Sámar Saber

We propose that catch-per-unit-effort (CPUE) data from fishing tournaments can derive a sentinel time series, serving as an indicator of unusual changes in unobserved abundance trends when stock assessments are uncertain. This proposition relies on strict tournament regulations that enforce consistent fishing tactics and standardized monitoring of catch and effort over time. However, three key challenges must be addressed to ensure its reliability: (i) correcting for daily bag limit effects on catch underreporting, (ii) accounting for between-boat variability and environmental factors affecting catchability at the trip scale, and (iii) modelling environmental effects on CPUE at the year scale. As a case study, we analyse a 19-year sentinel time series for the Mediterranean albacore (Thunnus alalunga), whose stock status remains uncertain due to data limitations. On average, 2.2 albacore were unreported per boat upon reaching the bag limit. No unusual changes were detected, as the series exhibited an apparent stability, with an average growth rate of 1.06 (95% credible interval: 0.77–1.48). Despite uncertainty, catchability peaked at an optimal wave height, while a hyperstability scenario was rejected by accounting for the boat consistency in tournament participation. Overall, our framework holds promise for improving the understanding of data-limited fisheries dynamics.

Skipjack is an important commercial species with a tropical distribution, although captures in the Mediterranean Sea have been recorded for decades. The western Mediterranean Sea, specifically the Balearic Sea, is a spawning area for several tuna species. We hypothesized that the western Mediterranean warming in the last few decades could lead to the expansion of skipjack tuna spawning areas from tropical areas to the Mediterranean Sea. We analysed 454 individuals (41.8-81 cm straight fork length) caught by sport fishing vessels in offshore trolling championships in Spanish Mediterranean waters during summer months from 2014 to 2019. Analysis of the gonadosomatic index and microscopic examination of the ovaries (n=192) showed that the skipjack is reproductively active in the western Mediterranean, particularly in the Balearic Sea. These results indicate that the skipjack has expanded its distribution and spawning area from tropical waters to the Mediterranean, probably owing to the gradual warming detected in the area in the last few decades. This new spawning activity in the area should be monitored in the near future to study the possible impact on other tuna species that share the distribution range and spawning area with skipjack tuna in the western Mediterranean.

Background: The bullet tuna (Auxis rochei) is an epipelagic fish with a worldwide distribution that is highly targeted by fisheries. Genetic diversity and population genetics are good indicators of population structure and thus, essential tools for fisheries management. Knowing which factors (biotic and abiotic) might be shaping such structure is crucial for management plans. In the present study, we assessed the population structure of the bullet tuna in the western and central Mediterranean Sea. Methods: We used two types of genetic data: the mitochondrial DNA control region and seven microsatellite loci. The analysis of 431 sequences with a length of 386 bp from the mtDNA CR and the results from 276 individuals were genotyped by seven microsatellite loci. Results: Both types of markers coincided in showing significant genetic differences between an Iberian Peninsula–Strait of Gibraltar stock in comparison with a North African stock. Conclusions: We argue that this differentiation pattern is likely caused by reproductive strategies such as coastal spawning, larval retention, and natal homing behavior. These results should endorse the implementation of management plans for a resource that currently is not being managed. Thus, to ensure sustainability, these new policies should consider the presence of at least two genetically identified stocks.

The number of studies of reproductive biology for Atlantic bluefin tuna carried out in the Gulf of Mexico is significantly lower than those undertaken in the Mediterranean Sea. Four spawning areas have been found for the eastern Atlantic bluefin tuna stock in the Mediterranean Sea, so it is not implausible that there is more than one spawning area in the Gulf of Mexico for the western Atlantic bluefin tuna stock. The individuals used in this study were caught as bycatch by the Mexican surface longline fleet between January and April 2015. A total of 63 individuals ranging between 192 and 293 cm L_F (mean = 238 ± 22.52 cm) were measured. Gonads from 46 fish (31 females and 15 males) were collected for histological examination. All the individuals were classified as mature; 25 were reproductively active (in spawning capable and spawning stages). The histological analysis indicates spawning activity in Mexican waters (the southern Gulf of Mexico). Spawning occurred in March and April, when the sea surface temperature was 25.57 °C ± 0.69 in March and 27.03 °C ± 0.69 in April. Information on the location of the spawning areas is necessary for a correct management of species. The present study provides the first histological evidence of reproductive activity in Mexican waters, and indicates a wider spawning area, beyond just the northern zone, potentially encompassing the entire Gulf of Mexico.

One the problems of fisheries assessment and management is the presence of multiple species in a single fishery. In such cases, each species may respond differently to management measures. In this study, we reported for the first time the presence of a second species, the frigate tuna (Auxis thazard), in the fishery for bullet tuna (A. rochei) off the Iberian Peninsula. The bullet tuna is heavily targeted by both artisanal and commercial fisheries, thus the presence of a second species may complicate the management. Seven frigate tuna individuals (1.6%; 95% CI 0.7-3.4%) were detected after analyzing more than 400 individuals, and they were unambiguously assigned to the species after the analysis of two unlinked genetic loci (nuclear gene Tmo-4c4 and the mitochondrial DNA control region). The unexpected presence of a species distributed in temperate and tropical waters such as the frigate tuna in this northern location may be a consequence of global warming. However, continuous monitoring with a validated methodology for species identification is needed to confirm this hypothesis.

INTRODUCTION The small tuna species Auxis rochei (bullet tuna), Sarda sarda (Atlantic bonito) and Euthynnus alletteratus (little tunny) are widely distributed in Atlantic and Mediterranean waters. Although these species have a high socio-economic relevance for a considerable number of local communities at the regional level, the status of these fish stocks is unknown. Information on life history traits should be considered to develop realistic models to assess fish stocks; these models could then be used as the basis for the sustainable exploitation, management, and conservation of fish species. Sex ratio can be male- or female-biased with increasing length or age. In general males of marine fish species predominate in small length classes while females predominate in the large ones. However in species of the genus Thunnus, which are large tunas, males are more numerous in the larger length classes. Bullet tuna and Atlantic bonito from the Iberian Peninsula are belong to the same populations (Ollé et al., 2017., Viñas et al., in press), whereas no genetic studies have been addressed for little tunny. The aim of this study is to analyse the sex ratio for the three mentioned small tuna in the western Mediterranean Sea and Atlantic margin of south Iberian Peninsula. MATERIAL AND METHODS Individuals of bullet tuna, Atlantic bonito and little tunny were collected from 32 localities throughout the Mediterranean Sea (Spanish coast) and Atlantic Ocean (south of the Iberian Peninsula) between 2003 and 2017 (Figure 1). The individuals of the three species were caught by both commercial and recreational fisheries. Fork length (FL) was measured either to the nearest 0.1 cm (when fish was measured in the lab or at landings) or to the nearest 0.5 cm (fish measured on board). A total of 3929 fish (bullet tuna, 1280; Atlantic bonito, 1388; little tunny, 1261) were sampled. The sex was identified and classified as male, female and undetermined (individuals with small gonads, which sex cannot be identified visually) (Table 1). Sex ratio was calculated as the proportion of females by 2 cm FL class. The observed female proportions by length class were modelled by means of a binomial generalised linear model (GLM). Statistical differences between length classes were assessed by means of the chi-square test. RESULTS A total of 186 bullet tuna were classified as undetermined. The size of males ranged between 26.0 and 48.7 cm FL (n = 540) and females ranged between 24.0 and 47.5 cm FL (n = 554). Significant differences in the proportion of females were found between length classes (χ2 = 23.54, df = 12, p = 0.02). The sex ratio was 1:1 in the length class group between 34 and 36 cm FL. Females predominated in length class less than 26 cm FL. The analysis of the predicted sex-ratio by length showed a trend of abundance of males in the larger length classes (Figure 2a). A total of 197 Atlantic bonito were classified as undetermined. The size of males ranged between 25.6 and 79.3 cm FL (n = 520) and females ranged between 23.9 and 71.0 cm FL (n = 671). Significant differences in the proportion of females were found between length classes (χ2 = 39.87, df = 23, p = 0.02). The analysis showed that females predominated in almost all of the length class groups. The sex ratio was 1:1 in the length class group between 40 and 42 cm FL. Predicted sex-ratio by length is shown in (Figure 2b). A total of 384 little tunny were classified as undetermined. Two intersex fish were sampled in the trap located in Mazarrón (for details, see Macías et al., 2014). The size of males ranged between 30.6 and 101.0 cm FL (n = 414) and females ranged between 28.5 and 99.0 cm FL (n = 461). Significant differences in the proportion of females were found between length classes (χ2 = 73.49, df = 36, p = 0.0002). The analysis showed that males were more abundant in the larger length classes (> 96 cm FL). Predicted sex-ratio by length is shown in (Figure 2c). CONCLUSIONS Females predominated in small length classes; however, this result should be considered with caution because for the smallest fish (i.e. fish with tiny gonads), the ovaries are easier identified than testes. The analysis of sex ratio by length for both bullet tuna and little tunny indicated that females predominated in the smaller length classes and males in the larger length classes. Specifically, for the Atlantic bonito the trend showed that were consistently more females than males across all sizes classes.

Maggio, T., Allegra, A., Andaloro, F., Pedro Barreiros, J., Battaglia, P., Butler, C. M., Cuttitta, A., Miguel Rodrigues Fontes, J., Freitas, R., Gatt, M., Saadet Karakulak, F., Macias, D., Nicosia, A., Oxenford, H. A., Saber, S., Vasco Rodrigues, N., Yildiz, T., and Sinopoli, M. Historical separation and present-day structure of common dolphinfish (Coryphaena hippurus) populations in the Atlantic Ocean and Mediterranean Sea. ICES Journal of Marine Science, doi: 10.1093/icesjms/fsy174 .

The common dolphinfish (Coryphaena hippurus) is an epipelagic, mid-trophic level, highly migratory species distributed throughout the world’s tropical and subtropical oceans in waters greater than 20°C. Life-history variables, migratory behaviour, and genetic markers have been used to define major stocks in the central Atlantic Ocean and Mediterranean Sea. Here, we used the mitochondrial DNA gene NADH subunit 1 (688 bp) to test for differences between population groups. A total of 103 haplotypes were detected among 203 fish. Gene diversities in samples were large and similar among populations (mean h = 0.932; range 0.894–0.987), but nucleotide diversities varied widely among samples (range π = 0.004–0.034) and appear to reflect population histories. Principal component analysis revealed two large populations groups, and the analysis of molecular variation and pairwise values of Φ_ST resolved population structure within these groups. Populations in the eastern Atlantic and Mediterranean showed the largest amounts of divergence from one another (Φ_CT = 0.331). Adult movement and biophysical barriers to larval dispersal may explain contemporary differences between stocks, but the divergent populations in the Mediterranean Sea are likely due to isolations by cold temperature barriers during Pleistocene glaciations. The geographically large stock groupings require international cooperation in the harvest management and conservation of local dolphinfish populations.

In this manuscript, we test how an understanding of geographical variation in larval fitness in relation to temperature and habitat use could be a useful method to improve our understanding of recruitment and develop better indices of annual recruitment. On the basis of the assumption that growth and survival of tuna larvae are influenced by temperature, we have developed a potential larval survival index for Atlantic bluefin tuna (Thunnus thynnus) by combining empirical data from egg and larval rearing experiments with temperature data from hydrodynamic models. The experiments were designed to test the full range of temperature variability that bluefin larvae would experience in the field and provide a mechanistic understanding of the processes driving egg and larval survival. We then developed a biological model using the temperature-related growth expressions and a size-dependent survival function for the larvae. The biological model was applied to a time-series of spatially explicit temperature data for the western Mediterranean from the Strait of Gibraltar to 6°E, which includes the major recognized bluefin tuna eastern stock spawning area, the Balearic Sea. Our results show that areas with high probabilities of larval survival coincide with those that would be considered as optimal based on other data sources (ichthyoplankton surveys, spawning female locations from commercial fisheries data, and adult tracking data). However, evidence of spawning has been found in areas with suboptimal thermal habitats, as predicted by the model, which we discuss regarding sampling effort and salinity fronts. There was a good match between the survival index and recruitment indices from standardized CPUE fisheries data. These results have implications for our understanding of the recruitment process of the eastern stock of Atlantic bluefin tuna, since they suggest that the combined effects of temporal and spatial variability of the environment drive recruitment success, which has important implications for the management of the species.