WASHINGTON (Saving Seafood) — November 19, 2013 — Bluefin tuna remains one of the most challenging fish stocks to successfully manage. Part of that difficulty traces back to uncertainty over our scientific understanding of the species, and particularly how to predict and accurately model the growth of the bluefin population. Unfortunately, the Pew Charitable Trusts does no favors to bluefin management by suggesting that the International Commission for the Conservation of Atlantic Tunas (ICCAT) should discount valid theories on bluefin population growth (“The Highs and Lows of Rebuilding Western Atlantic Bluefin Tuna,” 10/9).
Currently, ICCAT manages bluefin tuna by considering two different projections of bluefin recruitment (a measure of the number of new bluefin that are born each year). One theory, a “high recruitment” hypothesis, holds that if the overall bluefin catch were reduced, the higher number of adults left in the ocean would be able to subsequently produce a bluefin population much larger than current levels. In this scenario, bluefin would currently be considered overfished, meaning that catch levels should be sharply reduced in order to allow the population to begin rebuilding. A second scenario, the “low recruitment” hypothesis, contends that increasing the number of adult bluefin will actually have little-to-no effect on increasing the size of the stock, and that instead the growth, or lack thereof, in the bluefin population is constrained by prevailing climatic and ecological factors. If this scenario is correct, bluefin are not currently overfished, and therefore the catch does not need to be reduced, as it will not dramatically influence the size of the future population.
ICCAT continues to work with both theories, which produce dramatically different results and differing opinions on whether the stock is overfished, because there is no conclusive evidence favoring either one. Because the Commission lacks evidence to the contrary, it still considers both theories to be scientifically valid.
This lack of certainty is reflected in ICCAT’s scientific reports, in which the organization continues to use estimates based on both scenarios. In its October 2013 Report of the Standing Committee on Research and Statistics, ICCAT concluded that it “has no strong evidence to favor either scenario over the other and notes that both are plausible (but not extreme) lower and upper bounds on rebuilding potential.” It further notes that, partly due to the ambiguities in bluefin recruitment, “considerable uncertainties remain for the outlook of the western stock.”
But contrary to ICCAT’s determinations, Pew contends that all available evidence points just one way, to their preferred high recruitment model. But there is enough available evidence to maintain the low recruitment hypothesis as a valid and plausible alternative. A case for the plausibility of the low recruitment scenario was laid out by Dr. Steve Cadrin, of the University of Massachusetts School for Marine Science and Technology and President of the American Institute of Fishery Research Biologists; Dr. Molly Lutcavage and Ben Galuardi, of the University of Massachusetts Large Pelagics Research Center; and Dr. Walt Golet, of the University of Maine, responding to claims from an earlier Pew piece that the low recruitment hypothesis was “unsupported.”
According to these scientists, there are several indications that environmental changes may have affected the bluefin stock. Bluefin recruitment has remained depressed since the 1970s, with a pattern of low-to-moderate bluefin recruitment having become the norm in the last 40 years. In that same period, there have been observed changes in ocean temperatures resulting from shifts in cyclical climate patterns. The stock has also contracted from its previously observed range, no longer appearing in places like Norway and Brazil where it was once abundant. These pieces of evidence suggest that long-term factors unrelated to the intensity of fishing pressure may at least be partly responsible for the decline in bluefin over the past several decades.
Pew’s assertion that there is no scientific backing for the low recruitment hypothesis is simply unfounded. ICCAT continues to consider all scientifically plausible theories – including both the high and low recruitment scenarios – because there is just not enough certainty in the available science to favor one theory over the other. Contrary to Pew’s recommendations, the best interests of bluefin management still dictate that until such certainty emerges, both recruitment scenarios should remain in consideration.
Read "The Highs and Lows of Rebuilding Western Atlantic Bluefin Tuna" from the Pew Charitable Trusts