Marine mammals and seabirds in Mauritanian waters : pilot study April 2004

The upwelling zones in Mauritanian coastal waters provide food-rich conditions for fish. Fish-stocks are exploited by a large international fleet of freeze-trawlers. Trawlers, fish and fish predators therefore congregate in a relatively narrow strip where the upwelling takes place. As an unwanted side-effect large animals, such as cetaceans, sharks and rays are accidentally caught in the nets. To be able to estimate the effect of by-catches on local populations of these groups informationon densities and occurrence in relation to season and environmental variables is needed. In April 2004 the Netherlands Institute for Fisheries Research (RIVO) and Alterra conducted a pilot survey of marine mammals and seabirds in cooperation with the Mauritanian Institute for Oceanographic and Fisheries Research (IMROP). As sea mammals often interact and form multi- species feeding associations with seabirds, the census included both sea mammals and seabirds. The main aim of the project was to investigate the suitability of the combination of a survey of marine mammals and seabirds with an acoustic survey that is carried out twice per year to assess pelagic fish stocks. The ultimate goal is to set up a monitoring program with several surveys per year. If the combination with the acoustic survey is useful and successful, the monitoring program can be carried out at relatively low cost. Between 16 and 28 April 2004 two observers joined the acoustic survey on board the Al Awam, the research vessel of IMROP. A total of 28 transects positioned East-West at distances of 10 nm apart were sampled, at depths varying from 20 m to 500 m. All marine mammals and seabirds encountered were noted, using standard seabirds at sea counting methods. Besides assessing numbers, also behaviour was recorded, following the observation protocol for ship-based seabird surveys in Europe. Observations were carried out when the ship was steaming. During fishing operations (2-5 times per day) observations were interrupted.In this report a short overview of the results is presented. Cetaceans were only rarely encountered during the survey. In total, ten groups or singles were observed during 91.5 hours of counts. The sightings included two large whales (Fin Whale and Humpback Whale), three species of small whales (Killer Whale, Long- or Short-finned Pilot Whale and unidentified Beaked Whale) and two species of dolphin (Common and Bottlenose Dolphin). In total 51.838 seabirds and 314 species were recorded of which Common/Arctic Tern, Black Tern, Wilson’s Storm-petrel and Sabine’s gull were the most abundant species. The general pattern can be characterised by a clear near shore-offshore gradient, with peak densities of most species at the drop-off of the continental shelf. The majority of birds was associated with trawlers