Assessing juvenile swordfish (Xiphias gladius) diet as an indicator of marine ecosystem changes in the northwestern Mediterranean Sea.

To preserve marine biodiversity, we need reliable early warning indicators that inform changes in marine ecosystems. As reliable samplers of mid-trophic level communities, studying the trophodynamics of large pelagic fish can contribute to monitoring these changes. Here, we combined stomach content and stable isotope analyses to reconstruct the diet of juvenile swordfish (Xiphias gladius) in the northwestern Mediterranean Sea, in a time-lapse of almost a decade (2012 and 2020). Overall, our study showed that swordfish fed on a wide range of fish and cephalopod species from both pelagic and demersal habitats. A dietary shift towards increasing consumption of cephalopods and decreasing consumption of Gadiformes had been observed between 2012 and 2020. Stable isotope approaches revealed that gelatinous organisms were also important prey, particularly for smaller-sized swordfish. We underline the importance of combining multiple and complementary approaches to better reconstruct the diet of generalist species. Our findings highlight the generalist and opportunistic diet of Mediterranean swordfish, which makes them good candidates for monitoring changes in the ecosystem.

The relative importance of biological and environmental factors on the trophodynamics of a pelagic marine predator, the blue shark (Prionace glauca).

Marine ecosystems have been significantly altered by the cumulative impacts of human activities. Pelagic sharks have become vulnerable to increases in mortality rates caused by fishing. The decrease in number of these top predators could have substantial cascading effects on wider marine communities. Concerns about these potential impacts, and the critical need for effective management, have led to an increased interest in assessing the trophic ecology of sharks. While stable isotope analyses have been used to provide relevant insights about the trophic ecology of sharks, the causal factors leading to trophic variation between individuals has been largely overlooked. Here, we investigated the relative effect of biological factors, geographic location, and environmental factors on the spatial trophodynamics of the blue shark (Prionace glauca). Specifically, stable isotope values of δ15N and δ13C, and the estimated trophic position (TP) were analysed for 180 blue sharks collected from south of the Canary Islands in the Atlantic Ocean, to the north-western Mediterranean Sea. The results showed that models which included combined variables explained the variation in δ15N, TP and δ13C values better than models which considered only stand-alone predictors. The independent contributions of environmental variables and biological factors seemed to be more important than geographic location for δ15N and TP. δ15N and TP increased in a curvilinear fashion with body size, and TP was higher for females. In the case of δ13C values, only an effect from sex was observed. Among environmental variables, chlorophyll-a, pelagic productivity, and sea-surface temperature proved to be reliable predictors, particularly for δ15N and TP, most likely due to their relationship with productivity and prey availability. This study provides new information on ranking the factors that influence the trophodynamics of the blue shark, namely the environment, the geographic location, and the biological factors of the species.

Understanding the role of ecological factors affecting mercury concentrations in the blue shark (Prionace glauca).

Human activities have increased environmental concentrations of pollutants in marine ecosystems, which can cause harmful effects on marine organisms. Top predators are particularly susceptible to bioaccumulation and biomagnification of pollutants through the food webs and are described as good sentinels for monitoring metal accumulation such mercury (Hg) in marine ecosystems. However, to be used as sentinels, it is important to understand the main ecological factors affecting the concentrations of pollutants in these organisms. In the present study, our main objective was to investigate the effect of body size, sex, trophic niche and geographic area on Hg concentrations in a top marine top predator, the blue shark (Prionace glauca). We analysed Hg in muscle samples from male and female blue sharks of different body sizes collected from the waters surrounding the Canary Islands and the South of Portugal, in the Atlantic Ocean, to waters of the north-western Mediterranean Sea. The results revealed that the sampling area was an important factor explaining Hg concentrations, showing higher values in the Mediterranean blue sharks. We also found a positive relationship between Hg concentrations and body size of blue sharks, indicating a bioaccumulation process of this pollutant in relation with body size. Moreover, we observed a relationship between Hg concentrations and δ13C values, a proxy of the use of inshore-offshore marine habitats. Individuals with depleted δ13C values that potentially foraged in offshore waters showed higher Hg values. Importantly, most of the analysed blue sharks presented Hg concentrations that exceeded the limits established by the European Union for human consumption.

Factors associated with the differential distribution of cetaceans linked with deep habitats in the Western Mediterranean Sea.

Deep-habitat cetaceans are generally difficult to study, leading to a limited knowledge of their population. This paper assesses the differential distribution patterns of three deep-habitat cetaceans (Sperm whale-Physeter macrocephalus, Risso's dolphin-Grampus griseus & Cuvier's beaked whale-Ziphius cavirostris). We used data of 842 opportunistic sightings of cetaceans in the western Mediterranean sea. We inferred environmental and spatio-temporal factors that affect their distribution. Binary logistic regression models were generated to compare the presence of deep-habitat cetaceans with the presence of other cetacean species in the dataset. Then, the favourability function was applied, allowing for comparison between all the models. Sperm whale and Risso's dolphin presence was differentially favoured by the distance to towns in the eastern part of the western Mediterranean sea. The differential distribution of sperm whale was also influenced by the stability of SST, and that of the Risso's dolphin by lower mean salinity and higher mean Chlorophyll A concentration. When modelling the three deep-habitat cetaceans (including Cuvier's beaked whale), the variable distance to towns had a negative influence on the presence of any of them more than it did to other cetaceans, being more favourable far from towns, so this issue should be further investigated.

Microplastic occurrence in deep-sea fish species Alepocephalus bairdii and Coryphaenoides rupestris from the Porcupine Bank (North Atlantic).

Microplastic occurrence in marine biota has been reported in a wide range of animals, from marine mammals and seabirds to invertebrates. Commercial and shallow-water fish have been the subject of numerous works on microplastic ingestion, given their importance in human diet and accessibility. However, little is known about microlitter occurrence in fish species inhabiting the dark ocean, in the bathyal zone and there is a high degree of uncertainty about microplastic distribution in offshore areas and the deep sea. In this study, bathydemersal species Alepocephalus bairdii and Coryphaenoides rupestris from the Porcupine Bank caught between 985 and 1037 m depth were inspected for microdebris. The stomach contents were digested by the alkaline method plus ethanol addition to avoid clogging. A filament of Polyethylene Terephthalate (PET) was found in the stomach of a specimen of A. bairdii, representing 4% of the total sampled specimens of this species (i.e. prevalence in n = 25). However, when considering potential microplastics, the prevalence increased to 28% in both, A. bairdii and C. rupestris. This work provides the first baseline study of microplastic items in fish from such depths in the Atlantic and suggests these species might be used as biomonitors in future research.

Sexual segregation in the foraging behaviour of a slightly dimorphic seabird: Influence of the environment and fishery activity.

Sexual segregation in foraging strategies has been little studied in marine species with slight sexual size dimorphism (SSD), particularly regarding the role of environmental conditions and fishery activities. Sexual differences in fishery attendance are of particular concern because uneven mortality associated with bycatch may exacerbate impacts in wildlife populations. Using a seabird species with slight SSD, the Scopoli's shearwater Calonectris diomedea, we assessed sexual differences in foraging strategies and evaluated whether annual environmental conditions and fishery activity shaped such differences. We used a 4-year dataset combining bird GPS tracking, stable isotope analysis, the North Atlantic Oscillation index (NAO, as main proxy of the annual environmental conditions), and fishing vessel positioning data (Vessel Monitoring System, VMS) from the North Western Mediterranean, a region under intense fishery pressure. From 2012 to 2015, we tracked 635 foraging trips from 78 individuals. Females showed a greater foraging effort, a lower fishery attendance, a lower trophic level, and a narrower isotopic niche width than males. Moreover, in years with unfavourable environmental conditions, both sexes showed a lower fishery attendance and increased foraging effort compared to the year with most favourable conditions. Our results revealed that environmental conditions influence space use, feeding resources and fishery attendance differently in males and females, overall suggesting competitive exclusion of females by males from main foraging areas and feeding resources, particularly in unfavourable environmental conditions. We highlight the importance of evaluating sexual segregation under disparate environmental conditions, particularly in species with slight SSD, since segregation may pass otherwise unnoticed if only years with similar environmental conditions are considered. The higher fishery attendance of males likely explains the male-biased bycatch ratio for this species. Thus, inter-sexual differences in foraging strategies can lead to an unbalanced exposure to relevant threats and have implications for the conservation of long-lived species.

La segregación sexual en especies con dimorfismo sexual poco acusado ha sido escasamente estudiada, particularmente en relación al papel de las condiciones ambientales y las pesquerías en las estrategias de búsqueda de alimento. Diferencias entre sexos en la asociación con pesquerías son de especial interés, ya que el impacto de las capturas accidentales sobre la dinámica poblacional podría magnificarse. En este trabajo exploramos las diferencias entre sexos en las estrategias de búsqueda de alimento en la pardela cenicienta (Calonectris diomedea) en el noroeste del Mediterráneo, y evaluamos si cambios anuales en las condiciones ambientales y las pesquerías modulan dichas diferencias. Utilizamos cuatro años de datos, integrando el seguimiento GPS de las aves, el análisis de isótopos estables, el índice NAO, y el seguimiento remoto de barcos pesqueros que operan en la zona (datos VMS). Entre 2012 y 2015 obtuvimos 635 viajes de alimentación de 78 individuos. Encontramos diferencias en las estrategias de búsqueda de alimento entre sexos a pesar del dimorfismo sexual poco acusado de esta especie. En comparación con los machos, las hembras mostraron mayor esfuerzo (viajes más largos en tiempo y distancia), menor asociación con barcos de pesca, menor nivel trófico (es decir, valores de δ15 N en plasma que apuntan a un menor consumo de descartes) y un nicho isotópico más estrecho. Nuestros resultados revelaron que las condiciones ambientales influyen en las diferencias sexuales en las estrategias de búsqueda y uso de recursos de alimentación, así como en la interacción con pesquerías. En general, esto sugiere cierta exclusión competitiva de machos hacia hembras, particularmente en años desfavorables. Incluir años con condiciones ambientales dispares en el análisis puede ayudar a determinar la segregación sexual en especies con dimorfismo sexual poco acusado. Además, en especies longevas, las diferencias sexuales en las estrategias de búsqueda de alimento pueden conllevar diferente exposición a ciertas amenazas como las capturas accidentales en artes de pesca, lo que debe considerarse en el diseño de estrategias de conservación.

Using opportunistic sightings to infer differential spatio-temporal use of western Mediterranean waters by the fin whale.

The fin whale (Balaenoptera physalus) is a cosmopolitan species with a resident population in the Mediterranean Sea. Due to its habitat, open seas often far from ports and airfields, and its long-distance migratory behaviour, studying and monitoring its distribution is costly. Currently, many opportunistic sightings (OS) reports are available, which provide a source of potentially useful, low-cost information about the spatio-temporal distribution of this species. Since 1993, the Spanish Institute of Oceanography has compiled a dataset comprising 874 records of OS of nine species of cetaceans in the western Mediterranean Sea and adjacent waters. The aim of this study was to use this dataset to investigate the differential use of these waters by the fin whale when compared with other cetaceans. We compared the presence of fin whales with the presence of any other cetacean species in the dataset. Binary logistic regression was then used to model these occurrences according to several spatio-temporal variables expected to reflect their habitat use. Several significant models reveal that fin whales are more prone than other cetaceans to use the waters over the slope of the Gulf of Lion in summer. This finding confirms that the Gulf of Lion is an area of importance for this species and suggests that the slope of the continental shelf could be particularly important. Our study shows how OS can be a source of useful information when appropriately analyzed.

Plastic debris straps on threatened blue shark Prionace glauca.

Juveniles of blue shark Prionace glauca caught in pelagic longlines targeting tuna and swordfish in the Atlantic Ocean and the Mediterranean Sea were found entangled with plastic straps around their gill region. The plastic debris were identified as strapping bands and caused several degrees of injuries on the dorsal musculature and pectoral fins. They were also obstructing the gill slits probably causing breathing issues. These records were uploaded in the web site seawatchers.org, and highlight the potential of citizen science in revealing the occurrence of such problems which could help to measure the effects of plastic debris on marine life.

Interannual differences for sea turtles bycatch in Spanish longliners from Western Mediterranean Sea.

Recent studies showed that regional abundance of loggerhead and leatherback turtles could oscillate interannually according to oceanographic and climatic conditions. The Western Mediterranean is an important fishing area for the Spanish drifting longline fleet, which mainly targets swordfish, bluefin tuna, and albacore. Due to the spatial overlapping in fishing activity and turtle distribution, there is an increasing sea turtle conservation concern. The main goal of this study is to analyse the interannual bycatch of loggerhead and leatherback turtles by the Spanish Mediterranean longline fishery and to test the relationship between the total turtle by-catch of this fishery and the North Atlantic Oscillation (NAO). During the 14 years covered in this study, the number of sea turtle bycatches was 3,940 loggerhead turtles and 8 leatherback turtles, 0.499 loggerhead turtles/1000 hooks and 0.001014 leatherback turtles/1000 hooks. In the case of the loggerhead turtle the positive phase of the NAO favours an increase of loggerhead turtles in the Western Mediterranean Sea. However, in the case of leatherback turtle the negative phase of the NAO favours the presence of leatherback turtle. This contraposition could be related to the different ecophysiological response of both species during their migration cycle.

Dolphinfish bycatch in Spanish Mediterranean large pelagic longline fisheries, 2000-2010.

The aim of this paper is to describe the dolphinfish bycatch rates in the longline fisheries of the Western Mediterranean and modelling the nominal bycatch abundance and distribution of dolphinfish from the Spanish Mediterranean as a function of technical, geographical, and seasonality factors. Our results indicate that the impact of the pelagic and semipelagic longline on the dolphinfish population is relatively low (1.083 fishes per 1000 hooks), in contrast with the greater effect on the target species population. We obtained a statistically significant logistic model, with the following factors: technical characteristics of the fishery, geographical location, and seasonality. Drifting surface longliners targeting albacore is the gear with the highest effect on Mediterranean dolphinfish population. The technical characteristics of the fishery and seasonality factors have an important role in explaining the absence or presence of dolphinfish bycatch in the different boat strata, gear types, and seasons. Moreover, sea surface temperature and lunar phases also present additional explanations. Lunar phase as SST has been frequently used as an explanatory variable affecting catch rates of dolphinfish.