Search

The chemical composition of otoliths (earbones) can provide valuable information about stock structure and connectivity patterns among marine fish. For that, chemical signatures must be sufficiently distinct to allow accurate classification of an unknown fish to their area of origin. Here we have examined the suitability of otolith microchemistry as a tool to better understand the spatial dynamics of skipjack tuna (Katsuwonus pelamis), a highly valuable commercial species for which uncertainties remain regarding its stock structure in the Indian Ocean. For this aim, we have compared the early life otolith chemical composition of young-of-the-year (

Pacific Ocean tuna is among the most-consumed seafood products but contains relatively high levels of the neurotoxin methylmercury. Limited observations suggest tuna mercury levels vary in space and time, yet the drivers are not well understood. Here, we map mercury concentrations in skipjack tuna across the Pacific Ocean and build generalized additive models to quantify the anthropogenic, ecological, and biogeochemical drivers. Skipjack mercury levels display a fivefold spatial gradient, with maximum concentrations in the northwest near Asia, intermediate values in the east, and the lowest levels in the west, southwest, and central Pacific. Large spatial differences can be explained by the depth of the seawater methylmercury peak near low-oxygen zones, leading to enhanced tuna mercury concentrations in regions where oxygen depletion is shallow. Despite this natural biogeochemical control, the mercury hotspot in tuna caught near Asia is explained by elevated atmospheric mercury concentrations and/or mercury river inputs to the coastal shelf. While we cannot ignore the legacy mercury contribution from other regions to the Pacific Ocean (e.g., North America and Europe), our results suggest that recent anthropogenic mercury release, which is currently largest in Asia, contributes directly to present-day human mercury exposure. [ABSTRACT FROM AUTHOR]

The chemical composition of otoliths (earbones) can provide valuable information about stock structure and connectivity patterns among marine fish. For that, chemical signatures must be sufficiently distinct to allow accurate classification of an unknown fish to their area of origin. Here we have examined the suitability of otolith microchemistry as a tool to better understand the spatial dynamics of skipjack tuna (Katsuwonus pelamis), a highly valuable commercial species for which uncertainties remain regarding its stock structure in the Indian Ocean. For this aim, we have compared the early life otolith chemical composition of young-of-the-year (<6 months) skipjack tuna captured from the three main nursery areas of the equatorial Indian Ocean (West, Central and East). Elemental (Li:Ca, Sr:Ca, Ba:Ca, Mg:Ca and Mn:Ca) and stable isotopic (δ13C, δ18O) signatures were used, from individuals captured in 2018 and 2019. Otolith Sr:Ca, Ba:Ca, Mg:Ca and δ18O significantly differed among fish from different nurseries, but, in general, the chemical signatures of the three nursery areas largely overlapped. Multivariate analyses of otolith chemical signatures revealed low geographic separation among Central and Eastern nurseries, achieving a maximum overall random forest cross validated classification success of 51%. Cohort effect on otolith trace element signatures was also detected, indicating that variations in chemical signatures associated with seasonal changes in oceanographic conditions must be well understood, particularly for species with several reproductive peaks throughout the year. Otolith microchemistry in conjunction with other techniques (e.g., genetics, particle tracking) should be further investigated to resolve skipjack stock structure, which will ultimately contribute to the sustainable management of this stock in the Indian Ocean. [ABSTRACT FROM AUTHOR]

Mercury, omega-3 (docosahexaenoic acid, DHA and eicosapentaenoic acid, EPA) and macronutrients (fat and proteins) were quantified on a wet weight (ww) basis in 20 species of fish taken as bycatch in tropical tuna fisheries. Based on a hazard quotient taking into account mercury and omega-3 contents, a benefit-risk assessment for the consumption of these pelagic species was conducted for three people categories: young children, children and adults. All fish bycatch were found to be an excellent source of proteins (min‒max = 14.4–25.2 g/100 g fillet), had low omega-6/omega-3 ratios (<1, except for silky shark), and had mercury content below the safety limits defined by sanitary agencies. Silky shark and Istiophoridae had the highest mercury contents (min‒max = 0.029–0.317 ppm ww). Omega-3 contents were the lowest in silky shark (0.2 ± 0.2 mg/100 g fillet) and the highest in striped marlin (3.6 ± 3.2 g/100 g fillet). Billfishes (Istiophoridae, including striped marlin), minor tunas (Scombridae), and Carangidae had the highest omega-3 contents (min‒max = 0.68–7.28 g/100 g fillet). The highest hazard quotient values obtained for silky shark and great barracuda reflected a lower nutritional benefit (i.e., low omega-3 source) than risk (i.e., mercury exposure), making them not advisable for consumption. Eight species had low hazard quotients, and among them cottonmouth jack and flat needlefish were found of high health interest (high protein, moderate fat contents, and low omega-6/omega-3 ratio). A daily serving portion of 85–200 g (according to people category) can be recommended for these species. Batfish, and to a lower extent pompano dolphinfish and brassy chub, can also be consumed safely and would provide greater health benefits than risks. These results advocate for a better access of these species to local populations. Image 1 • Mercury and omega-3 were quantified in 20 fish species from tuna fisheries bycatch. • All species are good protein sources, with low omega-6/omega-3 ratio. • Eight species are of high interest, especially cottonmouth jack and flat needle fish. • Silky shark and great barracuda are not advised for consumption (low omega-3, high mercury). Based on omega-3, fat, protein, and mercury contents, most fish bycatch from tuna fisheries would be good food sources to reduce nutritional insecurity for local populations. [ABSTRACT FROM AUTHOR]

Deep-pelagic species are central to marine ecosystems and increasingly vulnerable to global change and human exploitation. To date, our understanding of these communities remains limited mainly due to the difficulty of observations, calling for complementary innovative tools to better characterize their ecology. We used mercury (Δ 199 Hg, δ 202 Hg, Δ 201 Hg, and Δ 200 Hg), carbon (δ 13 C), and nitrogen (δ 15 N) stable isotope compositions to segregate deep-pelagic species caught on the continental slope of the Bay of Biscay (NE Atlantic) according to their foraging depth and trophic ecology. Decreasing fish Δ 199 Hg values with corresponding depth estimates from the surface to down to 1,800 m confirmed that mercury isotopes are able to segregate deep species over a large vertical gradient according to their foraging depth. Results from isotopic compositions also identified different mercury sources, likely reflecting different trophic assemblages over the continental slope, in particular, the demersal influence for some species, compared to purely oceanic species. Overall, our results demonstrate how mercury stable isotopes can inform the vertical foraging habitat of little-known species and communities feeding in the deep.

Climate change is expected to affect marine mercury (Hg) biogeochemistry and biomagnification. Recent modeling work suggested that ocean warming increases methylmercury (MeHg) levels in fish. Here, we studied the influence of El Niño Southern Oscillations (ENSO) on Hg concentrations and stable isotopes in time series of seabird blood from the Peruvian upwelling and oxygen minimum zone. Between 2009 and 2016, La Niña (2011) and El Niño conditions (2015-2016) were accompanied by sea surface temperature anomalies up to 3 °C, oxycline depth change (20-100 m), and strong primary production gradients. Seabird Hg levels were stable and did not co-vary significantly with oceanographic parameters, nor with anchovy biomass, the primary dietary source to seabirds (90%). In contrast, seabird Δ 199 Hg, proxy for marine photochemical MeHg breakdown, and δ 15 N showed strong interannual variability (up to 0.8 and 3‰, respectively) and sharply decreased during El Niño. We suggest that lower Δ 199 Hg during El Niño represents reduced MeHg photodegradation due to the deepening of the oxycline. This process was balanced by equally reduced Hg methylation due to reduced productivity, carbon export, and remineralization. The non-dependence of seabird MeHg levels on strong ENSO variability suggests that marine predator MeHg levels may not be as sensitive to climate change as is currently thought.