Menakhem Ben-Yami Published in World Fishing, August 2003

NATURE STUMBLES

Nature is a highly respected scientific journal. Articles published in Nature are assumed to be scrupulously peer-reviewed, and represent high-level scientific information. This May, however, Nature made a "fishy" slip.

It is hardly disputable that many of the world's fish resources are in a bad shape. We're frequently reminded of this condition by articles in the general media, scientific papers, and by NGOs, new fishery management steps, and, of course, the fishermen.

In a widely-publicized article in the May issue of Nature entitled "Rapid worldwide depletion of predatory fish communities", R.A. Myers and B. Worm, of the Dalhousie University in Nova Scotia, say in a nutshell that "Industrialized fisheries typically reduced community biomass by 80% within 15 years of exploitation" and that "large predatory fish biomass is today only about 10% of pre-industrial levels".

Devastation?
The authors say that industrialized fishing has devastated marine ecosystems, and try to prove their case by comparing the biomass of tuna and billfish populations of the mid 20th century with contemporary figures, using Japanese longlining global data from 1952 to 1999. "Since" - they wrote - "pelagic longlines are the most widespread fishing gear, and the Japanese fleet the most widespread longline operation covering all oceans except the circumpolar seas", these data should represent the state of the biomass of those species during the above period. In the authors' opinion, most scientists and managers may not be aware of the true magnitude of the change, because the majority of declines of the populations of those species occurred during the first years of exploitation, and before surveys were undertaken.

However, the Myers and Worm study is seriously misleading for quite a few reasons. It should remind us the story about Napoleon taking to task a town mayor as to why the bells didn't ring upon the emperor's arrival at the town gates: "Majesty, there're 10 serious reasons why we couldn't ring the bells"- said the mayor. "The first is that we've got no bells". "Enough" ­ said Napoleon - "don't tell me the others".

The Myers and Worm study seems to have no leg to stand on because the authors ignore the fact that, for all large-pelagics fisheries, including those of the various tuna species, an estimate based on longline catches couldn't bear witness on those species' biomass. The most that such an analysis could have said would be that the share of the big, old tunas in the total tuna population has decreased.

The authors' and their peer reviewers must've been (or have chosen to be) ignorant of the fact that Japanese longlines target and catch only the larger and older fish living in the deeper and cooler water layers. Any student of tuna populations knows that those are the purse seine fisheries that nowadays are taking most large pelagics. They operate in the upper water layer and catch younger, smaller fish. Hence, without purse-seine catch data, hardly a valid thing can be said about most tuna populations' biomass. If the authors wished to show total biomass decrease, they were way off their target.

Myers' and Worm's article provoked severe criticism from tuna experts. Dr. Gary Sharp is a distinguished researcher in tuna physiological ecology and tuna oceanography. Back in the in the 1978 Physiological Ecology of Tunas he had already mapped out the monthly distribution of tuna species in the Pacific. In 1978, after his arrival in Rome, the FAO/Indian Ocean Fisheries Development Program published his Indian Ocean tuna fisheries maps, long with explanations of the different distribution and vulnerability of tuna species and age groups by their individual temperature and depth preferences.

Longline
More recently, he wrote an entire Chapter on the subject: 'Tuna Oceanography, an Applied Science', in the "Tuna - Physiology, Ecology, and Evolution", 2000, edited by Barbara Block and E. Don Stevens. The Chapter describes the evolution of the tuna fisheries that changed the sequential longlining approaches, and subsequent market changes that affected these fisheries. The early longline fisheries were near surface, targeting abundant juvenile tunas, but later went farther offshore in pursuit oflarger and fatter individuals in deeper waters. All this is well documented, but apparently unread by the Nature article writers, and reviewers.

Sharp posted his reaction to Nature's May article on the Internet FISHFOLK List and had this to say:
"The absolute non-overlap of the surface fisheries, and the longline fisheries is what fisheries oceanography is all about. Myers and Worm have demonstrated little understanding of any of these distinctive fisheries".

Another nail was driven into the Myers and Worm study's coffin by tuna specialist Norm Bartoo of the US National Marine Fisheries Service (NMFS) Southwest Fisheries Science Center in La Jolla. "The paper may be criticized vigorously because of some serious errors in the analysis", Bartoo wrote.

Those, who unlike Napoleon, want to see all the reasons why that study shouldn't have been published in Nature, are referred to a detailed paper, "Comments on Myers & Worm", by John Hampton of the Oceanic Fisheries Programme at the Secretariat of the Pacific Community, John R. Sibert of the Pelagic Fisheries Research Program, University of Hawaii, and Pierre Kleiber, of the US National Marine Fisheries Service, Pacific Islands Fisheries Science Center. These scientists who specialize in tuna fisheries leave the Myers' and Worm' study without even a wooden pegleg to stand on. They take apart almost every methodological and factual aspect of the Myers and Worm article. "Fundamentally flawed", "incorrect", "too restrictive" are some of the epithets they use. They conclude that "Myers and Worm do the fisheries community a disservice by applying a simplistic analysis to the available data, which exaggerates declines in abundance and implies unrealistic rebuilding benchmarks".

Let's face it, commercial fisheries do modify marine ecosystems, including age-and-size composition of fish populations. No stock can both be commercially fished and maintain its "virgin" composition. Neither its biomass can both be maintained at the "virgin" level and commercially fished. The normal assumption is that a stock is most productive and can be sustainably exploited at about half of its pre-exploitation size. Over decades of fishing, the composition of tuna biomass has changed in favour of younger year-classes not represented in longline catch data.

Myers and Worm demonstrated ignore the workings of the world's tuna fisheries, and their conclusions (biomass decrease by 90%, etc., as based on the "analysis" of tuna longlining data) are all but fallacious, to say the least.

Until it has come to an issue, and its treatment of an area with which I've been involved for years, (and indeed I've written books and articles), never a doubt of Nature's wisdom crossed my mind. But this time, I'm afraid, an article, which received undeserved publicity because of the prestige of its host, does not seem to have been peer reviewed by the appropriate specialists. Anybody blushing?

 

ps,

Site editor's note:

It would probably pay for Meyers and Worm to sit down and read both of the Academic Press volumes by Sharp and Dizon (1978) and Block and Stevens (2001), as well as basic early 1970s Japanese research by S. Saito, S. Sasaki, Hanamoto and others (referenced in both volumes) that changed the world of longline fishing after they employed vertical longline techniques to find out where which and what sizes of tunas were most abundant and thus most vulnerable to longline gear. GDS, ed.