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Archived – Ontario study examines impact of net pens on wild stocks
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More wild fish found around freshwater cage sites researchers find
The program looked at fish abundances and stable isotopes in a selected number of fish at five different cage sites in the general vicinity of Manitoulin Island.
If wild freshwater fish in Lake Huron are consuming fish feed and faeces from trout farms of the lake, they're doing so only in such miniscule quantities that it's not altering the ratios of the three most indicative stable isotopes in their bodies, according to a recent study in the area.
According to the lead scientific researcher on the project, Dr. Tom Johnston of the Ontario Ministry of Natural Resources, the results are good news for the lake's fish farmers: the project shows that their operations do attract increased numbers of fish from the wild, but they appear to do so for reasons apart from the sites being seen as a potential food source.
"Our study wasn't designed to demonstrate whether or not it (the pen-site waste) is adversely affecting them, but it doesn't appear to be," said Johnston.
The study, which was funded through OMNR, the Department of Fisheries and Oceans and the University of Waterloo, detected no dietary shift in either the fish or in the zebra mussels, crayfish and larval may-flies the field personnel also collected from the lake floor at the sites for analysis.
Johnston said the program looked at fish abundances and the stable isotopes in a selected number of fish at five different cage sites in the general vicinity of Manitoulin Island.
The field program followed a pilot study carried out in 2002. Using a DFO research boat field personnel set bottom-grounded gillnets at farm sites and at reference locations about one-two kilometers away. The reference sites were chosen to closely match the fish habitat at each farm site for substrate and water depth.
Researchers discovered that wild fish like the pens for non-food reasons, perhaps for shelter against birds and other predators.
Some nets had smaller meshes, while others had larger meshes to catch only the larger fish, but care was taken to ensure consistency in which mesh-size was used where and when.
The two sets of nets were left in place from about 7 p.m. until about 9 a.m. the next day, when they were retrieved and the fish counted, noted for species, measured for length and weight, and some had samples taken for laboratory analysis of their isotope ratios.
Based on this sampling design, Johnston said, the fish farms were shown to have no effect on the fish's isotope ratios, but it was noted that abundances of fish both large and small were greater around the pens than at the reference sites, especially spot-tail shiners, which only grow to about 75-100 mm long.
Apparently, said Johnston, the fish like the pens for non-food reasons, perhaps for shelter against birds and other predators.
"That was pretty consistent across all the sites," said Johnston. "I'd say the abundance was anywhere between about 50 and 100% higher at the pen sites, but you have to remember that fish abundances estimated from gillnet catches are highly variable.
Research team: Tom Johnston, Michael Whittle, Michael Power. For information contact Tom Johnston (Email: tjohnston@laurentian.ca). Submitted by DFO (ACRDP).
Quentin Dodd
