Stocking of fish species have been instituted, often, as a means of preserving sport fisheries in response to diminished indigenous stocks and public pressure. The effects of these “stop-gap” measures have usually met with failure and have been considered to be expensive and a “bandage” solution masking and more importantly exacerbating the real problem.

In 1996 the Nottawasaga Steelheaders in conjunction with the University of Guelph’s Fish Genetics Laboratory completed a study of the genetic stock structure of wild Nottawasaga River Steelhead. This was the first of its kind, and was undertaken to determine the genetic diversity within this population and compare tributary strains to those of other tributaries in other systems.

Over several months, samplings were collected from juvenile (121) and adult (5) steelhead from six Nottawasaga tributaries and major confluences. A total of 18 different strains (genotypes) were identified within this watershed. This represents the highest documented number of steelhead strains found in any river system in the province of Ontario. Four of these strains were newly identified as being specific to only the Nottawasaga and Bighead River steelhead populations. The Upper Nottawasaga specifically exhibited the highest level of genetic diversity (89%) of any Great Lakes steelhead population. In fact this degree of diversity exceeds even those of steelhead populations of Lake Ontario’s Ganaraska River and New York’s Salmon River. In fact the entire Nottawasaga River steelhead population was genetically distinct from other tributaries in Ontario. This high level of genetic diversity reflects well on the abilities of Nottawasaga steelhead to overcome stressful environmental and climatic irregularities and has ensured high repeated spawner rates documented at Nicholson Dam.

Not only are Nottawasaga steelhead distinct from other river systems, but steelhead from tributary to tributary within the Nottawasaga are also genetically different. This so-called “local population structuring” further makes us aware of the complexity of this diversity. Wild fish from the Nottawasaga River drainage have what are termed “co-adaptive gene complexes” which enable fish from say, the Pine River tributary, to exhibit characteristics allowing them to survive that river well and not a neighbouring tributary, let’s say, the Mad River. These two drainage systems have differing geo-morphological characteristics and as a result, wild steelhead’s DNA have acquired years of coding, adapting and expression for that particular tributary from which they originated and live and not that of another.

The stocking of hatchery fish or fish from another drainage on top of an existing wild population leads to what biologists and geneticists refer to as “Out-breeding Depression”. This Out-breeding Depression means that when stocked fish mate with wild fish, their offspring are less likely to survive because they are lacking the co-adapted gene (DNA) complexes of their parents and which are deemed necessary for survival in that tributary environment. Even, the removal of eggs from wild fish to be reintroduced into the same system is detrimental. Fish will naturally seed their habitat to its maximum carrying capacity. Addition of more fish results in higher mortality due to excess competition for food and space thus reducing the total numbers of returning adults.