Where Do Derelict Nets in the Papahānaumokuākea Marine National Monument Come From?

A tangle of derelict nets float on top of the water located near Hawaii.


One of the biggest obstacles in solving the derelict fishing net problem in the Papahānaumokuākea Marine National Monument (monument) is identifying their source. The nets that NOAA pulls off from the monument’s coral reefs and shorelines have few identifying characteristics - they are different types, shapes, sizes, and colors, but there is no one fingerprint to tie a netback to a specific country or fishing vessel. Without knowing the source it is difficult to stop derelict fishing nets from entering the monument. The NOAA Marine Debris Program spoke with Amy MacFadyen, an oceanographer and modeler with NOAA’s Office of Response and Restoration, and Jim Potemra, an oceanographer at the University of Hawai'i at Mānoa to find out more about derelict nets and how they move inthe ocean. 


Q: Why are so many nets collecting on these tiny islands? 


AMF (Amy MacFayden): The monument is perfectly situated for accumulating marine debris. The islands lay in the path of the North Pacific Subtropical Convergence Zone, or STCZ. This is a southwest-to-northeast line in the Pacific Ocean where debris tends to concentrate due to a combination of oceanic and atmospheric forces. The STCZ is positioned north of Hawaii but shifts south in the winter, especially during El Niño years. In the summer, it shifts back north. The islands act as a big comb, snagging debris as the zone shifts north and south. 


Q: Where are the derelict nets coming from? 


JP (Jim Potemra): Unfortunately, we really can’t tell where the nets are coming from because there are no markings that identify them at all. With nets, there really is no way to tell definitively once they’re lost in the ocean. 


Q: Can modeling help determine their path? 


AMF: NOAA has modeled marine debris movement in the past with General NOAA Operational Modeling Environment (GNOME), a modeling tool developed to predict the movement of floating pollutants (like oil slicks) in water. We used GNOME to model the movement of debris that came from the 2011 tsunami in Japan to examine how long it might take to reach U.S. shorelines and what areas might expect to see tsunami debris. But in that case we knew the general starting point of the debris. We were able to use ocean current and wind data, plus the starting point, to show where the debris might be located at any given point in time. In this case, we only have the end point, so we’re missing critical information. 


JP: Yes, I think the point is we have three main constraints with modeling nets: 1) where it came from, 2) where it ended up, and 3) how long it took to get there. With modeling, we can get a pretty good idea of any one of these, but to do so we need the other two pieces of information. For example, if you can tell me the net’s origin and that it ended up in Hawaii (you know 1 and 2), a model can estimate how long it took to get there. At present, all we know is 2 (where it ended up), so we really can’t get 1 or 3. If you know how long it has been in the water, then modeling can backtrack its course and maybe narrow down the possibilities of the general area it came from. If you don’t know how old it is, it makes it really hard. These nets don’t degrade very well. They stay intact a long time and get tangled up and organisms accumulate on them. One factor that could help in figuring out how long it has been in the water is the biofouling, the gathering and growth of organisms on surfaces under water, on the net. 


Q: So right now we don’t know how old the nets are, but if we did, what could we learn


JP: If there’s biofouling and it indicates the net has been in the water for a matter of months, from that you can tell that it likely came from some ship that was in the Pacific gyre. If it’s been in the water for say, three years, it could have come from anywhere on the Western side of the Pacific Ocean. 


JP: It could take nets years to get from where they started because they’re not directly driven by the wind, but rather the ocean currents. Even then, it’s a little bit easier to track floating debris with higher “windage.” Ghost nets become an issue because they’re often floating below the surface. We’re less sure of the currents there. If the nets change depth, the currents can change. 


Q: What’s windage? 


AMF: Marine debris in general is moved by currents and wind. Where debris goes and how fast it gets there depends on the ocean conditions where the debris is located, as well as what is often referred to as the debris’ “windage.” Items that float above the ocean surface and have more area exposed to the wind (i.e. more “sail”) have higher windage. For example, an empty plastic bottle floating on the surface will “feel” the winds much more than a net which is floating beneath the surface. 


Q: Are there any scientific advances in modeling that could help? 


AMF: Certainly, ocean and atmospheric models continue to improve in ways that can be applied to the problem, and statistical modeling approaches can identify regions with the highest probability that the nets originated there on different timescales (i.e. weeks, months, years). But unless we can identify the timescale of how long the nets have been floating (and this is probably widely variable), it may not be very informative. 


JP: I think another advance is with computer speeds. We can now run many simulations in a short period of time and we have models that run on desktop computers and don’t need a super-computer. So, with more model runs we can hopefully get more accurate simulations. 


Q: Anything else? 


JP: It’s just a really tough problem, and the more you think about it, the more complex it gets. Another issue that most people have a hard time conceptualizing is the degree to which this is a problem. You’ll go out there to this remote place and pull tons of this stuff off a reef; that’s like going to Antarctica and finding two tons of soda cans. Most people think places like the Monument are a pristine environment since they are uninhabited by humans, but unfortunately they suffer from human impact. 


AMF: Marine debris is obviously a global problem, so until we find a way to locate these nets at sea, we’ll need to focus on a combination of removing them and raising awareness about the issue. Prevention is ultimately going to be the key.