Tunas off northwest Africa: The epipelagic diet of The Bigeye and Skipjack tunas
Introduction
Marine top predators are among the most threatened functional groups in the ocean, and tunas are one of the most targeted by the fishery industry. In 2018, around 5.2 million tonnes of commercial tunas were landed worldwide (ISSF, 2017), reaching up to 7.9 million tonnes when including all tuna and tuna-like species (ca. 9% of global total marine catches) (FAO, 2020). In the Atlantic Ocean, an average of half a million tonnes of tunas are captured per year (2000–2018; ICCAT, 2020). Such pressure has led to the depletion of some stocks, such as the Bigeye tuna, which is currently considered overfished in the Atlantic Ocean (ISSF, 2017). The decline of top predators impacts the structure of communities and the diversity of food webs, as well as the productivity and connectivity of the ecosystems (Doney et al., 2012). Impacts are not only observed directly on their prey but can produce cascading effects on other marine species and communities. For example, the increase in tuna fishing activity in the 1960’s in the eastern tropical Atlantic Ocean and the consequent decline of tuna populations in that area is thought to have caused a shift in the diet and a dramatic decline of the once massive Sooty tern Onychoprion fuscatus population of Ascension island which depends on interactions with sub-surface predators to locate and catch prey (Reynolds et al., 2019).
The Bigeye tuna Thunnus obesus and the Skipjack tuna Katsuwonus pelamis are the main targeted tuna species around the archipelago of Madeira (Gouveia et al., 2019), located in the subtropical NE Atlantic Ocean. In this archipelago there is a strong tuna fishing tradition taking place mostly from March to October (Gouveia and Mejuto, 2003). Here tunas are caught mostly using pole-and-line, which uses small pelagic fish as live bait. In Madeira, an average of 1667 (±481 SD) and 549 (±461 SD) tonnes of Bigeye and Skipjack tuna, respectively, are landed every year (2007–2017; Gouveia et al., 2019), comprising an average of 35% of local fisheries landings and even reaching 50% in some years (Hermida and Delgado, 2016). Contrary to the Bigeye tuna stock assessment, the Eastern Atlantic Skipjack tuna stock is not considered overfished, even though it accounts for nearly 50% of the catches in weight in the Atlantic Ocean (ISSF, 2017).
Despite the considerable economic importance of tunas in the subtropical and temperate NE Atlantic, more specifically in Madeira, Azores and the Canary Islands, few ecological studies on these species have been conducted in this region. This contrasts with the various studies conducted in the Pacific on their distribution (eg. Houssard et al., 2019; Lehodey et al., 1997), movements (eg. Schaefer et al., 2009), reproduction (eg. Hunter et al., 1986), and contamination (eg. Chen et al., 2014), and also in the South and NW Atlantic (Matthews et al., 1977; Matsumoto and Miyabe, 2002; da Silva et al., 2019). Large knowledge gaps on the ecology of these species remain in the Atlantic, such as migratory routes, spawning periods and foraging ecology. According to our literature search, only a single study on the foraging ecology of Skipjack tuna was conducted around the Canary Islands (Ramos et al., 1995), with no records for Bigeye tuna.
Tunas are opportunistic predators which means that their main prey shift according to region, reflecting prey availability in the different ocean compartments (Ménard et al., 2006; Gorni, 2016; Ohshimo et al., 2018). Therefore, knowledge on their diet will contribute to our understanding of food web dynamics and allow to infer broad community-scale changes in the abundance, availability, and diversity of poorly known mid-trophic prey. Such knowledge is required for ecosystem-based fisheries management and the conservation of large pelagic predators, including tunas.
Analysis of stomach contents is a direct and reliable way to assess the diet of tunas (eg. Glaser et al., 2015; Varela et al., 2019). However, the use of live bait during fishing events may raise concerns about what proportion of stomach content is bait. Indirect techniques, like determination of mercury concentration in tissues have allowed researchers to infer the diet of several predators for which sampling stomach contents is difficult (Layman et al., 2012; Teffer et al., 2014). Mercury concentrations can help distinguish between a mesopelagic/bathypelagic diet and an epipelagic one, as deep-ocean species have significantly higher concentrations of mercury than epipelagic species due to the higher rate of microbial mediated methylation of mercury in sub-thermocline low oxygen waters (Choy et al., 2009).
This study describes the diet of two important top predators, the Bigeye and the Skipjack tunas, in the pelagic region around the archipelago of Madeira, using stomach content analysis. To control for potential biases due to the use of live bait in this fishery, we also determined the mercury concentration in tissues of these two tunas and of two other abundant pelagic predators from the region with epipelagic diets, the Yellowmouth barracuda Sphyraena viridensis and the Longfin yellowtail Seriola rivoliana. Further comparisons of mercury concentrations with tunas from other ocean basins were also done.
Section snippets
Tuna sampling
Seventy-one bigeye and 61 Skipjack tuna stomachs collected from 16 and 10 fishing events, respectively, were obtained directly from local fishermen. The average fork length of Bigeye tunas was 77.0 ± 26.1 cm (60−170 cm) and of Skipjack tunas was 50.1 ± 3.8 cm (42.6–59.3 cm). Fish were captured using pole-and-line fishing vessels in Madeira island in 2016/2017 and 2016–2018 for Bigeye and Skipjack tuna, respectively. Stomachs were removed and kept frozen until sorting in the laboratory. A sample
Bigeye tuna diet
A total of 762 prey items were found in 69 stomachs of Bigeye tuna (2 stomachs were empty), of which 95% were identified to family, genus or species level, resulting in 24 species, 23 genus and 20 different families. Teleost fish were the most consumed prey, followed by cephalopods (Table 1). The Bigeye tuna presented an epipelagic diet (NF = 84% and W = 92%) with only 9% (W = 7.5%) of the prey belonging to mesopelagic layers. The Atlantic chub mackerel was the most consumed species overall (NF
Discussion
The use of stomach contents allowed us to document the diet of the two most fished tuna species, the Bigeye and the Skipjack tuna, in the waters around the archipelago of Madeira. Here, both species presented a diet based on epipelagic organisms, and in the case of the Bigeye tuna complemented by some mesopelagic prey. Some prey species might have been found in the stomachs analysed should our sampling be more intensive for both species. Still, this would not affect our conclusions as their
Authors' contributions
Joana Romero- Conceptualization, Design and Methodology, Participating in the collection of data, Analysis of the findings, and Drafting and Revising the manuscript. Paulo Catry – Conceptualization, Design and Methodology, Analysis of the findings, Drafting and Revising the manuscript. Margarida Hermida - Methodology, Participating in the collection of data, and Revising the manuscript. Verónica Neves - Analysis of the findings and Revising the manuscript. Bárbara Cavaleiro – Participating in
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by Fundação para a Ciência e a Tecnologia (FCT Portugal) through the project OceanWebs (PTDC/MAR-PRO/0929/2014) and Seamigrant (PTDC/BIA-ANM/3743/2014), the strategic projects UIDB/04292/2020 and UIDP/04292/2020 granted to MARE, and UIDB/50017/2020 and UIDP/50017/2020 granted to CESAM. PD/BD/127810/2016 was granted to JR by FCT. MH was supported by a postdoctoral grant from Regional Agency for the Development of Research, Technology and Innovation of Madeira (ARDITI),
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