Changing environmental temperatures impact the physiological performance of fishes, and consequently their distributions. A mechanistic understanding of the linkages between experienced temperature and the physiological response expressed within complex natural environments is often lacking, hampering efforts to project impacts especially when future conditions exceed previous experience. In this study, we use natural chemical tracers to determine the individual experienced temperatures and expressed field metabolic rates of Atlantic bluefin tuna (Thunnus thynnus) during their first year of life. Our findings reveal that the tuna exhibit a preference for temperatures 2–4 °C lower than those that maximise field metabolic rates, thereby avoiding temperatures warm enough to limit metabolic performance. Based on current IPCC projections, our results indicate that historically-important spawning and nursery grounds for bluefin tuna will become thermally limiting due to warming within the next 50 years. However, limiting global warming to below 2 °C would preserve habitat conditions in the Mediterranean Sea for this species. Our approach, which is based on field observations, provides predictions of animal performance and behaviour that are not constrained by laboratory conditions, and can be extended to any marine teleost species for which otoliths are available.

Atlantic bluefin tuna (ABT), Thunnus thynnus, is a globally important fishery resource. This oceanic top predator undertakes long-distance migrations from its nutrient-rich feeding grounds in the north Atlantic to oligotrophic spawning grounds. Larvae are born mainly in the corresponding ecosystems of its two well-established spawning areas: the Gulf of Mexico (GoM) and the Mediterranean Sea (MED). Stable isotopes analyses (SIA) of δ15N and δ13C have proven useful for understanding marine food webs and have evaluated larval ABT signatures in their respective spawning areas. This study is among the first to use larvae collected during the same spawning season (2014) from the GoM and one of the main spawning areas within the MED (the Balearic Sea) to standardize methodologies, compare larval growth, and relate SIA with larval postflexion growth using GAMs for the two spawning areas. The main findings identify significant population differences, with comparatively faster growth in GoM larvae. The GoM population had larger otoliths with wider increments, as well as higher δ15N values and higher trophic position for postflexion stages. Collectively, these findings highlight the importance of not only developing well-calibrated growth curves, but also integrating the distinct early life history dynamics for each spawning area to properly support ABT’s ongoing management efforts.

The commercially important Atlantic bluefin tuna (Thunnus thynnus), a large migratory fish, has experienced notable recovery aided by accurate resource assessment and effective fisheries management efforts. Traditionally, this species has been perceived as consisting of eastern and western populations, spawning respectively in the Mediterranean Sea and the Gulf of Mexico, with mixing occurring throughout the Atlantic. However, recent studies have challenged this assumption by revealing weak genetic differentiation and identifying a previously unknown spawning ground in the Slope Sea used by Atlantic bluefin tuna of uncertain origin. To further understand the current and past population structure and connectivity of Atlantic bluefin tuna, we have assembled a unique dataset including thousands of genome-wide single-nucleotide polymorphisms (SNPs) from 500 larvae, young of the year and spawning adult samples covering the three spawning grounds and including individuals of other Thunnus species. Our analyses support two weakly differentiated but demographically connected ancestral populations that interbreed in the Slope Sea. Moreover, we also identified signatures of introgression from albacore (Thunnus alalunga) into the Atlantic bluefin tuna genome, exhibiting varied frequencies across spawning areas, indicating strong gene flow from the Mediterranean Sea towards the Slope Sea. We hypothesize that the observed genetic differentiation may be attributed to increased gene flow caused by a recent intensification of westward migration by the eastern population, which could have implications for the genetic diversity and conservation of western populations. Future conservation efforts should consider these findings to address potential genetic homogenization in the species.

AbstractThere is growing concern over the conservation status of sharks and relatives exposed to fishing mortality. The Food and Agriculture Organization of the United Nations in 1999 adopted the International Plan of Action for the Conservation and Management of Sharks (IPOA), which provides nations with advice on adopting and implementing national plans. An assessment of global national and regional plans of action on sharks (NPOAs) found: most are out of date; limited use of specific, measurable and timebound objectives and activities; no outcome objectives; and few performance assessments. This makes most NPOAs inadequate for planning and assessing efficacy. Over 33% of the annual retained catch of sharks and relatives was from countries without NPOAs and less than 12% was from countries with current NPOAs. NPOAs identified fisheries management framework deficits, ecology knowledge gaps, institutional capacity and coordination shortfalls, and budget constraints as the largest obstacles to implementation and are improvement priorities. We recommend how to amend the IPOA to better support the adoption and effective design and implementation of NPOAs for evidence‐informed conservation and management.

An individual tagging model was implemented within the spatial, seasonal, multi-stock, multi-fleet operating models of the peer-reviewed Management Strategy Evaluation (MSE) framework for Atlantic bluefin tuna to evaluate the benefits of a harvest strategy that utilizes conventional gene tagging. A multi-year Brownie estimator was developed to test the accuracy and precision of exploitation rate estimates arising from gene tagging programs with various scenarios for spatial release distribution, release numbers and fishery exploitation rates. Harvest strategies that used the Brownie estimator were tested to evaluate yield and resource conservation performance relative to idealized management using perfect information. For the eastern stock, releasing 1,000 fish throughout the Atlantic and genotyping 27% of all landed fish at an estimated cost of US$2M was sufficient to obtain estimates of exploitation rate with a coefficient of variation of 20%. For the western stock, the same precision in exploitation rate estimates required the release of 1,300 fish and genotyping rate of 35% at an estimated cost of US$2.5M. Harvest strategies using the gene tagging data provided expected yield and resource conservation performance that was not substantially lower than a harvest strategy assuming using perfect information regarding vulnerable biomass. Reducing the number of releases most strongly affected the worst-case ‘lower-tail’ outcomes for West area yield and eastern stock biomass. Conventional gene tagging harvest strategies offer a promising basis for calculating management advice for Atlantic bluefin tuna that may be cheaper, simpler, and more robust than the current conventional stock assessment paradigm.

Atlantic bluefin tuna (Thunnus thynnus; ABFT) is one of the most iconic fish species in the world. Recently, after being very rare for more than half a century, large bluefin tunas have returned to Nordic waters in late summer and autumn, marking the return of the largest predatory fish in Nordic waters. By tagging 18 bluefin tunas with electronic tags (pop-up satellite archival tags), we show that bluefin tuna observed in Nordic waters undertake different migration routes, with individuals migrating into the western Atlantic Ocean, while others stay exclusively in the eastern Atlantic and enter the Mediterranean Sea to spawn. We additionally present evidence of possible skipped spawning inferred from behavioural analyses. In Nordic waters, ABFT are primarily using the upper water column, likely reflecting feeding activity. The results support the hypothesis that ABFT migrating to Nordic waters return to the same general feeding area within the region on an annual basis. These observations may have important implications for management because (1) tunas that come into Nordic waters might represent only a few year classes (as evidenced by a narrow size range), and thus may be particularly vulnerable to area-specific exploitation, and (2) challenge the assumption of consecutive spawning in adult Atlantic bluefin tuna, as used in current stock assessment models. Without careful management and limited exploitation of this part of the ABFT population, the species’ return to Nordic waters could be short-lived.

Half a century ago it was thought that artificial reefs (AR) would be the solution to numerous problems. After the disappearance of this panacea, people started to think about AR more from the point of view of their efficiency and usefulness. However, some issues persist. Will it be possible some time after the reef deployment to reach the goals initially outlined by the proponents? It is very important to get answers to this and other questions to know about the efficiency of reef deployment. However, answers to these questions are not always easy to obtain. The AR are submerged, often difficult to access, and the resources available for research are generally scarce and very well delimited in time. In this chapter, the authors approach the case study of artificial reefs deployed off the Algarve coast in the south of Portugal over time. The focus will be on the contribution of reef structures to human activities at economic and social levels, which in some ways have benefited from these improvements on the seabed.

Plastic dart tags are the main type of tag used in all major fish tagging projects but their potential for studying trophic relationships has never been addressed. The occurrence of dart tags in the stomach contents of large pelagic fishes and direct observations onboard tagging vessels reveal novel information on predation and cannibalism by tropical tunas in the Atlantic Ocean. The observations were all made during the Atlantic Ocean Tropical Tuna Tagging Programme (AOTTP), which targeted bigeye tuna (Thunnus obesus), yellowfin tuna (Thunnus albacares), skipjack tuna (Katsuwomis pelamis), wahoo (Acanthocybium solandri), and little tunny (Euthynnus alletteratus). During the programme (so far) 5 conventionally tagged fish have been reported in stomach contents from captures made in Brazil, São Tome and Principe, and Ivory Coast: including one yellowfin tuna and two skipjack tunas in a yellowfin tuna, one yellowfin tuna in a blue marlin (Makaira nigricans), and one yellowfin tuna in a non-identified shark. We use these observations to illustrate the potential role that dart tags could play in helping describe trophic interactions among marine pelagic fish species.

An objective of the Atlantic Ocean Tropical tuna Tagging Programme (AOTTP) was to estimate Type-I (immediate) and Type-II (long-term) tag-shedding rates for tropical Atlantic tunas from double-tagging experiments. Historical information on tuna tag-shedding studies conducted in different parts of the world was incorporated as prior distributions using a Bayesian approach to estimate the new tag-shedding parameters. Type-I and Type-II tag-shedding rates were respectively estimated at 0.007 and 0.084/yr for bigeye tuna, 0.021 and 0.051/yr for skipjack and 0.021 and 0.088/yr for yellowfin tuna. Using realizations derived from the MCMC posterior distributions, the shedding rate was estimated to reach 50% of the tags after seven and a half years at sea for yellowfin and after eight years at sea for bigeye tuna. The loss rate of conventional tags is lower for skipjack. Our results suggested that continuous Type-II shedding rate is size-dependant for yellowfin and bigeye (i.e., showing a three-fold increase between individuals less than 45 cm fork length (FL) at release and fishes larger than 65 cm FL). This study reinforces the need to account for tag-shedding along with other sources of uncertainty, such as reporting rate, in order to accurately estimate the exploitation and mortality rates derived from tagging data.