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Good News on Tuna (Mostly)

December 14, 2011

A new study at PNAS shows that most tuna stocks are doing OK, a marked difference from a study earlier this decade.  The earlier study had found that global community biomass of large pelagic fishes had been reduced by around 90% from pre-industrial abundance.  But this study was criticized for only using data on catches from one gear type.  This study updates things for 26 stocks of tuna and related species.  We now see that for these fish, populations have fallen only 52% and catches are mostly around MSY levels.

Here are two important charts:

In spite of this positive finding, the authors call for protective measures:

[T]hese population declines cannot continue without compromising yields in the near future: The majority of populations are fully exploited, which limits the further expansion of catches from these fisheries. Currently, fisheries catch around 10–15% of the tunas and their relatives each year globally. The global demand for tunas and their relatives is still increasing, as is the trajectory of fishing mortality. The largest declines in adult biomass have occurred in two groups of species with distinct life histories, the largest and less productive temperate tunas and the smallest and more productive mackerel species.

Further, there are some stocks that are in really rough shape, like the Atlantic and Southern bluefin tuna.  They go on to point out three problems in tuna management and offer one great recommendation:

We highlight three issues to be tackled with urgency to reduce the risk of tropical tunas and other scombrid populations deteriorating in the same way as the bluefin tunas and to minimize the considerable collateral damage and biodiversity consequences of these fisheries. First, tuna productivity is apparently declining; the current estimates of MSY for some tuna populations are lower than in the past, partly a result of the increased mortality of immature tunas in the past two decades from purse seine fisheries, which has consequently decreased the maximum potential yield of the fisheries.

Second, their high value and global demand, and the rising fishing capacity and mortality, are exacerbating the pressure on populations that are already fully exploited or, in some cases, overexploited. Management of tuna populations under the single-species approach appears to be largely successful for the less valuable tropical species but has not been effective for high-value bluefin tunas driven by the scale of international demand for and trade of high-valued tunas. In those cases, additional measures seem to be required. Here, we have a case where trade is overwhelming the, normally effective, scale of fisheries management. Hence, there appears to be a role for conservation tools, such as the Convention on International Trade in Endangered Species (CITES), to work alongside the existing management framework to ensure the recovery and future sustainable fishing of the most exploited populations.

Third, exploitation of productive species, such as tunas, at MSY is driving steep population declines and elevating the risk of extinction of some unmanaged and less productive bycatch species. Tuna fisheries are directly responsible for endangering a wide range of oceanic pelagic sharks, billfishes, seabirds, and turtles.

Many of these issues could be alleviated if fisheries management organizations treated MSY as an upper limit rather than a target reference point in their management objectives, a longstanding recommendation of several international United Nations Food and Agricultural Organization agreements and guidelines over the past 15 y. [bold added]

 

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4 Comments leave one →
  1. Pete Thompson permalink
    December 14, 2011 3:35 pm

    Particularly liked the “We now see that for these fish, populations have fallen ONLY 52% and catches are MOSTLY around MSY levels.”

    While this is encouraging on many fronts, you have buried the policy recommendation that needs to be repeated over and over and over at the end:

    “Many of these issues could be alleviated if fisheries management organizations treated MSY as an upper limit rather than a target reference point in their management objectives, a longstanding recommendation of several international United Nations Food and Agricultural Organization agreements and guidelines over the past 15 y.”

    Thanks for the post…encouraging, but still work to be done.

    • December 14, 2011 3:43 pm

      Well, that is what MSY means in most fisheries, a 50% biomass reduction. I just meant to point out that things aren’t near as bad as we thought.

      I also would take it a step further and say that our present understanding of MSY should not even be an upper-bound. Dr. Sidney Holt recently wrote three excellent guest posts on this blog reviewing the wobbly science underpinning our current MSY modeling. You might enjoy them. (Just search for ‘Holt’ and you should find them.)

      • Pete Thompson permalink
        December 14, 2011 4:00 pm

        Yes. I like Dr. Holt’s posts (I’ve actually only read two of them) and I agree that there needs to be some adjustment of the models.

        I am going to try to be more interactive on your posts in the coming weeks/months and hopefully others will join in. Look for more soon. (You can thank DCMC for my attention.) Later.

        pt

      • December 14, 2011 4:16 pm

        Thanks, Pete, I’d really enjoy having more of your comments. I usually put up 2-3 posts a week, but have so far held off posting to DCMC but every couple weeks. I don’t want to wear out my welcome.
        All the best,
        Mark

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