Hydrography and Jack mackerel stock in the South Pacific - Final report

This study aims (i) to identify the most likely population structure hypotheses of Jack mackerel, (ii) to identify management objectives for Jack mackerel, and (iii) to evaluate sustainable management strategies to achieve these objectives. These three elements were considered through literature review, statistical and population dynamics modelling. Two different conclusions were drawn: a conclusion towards most likely population structure and a conclusion towards most likely stock structure

3-D habitat suitability of jack mackerel Trachurus murphyi in the Southeastern Pacific, a comprehensive study

South Pacific jack mackerel, Trachurus murphyi, has an ocean-scale distribution, from the South American coastline to New Zealand and Tasmania. This fish, captured by Humans since the Holocene, is nowadays heavily exploited and its population has decreased substantially since the mid-1990s. The uncertainty associated to jack mackerel population structure currently hampers management. Several hypotheses have been proposed from a single population up to several discrete populations. Still no definitive answer was given. Determining how environmental conditions drive jack mackerel distribution can provide insights on its population structure. To do so, here we performed in three steps. First, we used satellite data to develop a statistical model of jack mackerel horizontal habitat suitability. Model predictions based on interaction between temperature and chlorophyll-a match the observed jack mackerel distribution, even during extreme El Niño event. Second, we studied the impact of oxygen and show that jack mackerel distribution and abundance is correlated to oxygen over a wide variety of scales and avoid low oxygen areas and periods. Third, on the basis of the above we built a conceptual 3D model of jack mackerel habitat in the Southeastern Pacific. We reveal the presence of a low suitable habitat along the Chilean and Peruvian coast, figuratively presenting a closed door caused by a gap in the horizontal habitat at ∼19–22°S and a shallow oxycline off south-centre Peru. This kind of situation likely occurs on a seasonal basis, in austral summer but also at longer temporal scales. A lack of exchanges at some periods/seasons partially isolate jack mackerel distributed off Peru. On the other hand the continuity in the habitat during most of the year explains why exchanges occur. We conclude that the more likely population structure for jack mackerel is a pelagic metapopulation

Analyse pluridisciplinaire de l’efficacité des Aires Marines Protégées, Le logiciel IPER : Estimation des Indicateurs de PERformance

National audienceno abstrac

A finer look into the twilight zone : comparing acoustic records from an animal-borne miniature sonar and a multifrequency echosounder [résumé]

ICES. Working Group of Fisheries Acoustics, Science and Technology (WGFAST), Somone, SEN, 25-/04/2022 - 28/04/2022An animal-borne miniature active echo-sounder has been recently deployed on southern ele-phant seals (Mirounga leonina) from the Kerguelen and Argentinian colonies. This high frequency sonar (1.5 MHz) has shown a strong potential in detecting small mid-trophic level targets (zoo-plankton and micronekton). Relative abundance and distribution can be assessed, allowing to observe temporal (diel migration) and spatial patterns of plankton. However, the interpretation of the collected data remains uncertain. To address this lack of information, we conducted in situ experiments onboard the Marion Dufresne vessel in the Southern Ocean (10th February to 6th March 2022). The microsonar was fixed on the rosette sampler at 13 locations and attached 16 times to a trawling net (4 mm mesh). Records will be analyzed applying a recent method devel-oped on elephant seals data to detect targets in the beam and estimate organisms abundance. The result will be compared with biological samples and backscattering layers detected by a multifrequency EK80 echosounder (18, 38, 70, 120 and 200 kHz), offering an acoustic landscape of the seals foraging area. This study will benefit to ongoing research regarding biological fields visited by the elephant seals, bringing precision on microsonar target detection capacities

Macrozooplankton and micronekton diversity and associated carbon vertical patterns and fluxes under distinct productive conditions around the Kerguelen Islands

International audienceMesopelagic communities are characterized by a large biomass of diverse macrozooplankton and micronekton (MM) performing diel vertical migration (DVM) connecting the surface to the deeper ocean and contributing to biogeochemical fluxes. In the Southern Ocean, a prominent High Nutrient Low Chlorophyll (HNLC) and low carbon export region, the contribution of MM to the vertical carbon flux of the biological pump remains largely unknown. Furthermore, few studies have investigated MM communities and vertical flux in naturally iron fertilized areas associated with shallow bathymetry. In this study, we assessed the MM community diversity, abundance and biomass in the Kerguelen Island region, including two stations in the HNLC region upstream of the islands, and two stations in naturally iron fertilized areas, one on the Plateau, and one downstream of the Plateau. The MM community was examined using a combination of trawl sampling and acoustic measurements at 18 and 38 kHz from the surface to 800 m. A conspicuous three-layer vertical system was observed in all areas - a shallow scattering layer, SSL, between 10 and 200 m; mid-depth scattering layer, MSL, between 200 and 500 m; deep scattering layer, DSL, between 500 and 800 m - but communities differing among stations. While salps (Salpa thompsoni) dominated the biomass at the productive Kerguelen Plateau and the downstream station, they were scarce in the HNLC upstream area. In addition, crustaceans (mainly Euphausia vallentini and Themisto gaudichaudii) were particularly abundant over the Plateau, representing a large, although varying, carbon stock in the 0–500 m water layer. Mesopelagic fish were prominent below 400 m where they formed permanent or migrant layers accounting for the main source of carbon biomass. Through these spatial and temporal sources of variability, complex patterns of the MM vertical distribution and associated carbon content were identified. The total carbon flux mediated by migratory myctophids at the four stations was quantified. While this flux was likely underestimated, this study identified the main components and mechanisms of active carbon export in the region and how they are modulated by complex topography and land mass effects