Strait Science: Sailing drones aid in pollock survey
The COVID-19 pandemic caused the cancellation of a number of scientific cruises to the seas off Alaska. Among those cancelled was NOAA’s annual trawl survey to get information about stocks of pollock, the USA’s number one fishery by weight. NOAA Fisheries’ Alex De Robertis spoke in UAF Alaska Sea Grant’s Strait Science series on how they managed to complete the survey without a big ship with crew.
“As the pandemic developed we realized it was going to be difficult to do ship-based surveys in the Bering Sea,” began De Robertis his presentation via Zoom. His Seattlebased group had been working with drones for the past five years so they decided to put together a project to fill in the gaps in the data.
Walleye pollock are a common fish species with large economic value. “They are the second largest fishery in the world, roughly five percent of global fish catch,” he said. The fishery is mostly on the Eastern Bering Sea shelf and in deeper water, 300 feet and more. A lot of the fishing for pollock takes place out of Dutch Harbor. Pollock are about two feet long and are a certified sustainable seafood, a well-managed fishery. The familiar McDonalds Fillet-O-Fish is pollock.
Management of the Bering Sea fishing stock is comprised of three major parts. The bottom trawl survey, the acoustic trawl survey and information which comes from the actual fishing trawlers. The bottom trawl survey and the acoustic trawl survey were both cancelled due to COVID-19. As the trawlers elected to fish, and the observers were on board, that part of the information would be available. These three data streams are combined together into population models and stock assessments, and those are used to set the fishing quotas.
It’s interesting to note that catcher processor vessels arrived in Dutch Harbor with entire crews testing positive for COVID-19.
De Robertis’ group turned to the ocean going drones they’d worked with for a partial solution to the lack of a survey. “They are essentially sailboats,” said De Robertis. “They’re about 20 feet long, have a wing on them which functions as a sail. That’s the propulsion. It has solar panels to generate the electricity. The sonar instruments are located in the keel.” Saildrones of Alameda, California had developed very high technology wind and solar powered robots which can be controlled via satellite. Their instructions can be update their instructions and they are reliable for a long time.
“The pandemic was happening, we weren’t able to travel,” said De Robertis. “Typically what happens with the drone vehicles is we transport them by freighter. We put them into shipping containers, ship them up to Dutch Harbor and release them there. But these they actually sailed from Alameda to Alaska. That took about 45 days. They move on the average at about 2 knots, roughly walking pace.” The three drones made their way to the Bering Sea shelf from San Francisco Bay and started surveying the pollock stocks on July 4. That meant the timing was roughly the same as if the NOAA ship had been doing the survey. Bad weather meant no survey work for the drones, but there were only a few days when weather was a problem. A manned ship also knocks off when the weather is difficult.
The survey by boat is normally comprised of 30 transects 20 miles apart. “It’s like going along with a lawnmower,” said De Robertis. “Basically what we did with the drones is what we would have done with the ship except we couldn’t.”
Trawling is important for the surveys not only for the sonar component but also because they’re able to catch fish and measure them for size and determine their age. “What you really want to know is fish biomass,
or weight,” said De Robertis. “That is what you use to set fishing quotas.” One advantage a ship has is the sonar is more specific and gives more accurate information. How do they know what species of fish they’re looking at? As the Bering Sea is dominated by walleye pollock that’s not a problem. If you see fish, it’s a pollock. The midwater fish are dominated by pollock. It’s one of the cleanest fisheries in the world. “There is a bycatch in the pollock fishery but it’s a very small amount,” he said. “Tons of pollock vs. one salmon. We have many years of information. This wouldn’t work on the equator where there are many species.”
Once the survey was complete the drones had to sail back to Alameda, once again under their own steam at 2 knots. That took another 45 days. The data was all on a memory stick which had to be removed from the drone. The stick is then uploaded onto the computer and the work can begin. “Pretty valuable memory stick, I guess, you could say,” he said.
“The short version to all this is everything went really well, as well as one could have hoped. Everything worked. All the instruments worked well.” The drones sailed unmolested through the Russian war games which drove the pollock fleet out. “It
didn’t seem to be a problem. I was sort of worried about it at the time.”
What the data from the drones showed was the pollock are expanding their footprint in the Bering Sea. Data from 2010 shows they are moving north with the warm water. The bottom trawl survey has shown the same thing. There are more pollock in the Northern Bering Sea. There’s an increase in the pollock population from 2018 of about 45 percent. It’s important to understand how much confidence to have in the drone survey. Half transects mean less sampling and the unit conversion isn’t so precise. “This whole project boils down to just two numbers: what is the estimate of total weight of fish and how much confidence to have in the estimate. We did all this basically for these two numbers.”
“This was a contingency plan in case surveys were cancelled,” said De Robertis. “They’d worked on these problems in the past and Saildrone did a great job. We launched the vehicles a week after we started the project. We processed the data in a similar way to the surveys because we want to make sure everything we do is the same. No changes in methodology.” According to De Robertis this is the first time drones have been used to gather data to inform fisheries management and set fishing quotas.