Monitoring the reproductive status of resident and migrant Atlantic bluefin tuna in the Strait of Gibraltar

Two distinct contingents of Atlantic bluefin tuna (ABFT), migrants and residents, occur in the Strait of Gibraltar (SoG). Long-term residents are commercially exploited all year round using hook and line gears, whereas migrants are caught by traps in springtime as they pass through the SoG towards Mediterranean spawning grounds. While reproductive features of migrants have been widely studied, the life history of residents remains poorly investigated. The relative contribution of this subpopulation to ABFT eastern stock productivity is therefore un - known. Reproductive traits of resident ABFT were monitored throughout the year, and were com pared with those of migrants crossing the SoG in spring. To assess maturation timing, gonads were classified into maturation stages based on histological features. Our results indicate bio - metric and reproductive differences between SoG residents and migrants. ABFT caught by hook and line gears were smaller on average than those collected from traps. The females sampled from the hook and line fishery in May and early June showed, on average, significantly lower GSI val ues than those caught by trap. In contrast, the GSI values of males sampled in May were not sig nificantly different between gears. Histological analyses showed a more advanced reproductive condition in trap-caught migrants. Delayed maturation schedules of residents in comparison to migrants would result in less egg production and poorer larval survival rates. The present findings encourage further investigation of reproductive schedules in unstudied subpopulations for a better understanding of ABFT dynamics.En prensa2,48

Efecto de las oscilaciones atmosféricas sobre las capturas de Grandes

Tunas and related species are considered Highly Migratory Pelagic animals (HMP) with a high fisheries interest and an important commercial value. Many authors have highlighted the effect of climatic oscillation such as NAO (North Atlantic Oscillation) and AO (Arctic Oscillation) on HMP. In this paper we analyzed the differential effect of these oscillations on three HMP: albacore (Thunnus alalunga), yellowfin tuna (Thunnus albacares) and swordfish (Xiphias gladius). Fisheries data for the Spanish fleet from North Atlantic area were obtained from International Commission for the Conservation of Atlantic Tunas (ICCAT). Data since 1950 for albacore, 1962 for yellowfin tuna and 1950 for swordfish, from different gears: troll (TROL), purse seine (PS), longline (LL) and bait boat (BB) were analyzed. We used the nonparametric test U Mann Whitney to test the relationships between species, gear and oscillation phases (positive or negative) of NAO and AO. Our results indicated that the landings of albacore caught with TROL increased with the positive phase of AO and AOw. The landing of swordfish caught with LL in the North Atlantic and Mediterranean Sea significantly increased with the positive phase of AO, AOw, and NAOw. Finally, the landing of yellowfin tuna caught with PS in the East Atlantic increased significantly with the positive phase of NAOw, and LL landing increased with the negative phase of NAOw. The inclusion of the climate effects on the standardization of abundance indexes used in the assessment of HMP could improve its results and catch projections.Postprin

Permeable frontiers in the open sea: The case of Swordfish in the Atlantic Ocean

There is a vivid debate about the border location between North and South Atlantic swordfish stocks. Climate oscillations, East Atlantic (EA) and North Atlantic Oscillation (NAO), have a major impact on the Northern Hemisphere climate and weather conditions. The initial hypothesis of present study was that if it is considering the southern frontier, each stock will be differentially affected by both climatic oscillations, which would imply the existence of a strong border. However, a similar effect on both sides of the border would result in a permeable barrier. The results suggest that the combined effects of EA and NAO affect both the North and the South Atlantic swordfish stocks in similar ways, and consequently, the location of the border may reside farther north than the current management boundary at 5°N.Postprint0,60

DIFFERENTIAL TEMPORAL AND SPATIAL DISTRIBUTION OF ADULT LOGGERHEAD SEA TURTLES FROM GULF OF CÁDIZ TO WESTERN MEDITERRANEAN SEA

The aim of this paper was to search for and model spatial and seasonal trends in occurrences (stranding or by-catches) of adult loggerhead turtles in the western Mediterranean area and Gulf of Cádiz (Atlantic) independently of their origin. Adult turtles were only bycaught on longlines from May to August. Adults were stranded in the eastern and western areas of the Strait of Gibraltar threshold throughout the year. In the Gulf of Cádiz (Atlantic), strandings were significantly concentrated in May and June, whereas in the Alboran Sea (Mediterranean), strandings mainly occurred in June and July. The probability of catching a mature loggerhead increases during June and July south of the Balearic Islands. The results suggest that adult loggerhead turtles move (migrate) from the Atlantic area (Gulf of Cádiz) to the Mediterranean (Alboran Sea) from May to June, and subsequently move to the Balearic Sea from June to July. These results are in line with those obtained by previous studies.Postprin

Genetic validation of the unexpected presence of a tropical tuna, bigeye tuna (Thunnus obesus), in the Mediterranean

Bigeye tuna (Thunnus obesus, Lowe, 1839) is one of the eight recognized species of the genus Thunnus. It is considered a tropical species distributed in the Atlantic, Pacific and Indian Oceans. To date, no validated presence of this species has been reported inside the Mediterranean Sea. This study, however, confirms, for the first time, the presence of three young individuals of this species within the Mediterranean Sea.Versión del editor1,83

FINAL REPORT FOR PHASE THREE OF THE ICCAT SHORT-TERM CONTRACT: SWORDFISH BIOLOGICAL SAMPLES COLLECTION FOR GROWTH, REPRODUCTION AND GENETICS STUDIES

This report details the third phase of biological sampling and associated analysis undertaken as part of an international swordfish biology program. The program was established in 2018 and sampling was conducted for swordfish in the North and South Atlantic and Mediterranean. Fish were sampled for size, sex, and maturity. Anal fin spines, otoliths, gonads, and tissues were obtained for ageing, growth, maturity and genetic analyses. These data will be used to inform ICCAT assessment and the ongoing management strategy evaluation process. In this report we examine sampling representativeness relative to spatial and temporal patterns in recent catch data. Samples were obtained from a broad temporal and spatial range, however, some improvements are required in spatial-temporal coverage.En prens

REPORT OF THE 2021 ICCAT SWORDFISH BIOLOGY WORKSHOP (Online, 22-26 March 2021)

“The results, conclusions and recommendations contained in this Report only reflect what was discussed by the Group of scientists that participated in the 2021 Swordfish Biology Workshop. Therefore, these should be considered preliminary until the Swordfish Species Group and the SCRS discuss and possibly adopts them at its annual Plenary meeting and the Commission revise them at its Annual meeting. Accordingly, ICCAT reserves the right to comment, object and endorse this Report, until it is finally adopted by the Commission.”En prens

Stereological comparison of oocyte recruitment and batch fecundity estimates from paraffin and resin sections using spawning albacore (Thunnus alalunga) ovaries as a case study.

Traditional histological protocols in marine fi sh reproductive laboratories using paraf fi n as the embedding medium are now increasingly being replaced with protocols using resin instead. These procedures entail differ- ent degrees of tissue shrinkage complicating direct comparisons of measurement results across laboratories or articles. In this work we selected ovaries of spawning Mediterranean albacore ( Thunnus alalunga ) as the subject of our study to address the issue of structural changes, by contrasting values on oocyte recruitment and fi nal batch fecundity given from the same tissue samples in both paraf fi nandresin.Amodernstereologicalmethod, the oocyte packing density (OPD) theory, was used supported by initial studies on ovarian tissue sampling and measurement design. Examples of differences in the volume fraction of oocyte stages, free space and connective tissue were found between the embedding media. Mean oocyte diameters were smaller in paraf fi nthaninresin with differences ranging between 0.5% in primary growth and 24.3% in hydration (HYD) stage oocytes. Fresh oocyte measurements showed that oocytes shrank as a consequence of the embedding process, reaching the maximaldegreeofshrinkageforoocytesintheHYDstage(45.8%inparaf fi nand26.5%inresin).Inordertoassess the effect of oocyte shrinkage on the OPD result, and thereby on relative batch fecundity (F r ), oocyte diameters corrected and uncorrected for shrinkage, were used for estimations. Statistical signi fi cant differences were found ( P b 0.05) between these two approaches in both embedding media. The average F r was numerically smaller in paraf fi n compared to resin (86 ± 61 vs. 106 ± 54 oocytes per gram of body mass (mean ± SD)). For both embedding media statistical signi fi cant differences ( P b 0.05) were seen between F r results based on either oocytes in the germinal vesicle migration stage or HYD stage. As a valuable adjunct, the present use of the OPD theory made it possible to document that the oocyte recruitment of spawning ovaries of Mediterranean albacore followed the typical pattern of an asynchronous oocyte development and indeterminate fecundityPostprint2,444

Assessing the responses of exploited marine populations in a context of rapid climate change: the case of blackspot seabream from Strait of Gibraltar.

Assessing the response of exploited marine populations in a context of rapid climate change: the case of blackspot seabream from the Strait of Gibraltar.— There is a growing concern over the decline of fisheries and the possibility of the decline becoming worse due to climate change. Studies on small–scale fisheries could help to improve our understanding of the effect of climate on the ecology of exploited stocks. The Strait of Gibraltar is an important fishery ground for artisanal fleets. In this area, b lackspot seabream ( Pagellus bogaraveo ) is the main species targeted by artisanal fisheries in view of its relevance in landed weight. The aims of this study were to explore the possible effects of two atmospheric oscillations, the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO), on the capture of blackspot seabream in the Strait of Gibraltar, to determine their association with oceanographic conditions, and to improve our knowledge about the possible effects of climate change on fisheries ecology so that fishery management can be improved. We used two types of data from different sources: (i) landings per unit of effort reported from a second working group between Morocco and Spain on Pagellus bogaraveo in the Gibraltar Strait area, for the period 1983–2011 , and (ii) the recorded blackspot seabream landings obtained from the annual fisheries statistics published by the Junta de Andalucía (Andalusian Regional Government). Our results indicate that the long–term landing of blackspot seabream in the Strait of Gibraltar is closely associated with atmospheric oscillations. Thus, prolonged periods of positive trends in the NAO and AO could favour high fishery yields. In contrast, negative trends in NAO and AO could drastically reduce yield.Postprint0,000

Albacore (Thunnus alalunga) larval index in the Western Mediterranean Sea, 2001-2015

Larval abundance indices express standardized abundances of larval densities from ichthyoplankton surveys. For more than two decades these indices have been used to assess the trends of the spawning stock biomass of various species in the Gulf of Mexico, being incorporated into the population models applied by ICCAT. Recently, the delta-lognormal models used for the calculation of the indices have been improved to incorporate the environmental variability and have been applied in the Balearic Sea to obtain a larval index for bluefin tuna (Thunnus thynnus). Here we apply the same methodological approach to calculate a larval index of albacore (Thunnus alalunga) from surveys conducted from 2001 to 2015 in the Balearic Sea, the most relevant spawning ground of this species in the Western Mediterranean. Results show a decreasing trend on albacore larval abundances and significant lower abundances from 2013 to 2015. This larval index, standardized for gears, sampling coverage, hour, salinity, date and sea surface temperature, attempt to provide information on the dynamic of the western Mediterranean stock of albacore, for which not much information available for assessment is availableLes indices d’abondance larvaire expriment les abondances standardisées de la densité larvaire à partir de prospections d’ichthyoplancton. Pendant plus de deux décennies, ces indices ont été utilisés pour évaluer les tendances de la biomasse du stock reproducteur de diverses espèces dans le golfe du Mexique et ont été incorporés dans les modèles de population utilisés par l’ICCAT. Récemment, les modèles delta log-normal utilisés pour calculer les indices ont été améliorés afin d’incorporer la variabilité environnementale et ont été appliqués dans la mer des Baléares afin d’obtenir un indice larvaire pour le thon rouge (Thunnus thynnus). La même méthodologie a été appliquée pour calculer un indice larvaire du germon (Thunnus alalunga) à partir des prospections réalisées entre 2001 et 2015 dans la mer des Baléares, la principale zone de frai de cette espèce de la Méditerranée occidentale. Les résultats présentent une tendance décroissante de l’abondance larvaire du germon et une abondance significativement plus faible de 2013 à 2015. Cet indice larvaire, standardisé pour les engins, la couverture de l'échantillonnage, l’heure, la salinité, la date et la température à la surface de la mer, vise à fournir des informations sur la dynamique du stock de germon de la Méditerranée occidentale, au sujet duquel il existe peu d’informations pour l’évaluationLos índices de abundancia de larvas expresan abundancias estandarizadas de densidades de larvas a partir de prospecciones de ictioplancton. Durante más de dos décadas estos índices se han utilizado para evaluar las tendencias de la biomasa reproductora del stock de varias especies en el golfo de México, y se han incorporado en los modelos de población aplicados por ICCAT. Recientemente los modelos delta lognormal utilizados para calcular los índices se han mejorado para incorporar la variabilidad medioambiental y se han aplicado al mar Balear para obtener un índice de larvas para el atún rojo (Thunnus thynnus). En este caso se ha aplicado el mismo enfoque metodológico para calcular un índice larvario de atún blanco (Thunnus alalunga) a partir de prospecciones realizadas desde 2001 a 2015 en el mar Balear, la zona de reproducción más importante para esta especie en el Mediterráneo occidental. Los resultados muestran una tendencia decreciente en la abundancia de larvas de atún blanco y unas abundancias significativamente inferiores de 2013 a 2015. Con este índice de larvas, estandarizado para los artes, la cobertura de muestreo, la hora, la salinidad, la fecha y la temperatura de la superficie del mar se intenta proporcionar información sobre la dinámica del stock de atún blanco del Mediterráneo occidental, para el que no se dispone de mucha información con vistas a su evaluaciónVersión del edito