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

Fish scale stable isotopes as potential indicators of nutrient pollution: Exploring the response of roach (Rutilus rutilus) scale δ15N and δ13C to a gradient of land use disturbance

To examine the suitability of fish scales as potential tracers of nutrient pollution, we analysed the nitrogen and carbon stable isotope values (δ¹⁵N and δ¹³C) in scales of a generalist fish species, roach Rutilus rutilus, collected from 22 Czech reservoirs covering wide gradients of catchment land use and nutrient enrichment. Using generalised additive mixed models in the first step and generalised linear mixed models in the second step, we evaluated the response of roach scale stable isotope values to catchment land use variables (percentage of agricultural land and human population density) and in-reservoir water quality variables. Roach scale δ¹⁵N values varied by 15 ‰ among the reservoirs and were strongly, linearly, and positively associated with the percentage of agricultural land in the reservoir catchments, pointing to agriculture as the dominant source of nitrogen pollution in the investigated systems. Roach scale δ¹³C values differed by 8 ‰ among the studied reservoirs and were not related to catchment land use variables or in-reservoir primary production (chlorophyll-a levels). Possible variation in roach foraging strategies (littoral versus pelagic) between reservoirs or the contrasting effects of eutrophication-related autotrophic and heterotrophic processes on baseline δ¹³C values may explain the lack of relationships between roach scale δ¹³C values and the explanatory variables. In summary, our findings show that fish scale δ¹⁵N values are sensitive bioindicators of catchment-derived anthropogenic nitrogen inputs to freshwater ecosystems. Because scales can be sampled in a nonlethal way and δ¹⁵N analysis is relatively inexpensive, we suggest that measuring the δ¹⁵N values of fish scales could be an effective method for monitoring nitrogen pollution in aquatic environments.

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.

Compound-specific 15N analysis of amino acids: A tool to estimate the trophic position of tropical seabirds in the South China Sea

Compound-specific 15N analysis of amino acids (AAs) is a powerful tool to determine the trophic position (TP) of organisms. However, it has only been used in a few studies of avian ecology because the AA patterns in the consumer-diet nitrogen trophic discrimination factor (TDFG lu-Phe = ∆15 NG lu-∆15 NP he) were unknown in birds until recently, and tropical seabirds have never been investigated with this methodology. Here, we explore the application of this method to tropical seabirds. In this study, we recovered the fossilized bones of tropical seabirds from ornithogenic sediments on two coral islands in the Xisha Islands, South China Sea, as well as the bones and muscle of their predominant food source, flying fish (Exocoetus volitans). Compound-specific 15N and 13C analyses of AAs in both seabird and fish bone collagen were conducted. The TP of flying fish was calculated based on a widely used single TDFG lu-Phe approach. We then calculated the TP of tropical seabirds in three different ways: (a) according to the composition of their diet; (b) based on the single TDFG lu-Phe approach; and (c) using a multi-TDFG lu-Phe approach. The results of the multi-TDFG lu-Phe approach were much closer to the results based on the composition of the seabird diet than the results of the single TDFG lu-Phe approach, confirming its applicability for tropical seabirds. For seabird bone samples of different ages, TP determined from the multi-TDFG lu-Phe approach was most similar to that of bulk δ15N of bird collagen, with seabirds occupying higher TPs during the Little Ice Age, as previously shown. In addition, the 13C Suess effect was reflected in the AAs δ13C in our samples. This study applied a compound-specific 15N analysis of AAs to determine the TP of tropical seabirds that has potential to extend to all tropical seabirds many of which are widely distributed and play a key role in the evolution of coral island ecosystems.

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.