Right about now, millions of walleye pollock are gathering in the Shelikof Strait, near Kodiak Island. They mass there every year towards the end of winter to prepare for spawning.
And soon, scientists will follow them to do their annual winter trawl survey.
“It’s timed to be there and survey the pollock just prior to the peak of spawning,” said fisheries biologist Lauren Rogers, with the National Oceanic and Atmospheric Administration. “So that timing of when pollock are going to be migrating to the spawning grounds or away from the spawning grounds is then going to be particularly important for that survey.”
But the timing of pollock spawning is becoming more unreliable, as human-caused climate change warms the ocean. That means the scientific surveys that are used for fisheries management could become unreliable too.
Between 2017 and 2019, surveys done by the Alaska Fisheries Science Center across the Gulf of Alaska produced wildly different estimates of pollock biomass. Summer surveys across the feeding grounds showed near-record lows, while the winter survey in Shelikof Strait showed record highs.
To understand the mismatch, Rogers and her collaborators worked backwards using surveys of larval pollock.
“We have the offspring’s information and then we can go back to what the parents must have been doing in order to put them in the world when they were put in the world,” Rogers said.
Rogers’ previous research on pollock showed that spawning can occur earlier when ocean temperatures are warmer. And according to a new paper, published this month, earlier spawning times account for much of the discrepancy in the Shelikof Strait survey.
In 2017 and in 2019, peak pollock spawning happened more than 2 weeks earlier than the long-term average. That was likely shaped, in part, by marine heat waves that hit the Gulf around that time. The new paper offers a tool that can be used to account for changes in spawning timing when building stock assessments.
But Rogers says spawning time isn’t the only thing fisheries surveys need to pay attention to — warming oceans may be changing fish in all kinds of ways. Recent studies in the Bering Sea, for instance, showed that as sea ice melts, populations of pollack are moving further north, out of the typical survey areas.
But Rogers says fisheries managers can try to adjust for these changes.
“If we have an understanding of the links between changes in the climate and changes in spawn timing, or migration timing, or distribution shifts or growth rates, then we can start to use that information when we’re interpreting our biomass estimates or when we’re doing our stock assessments,” Rogers said.
It’s not just a simple matter of doing the Shelikof survey earlier or moving the Bering Sea surveys north. Rogers said there are logistical challenges that come with doing that, and climate-driven changes will be unpredictable from year to year.
“We need to be monitoring to track changes as they’re happening,” Rogers said. “Planning for a continual shift is not going to allow us to respond as proactively as we need to.”
So to keep up, she says fisheries managers need to consider the latest climate science. It’s the only way to follow the fish in a rapidly changing ocean.
