Saving Seafood

University of Washington: Bottom-trawling techniques leave different traces on the seabed

July 18, 2017 — The following is excerpted from an article published yesterday by the University of Washington:

Fishing fleets around the world rely on nets towed along the bottom to capture fish. Roughly one-fifth of the fish eaten globally are caught by this method, known as bottom trawling, which has been criticized for its effects on the marine environment.

An international group has taken a close look at how different types of bottom trawling affect the seabed. It finds that all trawling is not created equal — the most benign type removes 6 percent of the animal and plant life on the seabed each time the net passes, while most other methods remove closer to a third. A University of Washington professor is among the main authors on the study, led by Bangor University in the U.K. and published July 17 in the Proceedings of the National Academy of Sciences.

The meta-analysis looks at 70 previous studies of bottom trawling, most in the Eastern U.S. and Western Europe. It looks across those studies to compare the effects on the seabed of four techniques: otter trawling, a common method that uses two “doors” towed vertically in the water or along the bottom to hold the net open; beam trawls, which hold the net open with a heavy metal beam; towed dredges, which drag a flat or toothed metal bar directly along the seafloor; and hydraulic dredges, which use water to loosen the seabed and collect animals that live in the sediment.

“We found that otter trawls penetrated the seabed 2.4 cm (0.94 inches) on average and caused the least amount of depletion of marine organisms, removing 6 percent of biota per trawl pass on the seabed,” first author Jan Geert Hiddink at Bangor University said in a statement. “In contrast, we found that hydraulic dredges penetrated the seabed 16.1 cm (6.3 inches) on average and caused the greatest depletion, removing 41 percent of the biota per fishing pass.”

Depending on the type of fishing gear, penetration depth and environmental variables such as water depth and sediment composition, it took from 1.9 to 6.4 years for the seabed biota, or marine plants and animals, to recover.

“These findings fill an essential science gap that will inform policy and management strategies for sustainable fishing practices by enabling us to evaluate the trade-off between fish production for food, and the environmental cost of different harvesting techniques,” said Ray Hilborn, a UW fisheries professor and one of four co-authors who designed the study.

“There’s a common perception that you trawl the bottom and the ecosystem is destroyed,” Hilborn said. “This study shows that the most common kind of trawling, otter trawling, does not destroy the marine ecosystem, and places that are trawled once a year really won’t be very different from places that are not trawled at all.”

Read the full report here

CFOOD: Do “Catch Reconstructions” really Implicate Overfishing?

January 22, 2016—The following is commentary from Michel J. Kaiser of Bangor University and David Agnew of the Marine Stewardship Council concerning the recently published article, “Catch Reconstructions Reveal that Global Marine Fisheries Catches are Higher than Reported and Declining” by Daniel Pauly and Dirk Zeller in Nature.

A new paper led by Daniel Pauly of the University of British Columbia that found global catch data, as reported to the FAO, to be significantly lower than the true catch numbers. “Global fish catches are falling three times faster than official UN figures suggest, according to a landmark new study, with overfishing to blame.”

400 researches spent the last decade accumulating missing global catch data from small-scale fisheries, sport fisheries, illegal fishing activity and fish discarded at sea, which FAO statistics, “rarely include.”

“Our results indicate that the decline is very strong and is not due to countries fishing less. It is due to countries having fished too much and having exhausted one fishery after another,” Pauly says.

Despite these findings, Pauly doesn’t expect countries to realize the need to rebuild stocks, primarily because the pressures to continue current fishing effort are too strong in the developing world. But this study will allow researchers to see the true problems more clearly and hopefully inform policy makers accordingly.

Comment by Michel J. Kaiser, Bangor University, @MicheljKaiser

Catch and stock status are two distinct measurement tools for evaluating a fishery, and suggesting inconsistent catch data is a definitive gauge of fishery health is an unreasonable indictment of the stock assessment process. Pauly and Zeller surmise that declining catches since 1996 could be a sign of fishery collapse. While they do acknowledge management changes as another possible factor, the context is misleading and important management efforts are not represented. The moratorium on cod landings is a good example – zero cod landings in the Northwest Atlantic does not mean there are zero cod in the water. Such distinctions are not apparent in the analysis.

Another key consideration missing from this paper is varying management capacity. European fisheries are managed more effectively and provide more complete data than Indian Ocean fisheries, for example. A study that aggregates global landings data is suspect because indeed landings data from loosely managed fisheries are suspect.

Finally the author’s estimated catch seems to mirror that of the official FAO catch data, ironically proving its legitimacy. “Official” FAO data is not considered to be completely accurate, but rather a proportionate depiction of global trends. Pauly’s trend line is almost identical, just shifted up the y axis, and thus fails to significantly alter our perception of global fisheries.

Michel J. Kaiser is a Professor of Marine Conservation Ecology at Bangor University. Find him on twitter here.

Comment by David Agnew, Director of Standards, Marine Stewardship Council

The analysis of such a massive amount of data is a monumental task, and I suspect that the broad conclusions are correct. However, as is usual with these sorts of analyses, when one gets to a level of detail where the actual assumptions can be examined, in an area in which one is knowledgeable, it is difficult to follow all the arguments.  The Antarctic catches “reconstruction” apparently is based on one Fisheries Centre report (2015 Volume 23 Number 1) and a paper on fishing down ecosystems (Polar Record; Ainley and Pauly 2014). The only “reconstruction” appears to be the addition of IUU and discard data, all of which are scrupulously reported by CCAMLR anyway, so they are not unknown. But there is an apparent 100,000 t “unreported” catch in the reconstruction in Figure 3, Atlantic, Antarctic (48). This cannot include the Falklands (part of the Fisheries Centre paper) and it is of a size that could only be an alleged misreporting of krill catch in 2009. This is perhaps an oblique reference to concerns that CCAMLR has had in the past about conversion factors applied to krill products, or perhaps unseen (net-impact) mortality, but neither of these elements have been substantiated, nor referenced in the supporting documentation that I have seen (although I could not access the polar record paper).

The paper does not go into much detail on these reasons for the observed declines in catches and discards, except to attribute it to both reductions in fishing mortality attendant on management action to reduce mortality and generate sustainability, and some reference to declines in areas that are not managed. It is noteworthy that the peak of the industrial catches – in the late 1990s/early 2000s – coincidentally aligns with the start of the recovery of many well managed stocks. This point of recovery has been documented previously (Costello et al 2012; Rosenberg et al 2006; Gutierrez et al 2012) and particularly relates to the recovery of large numbers of stocks in the north Pacific, the north Atlantic and around Australia and New Zealand, and mostly to stocks that are assessed by analytical models. For stocks that need to begin recovery plans to achieve sustainability, this most often entails an overall reduction in fishing effort, which would be reflected in the reductions in catches seen here. So, one could attribute some of the decline in industrial catch in these regions to a correct management response to rebuild stocks to a sustainable status, although I have not directly analyzed the evidence for this. This is therefore a positive outcome worth reporting.

The above-reported inflection point is also coincident with the launch of the MSC’s sustainability standard. These standards have now been used to assess almost 300 fisheries, and have generated environmental improvements in most of them (MSC 2015). Stock sustainability is part of the requirements of the standard, and previous analyses (Gutierrez et al 2012, Agnew et al 2012) have shown that certified fisheries have improved their stock status and achieved sustainability at a higher rate than uncertified fisheries. The MSC program does not claim responsibility for the turn-around in global stocks, but along with other actions – such as those taken by global bodies such as FAO, by national administrations, and by industry and non-Governmental Organisations – it can claim to have provided a significant incentive for fisheries to become, and then remain, certified.

David Agnew is the Director of Standards at the Marine Stewardship Council, the largest fishery sustainability ecolabel in the world. You can follow MSC on twitter.

Read the commentary at CFOOD