A decade ago, a “dolphin safe” campaign tapped into the public consciousness, imploring consumers to support tuna fishing practices that reduced accidental catches of dolphins.
The new fishing techniques prompted by the campaign ensured that the cetaceans would be released alive from large “purse-seine” nets, which had been used since the 1950s in the eastern tropical Pacific Ocean and had led to massive dolphin mortalities.
By the end of the 1990s, scientists confirmed that such bycatch deaths were dropping. Yet despite the decline in mortalities, the dolphin population has not recovered at a rate hoped since the implementation of dolphin-safe practices.
Now, a researcher at Scripps Institution of Oceanography at UC San Diego and her colleagues have pegged what may be hindering the recovery.
A study by Katie Cramer of Scripps Oceanography and Wayne Perryman and Tim Gerrodette of the National Oceanic and Atmospheric Administration’s (NOAA’s) Southwest Fisheries Science Center in La Jolla shows that fishing activities have disrupted the reproductive output of the northeastern pantropical spotted dolphin and possibly other species.
“The results of this study clearly show that depleted dolphin populations have failed to recover in part due to a decline in reproductive output, and that fishing has had an effect on reproduction,” said Cramer, a graduate student researcher in the Scripps Center for Marine Biodiversity and Conservation. “This shows that the fisheries indeed are still having an impact.”
The new conclusions are based on broad surveys conducted by the NOAA Fisheries Service between 1987 and 2003 designed to assess the size and health of dolphin populations in the eastern Pacific Ocean. The surveys included military reconnaissance photos of more than 20,000 animals.
Cramer, who participated in helicopter surveys between 1998 and 2003, and her colleagues used the image database to analyze entire dolphin schools, focusing in particular on mother-calf pairs. She and her colleagues compared the data with the number of fishing events in which a dolphin school was chased by speedboats and encircled in a purse-seine net in order to capture the large yellowfin tuna that often swim with dolphin schools.
The scientists found a strong link between the amount of fishing and reproductive output in a given year for the northeastern pantropical spotted dolphin, the species most frequently encountered by the fishery. Both the proportion of adult animals in the photographs with calves, and the body length of calves that had disassociated from their mothers, which indicates when they stopped nursing, declined with increasing fishing activity.
Together, the results showed that fishing had a negative impact on calf survival rates and/or birth rates. This could be caused when fishing operations separate mothers from their suckling calves, interfere with the conception or gestation of calves, or a combination of the two.
“The link between fishing activity and … reproductive output indicates that the fishery has population-level effects beyond reported direct kill,” the authors wrote in their report, published in Marine Ecology Progress Series.
The study also notes that reproductive output of the eastern spinner dolphin also declined, but a direct link to fishing was inconclusive.
In continuing study, researchers at the Southwest Fisheries Science Center are attempting to understand the exact mechanism that leads to reduced reproductive output.
—Mario C. Aguilera