A new paper published in Marine Mammal Science by NOAA's Jay Barlow of the Marine Mammal and Turtle Division, Southwest Fisheries Science Centre indicates that the abundance estimates for Eastern Tropical Pacific dolphin species have been misjudged and are likely greatly underestimated.
It reviews the methods used to estimate cetacean populations and finds that they are significantly impacted by different Beaufort conditions (a measure of sea conditions). The findings in this paper raises serious questions about the reliability of the series of population assessment surveys conducted by NOAA between 1986 and 2006, and effectively calls into question the long-held NOAA assertion the Spotted and Spinner dolphin stocks are "depleted" in the Eastern Tropical Pacific.
Cetacean populations are estimated using a line-transect method and it had been assumed that animals on the track line are always detected, even in poor sighting conditions. A previous paper by Mr Barlow had found that this assumption was largely true for large schools of dolphins, but his new paper, based on more data, shows that abundance estimates are low, even for large schools and even in good conditions of one or two on the Beaufort scale for many species.
The new paper's results show that for Spotted and Spinner dolphins, the downward bias would represent a 27 per cent reduction in sightings under for Beaufort 1 conditions, a 47 per cent reduction under Beaufort 2, a 61 per cent reduction under Beaufort 3, a 72 per cent reductions under Beaufort 4, a 79 per cent reduction under Beaufort 5, and an 85 per cent reduction in sightings under Beaufort 6 conditions. On average, Beaufort conditions for the surveys were well above the ideal 0-2 conditions and were between 3.5 and 4.5. Stated another way, this means that there is a high likelihood Northeastern Spotted dolphin populations have been underestimated by more than 60 per cent, and Eastern Spinner dolphins have been underestimated by 73 per cent.
Additionally, the new paper calls into question the methods NOAA has used to conduct their surveys. For example, when cetaceans were seen within 3 nautical miles of the transect line, the survey effort was typically halted which, in practice, means stopping the count – a serious deviation from normal transect procedures. The paper goes on to explain that the ship was then manoeuvred away from the original transect line to approach the animals so that the observers could better determine the species composition and estimate the average group size. This implies interrupting the counts, closing down the sighting effort and leaving the original survey transect line.
The practical consequence of stopping the counting and the effective searching effort shut out the opportunity for spotting dolphin groups swimming ahead in the original track line. Other pods present would be neither observed nor counted. As a result, and because they stop the survey in order to change the ship's direction, the "true" dolphin abundance in the surveyed area is reduced within the transect line and zone surveyed. Further evidence of these faults in the methodology is included in two other oft-cited papers, Gerrodette and Forcada (2005) and Gerrodette et al (2008).
This new research clearly illustrates that, given the Beaufort conditions prevalent during most of the NOAA cruises and the flawed methodology employed, population estimates likely do not reflect true population sizes. At the same time, the conclusions of Jay Barlow's paper substantially counter and undermine the various non-recovery hypotheses that NOAA has presented for the so-called "depleted" Spotted and Spinner stocks including speculative claims of exceedingly high levels of "cryptic mortality" to explain their claims that these two stocks have failed to "recover at the expected rate." It is impossible to determine if cetaceans are dying if you do not have an accurate picture of the population at any given time.
This article was first published in Campaign for Eco-Safe Tuna – www.ecosafetuna.org
