Simulating nutrient release from parental carcasses increases the growth, biomass and genetic diversity of juvenile Atlantic salmon

Abstract

1. The net transport of nutrients by migratory fish from oceans to inland spawning areas has decreased due to population declines and migration barriers. Restoration of nutrients to increasingly oligotrophic upland streams (that were historically salmon spawning areas) have shown short-term benefits for juvenile salmon, but the longer term consequences are little known. Here we simulated the deposition of a small number of adult Atlantic salmonSalmo salar carcasses at the end of the spawning period in five Scottish upland streams ('high parental nutrient' treatment), while leaving five reference streams without carcasses ('low parental nutrient' treatment). All streams received exactly the same number of salmon eggs (n = 3,000) drawn in equal number from the same 30 wild-origin families, thereby controlling for initial egg density and genetic composition. We then monitored the resulting juvenile salmon and their macroinvertebrate prey, repeating the carcass addition treatment in the next spawning season.3. Macroinvertebrate biomass and abundance were five times higher in the high parental nutrient streams, even 1 year after the carcass addition, and led to faster growth of juvenile salmon over the next 2 years (but with no change in population density). This faster growth led to more fish exceeding the size threshold that would trigger emigration to sea at 2 rather than 3 years of age. There was also higher genetic diversity among surviving salmon in high parental nutrient streams; genotyping showed that these effects were not due to immigration but to differential survival.4. Synthesis and applications. This 2-year field experiment shows that adding nutrients that simulate the presence of small numbers of adult salmon carcasses can have long-term effects on the growth rate of juvenile salmon, likely increasing the number that will migrate to sea early and also increasing their genetic diversity.However, the feasibility of adding nutrients to spawning streams as a management tool to boost salmon populations will depend on whether the benefits at this stage are maintained over the entire life cycle. K E Y W O R D S fisheries management, growth rate, marine derived nutrients, migration, oligotrophic, phosphorus, salmon, smolt | 1939 Journal of Applied Ecology MCLENNAN Et AL.

Metabolic Rate Interacts with Resource Availability to Determine Individual Variation in Microhabitat Use in the Wild

“…As part of a larger study examining the ecological effects and evolutionary consequences of nutrient levels for freshwater ecosystems (Auer et al 2018a;McLennan et al 2019), we planted embryos from full-sibling Atlantic salmon families in equal distribution and density across 10 replicate tributary streams of the River Conon in northern Scotland (see app. A for description of study sites; apps.…”

Nutrient limitation in Atlantic salmon rivers and streams: Causes, consequences, and management strategies

“…Modelling by Benjamin et al (2020) has demonstrated the potential for this method with chinook salmon ( Oncorhynchus tshawytscha ), with increases in potential smolt output and size. However, in some cases faster growth may result in salmon reaching the size that triggers smolting a year earlier, at a smaller smolt size (McLennan et al, 2019). In such cases, nutrient additions may result in lower per capita chances of survival at sea, but increased numbers within a cohort surviving to become smolts, because of less time in the river and reduced inter‐cohort competition.…”

“…Although the populations of spawning migrants (and hence the nutrients transferred) tend nowadays to be on a larger scale in species of Pacific compared with Atlantic salmon, there is evidence that Atlantic salmon populations were once far larger, even before the declines documented over the last century (Lenders et al, 2016) so that their baseline ‘natural’ population size (and hence level of nutrient transfer) is unclear. Nonetheless, even current populations of Atlantic salmon are capable of delivering significant levels of marine‐derived nutrients to tributary streams, with positive impacts on algal growth, invertebrate populations and juvenile fish growth (Nislow, Armstrong & McKelvey, 2004; McLennan et al, 2019).…”

“…This includes consideration of the impact of declines in resource subsidy in upland streams resulting from decreased spawner abundance. Lower nutrient inputs from spawners results in reduced growth rates of juvenile salmon (Auer et al, 2018; McLennan et al, 2019), and potential alterations to marine survival arising from changes in size attained by the time of smolt migration, as this is correlated with return rates (Armstrong et al, 2018; Gregory, Armstrong & Britton, 2018). This has led to the suggestion that nutrient restoration in spawning streams that have experienced cultural oligotrophication could be used as a conservation tool to manage and enhance important fish populations.…”

Atlantic salmon in regulated rivers: Understanding river management through the ecosystem services lens

“…Categorization and assessments of ES can be carried out to give an overview of a system, but are usually carried out in relation zones (Gende et al, 2002). Atlantic salmon are iteroparous, but between 20% and 100% of the adults nevertheless die after spawning (Nyqvist, 2016), and their carcasses may constitute a relevant nutrient addition for many rivers (McLennan et al, 2019). Moreover, during and after spawning, Atlantic salmon gametes are eaten by riverine organisms (Samways et al, 2017), also contributing to the linkage between the marine and freshwater ecosystems.…”