CANADIAN CONFERENCE FOR FISHERIES RESEARCH (CCFFR)
THE J.C. STEVENSON MEMORIAL LECTURE

Ecosystems are ferocious monsters with vast amounts of energy flowing through a web of channels that is difficult to describe, often unpredictable, and sometimes downright hostile. The link between energy flow, (trophodynamics) and the flow of matter, such as nutrient elements and contaminants, through aquatic food webs is vital to our understanding of ecosystem function. Our ability to achieve complex management objectives that balance issues of productivity (water quality and fisheries yield), nutrient enrichment, contamination, habitat quality, and community composition requires this understanding. However, whenever Hercules was able to cut off one of the monster's ferocious heads, two grew back in its place. Thus while we are confronting eutrophication, contaminantion, and fisheries collapses, armed with all of the novel science we can muster, when we look up we see a proliferation of new problems: new species have invaded and proliferated, and the climate and hydrology are changing.

Our group, unlike Hercules, has not had to battle the Hydra alone. Our contribution has focussed on two key ecological variables (1) the trophic position and (2) the bioenergetic efficiency of the consumer and the food chain below it, as major determinants of the degree to which persistent contaminants such as PCBs, DDT, radiocesium, and methyl Hg bioaccumulate in fish. These factors can, in turn, be influenced by other ecological factors, such as the introduction of exotic species that lengthen the food chain, the presence of ecotypes that functionally diversify the web, and by the size structure of the underlying prey community, which can, in turn, be influenced by a number of factors such as toxicity. Our studies have emphasized the use of isotopic tracer techniques (C and N stable isotopes, and contaminant mass-balance models) for dealing with food-web and bioenergetic variability. Recently we have applied in situ metabolic enzyme measurements (Lactate dehydrogenase in muscle) to the in study of fish bioenergetics and ontogenetic diet shifts, and shown how they can be used to calculate activity costs of fish, and thereby trophic efficiency. The long-term goal of my research program is to link the make-up of the community to the efficiency of energy and material transfer through the trophic web. It is my view that the efficiency with which energy and matter are transferred through the food web to higher consumers has important influences on both fish productivity and contamination, and that transfer efficiencies are highly variable among systems due to the variable makeup of the food webs. It is also my view that our past history in the area of ecosystem management has been to wage war on the monster, and I fear that the monster is defeating us. In the future, we will have to make peace with it, if we are to coexist.