Past and distant whaling and the rapid decline of sperm whales off the Galapagos Islands

It is generally expected that exploited whale populations should rebuild following the end of whaling. Using photographic identification of individuals during a series of field projects, we studied female and immature sperm whales (Physeter macrocephalus) that visit the waters off the Galapagos Islands, Ecuador. Analysis of mark-recapture data, using a likelihood model, indicates the population decreased at a rate of about 20% per year (95% c.i. 7-32%) between 1985-1995. During the study period the animals were not hunted and were not obviously the subject of other immediate anthropogenic threats. Rates at which research vessels encountered whales also fell over this interval The decline seems to be due principally to migration into waters off the Central and South American mainland. The population also has a very low recruitment rate, about 0.05 calves/female/year, as indicated by rates of observation of calves. Although other causes, cannot be ruled out, both the high emigration rate and low recruitment rate are probably related to heavy whaling in Peruvian waters which ended in 1981. Whales from the Galapagos are moving east to fill productive but depopulated waters near the coast, and the virtual elimination of large breeding males (in their late twenties and older) from the region has lowered pregnancy rates. The case of the Galapagos sperm whales strongly suggests that exploitation can continue to have substantial negative impacts on the size and recruitment rate of an animal population well outside the range of the hunt and for at least a decade after it has ended.