The importance of seasonal macrophyte cover for the behaviour and performance of brown trout (Salmo trutta) in a groundwater‐fed river

Abstract

Groundwater‐fed rivers, such as the chalk streams of southern England, exhibit high levels of stability (e.g. flow and temperature) and physical homogeneity (e.g. depth and substrate grain size). However, growth of instream macrophytes is highly variable depending on season, providing an important but ever‐changing source of cover for stream‐dwelling salmonids, such as brown trout (Salmo trutta). In this study, the behavioural ecology of brown trout inhabiting a chalk stream was assessed during periods that included summer and winter. In a reach of the River Lambourn (Berkshire, UK), a combination of physical habitat mapping, electric‐fishing, passive integrated transponder and radio telemetry was used to quantify trout: (1) density relative to physical and thermal characteristics; (2) movement patterns; and (3) performance, in terms of growth. Trout density was positively related to depth during winter (February) and spring (May), but not at the end of summer (September). Despite no statistical relation between trout density and macrophytes, periods of strong and no association between density and depth coincided with sparse and extensive macrophyte cover throughout the study reach, respectively. Despite being greater for some fish in winter compared to summer, the daily distance moved was generally low (<3.5 m/day). While growth was mostly positive, less mass was gained, and performance deviated farther from optimal levels predicted by a growth model, during periods that included winter. A number of factors probably contributed to lower growth in winter, including costs of reproduction, temperatures—which deviated farther from those optimal for growth—and/or an inability to maximise energy intake, e.g. due to time spent holding position in deeper areas as cover provided by macrophytes declined. Despite the lack of extremes in chalk stream environments, the behavioural ecology of brown trout appears to be influenced by seasonal variation in instream cover provided by macrophytes. This emphasises the importance of balancing the management (e.g. cutting and removal) of macrophytes with the ecological benefits they provide.

From individuals to communities: Habitat complexity affects all levels of organization in aquatic environments

“…Another relevant but often overlooked aspect is the temporal variation of HC at the given spatial scale. This is particularly important in highly dynamic environments such as tidal shore areas but also in environments that vary seasonally such as ponds, streams, or seagrass meadows with rich submerged macrophytes (Gallardo et al, 2017; Henderson et al, 2017; Vowles & Kemp, 2019). Organisms in these environments must either tolerate a wide range of HC, possess physiological and behavioral adaptations to the more or less rapidly changing HC, or the whole community composition may covary with HC over time, for example, through seasonal species turnover (Gallardo et al, 2017), even if the ultimate drivers of the turnover may be unrelated to HC.…”

The influence of flow velocity on the response of rheophilic fish to visual cues

“…Indeed, physically complex habitats, such as patches of submerged vegetation, are utilised by fish to reduce predatory encounters [27]. In southern English chalk streams, trout density is positively related with water depth in winter and spring when availability of instream macrophyte cover declines [28]. This behaviour appears to reflect a predator avoidance (risk minimising) strategy similar to that described for trout in northern boreal river systems where diurnal activity and habitat use change seasonally to minimise fitness costs in response to harsh winter conditions that may elevate predation risk [29].…”