Monday, December 18, 2006

Are big fish disappearing or not?

Depends on what you mean by "disappearing." Would you say that tuna are disappearing if only about 1 out of 3 are left?

A new study reports the mainstream view from fisheries scientists. They say that 64% of yellowfin tuna are gone, but that's not a problem. In fact, removal of 64% is the goal.

The study's authors say that fisheries management is working and yellowfin tuna are being managed appropriately and sustainably. They also note that previous studies reporting problems were "half-baked" and based on "cherry-picked" data, and that these issues "cannot be reduced to sound bites." Hmmm...I have no doubt that the study is good, even if their metaphors are badly mixed and their criticism is self-referential.

How do the two studies really differ? The new study shows that the biggest tuna have declined by 80%. This is actually quite similar to the 90% decline of large fish found previously. Both studies agree that the largest fish are at least 80% gone. That's a striking similarity. In the new study, the sharp criticism of previous results is based on relatively modest declines of a few smaller species such as skipjack tuna.

Interesting, the methods are different, but the findings are fairly similar when comparing big fish to big fish. By far the largest difference is the threshold for concern. So decide for it ok to take most of the big fish out of the ocean?

The real conflict here is over the goals of fishery management. At stake is the future of our oceans and fish.


Anonymous said...

Although I'm inclined to leap to the defence of my colleagues, and to argue with you over things like your implication that the phrase "cherry-picked data" is self-referential, and your definition of "large fish" (which has to be more restricted than other definitions in order to make your point), I would rather draw attention to the fact that you have hit the nail precisely on the head (sorry for adding to the metaphors) when you say that "The real conflict here is over the goals of fishery management".

For some decades the goal agreed by society for most fisheries management has been to maximise production of human-usable fishery species at levels that can be maintained indefinitely. With that goal, the Western Pacific tropical tunas ARE being managed appropriately (although I'll reserve judgement on the future sustainability of bigeye and yellowfin until I see whether the recent meeting of members of the Western and Central Pacific Fisheries Commission has been able to accept the advice of its own Scientific Committee and implement measures to reduce fishing mortality significantly on those species). The maximum usable production, while still maintaining sustainability, for many fishery species can indeed usually be obtained at biomass levels which are much lower than "virgin" biomass levels, provided that periodic differences in recruitment & the environment can be tracked and predicted etc.

However, society is now developing different goals, which do not just require fishery managers to produce the maximum amount of fish over the longest period, but which require the ecosystem to remain more or less unchanged (although society has yet to define how much change is permitted, and we have yet to agree on how much natural change occurs anyway as a result of natural processes), and for various other factors to be taken into account, such as the well-being of certain iconic species, and the protection of habitat, particularly the setting-aside of "wilderness" areas that should be minimally impacted by humans.

So we're not really arguing about biology or ecology here. When it comes to the science we agree with each other much more than we disagree.

The science argument is just a proxy for the real debate, which is about what each society WANTS out of its "stakeholder space" in the ocean, amongst the range of achievable options. However, we're so conditioned to requiring scientific evidence before taking any decisions that we tend to forget that asking society what it WANTS does not require oceanic ecosystem scientific evidence (except in suggesting what combination of goals is likely to be achievable).

This, I think, is the debate that the ecosystem approach to fisheries management will facilitate. Its one of the few mechanisms from which both fisheries managers and environmentalists can see gains:- for fisheries managers because it enables them to get a better handle on non-fisheries impacts, and on the fishery as a whole; for environmentalists because they can see their main goals - iconic species protection and protected areas - actually being implemented.

And this decision about social goals for the ecosystem is not something that is made once and then carved in stone. It has to be continually revisited in the light on new knowledge, particularly knowledge about the responses of the system to management decisions, and to natural changes in ecosystems and societies themselves.

Whoops. Better go. I'm forbidden from doing anything that looks like work (apart from turkey-stuffing) during the Christmas vacation.

Anonymous said...
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Mark Powell said...

Thanks Tim,

The "self-referential" bit refers to the soundbite saying that these issues are too complex for soundbites.

I think we have some scientific disagreements amongst us, I think there is doubt absout the standard view that "THE GOOD DEPLETION" really works perfectly all of the time everywhere. Some species don't fit the theory, and there are extra factors not included in the theory like productivity being enhanced by biocomplexity and biocomplexity being reduced by loss of apparently "minor" populations through THE GOOD DEPLETION. One decade's minor population can be another decade's big producer.

Also, I removed the duplicate posting, at your request.


Anonymous said...

I'm not sure what you mean by the "good depletion" Mark. If it means what I think it means, then I'd be the first to point out that surplus production theory is not the ultimate answer to everything. However, it surely has to be admitted that you can't have any "production" at all without at least some "depletion", unless you start transcending the limits that the ecosystem puts on your "production species"? Say, by farming, by adding forage species, by removing another predator, by changing the habitat etc.

Fishing can be treated as predation within the system, and other human impacts as habitat changes, and all factored together in understanding the already variable, changing ecosystem. Just how much ecosystem change can be allowed as a result of our predation? Just when can an ecosystem be considered "degraded". These are the real questions, I reckon, and (at this stage of human understanding and knowledge) they are more questions that have to be answered by social choice rather than questions that can be answered by science.

Hence the importance that the popular press is taking in ocean management today.

Mark Powell said...


Now we're really getting somewhere.

The Good Depletion is the planned depletion of fish populations to 20-40% of their unfished biomass, based on an outdated belief that this is good management. We have much evidence of neglected problems in the outdated models that predict this is a good idea.

It's true that fishing takes fish out of the ocean, so some depletion always results from fishing. But depletion goes too far when it nearly eliminates big old fish and reduces the geographic range of fish populations. This depletion harms spawning potential, risks recruitment failure (an all too common problem these days, for "unexplained" causes), and risks loss of "minor" population segments that may be big producers under different oceanographic conditions (in the next decade?).

Thus "The Good Depletion."


Anonymous said...

You often focus on the elimination of big fish and the reduced geographic range of fish populations, and the need to include these factors in our assessment models

But what happens if we get it all these details worked out for a given species and come up with an ideal management plan for that species. Perhaps it would includes a network of marine reserves covering 20% of all habitat throughout the full geographic range, where the size and locations of the reserves are perfectly tuned to protect the large, old fish, allow enough recruits remain in the reserves to replace the large, old fish, and allow lots of dispersal to areas outside the reserves? What level of depletion for the population outside the reserves would you allow? In this ideal scenario, no amount of fishing outside the reserves would deplete the big fish or reduce the geographic range of the species, so would the target for fishers be based on maximizing yield, even if that meant removing almost all the fish that left the reserves? If, under this scenario, spawning potential would be preserved, recruitment failure avoided, and yield maximized at 40% of unfished biomass (no depletion inside reserve and 25% outside), would that be OK?

I suppose my point is that the choices of target biomass levels are somewhat separate from the specific issues of preservation of large fish and maintenance of geographic ranges, which seem to me to be just revisions in our estimates of how to maximize sustainable yield (or the probability of sustaining a given yield). This is not to say that you can't find lots of other reasons to advocate for higher biomass levels: ecosystem stability, the existence value of big fish, etc.

Mark Powell said...


Great comments and questions, thanks!

If looking only at single-species management, I think it would be fine (probably appropriate) to maximize yield outside of reserves if an ideal reserve network existed and it protected population structure, geographic range, and avoided loss of minor population segments (the latter hard to measure, so let's have range as a surrogate for minor population segments).

I think we should also consider ocean ecosystems, so maximizing yield would probably not be appropriate outside of our ideal reserve network. This is especially true for important forage species (including forage for mammals). I would adjusting biomass targets upwards from maximum yield to maintain food web and ecosystem structure, with upwards adjustments bigger for critical forage species (eaten by lots of animals or by endangered animals).

I AGREE that preservation of big fish and range are revisions in our estimates of how to maximize yield in a single-species perspective (by sustaining the population's productivity over the long term, avoiding collapse, etc.).

Again, good points. Ian, what do YOU think our biomass targets should be?


Anonymous said...

Ah, but it's so much easier to criticize others than to come up with a good solution of one's own.

I think we have a disconnect in our single-species management system where reference points which are intended to be proxies for MSY, if applied properly to the least productive species, would lead to a level of fishing in multi-species fisheries well below the maximum sustainable yield for the system as a whole. If we truly believed that the species are all independent and cared only about yield, then we wouldn't mind depleting the least productive species to a very low level in order to fish the productive ones harder. If, instead, we think that the species are interconnected in such a way that we need to make sure no species, regardless of it's commercial value, should be severely depleted, then we shouldn't claim that our target biomass is a proxy for MSY, it should be a proxy for ecosystem health.

I can't claim to know what that biomass target would be, but it would surely depend in large part on what we want our oceans to look like.

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