Ecosystem study unlocks the mystery of black cod survival

A juvenile black cod is tagged and released back into the ocean. (NOAA photo)

Over the past couple of decades black cod — or sablefish — has become one of Southeast Alaska’s most commercially-important species. Longliners target them in deep waters off the continental shelf, during the same season as halibut.

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Although stocks are strong, biologists don’t fully grasp black cod population ecology. A research partnership in Sitka hopes to change that.

Just about all commercially-utilized species in Alaska are studied intensively, and black cod are no different. There are fisheries surveys that go out and count and measure fish, and determine their age. And there are landings — tons of black cod sampled by state biologists at the dock.

But in one very important way, black cod are different from, say, salmon. Jamal Moss is a research fisheries biologist with NOAA’s Auke Bay Laboratories.

“Black cod abundance doesn’t seem to be related to the number of spawning adults out there,” Moss said.

So what makes it a good year for black cod — or, in Moss’s words “how do the stars align” to increase the chances that young black cod will grow to maturity?

“All signs point to that happening during their first year of life in the ocean,” Moss said.

Moss is back in Sitka where he’s teamed up with the Sitka Sound Science Center to survey the ecosystem that produces black cod. It’s called the “Gulf of Alaska Assessment.”

“Basically we go out there and measure the physical properties of the ocean. Temperature. We also look at phytoplankton — the small plants that live in the ocean — as well as zooplankton, larval marine fish, juvenile marine fish that are not larvae anymore, but are free swimming, and everything else we catch,” Moss said. “And that could range from jellyfish all the way up the line to salmon sharks and most recently, Pacific sunfish.”

Sunfish? More on that in a moment. Moss believes that the missing piece to understanding black cod survival is somewhere in the ocean environment — especially the kind of food, and the quality, that the juvenile fish need to survive.

As school children — school children in Alaska anyway — we’re taught all about the life cycle of salmon. And black cod, which are bottom dwellers, seem remote and mysterious by comparison. But it turns out that black cod have a fascinating beginning.

“After they hatch the larvae rise to the surface and they actually spend most of their first year in shallow waters — out in the ocean, but at shallow depths, let’s say the top three or four fathoms — and feed on plankton and other marine fish and critters,” Moss said. “And then they move closer to shore and typically rear in near-shore habitats before moving out into deeper water.”

In Sitka, one of these black cod nurseries is St. John the Baptist Bay, near Salisbury Sound.

The objective of the assessment — and its companion tagging study — is better management of an important commercial species, but Moss said the “bonus” is a deeper knowledge of the changing ocean environment, and how it affects all species.

And the ocean is changing. Beginning in 2014 oceanographers detected a massive area of the Pacific Ocean that remained at higher-than-normal temperatures, and is just now dissipating. Nicknamed “The Blob,” the phenomenon was created by a rare combination of ocean conditions, rather than by climate change. Nevertheless, Moss’s surveys during The Blob produced an atypical data set.

“We had pomfret, which are a pelagic fish that are typically offshore. We saw those fish come inshore,” Moss said. “They were eating a lot of the juvenile rockfish — Pacific Ocean perch in particular, in ‘14 and ‘15. We also saw Pacific sunfish — 800-pound fish that we were bringing on deck at times in our trawls. They don’t eat very high in the food chain — they mostly eat jellies and other things — but that was very interesting. And we saw more blue sharks come up into the waters.”

All of which can affect the growth and survival of black cod. Moss said we need to understand what tradeoffs are happening in the ecosystem, and if they favor traditional commercial species, or other fish “that are important ecologically, but maybe not commercially.”