Abstract
Marine to freshwater resource subsidies are characterised by the allochthonous transfer of energy and nutrients from the marine environment to a recipient freshwater system. These nutrients are known as marine derived nutrients, where this process of pulsed resource subsidies across ecosystem boundaries can generate ephemeral fluxes of energy that often enhance in situ productivity within the recipient ecosystem. Currently, the most comprehensively recognized process of marine resource transfer into freshwater systems involves the migrations of diadromous fish. These migrations and their effects have been studied globally, and the subsidies to riverine predators have been documented through the use of stomach content and stable isotope analyses. However, not many of these studies have focussed on Aotearoa’s (New Zealand) South Island lowland river systems, where these environments have been heavily degraded by continuous anthropogenic effects. In the present study, I aimed to further the knowledge on diadromous fish migrations and their importance to lowland river freshwater predators. In tandem, I aimed to conduct one of the first studies to use mucus as a non-lethal stable isotope “tissue” on wild salmonids and shortfin eels within Aotearoa, as recent reports indicate mucus is a potential substitute for lethally sampled tissues of wild fish.
The study was conducted along the lower Ōreti River, Southland, New Zealand, at three spatially separated sites during winter, spring, and summer. Brown trout (Salmo trutta) and shortfin eel (Anguilla australis) were used as the focal predators while migratory common smelt (Retropinna retropinna) were used as the seasonal prey. Fish were sampled at each site, during each season, where stomach contents and isotope samples of muscle, liver, and mucus were used to assess the importance of smelt to the diet of brown trout and shortfin eel while all tissues were compared against each other to observe isotopic differences.
Stomach content data revealed that brown trout and (to a lesser extent) shortfin eels changed their diet in response to a more seasonally abundant food source during the migratory smelt run. Results indicated that smelt were highly important to the diet of brown trout, but only moderately in shortfin eel during spring and summer, where importance was highest at the Estuary and decreasing towards the Upper Reach of the lower Ōreti River. Stable isotope data also indicated high proportional contribution of smelt to both brown trout and shortfin eel during spring while aligning with the stomach content data and revealing the assimilation of marine derived nutrients during the smelt migration. However, this was not observed in summer for brown trout where these results could be explained due to a change in the smelt’s isotopic values. Stable isotopes further revealed that brown trout δ15N increased at the Estuary and Middle Reach sites as the result of a seasonal shift, similarly for shortfin eel. Tissues were also comparable between each other, highlighting the potential use for mucus as a non-lethal bio tracer in substitution for traditional lethal tissues that are commonly used for dietary stable isotope studies. Furthermore, these results demonstrate how stomach content analysis and stable isotopes complement each other in producing accurate dietary patterns. In the present study, it was clear that smelt were an important dietary item to lowland predators during their migration. If estuaries continue to show declining trends in ecosystem health, this could have consequences for local communities and the surrounding biota.