. The presence of the aquaculture operation had a dramatic positive impact on prey species of fish (minnows and sculpin). As well, the farm was a boon to a predator species, lake trout, that has been the subject of restoration programs in Canadian and American Great Lakes waters. Lake trout numbers more than doubled during the 2003-07 study period, before crashing back to pre-experiment levels just two years after the study ended.
Not all the news was good, though. The aquaculture operation had a long-term negative effect on the ideal “oxythermal habitat” (less than four milligrams of oxygen per liter of water and temperatures of less than 15 celsius). White sucker numbers suffered, as did freshwater shrimp (Mysis). “The continued depression of Mysis densities and optimal oxythermal habitat availability nearly a decade following operations suggest potentially long-term impacts at this magnitude,” the study notes.
Asked if the results suggested a net benefit to the environment from open-cage freshwater aquaculture, the study’s lead author, Michael D. Rennie, Canada Research Chair in Freshwater Ecology and Fisheries at Lakehead University, replied: “I think the perspective of net benefit depends on what you are talking about.” He says lake trout did well over a five-year period, which was almost entirely due to increased growth rates, “but whether that’s representative of long-term I don’t think our study can say. There were tons of minnows in the fall but there’s strong evidence of massive overwinter mortality. White sucker generally seemed to do poorly. On the plus side, the ecosystem does seem to have largely recovered over the past 10 years since aquaculture ended.”
That recovery was twice the duration of the experiment itself, which operated a fish farm at a scale “that would never happen on the Great Lakes.” Rennie stresses: “The ELA experiment was at a level that’s well outside what would ever be approved on the Great lakes—in other words, the size of the farm relative to the size of Lake 375 is massive compared to anything that would ever be approved on the Great Lakes. The point in Lake 375 was to understand pathways of effect and to match nutrient loading to other ELA experiments that focused on nutrients, which it did very well.”
“To me,” Rennie adds, “the most instructive part of the whole-lake experiment is in identifying key species to watch that might respond to aquaculture—when we hit the ecosystem with a hammer, we saw these particular species respond in this way.” With a fish farm operating in the Great Lakes, he says, we might want to monitor species like lake trout and white suckers in the area, as well as other species like Mysis and minnows, depending on the farm location, to see if the farm is having an environmental impact.