Forecasting Sitka’s herring biomass is a thorough but imperfect science

Fisheries scientist Sherri Dressel reviews the data that shaped this year’s herring forecast in Sitka Sound. “It’s not a perfect world,” she told the 48 permit holders, “But Sitka has the best data set in the Pacific.” (Emily Russell/KCAW)

As seiners converge in Sitka for the annual sac roe herring fishery, the Alaska Department of Fish and Game is holding firm on its harvest target of 14,649 tons.

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This is despite having fallen short of that target in three of the last five years.

ADF&G managers held a preseason meeting with permit holders and processors to go over the data behind this year’s forecast.

Juneau-based biologists Kyle Hebert and Sherri Dressel walked their audience through the decades of data that have been compiled for the sac roe fishery.

Hebert talked about egg deposition and age distribution — two key components that go into the forecast model. ADF&G measured 79 nautical miles of spawn in Sitka Sound last year — in two distinct spawning events. Divers swam over 60 transects, measuring the width and density of spawn.

Department biologists also study the condition of the herring at the time of harvest. Hebert suggested that it was a little subjective — he called it “a gross metric” — but the herring in 2016 were in good shape.

“I think what we can take from this is that there are no obvious red flags that there’s a serious problem with food availability, nutrition, or something like that,” Hebert said.

But what about problems — or at least changes — in the environment that affect the herring population?

Sherri Dressel is a fisheries scientist and biometrician. She told the room that she basically counts backwards from the trillions of eggs deposited during the spawn to come up with a rough estimate of females, and from there an estimate of males, to begin the process of forecasting biomass.

“So our goal has been to at least over- and under-forecast, so at least it’s balanced,” Dressel said. “If we had it in a perfect world, we’d be spot on. But that’s not totally possible.”

Dressel said that, despite not always being on target, the department’s forecasts have tracked on a line close to actual abundance.

Over the past few years humpback whales have been overwintering in Sitka Sound, and it’s commonly assumed that they’re feeding on herring.

One permit holder asked if there was a correlation between water temperature, whale populations, and herring biomass.

“In 2012 the water temp’s cold, the biomass is up,” they said. “Could that be due to whales leaving the area?”

Dressel explained that a natural temperature cycle in the ocean called the Pacific Decadal Oscillation indeed correlated to herring abundance, with warmer water temperatures in the Gulf as high as 4-6 degrees correlated to fewer herring.

But Dressel said whales have been on the increase since hunting ended in the 1970s, and the model accounted for their predation, along with everything else out there that feeds on herring.

“In our assessment we don’t account for whales directly, but they’re included in natural mortality, so that when we don’t see fish come back, that includes everything that was eaten by whales and birds,” Dressel said.

The scientific presentation took the better part of an hour. Another permit holder appreciated the efforts of the department to manage the Sitka fishery, and asked if any other fishery was comparably studied.

Dressel said that Canadian biologists had strong expertise in modeling, but Sitka was probably the most comprehensively researched herring fishery anywhere.

“And part of that’s because we can get to the areas,” Dressel said. “Sitka’s the best because it’s closest to shore, it goes the longest, and we’ve got the most samples. We’ve probably got the best data set in Sitka of anywhere I know of in the Pacific.”

Besides receiving a refresher on the science behind the sac roe fishery, the fleet heard presentations from Alaska wildlife troopers on enforcement, and from the Coast Guard. Steve Ramp, with the Marine Safety Detachment cautioned the participants of the sometimes rough-and-tumble fishery that, due to a mechanical issue with their patrol boat, the Coast Guard would be monitoring activity from unmarked vessels. “You may not see them,” he said, “but they will be there.”