Shelter availability affects behaviour, size‐dependent and mean growth of juvenile Atlantic salmon

Abstract

1. Anthropogenic disturbances of the physical habitat and corresponding effects on fish performance are key issues in stream conservation and restoration. Reduced habitat complexity because of increased sediment loadings and canalization is of particular importance, but it is not clear to what extent fish populations are influenced directly by changes in the physical environment, or indirectly through changes in the biotic environment affecting the food availability. 2. Here, we test for the direct effect of habitat complexity on the performance (growth) of juvenile Atlantic salmon by manipulating shelter availability (interstitial spaces in the substrate) across 20 semi-natural stream channels without altering the substrate composition, and stocking each channel with a common density of fish. A simple method for measuring salmonid shelters using flexible PVC tubes was developed and tested. Daytime sheltering behaviour and growth rates were compared across the channels differing in shelter availability. 3. Measured shelter availability was strongly negatively correlated with observed number of fish not finding shelters and mass loss rates of the fish (growth performance) increased with decreasing number of measured shelters. Number and mean depth of interstitial spaces explained up to 68% and 24% of the among-channel variation in sheltering behaviour and growth performance, respectively. Furthermore, negative effects of shelter reduction increased with fish body size. Thus, changes in habitat structure may even influence the size selection gradients. 4. Shelter availability is an easily measured variable, possibly affecting the population demographics and long-term evolutionary processes, and is therefore a key habitat factor to be considered in stream restoration and habitat classification.

Grass-dominated stream sites exhibit low fish species diversity and dominance by guppies: an assessment of two tropical pasture river basins

“…Habitat complexity influences movements of prey and predators through hazardous sites (Gilliam & Fraser, 2001) with higher fish vagility in areas where habitat heterogeneity is low, such as channelized and lowland streams (Gorman, 1986). Indeed, reduced habitat complexity may create an environment less favorable for large-sized juveniles and reinforce negative impacts of degradation of stream habitats, as Finstad et al (2007) observed for salmonid populations.…”

Stress responses of juvenile brown trout under winter conditions in a laboratory stream

“…Access to shelter in low embedded bottom substrates with stream wood is a key habitat factor for overwintering juvenile salmonids (Finstad et al, 2007;Stickler et al, 2008;Linnansaari et al, 2009), and in its absence, surface ice may provide overhead cover (Linnansaari et al, 2009;Watz et al, 2016). This study showed that, when denied access to winter shelter, juvenile brown trout display a stress response, mainly by increasing their ventilation frequency.…”

“…Juvenile salmonids seem to cope relatively well with low temperatures and river ice formation per se (Linnansaari & Cunjak, 2013), but lack of access to shelters against predation from endothermic piscivores may lead to stress (Finstad et al, 2007;Watz et al, 2015) and has shown to be a major factor that influences overwinter mortality (Huusko et al, 2007;Brown et al, 2011).…”

Selective segregation in intraspecific competition between juvenile Atlantic salmon (Salmo salar) and brown trout (Salmo trutta)

“…Heggenes et al (1999) found that small trout (<7 cm) were abundant in shallow areas with water velocities ranging from 10 to 50 cm 3 Ás À1 , while larger trout (>7 cm) were found in deeper and more slowflowing habitats. Shelter or cover is related to the substrate composition (Finstad et al 2007;Venter et al 2008), and survival has been connected to access to shelter and cover for salmonids (Chapman 1966;Hunt 1976;Marschall & Crowder 1995;Finstad et al 2007). In our study, trout has a higher variation in habitat choice dependent on species, density and fish size.…”