Most canned tuna sold in the US is labeled “Dolphin-Safe”. In the US and most of Europe, this label means that the tuna in the can were caught without chasing and encircling dolphins. In Latin American countries, however, canned tuna may be called dolphin-friendly or dolphin-safe if dolphins were chased and encircled but no dolphins were reported to be killed. An important question behind the different definitions is whether or not chasing and encircling dolphins to catch the yellowfin tuna associated with them have significant harmful effects on dolphins, even if they are not killed. Research at the Protected Resources Division of the Southwest Fisheries Science Center, NOAA Fisheries, has been focused on this question for more than a decade.
There are clearly harmful effects for individual dolphins. Several hundred dolphins are killed each year in the eastern tropical Pacific purse-seine fishery, but this number is a small fraction (<0.1%) of the total population size. The effect of this direct kill on the population as a whole, therefore, is small. Indirect effects of chase and encirclement, however, might be large enough to affect the dynamics of the population. For example, some dolphin mothers and calves are separated during the chase, encirclement and release process, leading to unobserved mortality of the nursing calves. For mammals in general, stressful situations negatively affect reproduction. Estimating the indirect effects of chase and encirclement is challenging because the affected dolphins live far offshore and are spread over an area the size of North America.
A recently published paper is the first to indicate a population-level effect of the fishery due to these indirect effects (Cramer et al., 2008). High resolution aerial photographs of dolphin schools over a 17-year period provided data on reproduction for the 2 dolphin stocks most affected by the fishery. The authors showed that for both stocks the proportion of calves declined over time, and that for one stock, the proportion of calves and the size at which calves were weaned were negatively associated with the amount of chase and encirclement.