Population and Habitat Restoration Preamble
Diadromous fish populations are particularly difficult to understand, model, and manage because they traverse multiple habitats that present not only environmental, ecological, reproductive, and physiological challenges, but also frequently convey them across multiple management jurisdictions. Our knowledge of population-level effects is also dependent on the quality and extent of biological, population, and demographic data. For some species, such as Pacific salmon, populations are routinely monitored, life cycles are fairly well understood, and population trends are, in general, well documented. However, for other species such as anguillid eels, our understanding of life history is incomplete, population- level data are meager, and the trends in abundance are less clear. Application of “one size fits all” models, or models that assume closed, nonmigratory populations, therefore have little application to highly migratory diadromous species with life history traits that vary both inter- and intraspecifically. Similarly, classic population metrics and models, most of which were originally developed for stocks of exploited marine fishes, may not apply easily to populations that migrate between diverse and often multijurisdictional environments.
The habitat of diadromous fishes also plays a significant, if not primary, role in the sustainability of their populations; without adequate habitat in marine, estuarine, and freshwater environments, natural populations are generally unviable. Diadromous fishes are uniquely dependent on multiple habitats and, equally important, connectivity between those habitats. As such, provision and maintenance of migratory corridors can be as significant as habitat itself in the sustainability of diadromous fishes. Often, particularly in freshwater, essential habitats can be drastically curtailed or altered by both natural and anthropogenic causes. Restriction of access to, or alteration of, historic habitat by dams is the classic example of how barriers to migration can interfere with critical life history cycles. Dams not only create a migratory barrier, but can also alter habitat adjacent to the dam so that it is unsuitable for reproduction or growth, in both upstream and downstream directions. In such cases, simple provision of passage (often only in the upstream direction) is not enough to ensure connectivity and population viability. However, our knowledge of the details of habitat and connectivity requirements for many species is often limited.
Under circumstances of habitat loss, increased natural and anthropogenic mortality, and genetic bottlenecks, populations of diadromous fishes are often targeted for restoration through supplementation of hatchery-reared fish. In some cases, compensation by culture techniques can be justified, especially if population sizes are critically low or habitats are extremely limited. However, the response of specific stocks of a species to artificial propagation will vary with life history characteristics, dynamics of the local genetic structure, aspects of fish health, and environmental productivity. At best, culture techniques should be viewed as short-term measures to sustain populations until adequate habitat and genetic diversity can be restored. Newly developed guidelines to minimize the adverse effects of fish culture suggest that stocks be supplemented by using progeny from local stocks (especially for species with strong homing behavior) and by carefully considering the effects of stocking on genetic population viability. However, these guidelines have yet to be implemented widely by fisheries managers and at a global scale.