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

Abstract

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 fresh waters is the widespread use of pelletised marine fishmeal (“pellets”) by freshwater anglers as they target large‐bodied cyprinid fishes, such as European barbel Barbus barbus. 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. In experimental mesocosms, B. barbus fed low volumes of pellets (c. 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. 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. 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.

Tissue-specific turnover rates of the nitrogen stable isotope as functions of time and growth in a cyprinid fish

“…When scales from larger, wild fish are used then it is recommended that only material collected the edge of the scale is used as it represents the most recent growth (cf. Bašić & Britton, 2016;Gutmann Roberts et al, 2017).…”

“…Thus, their use here provides a strong model to determine turnover rates based on these food items. Barbus barbus is also an important riverine species in many European rivers (Britton & Pegg, 2011), being a benthic foraging species that often specialises in feeding on baits introduced by anglers (Gutmann Roberts et al, 2017). They are also invasive in a number of European rivers, with concern over the interactions of their populations with indigenous fishes (Bašić et al, 2016).…”

Stable isotope discrimination factors of omnivorous fishes: influence of tissue type, temperature, diet composition and formulated feeds

“…Jackson et al, 2016;Copp et al, 2017) and trophic subsidies influence consumer diets (e.g. Bašić et al, 2015;Gutmann-Roberts et al, 2017). Their application has been enhanced by the development of 'mixing models' that estimate the diet composition of consumers from the stable isotope ratios of their prey (Phillips et al, 2001(Phillips et al, , 2014.…”

“…Moreover, for omnivorous fishes that have mixed diets comprising animal and plant items, the varying proportions of these items could have potentially strong effects on their discrimination factors (Bastos et al, 2017). For example, in some cyprinid fish populations in recreational freshwater fisheries, some individual fish specialize on feeding on angler trophic subsidies based on formulated feeds containing marine-derived fishmeal, whilst others specialize primarily on natural prey Gutmann-Roberts et al, 2017). The influences of these angling baits on discrimination factors are thus potentially important for trophic studies completed in freshwaters where there are high angling activity and consequent individual trophic specializations in their target species (Gutmann-Roberts et al, 2017).…”

“…However, these formulated feeds are increasingly present as strong trophic subsidies in many European freshwaters, primarily through their use as baits by anglers in both lentic and lotic habitats (Jackson et al, 2013;Bašić et al, 2015;Gutmann-Roberts et al, 2017), but also from aquaculture (e.g. Grey et al, 2004).…”