Ocean going robots estimate Alaska Pollock stocks

Data collected by Saildrones helps determine sustainable catch in coming season

Saildrones like this one from Alameda, California, were employed by NOAA Fisheries to collect data to determine the abundance of Pollock in the eastern Bering Sea. The annual estimate of abundance is normally conducted by human crews, but was ruled out for health and safety reasons because of the global COVID-19 pandemic. Photo courtesy of NOAA Fisheries

In the midst of a global pandemic that ruled out the usual survey of Pollock in fishing waters off of Alaska, ocean-going robots came north from California to determine the abundance of the versatile whitefish from the Bering Sea/Aleutian Islands and Gulf of Alaska.

Three Saildrones left Alameda, California, in mid-May, sailing 2,200 nautical miles to the Bering Sea, where from July 4 through Aug. 20 they surveyed an area typically covered by a research vessel to collect information to estimate Pollock abundance, plus oceanographic and atmospheric information for weather forecasting.

“We were impressed with how well the Saildrones performed,” said NOAA Fisheries scientist Alex DeRobertis of the Alaska Fisheries Science Center. “This was particularly helpful in a year when there are no other sources of survey information for Pollock. From a technical standpoint, things went as well as one could have hoped. The Saildrones and the acoustic equipment we use to count fish performed flawlessly while at sea for five months.”

DeRobertis acknowledged that the Saildrones can’t, however, do everything that ship-based surveys can do.

“For example,” he said “We can’t tell the species/size/age of the fish from the acoustic measurements alone. In the case of Pollock surveys, we rely on trawl sampling for this, which is not possible in the case of the Saildrones.”

NOAA Fisheries normally conducts two types of surveys for Pollock, bottom trawls and acoustic trawls. The Saildrone project was an attempt to mitigate the loss of survey information caused by the pandemic-induced cancellation of the acoustic-trawl surveys in the late spring. Researchers then used drones for the acoustic part of the survey and results of previous surveys to estimate total biomass (weight) of Pollock.

“We were not able to estimate the abundance at age of the population, which is an important piece of information in stock assessments,” DeRobertis said.

The drone-based work did provide useful information in a situation where it was not possible to do so by other means, he said. The surveys also collect other information such as subsurface environmental conditions like temperature, and samples of fish that can be analyzed directly to provide useful information.

Autonomous vehicles have made great technological advances in the last few years.

“I think that rather than thinking of them as replacement for ships, it makes sense to think of them as potential complements to ship-based surveys,” he said. “From my perspective, the challenge in terms of fisheries is how to use these new tools to provide useful information. We tried to do just that in this project.”

NOAA Fisheries received the data needed in mid-October, giving researchers there four weeks to analyze the information and determine whether it could be incorporated into the stock assessments used to estimate Pollock abundance in that area of the Bering Sea. The analysis was needed in time to inform scientific discussions in mid-November by the North Pacific Fishery Management Council.

This is when annual assessments of commercial fish stock abundance and trends (up or down) in the Bering Sea Aleutian Islands and Gulf of Alaska are completed. Resource managers rely on these assessments to develop fishery management measures for the upcoming fishing season.

Researchers then used models to estimate the additional uncertainty due to the reduced amount of sampling. They also applied a method to estimate and adjust for the proportion of fish near the seafloor that were missed by the Saildrone’s wider beam.

Pollock are the most common midwater fish species found throughout the Bering Sea. By weight Pollock comprised about 98 percent of the catch in previous NOAA Fisheries surveys.