Evidence that Pacific tuna mercury levels are driven by marine methylmercury production and anthropogenic inputs.

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.

Adaptive spatiotemporal management to reduce shark bycatch in tuna fisheries.

Purse-seine tropical tuna fishing in the eastern tropical Pacific Ocean (EPO) results in the bycatch of several sensitive species groups, including elasmobranchs. Effective ecosystem management balances conservation and resource use and requires considering trade-offs and synergies. Seasonal and adaptive spatial measures can reduce fisheries impacts on nontarget species while maintaining or increasing target catches. Identifying persistently high-risk areas in the open ocean, where dynamic environmental conditions drive changes in species' distributions, is essential for exploring the impact of fisheries closures. We used fisheries observer data collected from 1995 to 2021 to explore the spatiotemporal persistence of areas of high bycatch risk for 2 species of oceanic sharks, silky shark (Carcharhinus falciformis) and oceanic whitetip shark (Carcharhinus longimanus), and of low tuna catch rates. We analyzed data collected by fisheries scientific observers onboard approximately 200 large purse-seine vessels operating in the EPO under 10 different flags. Fishing effort, catch, and bycatch data were aggregated spatially and temporally at 1° × 1° cells and monthly, respectively. When areas of high fishing inefficiency were closed the entire study period and effort was reallocated proportionally to reflect historical effort patterns, yearly tuna catch appeared to increase by 1-11%, whereas bycatch of silky and oceanic whitetip sharks decreased by 10-19% and 9%, respectively. Prior to fishing effort redistribution, bycatch reductions accrued to 21-41% and 14% for silky and oceanic whitetip sharks, respectively. Our results are consistent with previous findings and demonstrate the high potential for reducing elasmobranch bycatch in the EPO without compromising catch rates of target tuna species. They also highlight the need to consider new dynamic and adaptive management measures to more efficiently fulfill conservation and sustainability objectives for exploited resources in the EPO.

Gestión espaciotemporal adaptativa para reducir la captura incidental de tiburones en la pesca del atún Resumen La pesca con cerco del atún tropical en el Pacífico Tropical Oriental (PTO) resulta en la captura incidental de varios grupos de especies sensibles, incluidos los elasmobranquios. La gestión eficiente del ecosistema equilibra la conservación y el uso de recursos y requiere que se consideren las compensaciones y las sinergias. Las medidas espaciales adaptativas y estacionales pueden reducir el impacto de las pesquerías sobre las especies accesorias mientras mantienen o incrementan la captura intencional. La identificación de las áreas con alto riesgo persistente en mar abierto, en donde las condiciones ambientales dinámicas causan cambios en la distribución de las especies, es esencial para explorar el impacto del cierre de las pesquerías. Usamos datos de observadores de las pesquerías recolectados entre 1995 y 2021 para explorar la persistencia espaciotemporal de las áreas con alto riesgo de captura incidental para dos especies de tiburón (Carcharhinus falciformi y C. longimanus) y con tasas bajas de captura de atún. Analizamos los datos recolectados por los observadores científicos de las pesquerías a bordo de aproximadamente 200 embarcaciones grandes de pesca con cerco que operaban en el PTO bajo diez banderas diferentes. Agregamos los datos sobre el esfuerzo de pesca, captura y la captura incidental de forma espacial y temporal en celdas de 1° x 1° y mensual, respectivamente. Cuando las áreas con gran ineficiencia pesquera se encontraban cerradas durante toda la investigación y el esfuerzo se reasignaba proporcionalmente para reflejar los patrones históricos de esfuerzo, el esfuerzo anual de captura de atún parecía incrementar en un 1­11%, mientras que la captura incidental de las dos especies de tiburones disminuía en un 10­19% (C. falciformi) y 9% (C. longimanus). Antes de que de redistribuyera el esfuerzo de pesca, la reducción de la captura incidental se acumuló hasta el 21­41% (C. falciformi) y 14% (C. longimanus). Nuestros resultados son congruentes con resultados previos y demuestran el gran potencial de reducción de la captura incidental de elasmobranquios en el PTO sin poner en peligro las tasas de captura de las especies de atún. Los resultados también enfatizan la necesidad de considerar medidas adaptativas nuevas y dinámicas para cumplir de forma más eficiente los objetivos de conservación y sustentabilidad para la explotación de recursos en el PTO.

A guide to eliciting and using expert knowledge in Bayesian ecological models.

Expert knowledge in ecology is gaining momentum as a tool for conservation decision-making where data are lacking. Yet, little information is available to help a researcher decide whether expert opinion is useful for their model, how an elicitation should be conducted, what the most relevant method for elicitation is and how this can be translated into prior distributions for analysis in a Bayesian model. In this study, we provide guidance in using expert knowledge in a transparent and credible manner to inform ecological models and ultimately natural resource and conservation decision-making. We illustrate the decisions faced when considering the use of expert knowledge in a model with the help of two real ecological case studies. These examples are explored further to examine the impact of expert knowledge through 'priors' in Bayesian modeling and specifically how to minimize potential bias. Finally, we make recommendations on the use of expert opinion in ecology. We believe if expert knowledge is elicited and incorporated into ecological models with the same level of rigour provided in the collection and use of empirical data, expert knowledge can increase the precision of models and facilitate informed decision-making in a cost-effective manner.