Density‐dependent growth rate in an age‐structured population: a field study on stream‐dwelling brown troutSalmo trutta

Abstract

A field experiment during autumn, winter and spring was performed in a small stream on the west coast of Sweden, aiming to examine the direct and indirect consequences of density-dependent intercohort competition in brown trout Salmo trutta. Individual growth rate, recapture rate and site fidelity were used as response variables in the young-of-the-year (YOY) age class, experiencing two different treatments: presence or absence of yearlings and over-yearlings (age > or = 1+ year individuals). YOY individuals in stream sections with reduced density of age > or = 1+ year individuals grew significantly faster than individuals experiencing natural cohort structure. In the latter, growth rate was negatively correlated with density and biomass of age > or = 1+ year individuals, which may induce indirect effects on year-class strength through, for example, reduced fecundity and survival. Movement of YOY individuals and turnover rate (i.e. proportion of untagged individuals) were used to demonstrate potential effects of intercohort competition on site fidelity. While YOY movement was remarkably restricted (83% recaptured within 50 m from the release points), turnover rate was higher in sections with reduced density of age > or = 1+ year individuals, suggesting that reduced density of age > or = 1+ year individuals may have released favourable microhabitats.

Density dependence and its impact on individual growth rates in an age‐structured stream salmonid population

“…Growth rates in the subyearling age class were overall negatively related to the densities of older fish, but not vice versa. These results support recent research on the sequence of regulation of salmonid populations, notably that density dependence can be manifested in growth rates at relatively low population densities (Achord et al 2003, Einum et al 2006, Lobón-Cerviá 2007 and that inter-cohort competition can occur in age-structured populations (Nordwall et al 2001, Kaspersson andHö jesjö 2009). Our results showed no conclusive effects of temperature on growth rates, either as a standalone factor or in interaction with age class density.…”

“…Elliott (1985) found no evidence of competition between age classes of brown trout in England, whereas Nordwall et al (2001) reported on depressed growth in younger age classes with increased densities of older age classes in brown trout in Sweden. In two recent studies on brown trout in Sweden, Kaspersson and Hö jesjö (2009) found that the presence of yearling fish was associated with an overall reduction in subyearling growth rates, and Kaspersson et al (2013) reported on reduced growth rates, rather than increased mortality rates and movement, in small individuals when experimentally adding large brown trout to stream sections. Oppositely, Kvingedal and Einum (2011) found that subyearling brown trout densities in Norway were associated with depressed size-at-age for both subyearling and yearling fish, and that yearling density did not cause depressed sizes in either age class.…”

“…However, with the potentially strong influence of density on individual growth rates, and the consequences for life history expression and fitness, it is important to understand the extent to which density dependence is manifested at the individual level (Sogard 1997, DeAngelis and Mooij 2005, Kaspersson et al 2013. These effects can be particularly important in age-structured populations, in which the presence of one cohort can alter the behavior of another (Rosenfeld andBoss 2001, Kaspersson andHö jesjö 2009), and in populations experiencing abiotic conditions which can exacerbate the effects of competition (Crozier et al 2010, Parra et al 2012). Here we study the effects of population density on individual growth rates in juvenile age classes of steelhead (Oncorhynchus mykiss) in a population listed as threatened under the Endangered Species Act (NMFS 2010).…”

“…Here we study the effects of population density on individual growth rates in juvenile age classes of steelhead (Oncorhynchus mykiss) in a population listed as threatened under the Endangered Species Act (NMFS 2010). We hypothesize that individual growth rates in both subyearling and yearling steelhead are negatively related to the density of their age class , Lobón-Cerviá 2007, and that subyearling growth rates are more negatively affected by yearling densities than vice versa (Jenkins et al 1999, Kaspersson andHö jesjö 2009).…”

Habitat exclusion and reduced growth: a field experiment on the effects of inter-cohort competition in young-of-the-year brown trout

“…Indeed, previous experimental studies of inter-cohort interference suggest that the superior competitive ability of older cohorts can reduce both survival and growth rate of younger individuals (e.g. Crowley et al 1987;Nordwall et al 2001;Webster 2004;Kaspersson and Höjesjö 2009;Samhouri et al 2009). …”