Stable isotope turnover rates and fractionation in captive California yellowtail (Seriola dorsalis): insights for application to field studies

Tracing exploitation of egg boons: an experimental study using fatty acids and stable isotopes

Coordinated spawning of marine animals releases millions of planktonic eggs into the environment, known as egg boons. Eggs are rich in essential fatty acids and may be an important lipid subsidy to egg consumers. Our aim was to validate the application of fatty acid and stable isotope tracers of egg consumption to potential egg consumers and to confirm egg consumption by the selected species. We conducted feeding experiments with ctenophores, crustaceans and fishes. We fed these animals a common diet of Artemia or a commercial feed (Otohime) and simulated egg boons for half of them by intermittently supplementing the common diet with red drum (Sciaenops ocellatus) eggs for 10-94 days. Controls did not receive eggs. Fatty acid profiles of consumers fed eggs were significantly different from those of controls 24 h after the last egg-feeding event. Consumers took on fatty acid characteristics of eggs. In fishes and ctenophores, fatty acid markers of egg consumption did not persist 2-5 days after the last egg-feeding event, but markers of egg consumption persisted in crustaceans for at least 5-10 days. Additionally, consumption of eggs, which had high values of δ15N, led to δ15N enrichment in crustaceans and a fish. We conclude that fatty acids and nitrogen stable isotope can be used as biomarkers of recent egg consumption in marine animals, validating their use for assessing exploitation of egg boons in nature.

Isotopic turnover in polar cod (Boreogadus saida) muscle determined through a controlled feeding experiment

Polar cod (Boreogadus saida) is an important trophic link within Arctic marine food webs and is likely to experience diet shifts in response to climate change. One important tool for assessing organism diet is bulk stable isotope analysis. However, key parameters necessary for interpreting the temporal context of stable isotope values are lacking, especially for Arctic species. This study provides the first experimental determination of isotopic turnover (as half-life) and trophic discrimination factors (TDFs) of both δ¹³ C and δ¹⁵ N in adult polar cod muscle. Using a diet enriched in both ¹³ C and ¹⁵ N, we measured isotopic turnover times of 61 and 49 days for δ¹³ C and δ¹⁵ N, respectively, with metabolism accounting for >94% of the total turnover. These half-life estimates are valid for adult polar cod (>3 years) experiencing little somatic growth. We measured TDFs in our control of 2.6‰ and 3.9‰ for δ¹³ C and δ¹⁵ N, respectively, and we conclude that applying the commonly used TDF of ~1‰ for δ¹³ C for adult polar cod may lead to misrepresentation of dietary carbon source, while the use of 3.8‰ for δ¹⁵ N is appropriate. Based on these results, we recommend that studies investigating seasonal shifts in the diet of adult polar cod sample at temporal intervals of at least 60 days to account for isotopic turnover in polar cod muscle. Although isotopic equilibrium was reached by the fish in this study, it was at substantially lower isotope values than the diet. Additionally, the use of highly enriched algae in the experimental feed caused very high variability in diet isotope values which precluded accurate calculation of TDFs from the enriched fish. As a result of the challenges faced in this study, we discourage the use of highly enriched diets for similar experiments and provide recommendations to guide the design of future isotopic turnover experiments.

Post-Mortem Dental Profile as a Powerful Tool in Animal Forensic Investigations—A Review

Veterinary forensics is becoming more important in our society as a result of the growing demand for investigations related to crimes against animals or investigations of criminal deaths caused by animals. A veterinarian may participate as an expert witness or may be required to give forensic assistance, by providing knowledge of the specialty to establish a complete picture of the involvement of an animal and allowing the Courts to reach a verdict. By applying diverse dental profiling techniques, not only can species, sex, age-at-death, and body size of an animal be estimated, but also data about their geographical origin (provenance) and the post-mortem interval. This review concentrates on the dental techniques that use the characteristics of teeth as a means of identification of freshly deceased and skeletonised animals. Furthermore, this highlights the information that can be extracted about the animal from the post-mortem dental profile.

Trophic Dynamics and Feeding Ecology of Skipjack Tuna (Katsuwonus pelamis) off Eastern and Western Taiwan

The skipjack tuna (Katsuwonus pelamis) is a mesopredator fish species with seasonal abundance in waters off Taiwan. Regional ecological and life-history information has been historically lacking for this species. In recent years, stable isotope analysis (SIA) of carbon and nitrogen has been used to assess predator feeding ecology and broader ecosystem trophic dynamics. This study evaluated comparative skipjack feeding ecology in distinct regions off Taiwan, combining traditional stomach content analysis with SIA of individuals off western (n = 43; 2020) and eastern (n = 347; 2012–2014 and n = 167; 2020) Taiwan. The stomach content analysis showed the most important prey to be ponyfish (Photopectoralis bindus) in western Taiwan and epipelagic squids (Myopsina spp.) and carangids (Decapterus macrosoma;) in eastern Taiwan from 2012 to 2014 and epipelagic carangids (Decapterus spp.) and flying fishes (Cheilopogon spp.) in eastern Taiwan in 2020, suggesting that the skipjack tuna is a generalist predator across regions. In contrast, time-integrated diet estimates from Bayesian mixing models indicated the importance of cephalopods and crustaceans as prey, potentially demonstrating more mesopelagic feeding in less productive waters during skipjack migrations outside the study regions. Skipjack off western Taiwan had a slightly higher estimated trophic position than in the waters off eastern Taiwan, potentially driven by the varying nutrient-driven pelagic food web structures. Skipjack SI values increased with body size off eastern Taiwan but not in western waters, suggesting that opportunistic predation can still result in different predator–prey size dynamics between regions.