More than bags of nutrients: Weighing ecological and social costs of losing migratory fishes versus the management paradigm of compensatory mitigation
Fish migrations connect aquatic habitats around the world, and interactions and mortality associated with these extends their ecological influence to terrestrial habitats as well. In temperate settings, dramatic examples include the spawning migrations and mass mortality of anadromous species (e.g., Pacific salmon, steelhead and lamprey), but many non-anadromous species also exhibit migrations with attendant ecological roles. Fish migrations have a long history of providing diverse provisioning and cultural ecosystem services to humans, but are imperiled phenomena due to extirpation or loss of life-history forms. In turn, ecological roles that were multi-dimensional have been lost or homogenized. Often these changes have spanned several human generations, and knowledge of historic magnitudes of migrations is passing into legend, constituting a vanishing frame of reference for natural resource managers and the public. In its place, a paradigm of mitigation has arisen, whereby credit is assigned for attempts to offset, compensate for, or alleviate losses. However, this paradigm is rooted in assumptions. Do these align with empirically-derived ecological understanding, and do they reflect the full suite of human values associated with these fishes? Here, we address these questions with a focus on Pacific salmon, based upon results of our investigations and synthesis of ecological literature.
Mitigation for lost fish migrations often includes supplementation (e.g., via hatcheries). This may assume that natural migration and mortality of salmon has no ecological or social value, and/or that ample habitat and food resources exist in freshwater habitats to sustain rearing of added juveniles. In apparent contradiction, the assumption that populations are limited by space and/or food underpins two other key forms of mitigation: restoration of freshwater habitats and artificial additions aimed at compensating for loss of salmon-derived nutrients (translocated carcasses, pelletized tissue, or inorganic fertilizer). Results from our series of studies generally call into question the assumption that rearing habitat or food limit the size of endangered salmon runs in the upper Columbia and Snake River basins, and literature analysis suggests nutrient additions may not be informed by understanding of biogeochemical processes. Our investigations (and the literature) also highlight pathways by which natural salmon runs influence both aquatic and terrestrial ecosystems, pathways not mimicked by mitigation actions. Indeed, adding bags of nutrients provides no ecological or cultural substitute for migrating salmon. As migrations of other fishes are compromised around the world, frameworks erected to mitigate for losses deserve critical evaluation before science and society assume that they do so.