Trophic consequences for riverine cyprinid fishes of angler subsidies based on marine derived nutrients.

Abstract

1. The crossing of freshwater ecosystem boundaries by marine derived nutrients (MDN) is usually associated with migratory salmonid fishes returning to natal rivers. An alternative source of MDN in freshwaters is the widespread use of pelletized marine fishmeal (‘pellets’) by freshwater anglers as they target large bodied cyprinid fishes, such as European barbel Barbus barbus. 2. Here, the trophic consequences of MDN from pellets for riverine cyprinid fishes were tested. Approaches used stable isotope analyses in controlled and wild scenarios, using B. barbus and chub Squalius cephalus as model species. The isotopic niche, measured as standard ellipse area, was used to assess trophic niche size, and mixing models predicted the extent to which MDN contributed to fish diet. 3. In experimental mesocosms, B. barbus fed low volumes of pellets (approximately 3 per fish) for 130 days had isotopic niche sizes that were up to four times larger than a control and ‘medium’ (6 per fish) and ‘high’ pellet (12 per fish) treatments. Somatic growth rates were significantly higher in the ‘medium’ and ‘high’ treatments. In pond enclosure experiments, when juvenile B. barbus and S. cephalus were fed pellets daily for 100 days, there was a substantial and significant shift in the position of their isotopic niche compared to controls with no pellets fed. However, for each species, there were no significant differences in their somatic growth rates in the presence/ absence of pellets. 4. In a lowland river, high proportions of MDN contributed to the diet of B. barbus and S. cephalus captured by angling, but with substantial individual variability in those captured by electric fishing. Across all B. barbus > 400 mm, MDN dietary contributions ranged between 9 and 71%. This suggested some individual diet specialisations within their population that was associated with feeding on this angler subsidy and that also resulted in a significant increase in the size of their population isotopic niche. 5. These results suggested that when pellets containing MDN are used in freshwater angling, they are consumed and assimilated by cyprinid fishes, influencing individual and population trophic positions, and isotopic niche sizes and dietary specialisations. The results also suggested that the extent to which individuals specialise in feeding on pellets potentially influences their vulnerability to capture by anglers.