Fishing down marine food webs?
Menakhem Ben-Yami shares his opinion on Pauly and Zeller’s recent paper on fish catches.
In a recently published paper, Professor Daniel Pauly and Dr Dirk Zeller of the University of British Columbia wrote that the FAO-reported global catch is significantly higher than the true catch figures, and that world fish catches are falling three times faster than official UN figures suggest.
According to them, new studies indicate a fast decline of catches because countries have been fishing too much, while exhausting one fishery after another. Well this is not the first time that Professor Pauly has told us such bad news - he started some 20 years ago.
Professor Pauly is a world-famous fishery scientist, an author and co-author of some 500 scientific papers. In the late 1990s he became famous when he came up with his ‘Fishing down Marine Food Webs’ theory. Accordingly, the world's fishermen would keep fishing, until all there is left to catch would be some small organisms well down the food ladder.
Now, some 20 years later, we're witnessing that the status of the world's fish stocks, although far from satisfactory, is also far off Professor Pauly's forecast. As the saying goes, "prophecy is a risky undertaking, especially with regard to the future".
So, according to FAO and NOAA reports, while in some areas overfishing is evident, in others, due to appropriate management means and/or external factors, fish populations are apparently at a sustainable level. In some cases, formerly depressed, or even ‘collapsed’ populations, have returned to fishable status. This even includes totally ‘disappeared’ stocks such as the Newfoundland cod, whose recovery after over 20 years of no fishing at all has been recently reported. But, Professor Pauly insists that if the world fisheries data do not fit his predictions, it simply indicates that the data must be wrong…
If the fish we're now catching are smaller than they used to be 50-100 years ago, it has little to do with ‘fishing down the webs’. The cause rests with our 150 year old axiomatic inertia of selective fishing (disputed on this page in April 2005 and October 2014). The problem is that by divesting fish populations of the biggest and most prolific individuals, while releasing the smaller ones, throughout many generations, we generate a process of dwarfing their age composition. Whether this causes a genetic change, or if it is reversible remains to be seen. In any case, the result is that the smaller and younger spawners of today produce smaller progeny than 50 years ago.
It has been, therefore, recommended to reverse the present practice of selective fishing by reducing the mesh size in trawl nets so that, age-and-size-wise, the catches profile the whole fished populations. In gillnets, for example, this can be achieved by replacing them with small-mesh trammel nets.
In trawling, there's also a technological angle to such proposition. The widely propagated view that a trawl is vacuum-cleaning all fish in its path and leaving behind a trail of devastation, is just another fiction, propagated by anti-trawling interests and popular, often gullible, press. As underwater measurements, films and photos show, a trawl net with small meshes in its rear parts (throat and codend) encounters heavy resistance of the water, which, apart from affecting the towing speed, creates a sort of pressure wave in its centre, where the velocity of the water is lower than along the sides of the net where filtration is better. Hence, many fish in its path, especially the large ones, are able to escape capture.
In addition, on soft grounds, a passing trawl, by upsetting the very top layer of the sea bottom, is instrumental in oxygenating it, which prevents formation of a smelly, anoxic layer, where primary producers (autotrophs) cannot subsist.
Another factor, which seems to be missing in Professor Pauly's considerations, is the cyclic nature of the state of most fish stocks. Throughout recorded history, world catches have been depending on the seawater temperature that has been fluctuating in a more or less recurring fashion.
According to a story published on 22 January by CFOOD, whose ‘declared mission is to identify and refute "erroneous stories about fisheries sustainability that appear in mainstream media"’ several top marine scientists from around the world, including Professor Ray Hilborn of the School of Aquatic and Fishery Sciences, University of Washington, are of the opinion that the recent Pauly and Zeller paper tells nothing really new about world catch, or about the status of fish stocks.
Professor Hilborn wrote that bycatch, illegal catch and artisanal catch were under-represented in the FAO catch database, and that bycatch has declined dramatically. The authors' claim, picked up by media that the world fish stocks are in bad shape is mistaken. Fish stocks are in fact stable in some places, increasing in others and declining in yet others.
Professor Michel J. Kaiser, of Bangor University at North Wales, UK, points out that zero cod landings in the Northwest Atlantic where there's moratorium on cod fishing, does not mean there are zero cod in the water, a point neglected in Pauly and Zeller's paper.
In any case, it seems that Pauly's and Zeller's recent attempt at revival of the ‘Fishing down Marine Food Webs’ theory cannot change the bare reality that all the brouhaha it had caused was unjustified.
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