Influence of mesoscale features on micronekton and large pelagic fish communities in the Mozambique Channel

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Abstract

We investigated the diversity and distribution of two communities, micronekton organisms and large predatory fishes, sampled in mesoscale features of the Mozambique Channel from 2003 to 2009, by combining mid-water trawls, stomach contents of fish predators and instrumented longline fishing surveys. The highest species richness for assemblages was found in divergences and fronts rather than in the core of eddies. Despite an unbalanced scheme, diversity indices did not differ significantly between cyclonic and anticyclonic eddies, divergences and fronts. We found that eddies and associated physical cues did not substantially affect the distribution of micronektonic species which are mainly driven by the diel vertical migration pattern. Top predators exhibited a more complex response. Swordfish (Xiphias gladius) associated better with mesoscale features than tunas, with a clear preference for divergences which is consistent with the diel vertical migrations and occurrence of its main prey, the flying squids Sthenoteuthis oualaniensis (Ommastrephidae). On the other hand, the probability of presence of yellowfin tuna was not tied to any specific eddy structure. However, the highest values of positive yellowfin CPUEs were associated with low horizontal gradients of sea-level anomalies. We also showed a non-linear response of positive yellowfin CPUEs with respect to the depth of the minimal oxygen content. The larger the distance between the hooks and the minimal oxygen layer, towards the surface or at greater depths, the higher the CPUE, highlighting that yellowfin congregated in well-oxygenated waters. Micronekton sampled by mid-water trawls and stomach contents exhibited different species composition. The highly mobile organisms were not caught by trawling whereas they remain accessible to predators. The combination of stomach contents and mid-water trawls undoubtedly improved our understanding of the micronekton assemblage distribution. Our results provide some evidence that mesoscale features in the Mozambique Channel do not strongly affect the distribution of the mid-trophic level organisms such as micronekton and most of the large predatory fishes, and hypotheses are proposed to support this result.

Introduction

The Mozambique Channel is characterized as a highly dynamic oceanic environment where the predominant mesoscale oceanographic process is the formation of mid-ocean eddies (Biastoch and Krauss, 1999, Schouten et al., 2002, De Ruijter et al., 2002, Quartly and Srokosz, 2004). On average, 4–5 eddies per year propagate southwards from the central Mozambique Channel (15°S) towards the Agulhas retroflection region (35°S) along the western edge of the Channel, as depicted by the satellite altimetry imagery (Schouten et al., 2003). The circulation in the Mozambique Channel is dominated by large anticyclonic eddies that are very constant in size, ranging from 300 to 350 km diameter. The formation of these eddies is not related to any seasonal signal (Ridderinkhof and De Ruijter, 2003) but their intensity might respond to remote forcing controlling the intensity of the westward flow entering the North Mozambique Channel (Palastanga et al., 2006). Their internal dynamics and the interaction with the shelf as they pass off the Mozambican coast contribute to increase the inflow of nutrients in the upper layers and to enhance both the phytoplankton and zooplankton production in the offshore domain (Lamont et al., 2014, Huggett, 2014). However, large predator production, suggested by commercial fisheries data (Fonteneau, 2010), does not exhibit the same pattern.

Contrasting physical conditions in cyclonic and anticyclonic eddies can lead to different behavioral responses of the micronektonic communities, i.e. aggregation or avoidance, which in turn may impact the distribution of their predators. Micronekton comprises an assemblage of fishes, crustaceans, and squids, with size ranging from 2 to 20 cm, considered as a key trophic link between top predators and zooplankton (Brodeur and Yamamura, 2005). Large pelagic predators, targeted by high-sea fisheries such as purse seine and pelagic longline, are mainly represented by tunas (skipjack Katsuwonus pelamis, yellowfin Thunnus albacares, bigeye Thunnus obesus, albacore Thunnus alalunga and bluefin Thunnus thynnus), billfishes (from which swordfish Xiphias gladius is a key species) and, to some extent, sharks. Tunas and billfishes feed directly on micronekton, especially mesopelagic micronekton (Bertrand et al., 2002, Palko et al., 1981, Tsarin, 1997, Potier et al., 2007). As micronekton plays a central role in pelagic ecosystems, improving our knowledge of the processes affecting its distribution is essential to better quantify the yield of large oceanic fish stocks which are likely to be affected by patterns of food supply.

The influence of eddies on micronekton distribution is unclear. Both bottom up (controlling primary production) and aggregation effects (due to foraging activity) coexist (Goldthwait and Steinberg, 2008, Rii et al., 2008, Sabarros et al., 2009) and lag effects due to enrichment processes have also been considered (Domokos et al., 2007), which provide a rather complex set of interactions. Moreover, micronekton is not made up of passive organisms as they have locomotion capabilities to swim against currents. This implies that the micronekton distribution is a trade-off between active directed movements and physical processes. Catches of large predators by the fishing fleets are not evenly distributed throughout the Mozambique Channel (Fonteneau, 2010) and school aggregations have been documented in relation to mesoscale eddies (Tew-Kai and Marsac, 2010). In the open ocean, horizontal shears (not necessarily related to mesoscale activity) are known to concentrate large animals such as tunas, billfishes and sharks, presumably feeding on large aggregations of prey resources such as micronekton (Murphy and Shomura, 1972, Fiedler and Bernard, 1987).

The objective of this study was to perform an inferential analysis of the influence of mesoscale features on the spatial variability of both micronekton and top predator abundances in the Mozambique Channel, by pooling information collected by four different projects during 2003–2010, using research vessels and contracted commercial longliners. We investigated diversity and biomass indices of mid-trophic and top predator species in relation to mesoscale features, by combining three sources of data, mid-water trawls, stomach contents of predators and pelagic longline fishing surveys. These mesoscale features were defined by satellite imagery and physical oceanographic data collected during the cruises (see Lamont et al., 2014).

Section snippets

Material and methods

Location of the mid-water trawls and pelagic longline sets used for the study is illustrated in Fig. 1. The mid-water trawls were operated by research vessels made available by IPEV (Institut Paul Emile Victor, R.V. La Curieuse), IRD (Institut de Recherche pour le Développement, R.V. Antea) and FAO (Food and Agriculture Organization, R.V. Fridtjof Nansen) and top predator samples were collected from commercial longliners.

Species richness

Overall 178 species forming the mid-trophic levels, i.e. the top predators forage fauna, were collected from both mid-water trawls and stomach contents, while 32 species of predators were caught by longlines. The highest number of species was recorded in the divergence zone for top predators and mid-trophic species whatever the source of the data (Table 4). Common mid-trophic co-occurring species found in mid-water trawls and stomach contents were more numerous in frontal (25.3%) and divergence

Diversity of communities

The various indices used to depict richness and evenness of micronekton species and top predators do not differ significantly among mesoscale features. However, a common pattern can be distinguished on another descriptor such as the maximum number of species present in a given mesoscale feature. Divergences and fronts are gathering more species for all communities sampled in comparison to the core of eddies. In the mesoscale classification used in this paper, divergences and fronts are

Conclusion

The mechanism by which mesoscale features affect the distribution of micronekton and top predators is not straightforward. Complex interactions among physical cues, biogeochemical fluxes, biological enhancement and aggregation processes of micronekton and their predators coexist. The sample size used in this study is a limitation to disentangle the diverse effects and address appropriately the complexity of linkages. Previous studies carried out in this field of research in other regions were

Acknowledgments

We express our gratitude to the institutions and projects which provided vessels for the different cruises and contributed financially to the lease of these platforms: IRD, owner of the R.V. Antea, the Paul Emile Victor Institute (IPEV), owner of the R.V. La Curieuse, the Agulhas Somali Current Large Marine Ecosystem (ASCLME) project which allowed us to embark on the R.V. Fridtjof Nansen and the South West Indian Ocean Fisheries Project (SWIOFP) and the Fonds Français pour l’Environnement

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