Development of non-lethal sampling of carbon and nitrogen stable isotope ratios in salmonids: effects of lipid and inorganic components of fins

A non-lethal stable isotope analysis of valued freshwater predatory fish using blood and fin tissues as alternatives to muscle tissue

Stable isotope analysis (SIA) is widely used to study trophic ecology and food webs in aquatic ecosystems. In the case of fish, muscle tissue is generally preferred for SIA, and the method is lethal in most cases. We tested whether blood and fin clips can be used as non-lethal alternatives to muscle tissue for examining the isotopic composition of two freshwater predatory fish, European catfish (Silurus glanis) and Northern pike (Esox lucius), species of high value for many freshwater systems as well as invasive species in many others. Blood samples from the caudal vein, anal fin clips, and dorsal muscle obtained by biopsy punch were collected from four catfish and pike populations (14-18 individuals per population). Subsequently, these samples were analyzed for δ13C and δ15N. The effects of alternative tissues, study site, and fish body mass on the isotopic offset were investigated. Both species showed a correlation between the isotopic offset and the tissue type, as well as the study site, but no significant relationship with the body mass. The isotopic offsets between tissues were used to calculate the conversion equations. The results demonstrated that both blood and fin clips are suitable and less invasive alternative to muscle in SIA studies focused on European catfish and Northern pike. Blood provided better correspondence to muscle isotope values. However, our results clearly demonstrated that isotopic offsets between tissues vary significantly among populations of the same species. Therefore, obtaining a muscle biopsy from several individuals in any population is advisable to gain initial insights and establish a possible population-specific inter-tissue conversion.

Evaluation of muscle lipid extraction and non-lethal fin tissue use for carbon, nitrogen, and sulfur isotope analyses in adult salmonids

RATIONALE

Chemical lipid extraction or using alternative tissues such as fish fin as opposed to muscle may alter isotopic ratios and influence interpretations of δ¹³ C, δ¹⁵ N, and previously unassessed δ³⁴ S values in stable isotope analyses (SIA). Our objectives were to determine if lipid extraction alters these isotope ratios in muscle, if lipid normalization models can be used for lipid-rich salmonids, and if fin isotope ratios are comparable with those of muscle in adult salmonids.

METHODS

In six adult salmonid species (n = 106) collected from Lake Ontario, we compared three isotope ratios in lipid-extracted (LE) muscle with bulk muscle, and LE muscle with fin tissue, with paired t-tests and linear regressions. We compared differences between δ¹³ C values in LE and bulk muscle with predicted values from lipid normalization models and the log-linear model of best fit and determined model efficiency.

RESULTS

The δ¹⁵ N values in LE muscle increased (<1‰) relative to bulk muscle for most salmonids, with relationships nearing 1:1. There were either no differences or strong 1:1 relationships in δ³⁴ S values between species-specific bulk and LE muscle. One lipid normalization model had greater model efficiency (97%) than the model of best fit (94%). Fin had higher δ¹³ C values than LE muscle while δ¹⁵ N trends varied (<1‰); however, both isotope ratios had either no or weak linear relationships with fin and LE muscle within species. The δ³⁴ S values in fin were similar to those in LE muscle and had strong 1:1 relationships across species.

CONCLUSIONS

We recommend using the lipid normalization model to adjust for δ¹³ C values in lipid-rich muscle (C:N >3.4). LE muscle could be used without δ¹⁵ N or δ³⁴ S adjustments, but the minimal increase in δ¹⁵ N values may affect SIA interpretation. With high unexplained variability among adult species in fin-muscle δ¹³ C and δ¹⁵ N relationships, species-specific fin-muscle adjustments are warranted. No fin-muscle tissue adjustment would be required for δ³⁴ S values.

Trophic structure of apex fish communities in closed versus leaky lakes of arctic Alaska

Despite low species diversity and primary production, trophic structure (e.g., top predator species, predator size) is surprisingly variable among Arctic lakes. We investigated trophic structure in lakes of arctic Alaska containing arctic char Salvelinus alpinus using stomach contents and stable isotope ratios in two geographically-close but hydrologically-distinct lake clusters to investigate how these fish may interact and compete for limited food resources. Aside from different lake connectivity patterns ('leaky' versus 'closed'), differing fish communities (up to five versus only two species) between lake clusters allowed us to test trophic hypotheses including: (1) arctic char are more piscivorous, and thereby grow larger and obtain higher trophic positions, in the presence of other fish species; and, (2) between arctic char size classes, resource polymorphism is more prominent, and thereby trophic niches are narrower and overlap less, in the absence of other predators. Regardless of lake cluster, we observed little direct evidence of arctic char consuming other fishes, but char were larger (mean TL = 468 vs 264 mm) and trophic position was higher (mean TP = 4.0 vs 3.8 for large char) in lakes with other fishes. Further, char demonstrated less intraspecific overlap when other predators were present whereas niche overlap was up to 100% in closed, char only lakes. As hydrologic characteristics (e.g., lake connectivity, water temperatures) will change across the Arctic owing to climate change, our results provide insight regarding potential concomitant changes to fish interactions and increase our understanding of lake trophic structure to guide management and conservation goals.

Acid treatment of Atlantic salmon (Salmo salar) scales prior to analysis has negligible effects on δ13C and δ15N isotope ratios

There is debate in the literature as to whether scales of fishes require acidification to remove inorganic carbonates prior to stable isotope analysis. Acid-treated and untreated scales from 208 Atlantic salmon from nine locations on both sides of the Atlantic were analysed for δ13C and δ15N. Linear mixed-effect models determined the effect of acid treatment to be statistically significant. However, the mean difference was small (δ13C 0.1 ± 0.2‰, δ15N -0.1 ± 0.2‰) and not of biological relevance. This study concludes that Atlantic salmon scales do not need to be acidified prior to stable isotope analysis.

The influence of diet composition and tissue type on the stable isotope incorporation patterns of a small-bodied southern African minnow Enteromius anoplus (Cypriniformes, Cyprinidae)

RATIONALE

In trophic ecology, the use of stable isotope data relies on the general understanding of isotope turnover rates and diet-to-tissue discrimination factors (DTDFs). Recent studies on the application of stable isotope data have shown that isotope turnover rates and DTDFs can be influenced by many factors, including diet composition and tissue type. This study investigated the influence of diet composition and tissue type on stable isotope incorporation patterns in a small-bodied African minnow, the chubbyhead barb Enteromius anoplus.

METHODS

The isotopic incorporation patterns of carbon (δ¹³ C values) and nitrogen (δ¹⁵ N values) into white muscle and caudal fin tissues of the chubbyhead barb were examined using two isotopically different diets. Controlled-diet stable isotope feeding trials using a fishmeal-based diet (diet 1) and a soya-based diet (diet 2) were conducted over a 180-day period for the chubbyhead barb.

RESULTS

The two diets had contrasting isotopic incorporation patterns: diet 1 was associated with progressively high δ¹³ C and δ¹⁵ N values, whereas diet 2 was associated with progressively low δ¹³ C and δ¹⁵ N values over time for both muscle and fin tissues. The δ¹³ C turnover rates were similar for both tissues (56 and 61 days), whereas the δ¹⁵ N turnover rates differed between fin and muscle tissue in both diets (diet 1 = 4 and 130 days, and diet 2 = 72 and 300 days, respectively). The DTDFs were similar for both tissues in diet 1 (Δ¹³ C: -3.96 to -2.62‰, Δ¹⁵ N: 1.98 to 2.61‰) and diet 2 (Δ¹³ C: 4.05 to 5.24‰, Δ¹⁵ N: 8.45 to 9.69‰).

CONCLUSIONS

These results suggest that fin tissue can potentially be used as an alternative for muscle tissue in food web studies with a reasonable level of error. The isotopic turnover rate and DTDFs estimates for E. anoplus, however, require consideration of diet composition because different diets may differ in their isotopic incorporation patterns.

Validating fin tissue as a non-lethal proxy to liver and muscle tissue for stable isotope analysis of yellow perch (Perca flavescens)

Stable isotope ecology typically involves sacrificing the animal to obtain tissues. However, with threatened species or in long-term longitudinal studies, non-lethal sampling techniques should be used. The objectives of this study were to (1) determine if caudal fin tissue could be used as a non-lethal proxy to liver and muscle for stable isotope analysis, and (2) assess the effects of ethanol preservation on δ¹⁵N and δ¹³C in fin tissue of juvenile yellow perch Perca flavescens. The δ¹³C of caudal fin was not significantly different from liver (t₂₃ = -0.58; p = 0.57), and was more correlated with δ¹⁵N in liver (r² = 0.78) than muscle (r² = 0.56). Ethanol preservation enriched ¹⁵N and ¹³C for caudal fins, but by using our developed regression models, these changes in δ¹⁵N and δ¹³C can now be corrected. Overall, caudal fin tissue is a more reliable proxy to liver than muscle for δ¹⁵N and δ¹³C in yellow perch.

High variability in stable isotope diet-tissue discrimination factors of two omnivorous freshwater fishes in controlled ex situ conditions

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(15)N 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(15)N 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.

Small Tails Tell Tall Tales--Intra-Individual Variation in the Stable Isotope Values of Fish Fin

Background

Fish fin is a widely used, non-lethal sample material in studies using stable isotopes to assess the ecology of fishes. However, fish fin is composed of two distinct tissues (ray and membrane) which may have different stable isotope values and are not homogeneously distributed within a fin. As such, estimates of the stable isotope values of a fish may vary according to the section of fin sampled.

Methods

To assess the magnitude of this variation, we analysed carbon (δ¹³C), nitrogen (δ¹⁵N), hydrogen (δ²H) and oxygen (δ¹⁸O) stable isotopes of caudal fin from juvenile, riverine stages of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta). Individual fins were sub-sectioned into tip, mid and base, of which a further subset were divided into ray and membrane.

Findings

Isotope variation between fin sections, evident in all four elements, was primarily related to differences between ray and membrane. Base sections were¹³C depleted relative to tip (~ 1 ‰) with equivalent variation evident between ray and membrane. A similar trend was evident in δ²H, though the degree of variation was far greater (~ 10 ‰). Base and ray sections were ¹⁸O enriched (~ 2 ‰) relative to tip and membrane, respectively. Ray and membrane sections displayed longitudinal variation in ¹⁵N mirroring that of composite fin (~ 1 ‰), indicating that variation in¹⁵N values was likely related to ontogenetic variation.

Conclusions

To account for the effects of intra-fin variability in stable isotope analyses we suggest that researchers sampling fish fin, in increasing priority, 1) also analyse muscle (or liver) tissue from a subsample of fish to calibrate their data, or 2) standardize sampling by selecting tissue only from the extreme tip of a fin, or 3) homogenize fins prior to analysis.