Intracohort and intercohort spatial density dependence in juvenile brown trout (Salmo trutta)

Abstract

An increased mobility with age can be expected for many organisms, which will reduce the potential for spatial density dependence. Here we quantify the extent of spatial density dependence for two juvenile age classes of brown trout ( Salmo trutta ) that differ in their dispersal abilities. As predicted, spatial variation in intracohort density had a strong effect on the performance of underyearlings, but not on yearlings. However, rather surprisingly, local underyearling density influenced the energetics of the older age class to the same extent as their own. Thus, yearlings do not appear to prioritize growth rate per se as a cue in movement decisions. The spatial patterns of densities and performance are consistent with older fish distributing themselves primarily according to preferred abiotic variables that also influence energetics. Such decisions would remove any relation between local density of their own age class and growth performance. However, because of competitive effects from the patchily distributed younger age class, spatial homogenization of competition intensities appears to be constrained. Variation in yearling body mass also increased with density of their own age class, indicating that even though the average effect of intracohort competition was absent, the individual response was asymmetric.

Unravelling the effects of water temperature and density dependence on the spatial variation of brown trout (Salmo trutta) body size

“…Despite the fact that younger individuals cannot out-compete older ones, increasing density of younger trout results in increased metabolic costs associated with guarding and defending territories and thus reduces the growth of older trout. Similar results were found by Kvingedal and Einum (2011) in Atlantic salmon. In addition, higher densities of younger age-classes would depress growth of older ones through exploitative competition for food, since even if younger individuals cannot out-compete older ones they can consume prey that otherwise would have been available to older age-classes.…”

Biotic and abiotic regulation of a low‐density stream‐dwelling brown trout (Salmo truttaL.) population: effects on juvenile survival and growth

“…Also, the predation by avian predators, such as herons Ardea cinerea and kingfisher Alcedo atthis (which have both been observed along the river), may be increased during low-flow periods. Conversely, we did not detect any effect of the 0+ density on the growth of age 1+ as reported by Kvingedal & Einum (2011). Einum et al (2006) demonstrated that the 0+ density did not affect summer survival but strongly affected dispersal.…”

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

“…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. Parra et al (2012) reported on density-dependent effects both within and among cohorts of brown trout in northern Spain, and notably that the size of overyearling fish was negatively associated with densities of younger cohorts.…”