Density‐dependent effects on salmonid populations: A review

Abstract

We reviewed 199 published data sets (21 species) to assess the relative frequency of various density‐dependent processes in salmonids. We examined studies for the presence or absence of density‐dependence: in growth, mortality, fecundity and recruitment. Based on data from all studies pooled 71% showed density‐dependence in growth, 23% found density‐dependence in mortality, 2% detected density‐dependence in fecundity and 4% displayed density‐dependence in recruitment. Most studies at the population level (e.g. changes in abundance or means of growth, mortality or fecundity) demonstrated density‐dependence; however, these studies were not common, likely because they require long time series of data (at least 3–4 times mean generation time for the species). Our review indicated there was no evidence that the probability of finding density‐dependent effects differed among the 21 species examined, but the number of studies per species varied substantially (5 [minimum for analysis] – 20+); hence, this finding should be viewed as tentative. Most salmonid species showed evidence of density‐dependence in growth; however, few studies examined density‐dependent effects on fecundity or recruitment, and further study is needed on these phenomena. There was no evidence that density‐dependent effects were restricted to a particular habitat type or geographic region.

Captive-bred Atlantic salmon released into the wild have fewer offspring than wild-bred fish and decrease population productivity

“…One potential explanation is that in years with more captive-bred fish-which are more fecund than wild-bred fish-there are more initial fry in total and hence there is stronger competition among offspring for feeding territories and, hence, lower juvenile survival for both provenances. However, our population-level analysis of productivity accounted for density dependence [59] and still found an effect of captive-bred intrusion, which implies that either a higher fraction of captive-bred fish fail to spawn successfully, or their offspring survive less well relative to the offspring of wild-bred parents [53]. Tentative evidence for the latter explanation was provided by our additional analysis where LRS records of zero were excluded, and the analysis of grand-offspring numbers presented in electronic supplementary material, figure S5.…”

Density‐dependent growth and survival in salmonids: Quantifying biological mechanisms and methodological biases

“…Other density‐dependent processes, such as emigration, or fitness correlates, such as fecundity, can also be important (Grossman & Simon, 2019; Vincenzi et al, 2012) but have not explicitly been considered here. The primary reason is that they are under‐represented in the literature (Grossman & Simon, 2019), but this would be important to revisit in the future when more studies are available.…”

“…Other density‐dependent processes, such as emigration, or fitness correlates, such as fecundity, can also be important (Grossman & Simon, 2019; Vincenzi et al, 2012) but have not explicitly been considered here. The primary reason is that they are under‐represented in the literature (Grossman & Simon, 2019), but this would be important to revisit in the future when more studies are available. In addition, investigating interspecific competition in a density dependence context would provide great insight for the management of invasive species, but this could not be done here due to the rarity of appropriate studies at present (but see Korsu, Huusko, & Muotka, 2010, for a categorical meta‐analysis on non‐native salmonids).…”

“…To date, no review has quantified the relative importance of biological and methodological factors on the strength and shape of density‐dependent growth and survival across salmonid populations (e.g. Grant & Imre, 2005; Grossman & Simon, 2019; Rose et al, 2001; Vincenzi, Satterthwaite, & Mangel, 2012).…”

Density‐dependent marine survival of hatchery‐origin Chinook salmon may be associated with pink salmon

“…Density-dependent effects between pink salmon and other species, including salmon, have been documented by a number of studies. Density dependence can affect survival when resources are limited or predators are responsive to increased prey (Wells et al 2017), and it can be associated with reduced growth and increased age at maturation (Ruggerone and Nielsen 2004, Cline et al 2019, Grossman and Simon 2019. In the North Pacific Ocean, high pink salmon abundance has been thought to decrease zooplankton biomass, inducing trophic cascades down to the phytoplankton level (Shiomoto et al 1997, Batten et al 2018) that can depress the availability of prey resources for numerous species including salmon (Ruggerone et al 2003, Ruggerone and Nielsen 2004, Kaga et al 2013) and seabirds (Toge et al 2011, Springer et al 2018.…”