Study raises questions about the impact of sea farms on wild fish
In recent years the number of open-water aquaculture operations has quickly grown to accommodate consumers’ taste for seafood amid declines in many of the world’s fish stocks. The interplay between farmed and captive populations of fish has, to date, been poorly understood. Conventional wisdom held that aquaculture would relieve pressure on wild stocks by fulfilling consumer demand. This logic would seem to have been particularly relevant in the case of salmon, the most heavily farmed fish and an animal that has suffered significant declines in some wild populations over the past three decades. However recent findings by researchers at the University of Hawai‘i, the University of Alberta, the University of Victoria, and the Raincoast Research Society indicate that salmon farms harm wild salmon populations.
The research, conducted in the Broughton Archipelago of British Columbia, Canada, was published in the October 4, 2006 issue of the Proceedings of the National Academy of Sciences. The researchers found that infestation rates of sea lice dramatically increased in wild juvenile salmon migrating past salmon farms on their way to the open sea. The paper concluded that wild salmon fingerlings measuring two inches or less in length suffered increased mortality due to the sea lice infestations. “Sea-cage operations have no way of separating farmed animals from pathogens floating in the ocean. So once these pathogens penetrate the farms, the farms turn into pathogen incubators,” says Neil Frazer, a researcher at the School of Ocean, Earth Science and Technology at UH Mānoa and a co-author of the paper.
Salmon-specific sea lice, Lepeophtheirus salmonis and Caligus clemensi, graze on the skin of the fish. Infestations of one or two lice rarely kill adult salmon, which have protective scales. Salmon fry, however, do not yet have these scales. By breaking the skin, lice make it difficult for small fish to maintain their internal salinity at levels lower than that of salt water. The sores created by lice also provide a pathway into the body for other pathogens. Even one louse is sufficient to kill a very young pink or chum salmon, says Frazer.
To estimate infestation rates, the scientists sampled over 14,000 juvenile salmon along 111 miles (178.6 kilometers) of salmon migration routes. In the study area, the migration routes began at the heads of inlets, far from salmon farms, then passed the farms on the way to open ocean. The pattern of infestation showed that salmon farms were the primary source of sea lice larvae in those waters, greater than background levels by many orders of magnitude.
The scientists also sequestered 3,000 infected and uninfected juvenile salmon in observation pens to measure mortality rates. Using mathematical modeling techniques, they combined the mortality data and infestation data and found that wild juvenile salmon passing salmon farms suffered mortalities ranging from 9% to 97%. The wide range of estimates included all environmental conditions, since low temperature and low salinity retard sea lice development. Under typical coastal conditions in the study area, the measured increase in mortality is more than sufficient to significantly reduce wild populations, the researchers believe.
Frazer says that wild adult salmon carry many pathogens, but when the adults lay their eggs and die most of their pathogens die with them. During the six-month interval between the death of adult salmon and the emergence of the baby salmon, the pathogens die out because adult salmon are not present, and the pathogens cannot live without a host. Salmon farms eliminate this natural fallowing by keeping adult salmon present year-round in coastal areas.
Critics of the study, including researchers working for Canadian aquaculture organizations, claim that wild salmon populations in the Broughton Archipelago remain healthy and numerous, and that the study was flawed because it tended to select sickly animals over healthy ones due to the higher likelihood that less robust fingerlings would be caught in the seine nets used to collect samples. The researchers reply that wild populations increased greatly during a recent period when eleven farms were fallowed – indicating a strong population-level effect of farms – and that their sampling methods have been subjected to published scientific peer review. Says Frazer, “Basic physics shows that this problem isn’t unique to sea lice and it isn’t unique to salmon. Our findings suggest that removing sea cages from open-water locales is necessary to prevent declines of wild fish.”
Neil Frazer is a faculty member in the Department of Geology & Geophysics at UH Mānoa. He received his Ph.D. in seismology from Princeton University. His marine research interests include acoustical tracking of humpback whales and sperm whales, and the biosonar model for humpback whale song.
Photos: Alexandra Morton, Raincoast Research Society