Reports that say study shows fishing covers more than half of oceans greatly overstate impact, expert says

Novel approach

The report, published in late February in the journal Science​, took a novel approach to assessing the level of global fishing pressure. The authors used data from the AIS (automatic information system) transponders on ships, meant primarily to help avoid collisions, to track the individual vessels’ paths. The primary recipients of the pings of these transponders are other ships’ transponders, but the researchers discovered the data can be downloaded from the records of communications satellites.

Because ships move much differently when engaged in fishing operations such as dragging nets or setting and retrieving longline gear than they do when transiting from one location to another, the authors were able to gauge actual fishing activity. They collated all the AIS data they could capture in the 2012-2016 time frame and put it all on a map.

The authors' aims in the paper were to first assess what factors affected fishing pressure, such as market demand for end products, fuel prices, national holiday periods, etc., and secondarily to validate the use of this data for this purpose (the first time, so far as can be determined, this has been done). They came to this relatively mild and seemingly non-controversial conclusion:

“​We find that global patterns of fishing have surprisingly low sensitivity to short-term economic and environmental variation and a strong response to cultural and political events such as holidays and closures.​”​

Stunning fishing map

But it’s the map they generated, rather than the conclusions of the report itself, that has generated the most buzz in the mainstream media. The researchers overlaid the AIS data showing where and when fishing vessels were operating, with color gradations to show the intensity of activity. More than half of the world’s ocean surfaces were lit up with vessel tracks.

The reaction of The National Geographic​ was typical of the how the paper was reported in the mainstream press.

“​They found the footprint left by the industry was staggering. More than 55 percent of ocean surface is covered by industrial fishing, they found. That's more than four times the area covered by agriculture,​”​ the magazine wrote in an online report on the paper​​.

False conclusions drawn from report

The conversation around the paper fed into the growing perception that the world’s oceans are on the verge of a tipping point, leading inevitably to a collapse. This is just plain wrong, said Hilborn, who is a professor in the School of Aquatic and Fishery Sciences at the University of Washington. Hilborn has written and spoken on many occasions about what he considers to be overly simplistic views of the health of the oceans and the impact of human activity on it.

First off, Hilborn said the study, while breaking ground in the novel use of the AIS data, doesn’t say anything new in terms of  where ships are fishing and when. Most of the area on the map covered by the wakes of fishing vessels is associated with the deep sea tuna fishery.

“​The paper is wrong in saying that this gives a new understanding of where fishing takes place. Of the tracks on the map, 95% of that is associated with tuna purse seining, and we​’​ve known about that for a time,​”​ Hilborn said.

Coarse scale clouds picture

Second, Hilborn, while not seeking to be overly critical (this was a first pass at the AIS data, after all), did say that the scale of the map was too coarse to really draw the kind of conclusions some commentators have. The default measure to decide if a sector of ocean was going to go into the ‘subject to fishing activity’ or the ‘no fishing activity’ bucket was a half-degree square,’ or about 3,000 square kilometers for this measure at the equator. That was size of the pixels, so to speak, that were used to assemble the map. If even one fishing vessel was recorded as having fished in that area, it lit up on the map as being a region where fishing was taking place.

“​If the pixel was the whole world, you say fishing covered 100% of the oceans. The finer the scale you use to look at the data, the lower the footprint you come up with. If you used a square kilometer, for example, that would be about a 2.5 thousand times finer scale than the maps they present,​” ​he said.

“​This really tells us nothing about the impact of fishing. It just tells us where the fishing boats were,​”​ Hilborn said.

A couple of features of the map are striking. One is that fishing exclusion zones that have been declared around some Pacific Ocean island nations appear to be respected for the most part and show up as blank blobs on the map.  The second is one of the areas of greatest concern in terms of overfishing—the waters off of Southeast Asia—shows up as blank, too, because very little AIS data is available from that region.

Most of the fishing vessels in this area are too small to be required to have an AIS transponder installed, which could be potentially worrisome, Hilborn said. The area is heavily fished by a plethora of small operators and small boats, which makes getting reliable data difficult.

Terrestrial vs oceanic impacts

One key aspect about fishing pressure that Hilborn believes is missing from the debate about whether the oceans are overused is the tolerance for the impact of human activity. On land, it seems, commentators concerned about the environment will put up with a lot. 

In the oceans, they look on any human activity with unease. Hilborn said fishing in a given area does indeed introduce a stimulus into that ecosystem, but done responsibly, does not fundamentally alter it. But humans have fundamentally altered terrestrial ecosystems for several millenia now. 

Even the most ecologically sensitive agricultural practices alter local ecosystems beyond all recognition. The mixture of species is different, with many or most of the original resident plants and animals greatly reduced or extirpated. And grasslands have been greatly altered, too, via intensive animal husbandry. Yet products marketed under certifications or catch phrases such as ‘organic,’ ‘grass-fed’ or ‘pasture raised’ are seen as eco friendly, whereas oceanic ingredients increasingly come under suspicion.

“​The media coverage of this issue is really absurd. That the oceans are near collapse is a common narrative and it​’​s being fostered by a number of environmental groups that raise money by saying the oceans are in trouble. The fishing industry should turn around and say, look, we are now being fully transparent about where and when we fish,​” ​Hilborn said.

“​In the half of the world where we have the best data, fishing pressure has actually been declining. All food production has a cost. We just have to look more carefully at what those costs are,​” ​he said.

Source:​ Science​

Tracking the global impact of fisheries

23 Feb 2018; Vol. 359, Issue 6378, pp. 904-908 DOI: 10.1126/science.aao5646

Authors: Kroodsma DA, et al.