Trophic niche shifts in an invaded food web

Background/Question/Methods Feeding interactions between new and established species are central to understanding the dynamics of invaded food webs, and hence the consequences of species invasions for native communities. Despite being a common management practice for species of commercial and conservation importance, the consequences of fish stocking programmes for the recipient community are not often assessed. Atlantic salmon (Salmo salar) stocks are declining globally: International agreements require nation states to preserve (and enhance) stocks. Increasingly this will involve restocking programmes in Europe and North America.
We used the opportunity that fish stocking provides to investigate the impact of a controlled “species invasion” on the feeding interactions of top predators in a quantified stream food web.

Here we report the impact of the re-introduction of native Atlantic salmon to a river on the existing brown/sea trout (Salmo trutta) population. The salmon population had become extinct in this tributary of the river Frome (Dorset, UK) c10 years ago. As part of a larger project assessing the consequences of a “controlled species invasion” on food web structure and functioning, we assessed the diet, growth and production of the trout for one year. We then reared native stock salmon fry and used these to repopulate the stream, thus doubling the density of salmonid fish in the experimental reach throughout the whole of the following year.

Results/Conclusions

A significant shift in young of the year trout diet pre- and post- salmon introduction was observed, indicating a shift in the feeding niche of trout under competition with salmon of the same size. Density of autochthonous invertebrate prey declined in the second year when salmon were present and the total density of salmonids higher, potentially suggesting resource competition. Ontogenetic changes in diet overlap of the two species were observed indicating a separation of their trophic niche. Analysis of individual diet, from repeatedly sampled individuals marked with passive induced transponder (PIT) tags, supported these observations, but did not indicate any individual specialization. However, the presence of salmon did not impact upon the biomass intake or growth of 0+ trout. The observed diet suggested that the two fish species were segregating the habitat: Salmon were exploiting autochthonous (in-stream) production, particularly in fast flowing areas, whereas 0+ trout expanded their diet to include species from slower flowing, marginal habitats. The opportunistic diet of trout and reliance on allochthonous (terrestrial) inputs facilitated the coexistence of the two species.