Muscle and scale isotopic offset of three fish species in the Mediterranean Sea: Dentex dentex, Argyrosomus regius and Xyrichtys novacula

Estimating benthic trophic levels to assess the effectiveness of marine protected area management

Designating and managing marine protected areas (MPAs) can mitigate many ocean threats. Banning fishing activities within MPAs enhances the robustness of food-web dynamics and thus increases trophic resilience. Ecosystem function indicators, such as mean trophic level, are increasingly applied in conservation management. Stable isotope analysis is a common tool in trophodynamic studies as it provides information about food sources and trophic level within food webs. In contrast to the traditional top-down approaches in conservation management (mainly for fisheries), this study focuses on bottom-up responses to protection according to the target species in regional small-scale fisheries. The present study aimed to examine how MPA status affects trophodynamics in the rocky reefs of the Illas Atlánticas Marine-Terrestrial National Park (Galicia, NW Spain). Results showed no differences between inside and outside the MPA in species stable isotopic signatures or trophic level. However, these results should be considered with caution due to some limitations in the study design (small number of sites per location, biogeographic differences associated with the island nature of the MPA, or seasonal variability). Nevertheless, the lax fishing management, the lack of proper implementation (the MPA was established in 2002 without a management plan until 2019), and the small size of the studied MPA may result in ineffective conservation outcomes that could have been reflected in the stable isotopic content of the food web. The large number of "paper park" MPAs existing worldwide are not only detrimental to the perception of marine protection, but also provide poor protection of marine ecosystems. Subject to further studies accounting for both environmental and management factors on stable isotope signatures, trophic interactions can form a cost-effective tool for monitoring MPA effectiveness.

Effects of Tissue Preservation on Carbon and Nitrogen Stable Isotope Signatures in Syngnathid Fishes and Prey

Simple Summary

Stable isotope analysis (SIA) was used to assess the influence of various preservation methods (freezing, ethanol and formaldehyde) on syngnathid (seahorses and pipefishes) fins, seahorse newborns (seahorses), and prey (copepods and Artemia). The first available conversion models for syngnathids are provided, enabling their application to isotopic studies in the field and in the laboratory.

Abstract

Isotopic stable analysis (SIA) is a powerful tool in the assessment of different types of ecological and physiological studies. For that, different preservation methods for sampled materials are commonly used prior to isotopic analysis. The effects of various preservation methods (freezing, ethanol and formaldehyde) were analyzed for C:N, and δ¹³C and δ¹⁵N signals on a variety of tissues including dorsal fins (three seahorse and two pipefish species), seahorse newborns (three seahorses species), and prey (copepods and different stages of Artemia) commonly used to feed the fishes under rearing conditions. The aims of the study were: (i) to evaluate isotopic effects of chemical preservation methods across different types of organisms and tissues, using frozen samples as controls, and (ii) to construct the first conversion models available in syngnathid fishes. The chemical preservation in ethanol and, to a lesser extent, in formaldehyde significantly affected δ¹³C values, whereas the effects on δ¹⁵N signatures were negligible. Due to their low lipid content, the isotopic signals in fish fins was almost unaffected, supporting the suitability of dorsal fins as the most convenient material in isotopic studies on vulnerable species such as syngnathids. The regression equations provided resulted convenient for the successful conversion of δ¹³C between preservation treatments. Our results indicate that the normalization of δ¹⁵N signatures in preserved samples is unnecessary. The conversion models should be applicable in isotopic field studies, laboratory experiments, and specimens of historical collections.

Tissue-specific δ13C in ancient and modern tropical seabirds and flying fish in the Xisha Islands, South China Sea

We analyzed ¹³C characteristics in samples of bird bones, feathers, eggshell carbonate and membrane from modern specimens of red-footed booby (Sula sula) as well as fish muscle, scales, and bones from its predominant food source, flying fish (Exocoetus volitans), and muscle from its secondary food source squid (Loligo chinensis), as well as in ancient sub-fossil samples of seabird and flying fish at the Xisha Islands, South China Sea. δ¹³C is tissue-specific in both seabirds and flying fish due to the variance in turn-over among the tissues and differences in the type and content of amino acids across a diverse range of tissues. The δ¹³C discrimination factors also differed significantly among the various tissues between tropical seabirds and their prey. A Suess effect, caused by fossil fuel combustion and the emission of carbon with fewer ¹³C isotopes, was observed in the bird and fish tissue from ancient to modern time. Our study provides a multiple variability index for δ¹³C in organisms along a food chain, and verifies that tissue-specific ¹³C analysis is essential to identify diet and species and thus is a valuable tool for research on tropical seabird ecology.

The efficacy of scale sampling for monitoring trace element concentrations and stable isotopes in commercially harvested walleye (Sander vitreus)

Commercial and sport fishes are subject to rigorous monitoring for concentrations of elements that could pose threats to human health, with numerous advisories issued by authorities annually for those fisheries with high mercury (Hg) concentrations. In Lake Winnipeg, Manitoba, Canada, the commercial walleye fishery is valued at more than $20 million/year, but has historically been subject to Hg advisories. We used an information theoretic approach to evaluate the utility of non-destructive fish-scale sampling to predict As, Mn and Hg concentrations, as well as stable isotope values in walleye muscle by analysing paired samples. Hg concentrations in scales were significantly related to those in muscle (r(2) = 0.75), but the relationships were weaker for As and Mn. The δ(15)N values in scales predicted δ(15)N in muscle reasonably well (r(2) = 0.72), whereas scale δ(13)C and δ(34)S had less predictive power for estimating their respective muscle stable isotope values. For all three isotope values, sex was a marginal predictor, with parameter confidence intervals bounding zero. Analytical constraints currently limit the utility of non-destructively analysing scales for Hg, but hindcasting trophic changes using archived walleye scales may be useful in understanding shifts in nutrients and production, particularly in impacted lake systems.

δ13C and δ15N values in scales of Micropterus salmoides largemouth bass as a freshwater environmental indicator

We have investigated the effectiveness of using the Micropterus salmoides largemouth bass, which is a top predator found throughout the world, as the index of a hydrosphere environment and its food chain. To this end, we used stable carbon and nitrogen isotope analysis (SIA). Largemouth bass were collected from eight dam reservoirs and two ponds in Toyama Prefecture, Japan. Toyama is located in central Japan and features a variety of distinct geographical environments, a result of the 3000-m elevation that changes over short distances, and abundant water systems. The mean δ(13)C and δ(15)N values for the lipid-extracted muscle of largemouth bass from all sampling locations showed large variability, but there were only small standard deviations at each sampling location. The isotope ratios for largemouth bass express the characteristics of each investigated hydrosphere environment and food chain. A very high correlation (δ(13)C: Y(scale) = 0.96 X(muscle) + 1.58, R(2) = 0.98, δ(15)N: Y(scale) = 0.92 X(muscle) - 1.15, R(2) = 0.95) of SIA values was found between largemouth bass scales and lipid-extracted muscles, which suggests that the more easily analyzed scales are useful as SIA samples for the monitoring and comparison of hydrosphere environments throughout the world.

Isotopic turnover of a submersed macrophyte following transplant: the roles of growth and metabolism in eutrophic conditions

Stable isotopic turnover with isotopic change due to growth and metabolic tissue replacement associated with a change in environmental conditions is a critical aspect of the use of stable isotope analyses as time-integrating tracers of resource-consumer interactions. However, stable isotopic turnover in plants remains poorly understood compared with those of animals, and here we used continuous flow elemental analyzer-isotopic ratio mass spectrometry (EA-IRMS) to analyse the turnover of stable carbon and nitrogen isotopes in a submersed macrophyte (Vallisneria natans) after transplantation to hypereutrophic and mesoeutrophic treatments in a field mesocosm experiment. The direction and magnitude of the isotopic shifts of V. natans were suggested to be determined by the inorganic nutrient availability and its isotopic content in the different treatments. Based on the modelling results of turnover, the contribution of growth to the isotopic turnover was as high as those observed in various aquatic ectotherms. However, the contribution of metabolism was also considerable, especially for nitrogen in the hypereutrophic treatment, which was argued to be a response, co-occurring with growth inhabitation and biochemical disorder of V. natans, to the stress induced by the eutrophication. Our results indicated that isotope turnover in a macrophyte is a feasible technique for estimating its ecophysiological conditions in the natural environment, and that it may facilitate understanding of isotopic data in field studies of food web and habitat restoration under eutrophic conditions.

Relationship between isotope composition of sulphur in sulphate dissolved in river water and sulphur extracted from fish scales

More than 100 trouts captured in more than 20 rivers from Poland have been analysed. Only fish caught by trout fishermen were used for this study. Isotope compositions of sulphur (δ(34)S) and carbon (δ(13)C) of fish scales were examined together with δ(34)S from sulphate dissolved in river, as well as patterns of fish diet. We predominantly examined adult fish, at least 4 years old. A scatter of isotope values occurred among the samples obtained from fish caught in different rivers and also for fish caught in the same river. The scatter of the δ(34)S values was much larger than that of δ(13)C values. We noticed the relationship between δ(34)S of scales and δ(34)S of riverine sulphate. There is also a significant difference between isotope compositions obtained for different fish species, which can be attributed to their different diet.

Use of sulphur and carbon stable-isotope composition of fish scales and muscles to identify the origin of fish

δ34S and δ13C analyses were used to determine the origin of trout specimens. The isotope record of their scales and muscles are compared with a database previously obtained from wild- and reared fish coming from Polish rivers and pond farms. The comparison made it possible to find out whether the trout were wild or reared.