Myctophid assessment in relation to Oceanographic conditions: a three Dimension Density Distribution approach combining Modelling-, Acoustic- and Predators’ data – MyctO-3D-MAP

Lanternfishes (Family Myctophidae) are small luminous species that dominate the mesopelagic fishes of all the world’s oceans. Their large biomass and high nutritional quality make them among the few new fish resources available for commercial fisheries. However, myctophids play a pivotal role in the transfer of energy from zooplankton to higher trophic levels and represent key prey species for a broad range of predators including squids, fish, seabirds and marine mammals. Despite their ecological significance, remarkably little is known about their distribution and basic biology. Hence, given the indications of future exploitation, there is an urgent need to collect essential information on their ecology and biomass in relation to oceanographic conditions at various temporal and spatial scales before any fisheries develop. More than anywhere else, the mesopelagic ichthyofauna of the Southern Ocean is dominated by myctophids and includes their most important predators worldwide. This is especially true for the southern Indian Ocean, which holds large seabird and pinniped populations relying mainly on myctophids. The inter-disciplinary MyctO-3D-MAP project will bring together ecologists, oceanographers and modellers to investigate the 3-dimensions distribution of myctophid biomass and to define the physical and biological characteristics of the habitats of the few dominant myctophid species. Three main goals structure the MyctO-3D-MAP project: the first one is descriptive and documents the horizontal and vertical distribution of myctophids at different spatiotemporal scales, the second goal is explanatory and defines the habitats where the myctophids are and where they are not, and the third goal is predictive and addresses the consequences of environmental variability on fish distribution and habitats. To achieve this, the MyctO-3D-MAP project will implement an innovative approach that is structured in four main complementary work packages (WP1 coordinates the program): 1) bio-logging (WP2), i.e. the use of two air-breathing deep-diving myctophid predators (the king penguin and southern elephant seal) as autonomous ocean profilers to investigate the distribution of their prey and oceanographic parameters. Both species will be equipped with a new generation of data loggers allowing the detection of instantaneous prey capture events along their dives with the characteristics of the water column (depth, temperature, salinity, O2, Chl a, bioluminescence). 2) Myctophid survey cruises (WP3), i.e. first a dedicated research cruise (east of Kerguelen, in areas where instrumented predators will forage) using acoustic methods completed by net samplings and second, five acoustic cruises (southern Indian Ocean) during the logistical inter-island trips to quantify the distribution and biomass of myctophids at the latitudinal and seasonal scales. 3) Myctophid habitats (WP4), i.e. the physical oceanographic parameters provided from predators, XCTD (cruises) and satellite telemetry will be used to define the fish habitats, with a special emphasis on key structures presumably controlling the myctophid distribution, e.g. fronts, the Kerguelen shelf break and mesoscale eddies. 4) Modelling (WP5), i.e. a geostatistical model built from WP2-4 data will allow reconstructing the 3D distribution of myctophids. Its outputs will be used to assess the quality of an energy transfer and transport model (SEAPODYM) to predict the mesoscale distribution and abundance of myctophids in relation to environmental variability. Finally, an individually-based model coupled to SEAPODYM forced by IPCC scenarios will be used to assess the consequences of changes in fish habitats on top predators themselves. The overall approach of the MyctO-3D-MAP project is original and, to our knowledge, it has no equivalent at the national, European and international levels.