Bristol Bay sockeye are getting smaller, scientists say this year’s bigger and older fish are an anomaly
This season, Bristol Bay sockeye are coming back bigger, and older. The average fish weighs over a pound more than last year’s, thanks to an extra year spent in the open ocean. But looking at the last few decades, sockeye have been getting smaller, and fishermen are noticing.
“Decades ago, it seemed like the five plus pound fish were the typical return,” said lifelong setnetter Nathan Hill in Naknek. “And I feel like in the last, oh, I would say five plus years, there's been a smaller size, (and so) people have fished smaller gear.”
Smaller sockeye mean changing fishing practices, and implications for escapement.
Tim Sands is the area management biologist with the Alaska Department of Fish and Game for the Nushagak and Togiak Districts. He says smaller sockeye can slip through nets, and that escapement can add up.
“2018 was a record run here in the Nushagak district,” Sands said. “We had 20 million “one-twos” come back to the Wood River, and seven and a half million of them escaped.”
He says these “one year in freshwater” and “two years in the ocean” fish are on the younger side of Bristol Bay spawners, and therefore smaller.
“I think that was a real eye opener for fishermen,” he said. “And a lot of them started fishing smaller gear.”
So just how much have sockeye shrunk?
Daniel Schindler is a fisheries biologist with the University of Washington’s Fisheries Research Institute, where researchers have been studying salmon in the Nushagak watershed for over 70 years. Part of that research is analyzing decades of data on sockeye size.
They published a study this winter that found since the 1960s, the average weight of a Bristol Bay sockeye has decreased by ten percent, with most of that decline concentrated just in the last two decades.
“The reason fish are coming back smaller as adults has very little to do with freshwater rearing,” Schindler said.
He says in a given year, sockeye size is largely determined by two factors, the first being the age of the fish.
“Fish either typically spend two years or three years in the ocean growing up, and the fish that's been three years are generally bigger than the fish that spend two years,” he said. “So in any given year, the size of fish swimming up the rivers is mostly determined by the age composition.”
The second factor in sockeye size is how fast the salmon are growing, or what’s called the “size-at-age” of a salmon. Schindler says this growth rate is decreasing. That means a four year old fish today is smaller than a four year old fish in the 1990s.
And smaller size has physiological implications for salmon. First, biologists say smaller mouths can limit what kinds of food are available for salmon to eat. And smaller bodies may have more trouble burying eggs while spawning.
Lee Borden is a management biologist for Bristol Bay sportfishing with the Alaska Department of Fish and Game. He says that for how successfully a fish can reproduce, size matters. The larger a female fish is, the more fecund, or productive, the fish is.
“Smaller fish are less fecund, so you know less eggs,” Borden said. “And it’s not at a steady rate. A fish that's twice the size of another fish doesn't have twice the eggs, it might have 10 times the eggs. And because it's an exponential curve of fecundity.
So why might we be seeing this decrease in size?
Fisheries biologist Daniel Schindler says a big contributor is increased competition.
“Bristol Bay runs have built to these levels of abundance that we simply could not have imagined 20 or 30 years ago,” he said. “There's so many fish now that they're competing very strongly with each other and that grows more slowly when we have years with big returns.”
And it’s not just other Bristol Bay sockeye that the fish compete with. Schindler says over the last 50 years, there has been a buildup of hatchery fish in the Pacific ocean.
“There's more biomass of salmon in the North Pacific now than there probably ever has been,” he said. “That's because of this buildup of hatchery pink salmon and chum salmon, and they compete with Bristol Bay Sockeye causing them to grow a bit slower.”
Essentially, bigger runs on top of more hatchery fish means a smaller slice of the pie for each Bristol Bay sockeye.
Schindler also points to warming ocean temperatures as a factor in shrinking sockeye size, especially warmer summers. He says increased temperatures seem to restrict sockeye growth rates in the ocean.
Katie Howard is an ocean fisheries scientist with the Alaska Department of Fish and Game, studying the marine portion of salmon’s lives. She says, this smaller size goes beyond just salmon.
“We have seen more than just salmon getting smaller,” she said. “We've seen other fish species getting smaller, marine fish species.”
She says warming waters contribute to that smaller “size -at-age” seen in Bristol Bay’s sockeye.
“When the ocean warms, what we would expect for fish is that they grow really fast as young fish, so they get bigger as juveniles. But then they mature earlier and smaller,” Howard said.
Over the last few decades, larger runs and a declining size-at-age have driven down the average body weight of Bristol Bay sockeye, but this year appears to be an anomaly.
Nathan Hill, set netting over in Naknek, says this year he’s catching bigger, more robust fish—fish that were the norm decades ago.
“This year, it seems like there's really healthy sized salmon coming back. So not many small ones,” he said. “It's looking more like what it typically looks like, you know further back in time.”
Fisheries biologists are seeing the same. Stacy Vega is an area research biologist for commercial fisheries with the Alaska Department of Fish and Game, and oversees the collection of sockeye age, length, and weight data across Bristol Bay, essential to fisheries managers monitoring the run.
Vega says this year the run is largely composed of older fish, which is unusual for Bristol Bay.
“It's just not common to see this out of whack proportion,” she said. “Certainly is a little different than what we were expecting.”
And, older fish mean bigger fish, according to Vega. An extra year at sea helps sockeye pack on the pounds. But she says overall, the trend of declining fish size continues.
“The history and the trend that we've been seeing is smaller size-at-age. And that's what we expected to see. Seeing bigger fish is nice, it's good they're bigger than they were. But certainly the trend is, is smaller size-at-age,” she said.
Vega says one year of data doesn’t do much to change a four-decade long trend.
“This year's data point might be a little blip, little upward tick in the dataset,” she says. “But as far as how it's been going, we definitely have seen a declining size-at-age.”
The trade-off between run size and body weight is especially noteworthy within a fishery where biologists track salmon by quantity, and processors track fish by weight.
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