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The trophic ecology of the Pelagic Thresher shark (Alopias pelagicus) was evaluated based on chemical ecology using stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) in the vertebrae and muscles. Individuals were caught between August 2013 and October 2019 on both the coasts of Baja California Sur, Mexico. In Bahía Tortugas, the mean vertebrae (n = 35) values were 12.72 ± 1.06‰ (δ15N) and −14.79 ± 0.61‰ (δ13C), while in muscles (n = 32) these values were 16.63 ± 0.76‰ (δ15N) and −17.18 ± 0.39‰ (δ13C). In Santa Rosalía, the mean vertebrae (n = 125) isotopic values were 14.4 ± 1.59‰ (δ15N) and −14.18 ± 0.51‰ (δ13C), while in muscles (n = 43), these values were 18.08 ± 0.96‰ (δ15N) and −16.43 ± 0.34‰ (δ13C). These results show higher δ15N values in Santa Rosalía as an effect of baseline isotopic differences between the two regions, whereas the δ13C values were lower in Bahía Tortugas, suggesting offshore ecological behavior (p < 0.05). In Santa Rosalía, there were significant differences by sex for δ15N in muscle, whereas the δ13C showed ontogenetic shifts, indicating that neonates feed in coastal areas more commonly than juveniles or adults (p < 0.05). Neither sex nor ontogenetic differences were observed in Bahía Tortugas (p > 0.05), suggesting a high overlap between their isotopic niches. Therefore, Alopias pelagicus uses the same ecological niche throughout its life, and there is consistency between sexes. The mean trophic position for both tissues and regions was 4.5, which corresponds to a tertiary predator, without any differences between stages or sex. Due to their higher energetic needs, juveniles and females showed the greatest isotopic niche amplitude; thus, their ecological niche is the widest. [ABSTRACT FROM AUTHOR]

Research on the feeding habits of batoids provides information of intra and interspecific interactions of this taxon in marine ecosystems. We analyzed 832 stomachs of nine ray species collected from the artisanal fishery on the Northwest coast of Baja California Sur (NBCS). We identified 79 prey items assembled in five trophic groups (crustaceans, fish, mollusks, polychaetes, and sipunculidae). Results indicated that the red crab Pleuroncodes planipes was the most important prey in the diet of the seven batoid species. A low dietary similarity of the rays was observed, which evidenced different feeding strategies. In this regard, we identified four well‐separated trophic guilds. Each guild was characterized by the consumption of specific prey: (1) Hypanus dipterurus and Narcine entemedor that fed mostly on polychaetes; (2) Rostroraja velezi and Gymnura marmorata fed mostly on fishes (Clupeidae and Batrachoididae); (3) Hypanus longus, Myliobatis californica, and Platyrhinoidis triseriata fed on crustaceans (Munnidae) and Sipunculus worms; and (4) Pseudobatos productus and Zapteryx exasperata fed on crustaceans (Portunidae) and fishes (Synodontidae and Batrachoididae). We concluded that batoids off the NBCS show prey sharing between the different species. [ABSTRACT FROM AUTHOR]

Sharks are top predators and play an important role in the regulation of marine ecosystems at lower trophic position. Mustelus californicus, Sphyrna zygaena, and Isurus oxyrinchus prove to be important fishery resources along the western coast of Baja California Sur and cohabit the same coastal areas, probably sharing resources. However, our knowledge about ecological dynamics of multiple species coexisting and sharing similar habitat resources is still limited, particularly for predators such as sharks. Therefore, this study focuses on the analysis of trophic ecology of the sharks species, using carbon (13C) and nitrogen (15N) stable isotope values in muscle tissues coupled with trace element concentration (Hg, Se, and Cd) in muscle and hepatic tissues of sharks. The values of δ13C (M. californicus −17.3 ± 1.1‰, S. zygaena −17.9 ± 0.5‰, and I. oxyrinchus −18.3 ± 0.3‰) and δ15N (M. californicus 18.2 ± 1.1‰, S. zygaena 18.4 ± 0.9‰, and I. oxyrinchus 17.8 ± 1.1‰) indicated that these species feed in the Gulf of Ulloa all throughout the year, and for extended periods with similar habitat use and trophic niche. The above-mentioned statement is also a conclusion supported by the significant correlation between isotopic and trace element concentrations in the muscular tissues in all studied species. Thus, the results of the present study emphasize the habitat and niche characteristics of three sympatric sharks off the coast of Baja California Sur, Mexico. [ABSTRACT FROM AUTHOR]

Along the Pacific coast of Baja California Sur (PCBCS), the banded guitarfish (Zapteryx exasperata), shovelnose guitarfish (Pseudobatos productus) and bat ray (Myliobatis californica) are highly abundant. Their ecological roles as predators in demersal communities can be key in this ecosystem. To better understand their trophic relationship in the PCBCS, stable isotopes analysis of carbon (ẟ13C) and nitrogen (ẟ15N) were used. Muscle samples (n = 265) were collected from shovelnose guitarfish (n = 94), banded guitarfish (n = 87) and bat ray (n = 84). We observed high variability in stable isotopes values, ẟ13C and ẟ15N of shovelnose guitarfish ranged from −18.53 to −12.85‰ and 15.93‰ to 20.37‰, respectively; banded guitarfish from −18.12‰ to −13.57‰ and 14.41‰ to 19.26‰, respectively; and bat ray from −17.73‰ to −13.98‰ and 13.97‰ to 18.46, respectively. Statistically significant interspecific differences were found (p < 0.05) for ẟ13C and ẟ15N values, as bat ray showed a lower mean ẟ15N value and less negative mean ẟ13C value. Mature male (MM) bat ray) showed significantly higher ẟ15N values and shovelnose guitarfish (MM) significantly lower ẟ13C values compared to other cohorts. Isotopic niche analysis using Bayesian ellipses (SEAc) indicated shovelnose guitarfish occupies the widest isotopic niche compared with bat ray and banded guitarfish. Based on SEAc, the banded guitarfish overlapped 0.46 with the shovelnose guitarfish, while the bat ray overlapped 0.38 and 0.39 with banded and shovelnose guitarfish, respectively. Reported data suggest there is a relative overlap among all species, with probably greater amount of partitioning between the bat ray and the other two species. [ABSTRACT FROM AUTHOR]

Stable isotopes of C (δ13C) and N (δ15N) were used to explore the trophic structure and evaluate mercury (Hg) biomagnification in the food web of muscle of three commercially important ray species from the Pacific coast of Baja California Sur (PCBCS): the shovelnose guitarfish (Pseudobatos productus), banded guitarfish (Zapteryx exasperata), and bat ray (Myliobatis californica). The food web of these ray species predominately consisted of zooplankton, three species of fish, and five species of invertebrates. Mean δ15N values in all species ranged from 10.54 ± 0.18‰ in zooplankton to 17.84 ± 0.81‰ in the shovelnose guitarfish. Mean δ13C values ranged from − 22.05 ± 0.75‰ in the red crab to − 15.93 ± 0.78‰ in the bat ray. Mean total Hg concentration ([THg]) in all species ranged from 0.0009 ± 0.0002 mg kg−1 ww in zooplankton to 0.24 ± 0.19 mg kg−1 ww in the banded guitarfish. The food web magnification factor was 6.38 and significantly greater than 1.0. The present study describes [THg] biomagnification in the benthic food web of three ray species of the PCBCS. This provides an important baseline knowledge of the biomagnification dynamics and pathways of Hg in this environment for these multiple interacting species. [ABSTRACT FROM AUTHOR]

Total mercury concentrations ([THg]) were determined in muscle and liver of the bat ray ( Myliobatis californica ), shovelnose guitarfish ( Pseudobatos productus ) and banded guitarfish ( Zapteryx exasperata ). Generalized linear models (GLM) were used to determine the effects of size and sex in [THg] and showed that both are determinants of [THg] in these species. The [THg] in both tissues significantly increased with length especially in sexually mature organisms with a steeper slope for mature male than mature female. This may relate to elasmobranchs sexual dimorphism driven variation in growth rates. Median muscle [THg] was significantly greater than liver in each ray species but there were some individuals with higher liver [THg] than muscle. There were individuals with muscle [THg] higher than the advisory thresholds of 0.2 and 0.5 mg kg − 1 ww (2.4 and 11% of the bat ray; 2.1 and 10% of the shovelnose guitarfish; 12.6 and 45% of the banded guitarfish, respectively). [ABSTRACT FROM AUTHOR]

Total mercury (THg) concentrations were measured in muscle and liver of two placental viviparous sharks, the Pacific sharpnose shark (Rhizoprionodon longurio) and the brown smooth-hound (Mustelus henlei); as well as in the muscle, liver, and yolk of the yolk-sac viviparous speckled guitarfish (Pseudobatos glaucostigmus) in Baja California Sur. The aim was to determine which factors could be involved in maternal transfer and resultant maternal and embryonic THg concentration. Higher THg concentrations were found in pregnant females compared to embryos paired tissues. THg concentrations of embryo tissues decreased with total length (T L), except for the muscle of the Pacific sharpnose shark. THg concentrations of embryo muscle was positively related to THg concentration in the muscle of pregnant females. Embryos T L , muscle THg concentration of pregnant females, percentage of THg concentration in embryos, along with the reproductive strategy are relevant factors required to improve our understanding of THg concentration in embryo tissues. • Maternal transference of THg in elasmobranchs during gestation • Maternal transfer varies depending on the species. • Embryo T L is an important factor determining [THg] in muscle and liver. • Muscle [THg] in pregnant females is an important factor determining [THg] in embryo tissues. [ABSTRACT FROM AUTHOR]

Cadmium, selenium, and mercury concentrations were measured in muscle and liver of juvenile blue (Prionace glauca) and smooth hammerhead (Sphyrna zygaena) sharks caught on the west coast of Baja California Sur, Mexico, to evaluate the human health risk associated with its consumption. Cd and Hg were lower than the maximum allowable limit for human consumption established by the Mexican government (Hg = 1.0 μg g-1 and Cd = 0.50 μg g-1). Interspecific differences in trace elements accumulation denoted diet variations and physiological requirements of each shark species. Calculated biomagnification factor (BMF) values confirmed a prey-predator trophic transfer of elements. Not significant results of Selenium health benefit Index value (P. glauca = −0.46; S. zygaena = −0.02) signify no potential risks for human health. However, calculated Hazard Index values displayed possible health hazards to the children who consume blue shark meat regularly. The local population is advised regarding the ingestion rates of shark. [Display omitted] • P. glauca and S. zygaena sharks show similar distribution patterns of Cd, Se and Hg. • Hg and Se presented BMF values >1, indicating trophic accumulation process. • High HI values in children, demand regulatory measures in consumption of shark meat. [ABSTRACT FROM AUTHOR]

A study on mercury (Hg) and selenium (Se) concentrations in the liver and muscle of brown smooth-hound shark Mustelus henlei and its principal prey items, was conducted in the western coast of Baja California Sur, Mexico. Average Hg concentrations were found to be high in the muscle than in the liver; however, Hg concentrations were below the maximum permissible limits, and hence, the consumption of this species does not constitute a risk to human health. The mean Se concentrations were higher in the liver than in the muscle. The results of Hg: Se molar ratio revealed that Se counteracts the toxicity of Hg in hepatic tissues, whereas the contrary occurs in the muscle. Significant differences in Hg and Se accumulation were observed between females and males. Biomagnification factor values >1 demonstrate a biomagnification process from its principal prey species (i.e., red crab, Pleuroncodes planipes and Pacific mackerel , Scomber japonicus). [Display omitted] • Consumption of this shark species does not constitute a risk to human health. • The shark tissues contained Hg and Se levels lower than other shark species. • Mercury levels in shark muscles and liver are below Mexican permissible limits. • Biomagnification in prey indicates magnification along the food chain. • The prey red crab was the major contributor of Hg to M. henlei. [ABSTRACT FROM AUTHOR]