Density‐dependent diel activity in stream‐dwelling Arctic charr Salvelinus alpinus

Abstract

Intraspecific competition plays a significant role in shaping how animals use and share habitats in space and time. However, the way individuals may modify their diel activity in response to increased competition has received limited attention. We used juvenile (age 1+) Arctic charr Salvelinus alpinus to test the prediction that individuals at high population density are more active and distribute their foraging activity over a greater portion of the 24‐h cycle than individuals at low population density. Individually tagged fish were stocked in seminatural stream enclosures at low (2 fish/m2) and high (6 fish/m2) density. During each of two 2‐week experimental rounds, activity of all fish within each enclosure was recorded every 3 h over seven 24‐h cycles. At high density, fish were more active and distributed their activity over a greater portion of the 24‐h cycle, with increased activity particularly at crepuscular times. Fluctuations in ecological conditions (e.g., water temperature and light intensity) also affected activity. Fish at high density grew as fast as fish at low density. This study demonstrates that individuals exhibit a degree of behavioral flexibility in their response to changes in ecological conditions and suggests that intraspecific competition can cause animals to modify temporal aspects of their activity to gain access to resources and maintain growth.

Diel activity and foraging mode of juvenile Arctic charr in fluctuating water flow

“…In lotic environments, Arctic charr typically use slow habitats (Heggenes & Saltveit, 2007;Tunney & Steingrímsson, 2012; but see Fingerle et al, 2016), so it is not surprising that they restricted their activity to periods of LWF. Activity peaks followed switches to LWF conditions.…”

“…A string was tied across the top of each enclosure to deter avian predators, presumably without affecting the risk perceived by fish (Larranaga & Steingrímsson, 2015). Each enclosure had ten 1+ individuals (and four young-of-the-year), which is a high enough density to create competition for food and space in a 4 m 2 enclosure (Fingerle, Larranaga, & Steingrímsson, 2016;Larranaga & Steingrímsson, 2015).…”

Diel movement of brown trout,Salmo trutta, is reduced in dense populations with high site fidelity

“…It is generally agreed that individuals adjust their spatial distribution to avoid contact and competition with conspecifics (Kuefler et al., ; Leibold, ), which is consistent with our results. With increasing population density, an increase in competitive pressure can be expected (Harrison, Blount, Inger, Norris, & Bearhop, ; Rose, Cowan, Winemiller, Myers, & Hilborn, ), which often appears as spatial and temporal changes in a distribution (Einum, Sundt‐Hansen, & Nislow, ; Kronfeld‐Schor & Dayan, ), such as shown for Arctic charr (Fingerle et al., ; Guénard et al., ; Larranaga & Steingrímsson, ). Furthermore, higher aggressiveness and stress accompanying an increase in abundance have been recorded in juvenile brown trout (Kaspersson, Höjesjö, & Pedersen, ).…”

“…Juvenile Atlantic salmon (Salmo salar Linnaeus, 1758) were found to emigrate from familiar environments with high population densities to new environments with lower densities of conspecifics (Steingrímsson & Grant, 2003), but the effect of competition on movement and position maintenance was inconclusive compared to the effects of environmental parameters. Similarly, juvenile Arctic charr (Salvelinus alpinus Linnaeus, 1758) showed increased diel activity with increasing abundance as a result of competition for shelters (Larranaga & Steingrímsson, 2015) and food (Fingerle, Larranga, & Steingrímsson, 2016). Furthermore, high population density and decreased food availability promoted increased feeding activity, which was followed by lower growth in Arctic charr (Guénard et al, 2012).…”

Studying behavioural variation in salmonids from an ecological perspective: observations questions methodological considerations

“…This first example investigates how Artic charr activity patterns are affected by fish density and environmental factors in an Icelandic stream (Fingerle et al 2016). Four nylon mesh enclosures (4 9 1 m) were paired in two blocks (one high-density-, and one low-density treatment per pair) in a side-channel of a natural river.…”