Studying the environment and ecology of the deep diving elephant seals: 20 years of sensors development

Over the last two decades, the collection of hydrographic profiles from CTD attached to deep diving seals has been very successful in collecting data from the polar regions of the Ocean. A judicious choice of the species, sex and age of individuals makes it possible to obtain data in under-sampled regions such as areas of pack ice or continental shelves. Among marine mammals, elephant seals are of particular interest for this observation mission because they are capable of diving continuously at great depths (590 ± 200 m, and up to 2000 m) over long periods (for 25 ± 15 min on average and up to 80 min). Over the years, we have supplemented physical oceanographic measurements with new series of biological measurements such as, for example, the concentration of phytoplankton in the euphotic layer of the ocean using fluorescence and light sensors. However, until very recently, we were sorely lacking information on the intermediate trophic levels, i.e. the whole zooplankton and the small fish and squid connecting phytoplankton to seals. Accelerometers provide information on the number of head movements associated with attempts to capture prey during the dive and thus on the abundance and distribution of prey in the water column simultaneous sampling of oceanographic conditions. The conception and deployment of a miniature echo sounder allowed considerable progress in insonifying and detecting small size particles (>1-2 mm) such as marine snow, zooplankton, fish and squids and estimate their abundance. Detection of bioluminescence, using a high sensitivity and fast responding light sensor provides complementary information on these critically important but poorly known intermediate food web levels which represent the largest daily biomass migration on earth. Today, the greatest benefit of such approaches is to evaluate how physical conditions (temperature, salinity, light) influence the distribution, abundance of marine life from phytoplankton to marine megafauna.  Future development to investigate further ocean life will be presented.