The most recent dolphin counts in the eastern tropical Pacific Ocean suggest that two species decimated by deaths in tuna fishing nets may be making a comeback.
Populations of the northeastern offshore spotted dolphin and the eastern spinner dolphin plummeted by 80 and 70 percent, respectively, between 1960 and 1990, as they were caught in nets set for tuna. More than six million dolphins have died this way since the late 1950s.
Thanks to increasingly strong regulations to prevent these deaths passed in the 70s, 80s and 90s, dolphin deaths have declined drastically, with fewer than 1,000 dolphins now dying a year in nets.
"Because they were so good at reducing the mortality, we've been expecting the population to recover for quite some time now," said Jeremy Rusin, of the National Oceanic and Atmospheric Association's Southwest Fisheries Science Center in La Jolla, Calif.
It took longer than expected to see improvement, but the latest survey, conducted in 2006, estimates the northeastern offshore spotted dolphin population at 857,884, up from 822,157 in 2003, and 636,780 in 2000. NOAA estimated the eastern spinner dolphin population at more than 1.06 million in 2006, compared with 673,943 in 2003.
Although the counts show an increase, Rusin emphasized that the results do not indicate a conclusive trend. The large area covered by the survey -- almost the size of North America -- means the estimates carry large uncertainty.
"We need additional surveys to be sure that the increases we've seen this year are real increases, and not just the reflection of statistical uncertainty," Rusin said.
One group of dolphins, the western/southern spotted dolphins, continued to decline. This finding, coupled with the rise of the northeastern offshore spotted dolphin, may simply be explained by a migration of some individuals from one zone to another, Rusin said.