Predicting the contributions of novel marine prey resources from angling and anadromy to the diet of a freshwater apex predator

Abstract

Anadromous fishes can be important prey resources for piscivorous fauna in lowland rivers. Freshwater anglers exploiting large‐bodied cypriniform fishes use high quantities of pelletised marine fishmeal baits that can contribute substantially to fish diets. This marine‐derived energy pathway also potentially provides a marine prey resource for freshwater piscivores. However, large‐bodied cypriniform fishes are often in a size refuge against predation due to their large sizes. Stable isotope (δ15N and δ13C) analysis assessed how novel marine prey resources influenced the diet of a freshwater apex predator, northern pike Esox lucius, in an impounded river basin (lower River Severn, western England). Up to three groups of prey resources were present: anadromous European shad (Alosa spp.), cypriniform fishes with dietary specialisms based on marine fishmeal baits, and freshwater prey. The availability of these prey resources to E. lucius varied according to river connectivity and levels of angling exploitation in different river reaches. Where the three prey groups were present, E. lucius were more enriched in δ13C values (range: −24.74 to −16.34‰) compared to river reaches where aspects of the marine prey groups were absent (range: −28.30 to −21.47). In all reaches, δ13C increased as E. lucius length increased. In the reach where all prey groups were present, the isotopic niches of three E. lucius size categories were strongly partitioned; this was not apparent in reaches where the marine pathways were unavailable. Stable isotope mixing models suggested that freshwater prey were the most important prey item, contributing between 42% and 96% to the diet of individual E. lucius. However, where present, anadromous fishes and cypriniform fishes specialising on marine fishmeal baits were also important prey items, contributing substantially to the diet of larger E. lucius (length > 650 mm). The total dietary contributions of the marine resources varied considerably among the individual larger fish (22–58% of total diet). The presence of two marine resource pathways in a lowland river thus strongly influenced the diet of an apex predator, but with contributions being a function of their spatial availability, E. lucius body size and individual trophic specialisations. These results emphasise how the anthropogenic activities of river engineering and human subsidies can affect the trophic dynamics of apex predators.

Application of eDNA metabarcoding in a fragmented lowland river: Spatial and methodological comparison of fish species composition

“…In addition, the completion of fish surveys and angler catches in recent years on both rivers have provided biometric data on fish populations that have high utility for fisheries management (e.g. Amat Trigo, Roberts, & Britton, 2017; Nolan et al, 2019), aspects which cannot be provided by eDNA methods.…”

Non‐lethal sampling for the stable isotope analysis of the critically endangered European eel Anguilla anguilla: how fin and mucus compare to dorsal muscle

“…Natural chemical tags, such as stable isotopes, are widely used to investigate the spatial and trophic ecology of fishes (Trueman et al ., 2012), with the stable isotope ratios of carbon and nitrogen (as δ 13 C and δ 15 N) commonly applied to trophic ecology and food web studies (Fry, 2006; Perkins et al ., 2014). Applications include providing information on diet composition (Nolan et al ., 2019), dietary shifts (Vander Zanden & Rasmussen, 1999), consumer trophic positions (Post, 2002) and foraging patterns (Cunjak et al ., 2005).…”

“…Although muscle can also be sampled non‐lethally through the collection of a biopsy sample via a muscle plug, these samples can introduce bias in the SI data when used on smaller fish (Schielke & Post, 2010). As neither lethal sampling nor biased data are desirable, especially when working on threatened species, alternative tissues, such as fin and scale tissue, that can be collected non‐destructively are increasingly used (Hutchinson & Trueman, 2006; Nolan et al ., 2019; Sanderson et al ., 2009).…”

The Human Dimensions of Recreational Anglers Targeting Freshwater Species in Coastal Ecosystems, with Implications for Management

“…Inshore lagoon ecosystems with brackish water offer a suitable habitat for freshwater top predators for several reasons. First, these ecosystems may receive energy from marine food webs through spawning migrations of species such as spring-spawning Atlantic Herring Clupea harengus, which can foster growth (Winkler 1987;Nolan et al 2019b). Moreover, due to the runoff from tributaries and the reduced exchange with seawater, many lagoons are eutrophic or even polytrophic, often shallow and thus polymictic, and hence productive in terms of fisheries yield (Winkler 1989(Winkler , 1991.…”