Relationships between Pacific salmon and aquatic and terrestrial ecosystems: implications for ecosystem‐based management

Abstract

Pacific salmon influence temperate terrestrial and freshwater ecosystems through the dispersal of marine‐derived nutrients and ecosystem engineering of stream beds when spawning. They also support large fisheries, particularly along the west coast of North America. We provide a comprehensive synthesis of relationships between the densities of Pacific salmon and terrestrial and aquatic ecosystems, summarize the direction, shape, and magnitude of these relationships, and identify possible ecosystem‐based management indicators and benchmarks. We found 31 studies that provided 172 relationships between salmon density (or salmon abundance) and species abundance, species diversity, food provisioning, individual growth, concentration of marine‐derived isotopes, nutrient enhancement, phenology, and several other ecological responses. The most common published relationship was between salmon density and marine‐derived isotopes (40%), whereas very few relationships quantified ecosystem‐level responses (5%). Only 13% of all relationships tended to reach an asymptote (i.e., a saturating response) as salmon densities increased. The number of salmon killed by bears and the change in biomass of different stream invertebrate taxa between spawning and nonspawning seasons were relationships that usually reached saturation. Approximately 46% of all relationships were best described with linear or curved nonasymptotic models, indicating a lack of saturation. In contrast, 41% of data sets showed no relationship with salmon density or abundance, including many of the relationships with stream invertebrate and biofilm biomass density, marine‐derived isotope concentrations, or vegetation density. Bears required the highest densities of salmon to reach their maximum observed food consumption (i.e., 9.2 kg/m2 to reach the 90% threshold of the relationship’s asymptote), followed by freshwater fish abundance (90% threshold = 7.3 kg/m2 of salmon). Although the effects of salmon density on ecosystems are highly varied, it appears that several of these relationships, such as bear food consumption, could be used to develop indicators and benchmarks for ecosystem‐based fisheries management.

Resource pulses increase the diversity of successful competitors in a multi‐species stream fish assemblage

“…Our results indicate that there were diminishing returns for increasing egg abundance increasing egg consumption (i.e., type II functional response) for individual size classes of fish, but that higher egg numbers were needed to benefit diverse consumers. This aligns with the synthesis of Walsh et al (2020) who showed that ecological relationships between freshwater fish and salmon density tend to be asymptotic, and require extremely high salmon densities (7.3 kg/m 2 ) to reach 90% of their asymptote. Moore et al (2008) found that the relationship between salmon spawning density and the number of salmon eggs available for consumption is positively exponential and linked to nest superimposition by female salmon.…”

“…Salmon‐bearing streams are a classic example of ecosystems that can experience resource pulses (Schindler et al 2003, Moore et al 2008, Walsh et al 2020). Salmon spawn and die in streams annually, producing carcass tissues and eggs that are consumed by a variety of predators and scavengers (Naiman et al 2002, Deacy et al 2016) such as stream fishes (Lang et al 2006, Bailey et al 2019).…”

Non‐native Chinook salmon add nutrient subsidies and functional novelty to Patagonian streams

“…For example, seabirds and anadromous fish bring marine‐derived nitrogen and phosphorus into terrestrial and freshwater ecosystems (e.g. Graham et al., 2018; Hood et al., 2019; Walsh et al., 2020), and the widespread collapse of seabird and anadromous fish populations has resulted in the global transfer of phosphorus from sea to land declining over 96% from historic levels (Doughty et al., 2016). Whereas the loss of historic connections among ecosystems disrupts nutrient cycles and recipient food web productivity (Gresh et al., 2000), the emergence of novel resource linkages among ecosystems that occur due to the establishment of non‐native species could be equally disruptive.…”

Indigenous Systems of Management for Culturally and Ecologically Resilient Pacific Salmon (Oncorhynchusspp.) Fisheries

“…There is an urgent need to realign the scales of fisheries with biologically relevant scales (e.g., salmon populations or watersheds) to reduce conservation risks, create opportunities for sustainable harvest, and support salmon-dependent species and ecosystem processes (Healey 2009 , Ward et al 2009 , Gayeski et al 2018b , Freshwater et al 2020 , Walsh et al 2020 ). However, realignment of harvest toward greater terminal or in-river harvest will be of limited benefit if mixed-stock harvest rates remain high and fish are intercepted before reaching terminal fisheries, and terminal fisheries are not a panacea for sustainability if in-river fisheries overharvest returning salmon (Freshwater et al 2020 ).…”