Shadow competition in wild juvenile sea‐trout

Abstract

Shadow competition occurs in a group of sit-and-wait predators when those closer to a source of mobile prey reduce the feeding success of those further from the prey source. It was examined in territorial juvenile sea trout Salmo trutta in a small stream. The fry formed groups of two to six fish with adjacent territories and a social hierarchy within each group. It was hypothesized that: (i) as group size increased, the mean number of prey eaten per fish within a group decreased and the variability in prey consumption between fish increased; (ii) prey consumption by individual fish decreased with increasing distance from the food source; (iii) group size increased as the mean water velocity immediately upstream from a group, and hence potential drifting food, increased. Five groups of fry were fed on small shrimps released upstream from each group at a rate of one every 15 s over a 10 min period, this procedure being repeated over 5 days to provide five replicates per group. Experiments were performed three times in 1967, 1969 and 1974 to provide information on 45 groups of fry. The first and third hypotheses were supported, but the second was only partially supported. In 1967 and 1969, territory size and shrimp consumption by individual fry decreased with increasing distance from the food source. This also occurred in 1974, except during a critical period for survival when fry density was exceptionally high with large numbers of sea trout lacking territories. This resulted in sea trout fry with the largest territories eating fewer shrimps than those with medium-sized territories because they spent more time defending their territories against sea trout lacking territories. This study is the first to demonstrate shadow competition in a vertebrate species, but has also shown that territorial defence may modify the consequences of shadow competition when densities are high and there is strong competition for the acquisition of a territory.

Stream temperature and the potential growth and survival of juvenile Oncorhynchus mykiss in a southern California creek

“…One potential explanation is that the fish varied in their potential growth rate; a second is that the food induced different energetic costs in different fish; and a third is that the fish had unequal access to food. The latter may be likely, as unequal access to food (due to shadow competition) has been directly observed in juvenile Salmo trutta L. inhabiting small streams (Elliott, 2002). However, the hypothesis of unequal energetic costs may also contribute, as this has been observed in O. mykiss inhabiting laboratory streams (Li & Brocksen, 1977).…”

Multiple central‐place territories in wild young‐of‐the‐year Atlantic salmonSalmo salar†

“…However, an understanding of the benefit of multiple central-place foraging may require more subtle knowledge of the influence of fish on drifting prey, a controversial topic in stream ecology. For example, although many studies suggest that stream salmonids have no effect on overall drift rate (Allan 1982;Miyasaka & Nakano 1999;Meissner & Muotka 2006), others indicate that in space-related social hierarchies, dominant fish closer to a predictable food source may reduce prey availability for fish immediately downstream (Nakano 1995;Elliott 2002a). By patrolling large areas, YOY salmon may increase their prey encounter rate by excluding competitors from areas immediately upstream of many of its focal points, especially because many invertebrates drift short enough distances to originate from within a multicentral territory (McIntosh & Townsend 1998;Elliott 2002b).…”

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

“…However, interpopulation variations in body size have also been shown to be densitydependent (Jenkins et al 1999;Lobón-Cerviá 2007). Increased population density decreases the quantity and (or) quality of available per capita resources (e.g., Elliott 2002;Ward et al 2006) and increases competition for territories that differ in feeding opportunities and thereby in growth potential for the fish (Ward et al 2007). Clearly, the effects of density dependence on growth may derive from competition among members of either the same cohort or different cohorts.…”