Invasive crayfish impacts on native fish diet and growth vary with fish life stage

Abstract

Assessing the impacts of invasive organisms is a major challenge in ecology. Some widespread invasive species such as crayfish are potential competitors and reciprocal predators of ecologically and recreationally important native fish species. Here, we examine the effects of signal crayfish (Pacifastacus leniusculus) on the growth, diet, and trophic position of the chub (Squalius cephalus) in four rivers in Britain. Growth rates of 0+ chub were typically lower in sympatric populations with signal crayfish compared with allopatric populations, and this effect could be traced through to 2+ chub in one river. However, growth rates of older chub (5+ to 6+) were typically higher in the presence of crayfish. Sympatry with crayfish resulted in lower chub length-at-age and mass-at-age in half of the rivers sampled, with no change detected in the other rivers. Stable isotope analyses (δ¹³C and δ¹⁵N) revealed that both chub and crayfish were omnivorous, feeding at multiple trophic levels and occupying similar trophic positions. We found some evidence that chub trophic position was greater at invaded sites on one river, with no difference detected on a second river. Mixing models suggested crayfish were important food items for both small and large chub at invaded sites. This study provides evidence that invasive species can have both positive and negative effects on different life stages of a native species, with the net impact likely to depend on responses at the population level.

Keywords: Chub Squalius cephalus; Competition; Diet shift; Invasive species; Scalimetry; Signal crayfish Pacifastacus leniusculus; Stable isotopes.

Fig. 1

A comparison of calculated mean (±SE) yearly growth rates of chub sampled from uninvaded (closed symbols) and invaded (open symbols) sites on a the Rother, b Chad Brook, c the Cherwell, and d the Evenlode

Fig. 2

Chub fork length-at-age at uninvaded (solid circles and line) and invaded (open circles and dashed line) sites on a the Rother, b Chad Brook, c the Cherwell, and d the Evenlode. Chub mass-at-age at uninvaded (solid circles and line) and invaded (open circles and dashed line) sites on e the Rother and f Chad Brook

Fig. 3

Baseline corrected chub δ¹⁵N as a function of fork length for uninvaded (solid circles) and invaded (open circles) sites on a the Rother and b Chad Brook

Fig. 4

Isotope bi-plots indicating the mean (±standard error) for chub, crayfish, and the putative prey of both species, for a the Rother and b Chad Brook. For the Rother small fish were 1+ cyprinids, Phoxinus phoxinus, Cottus gobio, and Barbatula barbatula, aquatic invertebrates were Trichoptera, Gammarids, and Ephemeroptera, and terrestrial invertebrates were Formicidae, Arachnidae, Hemiptera, Diptera, and Coleoptera. For Chad Brook small fish were Phoxinus phoxinus, Cottus gobio, Barbatula barbatula, and Gasterosteus aculeatus, aquatic invertebrates were Gammarids, Calopteryx sp., Heteroptera, Limnaea, and Trichoptera, and terrestrial invertebrates Formicidae, Arachnidae, Diptera, Coleoptera, and Gastropoda

Fig. 5

The mean (±95 % CI) percentage of small and large chub diets comprised by each food source at uninvaded and invaded sites, as indicated by the SIAR mixing model. Normal distributions of isotope data were confirmed by visual inspection of the data