Maternal effects play a fundamental role in shaping early larval growth and survival in marine fishes. This study explores the relationship between maternal trophic ecology and larval growth in bluefin tunas, with a focus on Southern Bluefin Tuna (SBT) and an expanded dataset from multiple Atlantic Bluefin Tuna (ABT) populations incorporated into the General Bluefin Model (GBM). Daily growth and stable isotopes (S15N and S13C) were obtained from 80 pre-flexion SBT larvae and 355 pre-flexion larvae from the GBM dataset. Results revealed a significant negative linear trend between larval age and S15N values, consistent with the gradual attenuation of maternally inherited isotopic signatures during development. Faster growing larvae showed significantly lower S15N and S13C values, indicating that maternal trophic behaviour and prey sources critically influence larval growth potential. Maternal isotopic niche breadth, inferred from larval isotope data, was markedly narrower in groups with optimal larval growth, suggesting that specialized (stenophagous) maternal feeding strategies promote enhanced offspring performance. These patterns were observed consistently in two bluefin species across seven different populations, despite geographic and temporal variability, highlighting a robust ecological link between maternal foraging behaviour and larval development. From these findings, we introduce the hypothesis of an Optimal Maternal Feeding Isotopic Niche, representing a constrained isotopic range associated with increased larval growth and survival. This framework advances our understanding of the influence of maternal trophic ecology on offspring fitness and offers valuable insights for the conservation and management of highly migratory pelagic species with complex reproductive strategies.

This study analyzed the growth patterns and survival of Southern bluefin tuna (SBT, Thunnus maccoyii) larvae collected during January-February 2022 in their only known spawning area in the eastern Indian Ocean (IO). Otolith microstructure was examined to characterize both population-level and intra-population growth (OPT-optimal and DEF-deficient group), with special emphasis on the flexion process, as well as to provide insights into larval survival. SBT larvae began flexion at sizes and ages comparable to those reported in other bluefin tuna species. At the intra-population level, larvae reached flexion at the same age; however, optimal (OPT) larvae reached this stage in better physical condition, with greater length, weight, and body depth, likely increasing their chances of survival at later stages. The observed larval growth rates (0.38 mmd(-1)) exceeded those from a historical study in 1987 (0.33 mmd(-1)), likely due to a similar to 2 degrees C increase in sea surface temperature and shifts in prey availability. Larval survival appears to depend on a selective process based on growth, in which only a small proportion of individuals (<2%) exhibited width increment in otoliths similar to those of surviving larvae, allowing for faster development and earlier access to larger prey. These findings highlight the need for expanded research on the early life stages of SBT, particularly in the context of ongoing ocean warming and climate change.

This study examines the fate of species caught as bycatch in the tropical tuna purse seine fishery across the Atlantic and Indian Oceans from 2003 to 2022, focusing on the impact of the industry-implemented Code of Good Practices (CGP) and bycatch mitigation measures from Tuna Regional Fisheries Management Organisations (t-RFMOs). Data were sourced from scientific observers under both Spain's National Data Collection Framework (DCF, Regulation (EU) No 2017/1004) and the OPAGAC-AGAC observer program, which covers other flags as well. Following the adoption of CGP and t-RFMO regulations, statistical tests reveal significant improvements in the live-release rates of sharks and rays after 2014 in both oceans, but no significant differences in live-release rates for marine turtles, which were already high. Despite these positive trends, interactions with certain species, such as billfish, experiences an increase in discard rates during the last years of the study period. The study identifies coastal regions near Gabon and Angola in the Atlantic, and northern Indian Ocean fishing areas as potential hotspots for some sensitive species, which may be confirmed as information from other fisheries come to light.

The Atlantic bluefin tuna (ABFT) population ranges throughout the Atlantic Ocean and Mediterranean Sea, and consists of multiple contingents that use diverse habitats and show different movement patterns over the life cycle. Based on body size, elemental and isotopic data of C and N in muscle and liver, we analysed eastern-stock ABFT by comparing mid-sized individuals caught by hook-and-line gears with larger individuals harvested from traps in the Strait of Gibraltar (SoG). Our results show that trophic-related chemical markers have potential for separating temporarily sympatric contingents throughout the ABFT population range, reflecting size-dependent spatial distribution and differential patterns of residency and migration. We present evidence of long-term residency of ABFT in the SoG that persists until the estimated age of ∼5 years (size of ∼140 cm in straight fork length). This age apparently marks a turning point in the life history, where there occurs an ontogenetic switch in the migratory behaviour and distributional pattern. This study contributes new insight into our knowledge about size structure and residency-movement patterns in eastern ABFT. It shows distinct size-dependent migratory and spatial dynamics. The present results encourage further investigation on poorly studied ABFT contingents for a better understanding of the population dynamics towards more comprehensive management plans.

Tuna Regional Fishery Management Organizations (tRFMOs) are increasingly interested in spatiotemporal management as a tool to reduce interaction rates with vulnerable species. We use blue shark ( Prionace glauca ) as a case study to demonstrate the critical first steps in the implementation process, highlighting how predictions of global habitat for vulnerable life stages can be transformed into a publicly -accessible spatial bycatch mitigation tool. By providing examples of possible management goals and an associated threshold to identify essential habitats, we show how these key areas can represent a relatively low percentage of oceanic area on a monthly basis (16-24% between 50 degrees S and 60 degrees N), yet can have relatively high potential protection efficiency (similar to 42%) for vulnerable stages if fishing effort is redistributed elsewhere. While spatiotemporal management has demonstrable potential for blue sharks to effectively mitigate fishing mortality on sensitive life stages, we identify inherent challenges and sequential steps that require careful consideration by tRFMOs as work proceeds. We also discuss how our single-species framework could be easily extended to a multispecies approach by assigning relative conservation risk before layering habitat model predictions in an integrated analysis. Such broader application of our approach could address the goals of tRFMOs related to reducing the ecosystem effects of fishing and pave the way for efficient fisheries co-management using an ecosystem-based approach.

The structure, materials and designs of drifting Fish Aggregating Devices (dFADs) have generally remained rudimentary and relatively unchanged since they first came into use in the 1980 s. However, more recently, dFADs have been increasing in dimensions and the prevailing use of plastic components. Abandoned, lost or discarded dFADs can therefore contribute to the global marine litter problem. Transitioning to biodegradable and non-toxic materials that have a faster rate of decomposition, and are free of toxins and heavy metals, relative to synthetic materials, has been prescribed as an important part of the solution to reducing marine pollution from industrial tuna fisheries that rely on dFADs. This review of the current state of dFADs considers aspects related to the use of biodegradable materials in their construction, including; regulations related to dFAD materials, trials of biodegradable designs and materials and future alternatives. During the last decade, regulatory measures at tuna Regional Fishery Management Organizations (tRFMOs) have gradually moved towards the clear recommendation to use biodegradable materials in dFAD construction together with other measures limiting the number of active dFADs and the use of netting materials. However, to provide operational guidance, more clarity is needed, starting with a standardised definition of biodegradable dFADs among tRFMOs. Research involving dFAD natural and synthetic materials is required, along with improved data collection for monitoring the transition of dFAD materials against specified standards for biodegradable dFADs. In addition, alternative and complementary actions need to be explored to contribute to minimising adverse effects of dFADs on the environment. Acknowledging the current difficulties for the implementation of fully biodegradable dFADs in tuna fisheries, a stepwise process towards the implementation of commercially viable biodegradable dFADs should be considered.

Foraging patterns and trophic interactions of young-of-the-year Atlantic bluefin (Thunnus thynnus, ABFT) and skipjack (Katsuwonus pelamis, SKJ) tunas co-occurring in coastal waters off the Gulf of Cadiz (GoC, East Atlantic Ocean) were investigated using stomach content and stable isotope analyses. Both species fed primarily on epipelagic fishes such as European anchovy (Engraulis encrasicholus), sardine (Sardina pilchardus) and Atlantic chub mackerel (Scomber colias). In SKJ, a positive correlation was found between δ15N values of muscle and liver and straight fork length (SFL). Differences in liver δ15N values suggest that ABFT were recently foraging at higher trophic levels than SKJ. Isotopic niche areas estimated by standard ellipse areas (SEAC) and kernel utilization density (KUD) analyses showed a broader niche for SKJ compared to ABFT, indicating a more diverse diet in the former species. There was no significant niche overlap between both tuna species, suggesting trophic niche partitioning and reduced competition for the resources.

Drifting fish aggregating devices (dFADs) are human-made floating objects widely used by tropical tuna purse seine (PS) fisheries to increase catch of target species. However, dFAD use has several negative impacts, including increased potential for overfishing, higher juvenile tuna catch, higher bycatch compared to other PS fishing modes, ghost-fishing, and generation of marine litter. Based on these impacts, some stakeholders, especially environmental non-governmental organizations and other competing fishing industries, suggest that dFADs should be completely banned. We list the pros and cons of dFAD fishing; address how to improve current management; and suggest solutions for the sustainability of dFAD fishing in the long term. A dFAD ban would lead to major changes in the availability and sourcing of tuna for human consumption and decrease the licensing revenue received by many developing states. Most importantly, we argue that tools exist today to manage for, reduce or eliminate most of the negative impacts of dFADs (e.g., bans on discards, limits on active dFADs, biodegradable non-entangling constructions, time-area deployment closures, recovery programs, and full data transparency, among others). Management decisions based on sound scientific reasoning are needed to address the legitimate concerns surrounding dFAD use and ensure the sustainability of both pelagic and coastal ecosystems and tropical tuna PS fisheries.

Drifting Fish Aggregating Devices (dFADs) are currently made with synthetic and non-biodegradable materials contributing to the increase of marine litter and other potential ecosystem impacts. Tuna RFMOs have promoted the research and progressive replacement of existing FADs by non-entangling biodegradable FADs (bioFADs). Here, we present the results of the first large-scale biodegradable FAD project in the Indian Ocean to develop and implement the use of non-entangling biodegradable dFADs. The bioFAD tested were fully non-entangling without netting minimizing completely the risk of entanglement. Tested bioFADs significantly contribute to the reduction of the synthetic plastic-based materials, increase the use of biodegradable materials and reduce the total material weight used in FADs, reducing their overall ecosystem impacts. The results of testing 771 bioFADs in real fishing conditions, showed that the fishing performance regarding presence/absence of tuna around dFADs, first day of tuna detection, proportion of FADs occupied by tuna, biomass aggregation underneath the FADs and catch per set between bioFADs and conventional dFADs were similar. This provides support for the efficacy of bioFADs regardless of the degradation experienced by the biodegradable materials tested. Although some bioFADs lasted up to one year, the degradation of the biodegradable material was important and some bioFADs lost their original structure after the study period, suggesting the need to find alternative designs for bioFADs that will suffer less structural stress than those bioFADs made of biodegradable material but with conventional design. The lessons learnt in this large-scale trial will contribute to refining the future designs of biodegradable FADs.

The Canary archipelago, which is part of the Atlantic biogeographical region of Macaronesia, provides suitable habitat for tropical and temperate-water large pelagic species. Atlantic bluefin tuna (ABFT) Thunnus thynnus occur year round in the Canary Islands. Life history traits of ABFT, specifically foraging patterns, have not been studied thus far in this region. We investigated ABFT trophic biology, combining stomach content and stable isotope analyses. A high proportion (~77%) of the stomachs contained prey, indicating active foraging. The diet primarily consisted of fishes, among which the snipefish Macroramphosus sp. was the major prey. Cephalopods and crustaceans were less important diet components. Moreover, a well-preserved specimen of Cory’s shearwater Calonectris borealis was found in a stomach sampled in 2018. Besides natural prey, 16.7% of the stomachs contained plastic debris. Inter-annual isotopic differences were detected in liver tissue samples, reflecting a dietary shift in 2018, where δ15N values were lower than in 2016 and 2017. Isotopic niche width estimations from stable isotope Bayesian ellipse in R and kernel utilization density analyses suggested a more diverse diet in 2017. Corrected and Bayesian standard ellipse area values from muscle and liver data indicated that the diet of ABFT in the Canary Islands is more euryphagous than in the Strait of Gibraltar (East Atlantic) but more stenophagous than it is in the Gulf of Saint Lawrence (West Atlantic). The present results show that the Canary archipelago represents a foraging ground for ABFT in spring.

Blue shark (Prionace glauca) is amongst the most abundant shark species in international trade, however this highly migratory species has little effective management and the need for spatio-temporal strategies increases, possibly involving the most vulnerable stage or sex classes. We combined 265,595 blue shark observations (capture or satellite tag) with environmental data to present the first global-scale analysis of species’ habitat preferences for five size and sex classes (small juveniles, large juvenile males and females, adult males and females). We leveraged the understanding of blue shark biotic environmental associations to develop two indicators of foraging location: productivity fronts in mesotrophic areas and mesopelagic micronekton in oligotrophic environments. Temperature (at surface and mixed layer depth plus 100 m) and sea surface height anomaly were used to exclude unsuitable abiotic environments. To capture the horizontal and vertical extent of thermal habitat for the blue shark, we defined the temperature niche relative to both sea surface temperature (SST) and the temperature 100 m below the mixed layer depth (Tmld+100). We show that the lifetime foraging niche incorporates highly diverse biotic and abiotic conditions: the blue shark tends to shift from mesotrophic and temperate surface waters during juvenile stages to more oligotrophic and warm surface waters for adults. However, low productivity limits all classes of blue shark habitat in the tropical western North Atlantic, and both low productivity and warm temperatures limit habitat in most of the equatorial Indian Ocean (except for the adult males) and tropical eastern Pacific. Large females tend to have greater habitat overlap with small juveniles than large males, more defined by temperature than productivity preferences. In particular, large juvenile females tend to extend their range into higher latitudes than large males, likely due to greater tolerance to relatively cold waters. Large juvenile and adult females also seem to avoid areas with intermediate SST (~21.7-24.0°C), resulting in separation from large males mostly in the tropical and temperate latitudes in the cold and warm seasons, respectively. The habitat requirements of sensitive size- and sex-specific stages to blue shark population dynamics are essential in management to improve conservation of this near-threatened species.

The use of fish aggregating devices (FADs) in tropical tuna fisheries has increased significantly during recent decades. Concurrently, concern about juvenile tuna mortality, bycatch, and marine debris associated with FAD fisheries increased, and this led to the implementation of FAD management measures and more sustainable designs (e.g., non-entangling or biodegradable FADs, limits on active FADs, etc.). This document reviews data collection and reporting requirements of tuna-Regional Fisheries Management Organizations (t-RFMOs) on drifting FADs and summarizes the work carried out since 2010 under the Spanish FAD management plan to create an adequate standard data collection aimed at improving science-based decision making. The aim of this study is to assist in the strengthening of data collection systems through: (1) a review of the existing data requirements, (2) a review of the status of FAD data collection worldwide and identification of data gaps, and (3) recommendations aimed at improving FAD management through the strengthening of FAD data requirements. Due to the complexities of data collection, we summarize the difficulties faced when processing the data and propose concrete and practical solutions to improve both the data collection system and information quality.

REPLYING TO A. V. Harry & J. M. Braccini Nature https://doi.org/10.1038/s41586-021-03463-w (2021) Our global analysis1 estimated the overlap and fishing exposure risk (FEI) using the space use of satellite-tracked sharks and longline fishing effort monitored by the automatic identification system (AIS). In the accompanying Comment, Harry and Braccini2 draw attention to two localized shark–longline vessel overlap hotspots in Australian waters, stating that 47 fishing vessels were misclassified as longline and purse seine vessels in the Global Fishing Watch (GFW)3 2012–2016 AIS fishing effort data product that we used. This, they propose2, results in misidentifications that highlight fishing exposure hotspots that are subject to an unexpected level of sensitivity in the analysis and they suggest that misidentifications could broadly affect the calculations of fishing exposure and the central conclusions of our study1. We acknowledged in our previously published paper1 that gear reclassifications were likely to occur for a small percentage of the more than 70,000 vessels studied, however, here we demonstrate that even using much larger numbers of vessel reclassifications than those proposed by Harry and Braccini2, the central results and conclusions of our paper1 do not change.

An abstract is not available for this content. As you have access to this content, full HTML content is provided on this page. A PDF of this content is also available in through the 'Save PDF' action button.

REPLYING TO H. Murua et al. Nature https://doi.org/10.1038/s41586-021-03396-4 (2021) Our previously published paper1 provided global fine-scale spatiotemporal estimates (1° × 1°; monthly) of overlap and fishing exposure risk (FEI) between satellite-tracked shark space use and automatic identification system (AIS) longline fishing effort. We did not assess shark mortality directly, but in addition to replying to the Comment by Murua et al.2, we confirm—using regression analysis of spatially matched data—that fishing-induced pelagic shark mortality (catch per unit effort (CPUE)) is greater where FEI is higher. We focused on assessing shark horizontal spatiotemporal overlap and exposure risk with fisheries because spatial overlap is a major driver of fishing capture susceptibility and previous shark ecological risk assessments (ERAs) assumed a homogenous shark density within species-range distributions3,4,5 or used coarse-scale modelled occurrence data, rather than more ecologically realistic risk estimates in heterogeneous habitats that were selected by sharks over time. Furthermore, our shark spatial exposure risk implicitly accounts for other susceptibility factors with equal or similar probabilities to those commonly used in shark ERAs3,5.

Despite bycatch of elasmobranch (sharks and rays) being a major concern in most fisheries worldwide, there is a lack of knowledge on their spatio-temporal species distribution, biology (life stage and sex-ratios), as well as their at-haulback mortality rate. Observer data from the French and Spanish tropical purse-seine tuna fisheries operating in the eastern Atlantic and western Indian Oceans between 2005 and 2017 were analysed to investigate elasmobranch bycatch. Data included 24 elasmobranchs species and distribution patterns of catch per unit of effort (CPUE) by species and sex-ratio were found to vary with life stages, areas, seasons and fishing modes. In general, higher catches were found in FAD-associated sets (>40%) than free tuna school sets (<17%) although this can vary depending on the species. For the large majority of species, a high proportion of juveniles were caught (30.7–100%), apparent at-haulback mortality rates was high (24.3–63.9%) and finally sex ratios was unbalanced (13.3–66.7% of females). Areas and seasons identified from these different components should be of interest for the monitoring and management of elasmobranch bycatches.

This study provides sound evidence of the spawning activity of common dolphinfish (Coryphaena hippurus) in the Gulf of Cadiz (southwest Spain) during summer months. Along with waters off Côte d'Ivoire, this area adjacent to the Mediterranean Sea is the easternmost Atlantic spawning ground documented for the species. All the fish analysed (5 males and 15 females) were at spawning stage, except the smallest female (61.5 cm in fork length), which might not have yet reached sexual maturation. The oceanographic conditions during the samplings were similar to those occurring during the peak spawning season in the Gulf of Mexico. The study results are consistent with recent genetic studies indicating differentiation between Mediterranean and Atlantic dolphinfish populations, with the Strait of Gibraltar representing a boundary between two distinct reproductive units. The present results may contribute to improve the understanding of the dynamics of dolphinfish population and the management of eastern Atlantic and Mediterranean stocks.

The distribution of the spinetail devil ray Mobula mobular in the eastern tropical Atlantic remains poorly known compared to the Pacific and Indian Oceans. We used fishery-dependent data and generalized additive models to examine the environmental characteristics associated with the presence of M. mobular in the eastern Atlantic Ocean. Results revealed that the distribution of M. mobular is significantly associated with seasonal upwelling systems in coastal and pelagic areas. Our model predicted the presence of the species in areas where there is evidence of its occurrence, such as the Angolan upwelling system and the coast of Ghana. In addition, our model predicted new hotspot areas, including locations around the Mauritanian upwelling system, the Guinea coast, offshore Ghana and the south coast of Angola and Brazil, where sample sizes are limited. Those areas, as well as the environmental preferences depicted by the model, provide valuable information about the habitat and ecology of the spinetail devil ray. Future research lines derived from this study, as well as its limitations, are discussed. Furthermore, in light of our results we discuss the improvements that are needed to contribute to the conservation and management of this vulnerable species.

The Spanish tuna purse seine freezer fleet targeting tropical tuna in the Indian Ocean is one of the most important fleets in the world. The present study firstly describes the history and evolution of this fishery (including its current status, following the economic crisis and the upsurge of Somali piracy of this last decade), and secondly describes the effort of Spanish scientific institutions to collect data (including estimates of catch, effort, and length-frequency distributions by species) from distant fisheries. This monitoring has been carried out in collaboration with stakeholders and other regional scientific organizations since its origins in the early 1980s. During this period the monitoring have been adapted to the change in the fishery, improving both the scientific estimates of the exploited species, as well as our knowledge of the impact of the fishery on the ecosystem, which in turn has served to improve the management and sustainability of the fishery. Although, in general, data quality has improved over time, there are periods with poor data quality. Currently, the priority is to eliminate possible biases from sampling at port. Finally, our general assessment of the adequacy of past and present monitoring systems, is that the current estimation system (called T3) is an important tool throughout the historical series to provide total tropical tuna catches, but in the new context of the TAC proposed for the yellowfin tuna, it is necessary to separate this scientific tool from others used for the flag state authorities to manage the TAC.