| dc.description.abstract | Loss of riparian vegetation along rivers and streams due to human activities lead to significant changes in aquatic environments. Many fish species, including the brown trout (Salmo trutta), utilize rivers and streams throughout their life cycle and may therefore be exposed to these changes. Thus, this study examined how riparian vegetation influences the density, survival, and movement patterns of juvenile brown trout.
During the summer of 2023, 15 electro-fishing stations were selected across four tributary streams in the Gausa watershed, Innlandet, Norway. Electro-fishing was conducted in June and September, with the aim of tagging fish with Passive Integrated Transponders (PIT-tags) and estimating the density of young-of-the-year (0+) and older age classes (>0+) of brown trout. At all electro-stations, riparian vegetation and other environmental covariates were assessed to further be linked with the estimated densities. Manual antenna PIT-scanning was conducted five times in all tributaries between July and November 2023 to establish detection histories to further estimate survival probability and record potential movements. Riparian vegetation and habitat types were also mapped along the scanned stretches. The relationship between riparian vegetation and 0+ density, >0+ density, survival probability, movement probability, and in-stream movement distances were then examined. Covariates were included in all analyses to isolate the riparian effects. Riparian effects on juvenile densities were assessed using linear models, considering riparian conditions and other environmental variables related to stream habitat. Additionally, the >0+ density was included as a density-dependent factor in the analysis of 0+ density. The riparian effects on apparent survival probability was estimated using a Cormack-Jolly-Seber model structure, modelled as a function of the detection histories, standardized fish lengths, the average riparian conditions, standardized water discharge of the corresponding tributaries, and seasons. Two analyses were conducted for addressing riparian effects on in-stream movement patterns: “movement probability” and “movement distances”. Prior to analyzing movement probability, individuals’ positions in the stream, along with their previous documented positions, were linked to corresponding on-site riparian conditions. Movement probability was then assessed through logistic linear regression, as a function of corresponding riparian conditions, habitat type, individual total length, and season. Movement distances were analyzed using linear models with the same covariates as in the analysis of movement probability, along with the average riparian conditions of the corresponding tributaries.
Riparian shading on the water surface, riparian buffer width, the presence of larger trees, and overhanging canopy cover were found to have a negative effect on 0+ density. Overhanging non-woody vegetation had a weak positive correlation, while no notable effects were found of upstream vegetation. Riparian vegetation was not found to have an effect on >0+ density. The negative effects of extensive riparian vegetation on 0+ fish density can possibly be explained by reduced light influx and autochthonous production in the presence of riparian cover. 0+ individuals are known to have higher food requirements when compared to >0+ and has previously been shown to prefer aquatic invertebrates as a food source. Stretches of low riparian cover could consequently be favored. The absent effect on >0+ suggests that older age classes are evenly distributed across sections of varying riparian conditions, thus being less affected by the riparian effects. This could possibly be a result of older age classes favoring riparian cover and allochthonous terrestrial invertebrates. A negative correlation between riparian vegetation and survival probability was found when the extent of riparian vegetation was adjusted for seasonal variations. Survival probability was consequently found to be lower in summer than in autumn, post-senescence. The negative effect of riparian vegetation on survival probability aligned with the negative correlation found for 0+ density. As the young-of-the-year age class were likely the highest represented among the tagged individuals, the observed negative riparian effect on survival probability might be in support of that stretches with extensive riparian vegetation seemingly may provide poorer growth conditions for 0+. However, as the variable representing the riparian effects was adjusted for seasonal variations, other factors that vary with season may also have influenced the survival probability. A major flood in mid-August and subsequent decreasing water flow through the autumn may have played a central role, decreasing survival probability during summer. No correlations were found between riparian vegetation and movement patterns. Both movement probability and movement distances were best explained by seasonal differences, with longer movement distances during summer and higher movement probabilities during autumn. These findings might be explained by the observed variations in water discharge throughout the study period, and align with the findings on survival probability, suggesting seasonal differences. The high water levels and mid-august flooding event may have caused fish dislocations, explaining the elevated movement distances during summer. The elevated movement probability during autumn might be explained by the reduced water levels potentially leading to increased competition for remaining habitat, resulting in an increase in small scale movements.
The findings of this study suggest that various aspects of riparian vegetation may have different impacts on different age classes of brown trout. This underscores the importance of processing a comprehensive understanding of riparian vegetation in fish management. The study also sheds light on the effect of water flow and flooding on juvenile brown trout ecology. | |
