Bycatch of dolphins in the purse-seine fishery for tropical tuna in the eastern tropical Pacific Ocean (the "ETP") has been a marine conservation issue for decades (the tuna-dolphin issue). Between 1960 and 1990 eastern spinner dolphins were reduced to 30%, and northeastern offshore spotted dolphins to 20%, of original population size. Due to various conservation and management actions, including using Dolphin-Safe labels on cans of tuna sold in the U.S., the number of dolphins killed in the international fishery has been greatly reduced. Since the early 1990s, the number of dolphin deaths reported by observers on tuna boats has been very low relative to population size. Based on the reduced mortality, recovery of the populations has been expected. However, previous data have indicated that neither stock was recovering at an expected rate.
A recently released report, "Estimates of 2006 dolphin abundance in the eastern tropical Pacific, with revised estimates from 1986-2003", may change this picture. The new population estimates are the first indications that a recovery may be underway. The cruises were carried out in a large area west of Mexico and Central America (Figure 1). For northeastern offshore spotted and eastern spinner dolphins, the new population estimates for 2003 and 2006 are higher than past estimates (Figure 2), which is cause for optimism. However, additional data and further analyses will be needed before a firm conclusion can be reached. Because of the very large area to be surveyed (over 20 million square km, approximately the size of North America), the estimates can be highly variable. This means that trends cannot be reliably determined from one or two estimates alone; a longer time-series is needed. The slopes of the dashed trend lines shown in Figure 2 are not statistically different from zero, meaning that we are not highly confident that the trend is real. Furthermore, the abundance of one dolphin stock involved in the fishery (western/southern offshore spotted dolphins) shows a decline over the same period (Figure 2).
As often happens in science, the latest results raise new questions. Continued monitoring of dolphin populations at sea through comprehensive ecosystem research cruises (STAR) is critical to assess whether these populations will continue to grow. Population estimates need to be included in assessment models which can include information on reproduction through hormonal studies and photogrammetry, habitat changes, and indirect mortality from various causes. The decline of the western/southern spotted dolphins, in the absence of significant reported bycatch, raises the question of whether the stocks, which are the units on which management is based, have been correctly described.
Figure 1. The eastern tropical Pacific Ocean ("ETP") showing daytime survey effort during the 2006 cruise (broken dark lines). Other years had similar levels of transect effort.
Figure 2. Estimates of population size of four dolphin stocks (for the kind of dolphins taken as bycatch in the ETP purse-seine fishery). Vertical lines indicate uncertainty (a 95% confidence interval) for each estimate of abundance. Since 1998, northeastern offshore spotted, eastern spinner and whitebelly spinner dolphins show an upward trend, but western/southern offshore spotted dolphins show a downward trend. The trends of the dashed lines, however, are not statistically different from zero. The trend lines are shown for only the most recent five estimates because upward trends are only expected since about 1995, when the dolphin bycatch had been reduced enough for the populations to grow.