Reliance of brown trout on terrestrial prey varies with season but not fish density

The importance of terrestrial carbon in aquatic ecosystems is widely recognised, but patterns of terrestrial reliance can be variable. Fish often act as important links between terrestrial and aquatic ecosystems, but little is known about how resource seasonality and fish density influence fish reliance on terrestrial energy in lakes. We sampled a high-latitude subarctic lake in Finnish Lapland during the open-water season over three consecutive years to assess both patterns of terrestrial reliance and trophic niche structure of introduced brown trout (Salmo trutta), the only resident fish species. The small size of the study lake made it possible to sample the whole population by conducting a complete fish removal, allowing for a direct assessment of size structure and changes in brown trout density over time. We hypothesised that annual and seasonal shifts in the dietary niches of brown trout would directly track the availability of pulsed resources such as aquatic and terrestrial insects as well as rodents. We further expected that dietary niche shifts would be correlated with population density, leading to a smaller trophic niche size at lower densities. We therefore investigated the annual and seasonal patterns of resource use using measures of dietary niche and in particular of terrestrial reliance, derived from stomach content analysis and stable-isotope analyses (SIA) of liver and muscle, along a temporal gradient of declining fish density. According to stomach content, terrestrial reliance in brown trout was the highest in each year at mid-to-late summer, evidently following the peak abundance of terrestrial invertebrates and rodents. Surprisingly, we could not detect annual or seasonal shifts in terrestrial reliance from estimates provided by isotope ratios in muscle or liver. Furthermore, fish density did not appear to influence either terrestrial reliance or trophic niche size. However, trophic position derived from SIA of liver tissue decreased with decreasing densities, while fish condition increased. Large, consistent pulses of terrestrial invertebrates in mid-summer (or rodents during their peak years) are likely important for brown trout in the long term and could explain the lack of density-dependent correlation in terrestrial reliance. However, further studies are needed to link the abundance of pulsed resources to resource use by fish across wider gradients of lake size, productivity and fish density.