Uneven status of Pacific halibut revealed by annual data
Following the trend of the past several years, overall Pacific halibut biomass seems to be down again.
The most recent stock assessment presented to the International Pacific Halibut Commission for its interim meeting on Nov. 25-26 shows a coastwide decline in spawning biomass, though that decline isn’t even across all areas.
That’s a continuation of a trend seen in stock assessments since 2015. Particularly, surveys have indicated lower numbers of halibut in the central Gulf of Alaska.
According to the 2019 stock assessment, biologists estimate the spawning biomass at 194 million pounds. It’s not down by much overall, but the impact to regulatory areas isn’t evenly spread; the central Gulf of Alaska, or Area 3A, has been declining fairly steadily since 2004, while Areas 2 and 4 — from British Columbia southward and the Bering Sea/Aleutian Islands, respectively — have seen increases in the same time period.
“What you will see here shortly … is that we have mixed trends coastwide. However, (commercial catches per unit of effort are) relatively flat at the coastwide level, with some relatively brighter spots and some relatively not-so-good spots across the coast,” said Ian Stewart, the lead scientist on the IPHC’s stock assessment. “…We’re looking at a period of relatively low productivity for the Pacific halibut stock over the next three years.”
All signs are pointing, as they have before, to lower fishing yields in order to maintain the target level of intensity on the stock. Overall Pacific halibut landings increased in 2019, coastwide, by a little more than 1 million pounds. That increase is in commercial, with reported mortality for subsistence and recreational fishing flat, according to figures Stewart presented to the IPHC.
Each year, the IPHC surveys halibut in the management areas to gather data for a stock assessment that will inform the fishing limits set by the IPHC in February, prior to the next season’s opening.
This year, the biologists also had new data to work with for the assessment to gather more information about the stock: sex. Female halibut are bigger, and have long been estimated to outweigh male fish in the commercial catch by weight, but with definitive data on sex distribution in commercial catches, biologists were able to establish exactly what proportion of the catch was male or female.
The sex ratio information improves the IPHC’s understanding of the stock dynamics significantly, Stewart said.
Coastwide, catches are coming in at 82 percent female on average by total number of fish. That’s much higher than they expected, he said.
“We’ve always known that the commercial catch would be dominated by female by weight, because female Pacific halibut are much larger than males, but in terms of having 82 percent by number, that is quite a bit higher than we would have expected,” he said.
In some areas, it’s higher. Area 4, which covers the Bering Sea and Aleutian Islands, the catch was 92 percent female. Areas C, D and E, the Central Bering Sea, were 97 percent female.
“(The catch is) almost completely females in the Bering Sea fishery,” Stewart said.
Pacific halibut are broadcast spawners, meaning the females carry the eggs and lay them into the water column, where they are fertilized by males. Biologists don’t think it takes that many males to sufficiently breed to keep up the stock. However, there isn’t a good tool for fishermen to exclusively target male halibut at present, Stewart said.
In addition to having fewer halibut, their size at age has been declining as well. In the early 1990s, the average halibut weighed more than 30 pounds; since 2010-11, the average weight has been in the mid-20s.
However, there may be some promise of better numbers down the road. The surveys track halibut age classes as well. To date, the cohort of 1987 — the fish born that year — have been one of the strongest contributors to catches across the coast. In more recent years, the 2005 cohort has dominated catches, and because halibut are multi-year fish, they can be represented for many years as the fish age.
While some of the other cohorts have been weaker, scientists have been tracking the 2011 and 2012 age classes, which are now starting to show up in catches. Because they’ve been younger, they haven’t been contributing as much to the overall catches so far, but in the future, they may show up more.
The cohorts aren’t as strong as the 2005 or 1999 age classes, but it is good news, Stewart said.
One of the factors that biologists think affects recruitment among Pacific halibut is an environmental trend called Pacific Decadal Oscillation, or PDO. PDO describes an environmental phenomenon in the North Pacific similar to the El Nino Southern Oscillation which can last years and describes an oscillation of sea surface temperature and pressure.
When temperatures near the coast are higher and cooler in the interior, accompanied by below-average pressure, fishery biologists have noted that Pacific halibut tend to have better recruitment. The opposite is true when the temperatures are lower near the coast and higher near the interior, accompanied by above-average sea level pressures, recruitment tends to be lower.
From 2007-13, the PDO value was described as negative, correlated with lower recruitment. Since 2014, the PDO has had a positive value, which may be mean better recruitment, but scientists won’t know for several years yet, Stewart said.
The anomalously warm temperatures in the Bering Sea for the last two years may also play a part in Pacific halibut numbers in the future. For the last two years, scientists have noted extremely low sea ice cover in the Bering Sea, accompanied by much warmer sea surface temperatures than normal.
This summer, residents and the National Marine Fisheries Service noted unusual sea bird and marine mammal die-offs, potentially correlated with ecosystem changes. There have already been some changes to the Pacific cod distribution, and scientists noted a “modest increase” in the density of Pacific halibut in the northern Bering Sea this summer, Stewart said.
It’s hard to definitively say how the warmer temperatures and lack of sea ice will affect Pacific halibut, but scientists have their eyes on the Bering Sea, he said.
“We don’t know if (the conditions) are bad yet, but they’re certainly different,” he said.
Elizabeth Earl can be reached at [email protected].