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1590 route in the North American Great Lakes, are now receiving more urgent attention (Carlton and Geller 1993). Carlton and Geller (1993) have characterized these exotic additions as a form of ecological roulette. Principles of population utilization · Vulnerable, threatened, and endangered species must be rigidly protected from all anthropogenic stresses. By definition, the stocks of such species face various degrees of risk of extinction. Removal and (or) preven- tion of anthropogenic stresses would significantly reduce this risk. These organisms are irreplaceable segments of the her- itage of biodiversity that uniquely characterizes the biota of a region. Any activity that risks their extinction should be proscribed. In Canada, the selection of species at risk and their classification is decided by the Committee on the Status of Endangered Wildlife in Canada, which comprises representatives from federal, provincial, and private agen- cies. In April, 1993, 35 wild species, subspecies, or sepa- rate populations of fish were designated as vulnerable, 11 as threatened, and 3 as endangered (Committee on the Status of Endangered Wildlife in Canada 1993). However, the determinations of the committee have no legal status so that each jurisdiction must provide its own appropriate safeguards to risks imposed on these designated species, subspecies, or separate populations. · Exploitation of populations or stocks undergoing rehabilitation will delay, and may preclude, full rehabilitation. For stocks undergoing rehabilitation, any exploitation will delay full rehabilitation and increase the risk of fail- ure. If some harvesting occurs, it should be strictly con- trolled to ensure that mortality rates do not prevent reha- bilitation and are, in fact, low enough to let rehabilitation proceed at a reasonable rate. Allowing higher mortality rates will ensure the perpetuation of put-and-take hatchery- dependent fisheries that may masquerade as rehabilitation projects. If harvest for other species is allowed, fishing practices should minimize the capture of the stock under- going rehabilitation. This is difficult to achieve in mixed stock fisheries, which should be managed for the protection of the least sustainable individual stock within the mix, or a more selective means of harvesting individual stocks should be found. · Harvest must not exceed the regeneration rate of a population or its individual stocks. Exploitation can only be sustained if it is restricted to the annual generation of fish protein that exceeds the main- tenance requirements of a population, or its individual stocks. Extinction is inevitable if losses exceed the nat- ural capacity of a population to replenish itself. This is a common characteristic of all renewable resources: Can. J. Fish. Aquat. Sci. Vol. 52, 1995 sustainable use is equivalent to renewal rate (Daily and Ehrlich 1992). Similar behaviour is well known in eco- nomics (Costanza and Daly 1992): exhaustion of capital will occur if annual expenditures exceed interest income. Planned harvest rates should also be sensitive to other uses and stresses imposed on the stock and other ecosystem components. They should acknowledge natural temporal variation in the productivity of the target population and of the ecosystem of which it is a part. In natural populations, annual regeneration rates vary widely from year to year and cannot be accurately pre- dicted. Given the amplitude of such natural variation, pop- ulation sustainability can only be guaranteed if fisheries are managed in a cautious manner. Ludwig et al. (1993) offer some sombre reflections on the difficulties of achiev- ing sustainability of exploited fisheries resources. To pro- vide a sufficient margin of safety, total allowable catch should be set well below maximum annual regeneration rates. A variety of formulae have been developed in fish- eries science with the aim of establishing defensible limits on exploitation: MSY, OY, Fo.l, etc. (Parsons 1993). Mostly, these approaches have been developed for single-species stocks, ignoring the ecosystem contexts in which they exist. Some methods developed for multi species assem- blages take account of energy flow dynamics and species interactions. Stock exploitation limits that recognize intrin- sic ecosystem dynamics are likely to be more stringent than those obtained by assessing each species in isolation. The need for caution in exploiting long-lived species is paramount. Such species typically are slow growing, are late maturing, have a low reproductive potential, and have a low turnover or regeneration rate. They may spawn infrequently. Consequently, they require multiple oppor- tunities to spawn and recruit offspring and long recovery times when populations are depressed. For such species, protection of adults is absolutely critical to long-term pop- ulation stability. The need to exercise caution may be espe- cially critical for long-lived species that are terminal in the food chain because they exert a dominant role in the integration and maintenance of community structure (Evans et al. 1987; Johnson 1994). Pauly et al. (1989) referred to the decline of fisheries for such key species as ecosystem overfishing. · Direct exploitation of spawning aggregations increases the risk to sustainability of fish stocks. Stock sustainability requires that the number of adult fish escaping harvest (i.e., spawner escapement) be suffi- cient to maintain the reproductive output of the stock. As spawner escapement drops toward zero, the risk to stock sustainability increases rapidly. The impact of a fixed harvest on spawner escapement is minimized if harvested fish are taken at random from the stock and is maximized if harvested fish are taken directly from aggregations of spawning adults. For exam- ple, consider a quota of 10 000 fish applied to a stock con- taining 40000 fish big enough to satisfy fishers, of which 10 000 fish are big enough to spawn. If the quota is taken at random from the stock, the number of spawners will be