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Though fisheries have intensively used drifting fish aggregating devices (DFADs) over the last three decades to facilitate their catch of tropical tunas, assessing the consequences of the presence of DFADs at sea on tuna behaviour and biology is a challenge. The use of DFADs has resulted in a major increase in the number of floating objects, which are spatially heterogeneous at sea. To date, no scientific consensus exists regarding the effects of DFADs on the large-scale movements and behaviour of tuna, mainly due to the difficulty of disentangling the respective roles of DFADs and environmental factors. Some biological indices show that tuna condition is lower when associated to a floating object than in a free-swimming school. It is not possible, however, to elucidate whether this is the cause or result of the association, or if it affects the fitness of individuals in the long term. Further scientific progress would require (i) the collection of time series of indicators to monitor habitat change, individual behaviour, individual fitness and population dynamics, and (ii) experimental studies to identify the underlying behavioural and biological processes involved the associative behaviour. The extent of the modification of the surface habitat by the massive deployment of DFADs and the current uncertainty of the possible long-term consequences on the individual fitness and dynamics of tuna populations argue for the need for increased awareness of this issue by Regional Fisheries Management Organisations regulating tuna fishing.

Natal origin of 92 Albacore tuna (Thunnus alalunga) from the West Indian Ocean was investigated using trace elements in otoliths. Otoliths were collected from (i) 46 juveniles caught in 2 locations in South Africa (SA-North and SA-South) in 2018 and Tasmania (SWTS) in 2019, which are both known to be nurseries areas for the species, and (ii) 46 older fish caught in the Reunion island (SWI) during 3 sampling events in 2018. Otolithsconcentrations in seven chemical elements (B, Mg, P, Cu, Zn, Sr and Ba) were analysed by LA-ICP-MS at two points, near the core and at the edge, providing an elemental signal from the material deposited while the fish were close to their spawning grounds and from material deposited while they were in (or close) to their capture areas, respectively. Core and edge signatures for the same otolith were significantly different for most elements.At the edge, PCA identified different signatures (in Sr, Ba, and B) between the 3 main capture areas. Multi-elemental signatures for the Reunion samples were stable over the 3 sampling events. Those for South Africa samples split into 2 groups, the group of juveniles sampled in the Atlantic (SA- North) exhibiting signatures in B statistically different from those of all the juveniles from the Indian Ocean (SA-South and SWTS).The PCA on core signatures (from West Indian ocean and Atlantic ocean samples) identified some variation in spawning origin according to fish capture location. Based on the most relevant number of clusters, 2 putative spawning origins (SpO) with distinct chemical signatures in B and P were identified. All two SpO apparently contribute to the 3 sampling events investigated, but in varied proportions. Interestingly, although someoverlap was found between SA-North and SA-South signatures, the two juvenile sampling locations mostly had different spawning origin. SpO-2 was identified as the main spawning source for the albacore analysed (52%), principally in SA-S (76%), SWI-May18 (62%) and SWI-Dec18 (70%). SpO-1 apparently provides 47% of the fishsampled, principally in SA-N (69%) and SWI-Feb18 (85%). The samples collected do not allow to prove that these different spawning origins correspond to spatially discrete zones in the Indian or Atlantic Ocean. Nonetheless, these results bring evidence for trans-oceanic migrations in T. alalunga, which has strong implications for the management of its stocks

This document presents an updated summary of the French purse seine fleet targeting tropical tunas in the Indian Ocean. The statistics cover the period 1981-2020 and specifically focus on the activity of the last year of the fishery. In 2020, a total of 13 French vessels operated in the western Indian Ocean including 10 purse seiners and 3 support vessels. The total capacity weighted by the months of activity for each vessel is 10626t. The total nominal effort in 2020 was of 1805 fishing days and 2414 sets with 1898 sets on floating objects and 516 on free schools. The total catch of the French component of the EU purse seine fleet of the Indian Ocean was 58149t, being composed of 42.2%, 52.6%, 4.5% and 0.8% of yellowfin tuna, skipjack tuna, bigeye tuna and other species respectively. The most noticeable change in 2020 was the decrease of 17.7% for the total of catches in comparison to 2019, respectively 58149t and 70622t. Remarkably, yellowfin tuna and skipjack tuna proportion of catches in 2020 for free swimming school returned to the proportion prevailing before 2018, after 2 years of a different fishing strategy targeting skipjack tuna on free swimming school.

There is a distinct population of blue whales, Balaenoptera musculus, in the northern Indian Ocean. The taxonomic status of these animals has long been uncertain, with debate over whether this population represents a distinct subspecies, and if so which name should apply. They have most frequently been assigned to B. musculus brevicauda, but are currently considered to be B. m. indica. The movements of these blue whales within the northern Indian Ocean are poorly understood. This paper reviews catches (n = 1,288), sightings (n = 448, with a minimum of 783 animals), strandings (n = 64) and acoustic detections (n = 6 locations); uses ocean colour data to estimate seasonality of primary productivity in different areas of the northern Indian Ocean; and develops a migration hypothesis. It is suggested that most of these whales feed in the Arabian Sea off the coasts of Somalia and the Arabian peninsula during the period of intense upwelling associated with the southwest monsoon (from about May to October). At the same time some blue whales also feed in the area of upwelling off the southwest coast of India and west coast of Sri Lanka. When the southwest monsoon dies down in about October–November these upwellings cease. The blue whales then disperse more widely to eke out the leaner months of the northeast monsoon (during about December to March) in other localised areas with seasonally high productivity. These include the east coast of Sri Lanka, the waters west of the Maldives, the vicinity of the Indus Canyon (at least historically), and some parts of the southern Indian Ocean. The data are consistent with the hypothesis that at least some of the blue whales that feed off the east coast of Sri Lanka in the northeast monsoon also feed in the Arabian Sea during the southwest monsoon. These whales appear to migrate eastwards past the north of Maldives and south of Sri Lanka in about December–January, returning westwards in about April–May

Oral presentation about Tracking seabirds to identify potential Marine Protected Areas in the Western Indian Ocean

Oral presentation about Tracking seabirds to identify potential high-seas Marine Protected Areas in the western Indian Ocean

In the lack of a satisfactory formulation for the two-stanza growth curve of yellowfin (Thunnus albacares), estimates of growth rate by size class can be an alternative as an input for integrated models. From tagging data collected in the Atlantic and Indian Oceans, we show however that for growth rate that does not follow a classical linear decreasing trend over size, simple estimates of length increments by size are biased due to an averaging effect of the time spent at liberty. We simulate length at recapture to illustrate the magnitude of the bias which can be expected depending on the duration of the time at liberty. Our results suggest that for large time at liberty growth rates by size are smoothed in such a way that the Von Bertalanffy model could erroneously be considered as a good alternative. With the aim of removing this bias we used a generalized additive model (GAM) to calculate an instantaneous growth rate by size (i.e., fixing time at liberty at 1 day at sea). The predicted growth rates by size are lower than the apparent growth rates but are similar in magnitude and in shape with observed growth rates for moderate times at liberty (< 90 days at sea). In the light of our findings, we suggest that hard part reading analyses, which in general reject the two-stanza growth assumption, could have been affected by the same type of bias.

International audience; Oral presentation about Tracking seabirds to identify potential high-seas Marine Protected Areas in the western Indian Ocean

International audience; Oral presentation about Tracking seabirds to identify potential pelagic Marine Protected Areas in the western Indian Ocean

International audience; Oral presentation about Foraging ecology of tropical seabirds : a western Indian Ocean perspective

International audience; Ecologists primarily use δ15N values to estimate the trophic level of organisms, while δ13C, and even recently δ15N, are utilized to delineate foraging habitats. However, many factors can influence the stable isotopic composition of consumers, e.g. age, starvation or isotopic signature of primary producers. Such sources of variability make the interpretation of stable isotope data rather complex. To examine these potential sources of variability, muscle tissues of yellowfin tuna (Thunnus albacares) and swordfish (Xiphias gladius) of various sizes were sampled between 2001 and 2004 in the western Indian Ocean during different seasons and along a latitudinal gradient (23°S to 5°N). Size and latitude effects on δ15N and δ13C were investigated using linear models. Both latitude and size significantly affect the stable isotope values of the studied species but variations were much more pronounced for δ15N. We explain the latitudinal effect by differences in nitrogen dynamics existing at the base of the food web and propagating along the food chain up to top predators. This spatial pattern suggests that yellowfin and swordfish populations exhibit a relatively unexpected resident behaviour at the temporal scale of their muscle tissue turnover. The size effect is significant for both species and indicates an increase in prey size through ontogeny. However, this effect is more pronounced in swordfish as a consequence of their different foraging strategies, reflecting specific physiological abilities. Swordfish adults are able to reach very deep water and have access to a larger size range of prey than yellowfin tuna. In contrast, yellowfin juveniles and adults spend most of their time in thesurface waters and large yellowfin tuna continue to prey on small organisms. Consequently, nitrogen isotopic signatures of swordfish tissues are higher than those of yellowfin tuna and provide evidence for different trophic levels between these species. Thus, in contrast to δ13C, δ15N analyses of tropical Indian Ocean marine predators allow the investigation of complex vertical and spatial segregation, both within and between species, even in the case of highly opportunistic feeding behaviours. The linear models developed in this study allow us to make predictions of δ15N values and to correct for any size or latitude differences in future food web studies.

Squids of the family Ommastrephidae are a vital part of marine food webs and support major fisheries around the world. They are widely distributed in the open ocean, where they are among the most abundant in number and biomass of nektonic epipelagic organisms. In turn, seven of the 11 genera of this family (Dosidicus, Illex, Martialia, Nototodarus, Ommastrephes, Sthenoteuthis, and Todarodes) are heavily preyed upon by top marine predators, i.e., birds, mammals, and fish, and currently support fisheries in both neritic and oceanic waters (Roper and Sweeney, 1984; Rodhouse, 1997). Their commercial importance has made the large ommastrephids the target of many scientific investigations and their biology is consequently reasonably well-known (Nigmatullin et al., 2001; Zuyev et al., 2002; Bower and Ichii, 2005). In contrast, much less information is available on the biology and ecological role of the smaller, unexploited species of ommastrephids (e.g., Eucleoteuthis, Hyaloteuthis, Ornithoteuthis, and Todaropsis).

11 pages; International audience; The foraging behaviour of animals depends on the distribution, abundance and predictability of their food resources. In the marine environment, top predators such as seabirds are known to concentrate their foraging effort in specific oceanic features where productivity is elevated or prey concentrated. When marine productivity is low and prey distribution unpredictable, such as in tropical waters, selection should favour the evolution of flexible foraging strategies. By using GPS, Argos transmitters and activity recorders, we studied the foraging strategy of red-footed boobies (RFBs) Sula sula breeding on Europa Island in the Mozambique Channel, to examine the way a central place forager searches for prey in tropical waters. RFBs only foraged during the day-time and never sat on the water at night; thus, they consequently had a limited maximum foraging range of 148 km. Primary production within this range was low overall (average 0.165 mg chlorophyll m–3), with a high spatial and temporal heterogeneity due to the presence of strong geostrophic currents and surface mixing by storms. RFBs appear to adjust their foraging strategy to cope with this situation. Individual birds do not return to the same oceanic sites from one trip to the next, but generally head in the direction of zones where productivity is enhanced. Active foraging, i.e. periods when birds spent time on the sea surface or dived, occurred at the most distant part of the foraging trip. Despite their limited foraging range and the low productivity of surrounding waters, red-footed boobies are able to track zones of prey availability located in areas of enhanced primary production. Searching is probably favoured by the presence of sub-surface predators that chase prey to the surface. The foraging strategy of this tropical sulid appears to differ extensively from that of temperate or polar seabirds whose prey distribution and availability are more predictable.

Place: Oxford Publisher: Pergamon-Elsevier Science Ltd WOS:000556810400011; Based on satellite and in situ data, the dynamic characteristics and vertical structure of a surface intensified mesoscale eddy dipole recently expelled from the South East Madagascar Current (SEMC) is described for the first time. The dipole was surveyed 250 nautical miles south of Madagascar between 14 and 23 November 2016, during west-east and south-north transects carried out over the northern Madagascar Ridge. The dipole consisted of two counter-rotating vortices of similar size (100 km) and intensity (0.7 f), and an intense southwestward jet (150 cm s(-1)) in the frontal region between the two eddies. The cyclonic eddy was lying on the western side of the anticyclonic eddy. With azimuthal velocities reaching 100 cm s(-1) at the surface and decreasing slowly with depth (40 cm s(-1) at -600 m), this dipole was defined as a highly non-linear (Ro similar to 0.7) isolated eddy-type structure (c(beta) similar to 11 cm s(-1) and U/c(beta) similar to 0.7) capable of trapping and advecting water masses over large distances. The enhanced concentration of chlorophyll-a found in the cyclone relative to the anticyclone could be tracked back to the spin-up phase of the two eddies and attributed to eddy-pumping. The eddy cores were located above the pycnocline (1026.4 kg m(-3)), within the upper 600 m, and consisted of varieties of Subtropical Underwater (STUW) found within the SEMC. The STUW found in the anticyclone was more saline and oxygenated than in the cyclone, highlighting mixing with the inshore shelf waters from the southeastern coastal upwelling cell off Madagascar. Observations suggest that the dipole interacted strongly with the chaotic bathymetry of the region, characterized by a group of five seamounts lying between -240 m and -1200 m. The bathymetry blocked its westward advection, trapping it in the vicinity of one shallow seamount for more than 4 weeks, so enhancing the role of the eddy-induced velocities in stirring the surrounding water masses. Squeezed between the southern Madagascan shelf and the northern flank of the anticyclone, two filament-like dynamic features with very different water-mass properties could be observed on the south-north transect: i) one filament highly concentrated in chlorophyll-a demonstrating the capacity of the eddy to export shelf water offshore; ii) intrusions of southern-type STUW generally found south of the South Indian Counter Current (SICC) recirculating on the external flanks of the anticyclone. Although the observed circulation and hydmgraphy were largely constrained by the presence of the mesoscale eddy dipole, unmistakable fine-scale dynamics were also observed in the vicinity of the MAD-Ridge seamount, superimposed onto the mesoscale eddy flow.

The Indian Ocean Tuna Tagging Program provided a unique opportunity to collect demographic data on the key commercially targeted tropical tuna species in the Indian Ocean. In this paper, we focused on estimating growth rates for one of these species, yellowfin (Thunnus albacares). Whilst most growth studies only draw on one data source, in this study we use a range of data sources: individual growth rates derived from yellowfin that were tagged and recaptured, direct age estimates obtained through otolith readings, and length-frequency data collected from the purse seine fishery between 2000 and 2010. To combine these data sources, we used an integrated Bayesian model that allowed us to account for the process and measurement errors associated with each data set. Our results indicate that the gradual addition of each data type improved the model's parameter estimations. The Bayesian framework was useful, as it allowed us to account for uncertainties associated with age estimates and to provide additional information on some parameters (e.g., asymptotic length). Our results support the existence of a complex growth pattern for Indian Ocean yellowfin, with two distinct growth phases between the immature and mature life stages. Such complex growth patterns, however, require additional information on absolute age of fish and transition rates between growth stanzas. This type of information is not available from the data. We suggest that bioenergetic models may address this current data gap. This modeling approach explicitly considers the allocation of metabolic energy in tuna and may offer a way to understand the underlying mechanisms that drive the observed growth patterns.

International audience; The impact of mesoscale activity on phytoplankton and nutrient distribution in the Mozambique Channel was simulated by coupling a biogeochemical model (PISCES) with a regional oceanic model (ROMS). Examples of the effects of eddies on the biogeochemistry of the Mozambique Channel are presented to illustrate the complexity of the system. In the model, several cyclonic eddies were found with low concentrations of chlorophyll at their cores, which contrasts with previous studies in the open ocean. In addition, several anticyclonic eddies were simulated with high concentrations of chlorophyll at their cores. Phytoplankton growth within these mesoscale features (both cyclonic and anticyclonic eddies) occurred in response to nutrient injection into the euphotic zone by advection, and subsequent retention of surrounding nutrient-rich waters within eddies. Offshore nutrient distributions depended strongly on lateral advection of nutrient-rich water from the coastal regions, induced by eddy interaction with the shelf. The environmental conditions at the locations where eddies were generated had an important effect on nutrient concentrations within these structures.

International audience; A multidisciplinary programme, MESOBIO (Influence of mesoscale dynamics on biological productivity at multiple trophic levels in the Mozambique Channel) was undertaken in the Mozambique Channel within the framework of a scientific partnership between France and South Africa. MESOBIO focused on the signature of the highly energetic eddy dynamics in the Mozambique Channel. The Channel, which is known to be one of the most turbulent areas in the world ocean, has a great diversity of marine organisms and is the site of active pelagic fisheries. MESOBIO was mostly based on observations at sea during 12 multidisciplinary cruises between 2002 and 2010. Hydrographic measurements, sampling of biological organisms ranging from phytoplankton to top predators, and experiments on primary production and energy transfer through the food web, were conducted onboard various research vessels. The data were analysed in relation to eddy field characteristics for the periods of the cruises, including seasonal or inter-annual variability in mesoscale activity. A modelling approach was also developed within MESOBIO for both the circulation in the Channel and the biogeochemical response to eddy forcing. This paper introduces the suite of articles on the MESOBIO investigations by summarizing background knowledge for the different disciplines and the key issues that were addressed within the programme.

International audience; The oceanic circulation in the Mozambique Channel (MZC) is dominated by mesoscale cyclonic and anticyclonic eddies that are known to play a key role in biological processes of less-productive deep-sea ecosystems by converting physical energy into trophic energy and by restructuring and concentrating biomass across the eddy field. In this study, hydroacoustics was used to investigate the spatial distribution of micronekton according to four classes of mesoscale features and assess whether cyclonic eddies, anticyclonic eddies or eddy edges (divergence and frontal regions) impact the density of micronekton. Acoustic data were collected continuously with a Simrad EK60 split-beam echosounder during three surveys carried out in the MZC within the framework of the MESOBIO programme. First, micronekton ascents and descents during the crepuscular periods (dusk and dawn, respectively) were similar to the well-known process of diel vertical migration, with the largest changes in the shallow layer, much smaller in the deep layer, and almost non-existent in the intermediate layer. Additionally, the acoustic densities for the total water column were greater at night than during the day, suggesting that organisms migrate from layers deeper than the water column that was sampled (740 m). Second, there was evidence of differences in the acoustic responses of micronekton to mesoscale features During two of the three surveys, cyclonic eddies exhibited greater micronekton density than anticyclonic eddies for day and night. In contrast, during the last survey, the greatest micronekton density was observed in anticyclonic eddies. To explain this discrepancy, several hypotheses are proposed, including the eddy generation site and trajectory throughout the life of the eddy, eddy-eddy interactions, seasonality and difference in monsoon wind regime, the depth of influence of eddies and a low dependence of movements of larger micronektonic organisms on the mesoscale gradients. Furthermore, this study demonstrated that mesoscale features could be predicted using acoustic responses at several acoustic frequencies.