Diet-tissue discrimination factors (Δ 13 C and Δ 15 N) are influenced by variables including the tissues being analysed and the taxon of the consumer and its prey. Whilst differences in Δ 13 C and Δ 15N are apparent between herbivorous and piscivorous fishes, there is less known for omnivorous fishes that consume plant and animal material. Here, the omnivorous cyprinid fishes Barbus barbus and Squalius cephalus were held in tank aquaria and exposed to three diets that varied in their constituents ( plant based to fishmeal based) and protein content (13% to 45%). After 100 days and isotopic replacement in fish tissues to 98%, samples of the food items, and dorsal muscle, fin tissue and scales were analysed for δ 13 C and δ 15 N. For both species and all diets, muscle was always enriched in δ 15 N and depleted in δ 13 C compared with fin tissue and scales. Across the different diets, Δ 13 C ranged between 2.0‰ and 5.6‰ and Δ 15N ranged between 2.0‰ and 6.9‰. The diet based on plant material (20% protein) always resulted in the highest discrimination factors for each tissue, whilst the diet based on fishmeal (45% protein) consistently resulted in the lowest. The discrimination factors produced by non-fish diets were comparatively high compared with values in the literature, but were consistent with general patterns for some herbivorous fishes. These outputs suggest that the diet-tissue discrimination factors of omnivorous fishes will vary considerably between animal and plant prey, and these specific differences need consideration in predictions of their diet composition and trophic position.

Ecological applications of stable isotope data require knowledge on the isotopic turnover rate of tissues, usually described as the isotopic half-life in days (T 0.5 ) or the change in mass (G 0.5 ). Ecological studies increasingly analyse tissues collected nondestructively, such as fish fin and scales, but there is limited knowledge on their turnover rates. Determining turnover rates in situ is challenging, with ex situ approaches preferred. Correspondingly, T 0.5 and G 0.5 of the nitrogen stable isotope (d 15 N) were determined for juvenile barbel Barbus barbus (5.5 ± 0.6 g starting weight) using a diet-switch experiment. d 15 N data from muscle, fin and scales were taken during a 125 day post diet-switch period. Whilst isotopic equilibrium was not reached in the 125 days, the d 15 N values did approach those of the new diet. The fastest turnover rates were in more metabolically active tissues, from muscle (highest) to scales (lowest). Turnover rates were relatively slow; T 0.5 was 84 (muscle) to 145 (scale) days; G 0.5 was 1.39 9 body mass (muscle) to 2.0 9 body mass (scales), with this potentially relating to the slow growth of the experimental fish. These turnover estimates across the different tissues emphasise the importance of estimating half-lives for focal taxa at species and tissue levels for ecological studies.

Scales and fin tissue are strong surrogates of dorsal muscle in food web studies as they can provide estimates of muscle values within an acceptable level of error when species-specific methods are used. Their derived fractionation factors can also be applied to models predicting fish diet composition from δ(15) N and δ(13) C values.

Stable isotope (SI) analysis studies rely on knowledge of isotopic turnover rates and trophic-step discrimination factors. Epidermal mucus ('mucus') potentially provides an alternative SI 'tissue' to dorsal muscle that can be collected non-invasively and nondestructively. Here, a diet-switch experiment using the omnivorous fish Cyprinus carpio and plant-and fishbased formulated feeds compared SI data between mucus and muscle, including their isotopic discrimination factors and turnover rates (as functions of time T and mass G, at isotopic half-life (50) and equilibrium (95)). Mucus isotope data differed significantly and predictively from muscle data. The fastest d 13 C turnover rate was for mucus in fish on the plant-based diet (T 50 : 17 days, T 95 : 74 days; G 50 : 1.08(BM), G 95 : 1.40(BM)). Muscle turnover rates were longer for the same fish (T 50 : 44 days, T 95 : 190 days; G 50 : 1.13(BM), G 95 : 1.68(BM)). Longer half-lives resulted in both tissues from the fish-based diet. d 13 C discrimination factors varied by diet and tissue (plant-based: 3.11-3.28%; fishmeal: 1.28-2.13%). Mucus SI data did not differ between live and frozen fish. These results suggest that mucus SI half-lives provide comparable data to muscle, and can be used as a non-destructive alternative tissue in fish-based SI studies.

Predicting the ecological consequences of invasions by non‐native species is a fundamental aspect of their risk‐based management. As impacts can include the negative consequences of resource sharing with native species, the application of in situ cohabitation field experiments can test hypotheses relating to invasion ecology via competitive interactions and processes. As fishes are adaptable and tractable experimental organisms, they are strong model species for use in studies on competitive interactions. The trophic consequences of invasion by two globally invasive freshwater fishes, common carp Cyprinus carpio and goldfish Carassius auratus, were tested on the threatened native fish crucian carp Carassius carassius. Cohabitation experiments, completed in pond enclosures, used all species in allopatric and sympatric treatments using a substitutive design where the number of fish per treatment was kept constant. Stable isotope analysis (δ13C and δ15N) assessed alterations in the trophic ecology of each species across treatments, with growth rates used to assess any consequent impacts on the fish. When in sympatry with C. auratus and C. carassius, the Cy. carpio isotopic niche was at a significantly lower trophic position compared to allopatry. This resulted in niche overlap with C. auratus, while for C. carassius, their isotopic niche shifted to a higher trophic position compared with allopatry. The growth rate of Cy. carpio was always significantly higher in sympatry than in allopatry, whereas growth rates for C. carassius and C. auratus were significantly depressed in Cy. carpio presence. In contrast, the isotopic niche sizes and positions and growth rates of the Carassius fishes were not significantly different between allopatry and when they cohabited. Plasticity in the isotopic niche of Cy. carpio resulted in significant alterations in their trophic position between allopatry and sympatry and, when coupled with their depressed growth in allopatry, suggests the competitive processes driving this were intra‐specific rather than inter‐specific. This then resulted in detrimental impacts on cohabiting Carassius fishes. These results emphasise that ecological consequences of Cy. carpio in invaded freshwaters include impacts on the trophic ecology of native fishes.

Application of stable isotope data to trophic studies requires understanding of factors influencing the isotopic discrimination factor (D) between consumers and their prey resources. This is missing for many omnivorous species, despite their diet and environment potentially impacting D. The effects of temperature, diet (including formulated feeds) and tissue type on D 13 C and D 15 N were thus tested experimentally. A temperature experiment exposed three species to identical diets at 18 and 23°C, whereas a diet experiment exposed one species to four diets at 18°C. At 23°C, C:N ratios, D 13 C and D 15 N were generally elevated versus 18°C. After lipid correction, tissue/species-specific differences at 23°C in D 13C and D 15 N were up to 0.73 and 0.54% higher, respectively. Across the four diets, there were also significant differences in D 13 C and D 15 N between a natural diet and diets based on formulated feeds. D 13 C and D 15 N of muscle were 1.51 to 2.76% and 3.13 to 5.44%, respectively. Highest D for both isotopes was from a formulated feed based on plant material that resulted in lower dietary protein content and quality. Thus, diet and environment fundamentally affected the isotopic discrimination factors and these factors require consideration within trophic studies based on stable isotopes.

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The effects of number of fish that are aged and scale sub-sampling strategies on the precision of estimates of mean age-length increments from populations of Rutilus rutilus, Leuciscus leuciscus and Leuciscus cephalus were tested. Analyses used data derived from river fish communities in eastern England, U.K.. Regarding the number of fishes analysed in each age group, for each species and mean fork-length increment at age, significant relationships were detected between sample size (n) and the coefficient of variation of the mean (Z) and mean length increment x‾ and measured variance (s(2)). This enabled calculation of the number of scales for producing a mean length increment at age according to n=ax‾(b(-2))Zx‾(-2). Outputs indicated that the number of scales requiring ageing increased substantially as precision increased, but with little variation between species per age category. Ageing between seven and 12 scales per age group would thus provide estimates at 10% precision. As the ages of fishes are not known in advance of scale ageing, the effect of scale sub-sampling regime on precision was also tested using randomized strategies of 10 fish per 5 mm, five per 5 mm, three per 5 mm, 10 per 10 mm, five per 10 mm and three per 10 mm. These were applied to the datasets and the consequences of their reduction in the number of scales for precision were determined using Z=a(0.5)x‾((b/2)(-1))n(-0.5). When compared to no sub-sampling, three per 10 mm always significantly reduced data precision, whereas 10 per 5 mm never significantly reduced precision. These outputs can thus be applied to the design of fish sampling protocols where age and growth estimates are required, with the randomized sub-sampling likely to be the most useful strategy.