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Perspective the degradation and abandonment of sustainable fisheries. In such systems, aquaculture can contribute to the genetic deterioration and loss of natu~al stocks, enhance the spread of diseases, accelerate the rates of invasion of exotic species, and promote habitat destruction and degradation (Folke et al. 1994). Stocks are the repository of the genetic diversity within each species and are the building blocks on which fish- eries management is based. Hence, the focus on fish stocks in the development of conservation principles is appro- priate. Species and, by extension, stock diversity is a reli- able indicator of biodiversity in aquatic systems (Moyle and Leidy 1992). Thus, a focus on preserving fish stock diversity is a practicable strategy for protecting overall biodiversity of aquatic systems. As fish are often primary determinants of ecosystem structure ( Brooks and Dodson 1965; McQueen et al. 1986; Carpenter 1988), protecting extant fish assemblages should usually protect extant ecosystem structure. In an individual water body, a popu- lation may be composed of one or several stocks. We believe that primary conservation requirements are met when the sustainability of individual fish stocks is secured. Supporting principles To ensure ecosystem protection, degradation of the phys- ical environment (e.g., habitat destruction), the chemical environment (e.g., water quality deterioration), and the biological environment (e.g., transfer of non-native stocks, introduction of exotic species) should be prevented. To ensure sustainable use of fish stocks, those populations that can be exploited, and those that cannot, need to be identified. For populations that can be exploited, the amount of harvest must be limited and the manner of harvesting must be controlled. The principles that we believe are nec- essary to effectively guide such actions are outlined below. Principles of ecosystem protection · The sustainability of a fish stock requires protec- tion of the specific physical and chemical habitats utilized by the individual members of that stock. The existence of clean, undamaged environments is crit- ical to sustainability. The biological processes that under- lie sustainability require good substrate, adequate water supply, suitable thermal characteristics, and good water quality. Habitat alterations or modifications that decrease ecological productivity will undermine efforts to conserve stocks. The right to use ecological productivity does not bestow the right to abuse it. Attempts to conserve habitat often depend on simplified assumptions that (i) the level of productivity is roughly proportional to the supply of suit- able habitat and (ii) the loss of habitat units that are non- critical will not noticeably affect sustainable levels of the preferred use, exploitation. Recent research by Pulliam and Danielson (1991) challenges these simple models. Good habitats may facilitate surplus production but other, mediocre or poor, habitats may be necessary to accom- modate the accumulated biomass. Hence, the total habitat may be required to support the level of biomass needed 1589 to support harvest expectations. Preserving critical spawn- ing habitats may be pointless if there is nowhere for adults to live before becoming vulnerable to exploitation. In related studies, Danielson (1991, 1992) has shown how changes in the relative supplies of different habitat types can shape the outcome of interspecific interactions. Suther- land and Anderson (1993) showed, with simple models, how biased elimination of better or poorer habitat units in a fragmented landscape can determine the shape of the relationship between sustainable population size and over- all habitat supply. Given the temptation to measure the degree of conser- vation success within fisheries in terms of productivity or potential yield, there is a clear danger that ecosystem mod- ifications that increase production, such as climate warm- ing, eutrophication, or aquaculture, might be viewed as changes that are consistent with conservation objectives. However, if the prior, undegraded condition of an ecosys- tem provides the direction for rehabilitation efforts and the benchmark for achievement of sustainable conservation success, any action that moves an ecosystem away from that benchmark state may be clearly judged as degradation. · The sustainability of a fish stock requires mainte- nance of its supporting native community. Degradation of the biological environment, via the trans- fer of non-native stocks and the introduction of exotic species, must be prevented. Transfers of non-native organ- isms inevitably result in the loss of locally adapted gene pools. In the Great Lakes basin, the invasion of species like sea lamprey, Petromyzon marinus, alewife, Alosa pseudoharengus, and rainbow smelt, Osmerus mordax, and the introduction of species like common carp, Cyprinus carpio, have had considerable, negative impacts on native fish communities (Emery 1985; Crossman 1991). Hence, delfuerately adding species to increase species richness, or more likely to improve exploitation opportu- nities, cannot be considered a conservation action. The consequent alteration to the productivity of natural ecosys- tems and the loss of surplus production from native stocks undermines sustainability. In addition, the accompanying degradation of biological habitats represents an irreversible shrinkage in the reservoir of biodiversity that uniquely characterizes each region of Canada. The rates of deliberate introductions and accidental, but usually man-assisted, invasions are continuing to increase (Crossman 1991; Welcomme 1992; Carlton and Geller 1993). The history of introductions of organisms foreign to native ecosystems is replete with examples demonstrating severe, detrimental effects (Billington and Hebert 1991; Mills et al. 1991). Managing with exotics has been com- pared to a game of chance (Magnuson 1976), where posi- tive results cannot be guaranteed (Regier 1968). Because changes wrought by such introductions can be extreme and because net results are hard to predict, proposed intro- ductions should be forbidden, unless a situation-specific evaluation confirms that the risk to existing ecosystems is minor. Invasions present a more complex problem, although species transfers by ships' ballast water, the major