In recent years, halibut scientists have included ecosystem impacts in their assessments of the Pacific halibut stock. At the forefront – fish are behaving differently due to warming oceans.
Ian Stewart is lead scientist for the International Pacific Halibut Commission. He has referenced the massive warm blob from 2014 through 2017 and said remains of it appear to be hanging around.
“We’ve seen a continued presence of warm surface waters through the fall of 2018. Somewhat anomalous to what we’ve seen in previous years. Maybe not quite the magnitude of the previous blob, but definitely different from what would be the norm in the N. Pacific.”
Last year, Stewart says, the warm surface waters went deeper.
“Particularly of note, and relevant to halibut in region 4, which means halibut across the entire coast because much of the coastwide recruitment likely comes from region 4, is the fact that there was virtually no sea ice in the winter of 2018 in the Bering Sea. And that led to no cold pool – in the summer being a tongue of cold bottom water that extends southward, generally corresponding to the extent of ice cover in the winter time.”
The lack of that cold pool has caused big changes in the Bering Sea.
“It’s led to more than half the cod biomass being distributed in the northern Bering Sea north of the normal survey grid – and a shift as well, not quite as extreme, but also a northward shift in pollock distribution. We did see a shift as well in Pacific halibut on the order of about a 20 percent increase in density between 2017 and 2018 in the northern Bering Sea.”
Scientists also track Pacific Decadal Oscillations that show recurring patterns of ocean climate variability. Stewart says it’s used as an index of halibut productivity.
“A positive PDO tends to correspond to relatively warm and relatively productive conditions in the N. Pacific Ocean. On average, this tends to be correlated with the level of Pacific halibut recruitment, historically. We don’t understand a specific linkage. Many smart scientists have investigated this historically and found this correlation to have held up. In fact, we re-estimate this correlation each year within the stock assessment model to see if it’s still holding up and it seems to still hold up. We have seen a period, starting in 2014, of relatively positive values with 2018 moving back to almost a neutral value.”