36 Mediterranean monk seals have been found washed ashore on the beaches of Western Sahara and Mauritania since May 19. This mortality has been detected by the monitoring team of the Spanish Mediterranean monk seal LIFE/CEE project, which has been studying the population since 1994 in collaboration with the Parc National du Banc d'Arguin.
The origin of these individuals is the colony inhabiting the Peninsula of Cap Blanc which is the only Mediterranean monk seal population that still keeps the social and numerical structure of a colony. Recent population estimates from this population range about 220-300 individuals, excluding pups. A main objective of the LIFE/CEE project is to detect potential threats to the colony which, because its limited distribution range,it has been traditionally considered to be extremely vulnerable. Indeed, the whole reproductive segment of the population concentrates in only two caves along a 1.5km coastline.
The mortality has centered on adults only, with an even proportion of sexes. It is noticeable that, while lactating females die, their pups remain alive and apparently healthy condition. Seal corpses do not present external markings suggestive of an incidental catch in fishing gear and, unfortunately, the corpses examined were from individuals that had all been dead for some days and were too decomposed to give a clear postmortem picture. The only indication so far obtained of the causative agent of the mortality is the fact that analyses of sea water samples collected in the vicinity of the colony, and carried out by the Department of Genetics of the Veterinary Faculty of Madrid University, have in all cases showed presence of at least three highly toxic dynoflagellates: Alexandrium minutum, Gymnodinium catenatum, and Dynophisis acuta. The first two ones produce a potent PSP (Paralytic Shellfish Poisoning) toxin which is lethal in high concentrations. Density of cells was about 2000 cells per litter, a quantity considered to be high. Further analysis are underway to confirm the hypothesis of a link between the observed seal mortality and a bloom of these highly toxic dynoflagellates, but the so far observed pattern of mortality would fit well with such causative agent. However, this last option should be considered yet preliminary until seal tissue analyses are finished.
Luis Felipe Lopez Jurado, Dep. of Biology, University of Las Palmas of Gran Canaria