Curious by Nature: Do hatchery fish hamper wild king and coho?

“I have heard that the huge releases of hatchery pink and chum salmon are seriously reducing food for king and coho salmon. It would seem that the lower numbers of the higher value species would be a priority.”

 

- Phillip Gray

Record-low king salmon returns on the Taku River have readers rightly looking for answers. Spend any time around fishermen and you’ll hear plenty, with concerns about the state of Alaska’s hatchery programs a recurring theme.

So how do hatchery fish interact with wild king and coho salmon? Do they compete for food? With the Board of Fisheries meeting this week and next in Sitka to address king salmon conservation, among other issues, it’s timely to answer Gray’s question.

First some background on Alaska’s salmon hatcheries:

The state began its hatchery program, or its “modern salmon enhancement program” in the early 1970s, when state harvest levels plummeted to a record low. The idea was to supplement stocks mostly for the benefit of commercial fishermen.

According to the 2016 annual hatcheries report, 28 salmon hatcheries operate in Alaska. Most (24) of these facilities are private nonprofit corporations funded from a portion of the hatchery’s salmon harvest.

Two additional sport fish hatcheries are operated by the state, one by the National Oceanic and Atmospheric Association and one by the Metlakatla Indian Association. About 27 million adult salmon returned to these hatcheries in 2016, the lowest return since 1992.

The state marks nearly all hatchery salmon so they can tell them apart from wild stocks. This allows them to assess the strength of both types of salmon independently. Pink salmon accounted for 13 million of returning adult hatchery salmon in Alaska in 2016. Eleven million were chum, 2 million were sockeye, 800,000 coho and 72,000 Chinook.

Broadly speaking, state research seems to show that Alaska’s wild salmon stocks haven’t been adversely affected by hatchery salmon, according to ADF&G Aquaculture Section Chief Sam Rabung.

Rabung said that hatchery release numbers have remained stable for the last three decades, during which period catch rates have remained high. Pink and chum salmon fry are a prey species for Chinook and coho salmon, he added, so it’s worth noting that, at least in the early stages of their life cycles, hatchery pink and chum salmon are the prey for king and coho.

Researchers believe that problems with Taku River Chinook may be occurring in the early stages of their ocean lives, the Empire has learned in its reporting on Taku Chinook conservation. So if hatchery pink and chum are outcompeting Chinook and coho for food, as Gray worries, it might be happening during the early part of their lives.

NOAA salmon biologist Jim Murphy studies juvenile salmon and their diets in northern Southeast. He shared some of his research, compiled over the last 20 years in nearshore (inside) waters, which shows certain overlaps in salmon diets.

Murphy attached a handy slide of pie charts to illustrate the differences (check C4 for the image.) Similarities are strongest, at first glance, between coho and Chinook diets. Both rely heavily on fish, though juvenile coho are markedly different from king salmon in that almost half their diet is made up of decapods, an order of crustaceans whose name literally translates as “ten-footed.”

“Dietary overlap is greatest for juvenile pink, chum, and sockeye; however, sockeye tend to rely more on Euphausiids (krill), and chum salmon predominately feed on Oikopleura (a gelatinous species of zooplankton). Chinook salmon predominately feed on fish; therefore there is little overlap in the diet of Chinook and chum salmon,” Murphy said.

These feeding behaviors continue after salmon leave inside waters and head to the Gulf of Alaska and the Bering Sea, Murphy said. But there, other species must be factored in.

“The role of salmon stocks from other regions and other pelagic nekton (near-surface active swimmers), such as squid, need to be considered in the trophic (feeding) interactions of salmon in offshore habitats,” Murphy said.

So it seems chum do not compete significantly with Chinook or coho for food, at least in the vulnerable early stages of their marine life. But what about pink salmon? Could pink salmon releases be blamed for lessening the supply of food for Chinook and coho?

At least for nearshore waters around Juneau, we can say that hatchery pink salmon probably aren’t dipping into the Chinook picnic basket, so to speak, as the local hatchery, Douglas Island Pink and Chum, hasn’t released pink salmon for at least the last 15 years, said operations manager Brock Meredith. (DIPAC decided to keep the “Pink And” in the acronym because, otherwise, they’d have a pretty embarrassing acronym).

But there is some interesting research that shows how pink salmon might affect coho salmon, according to ADF&G biologist Leon Shaul. Pink salmon spawn in distinct even-odd year groups: those that spawn in odd-numbered years are genetically unrelated to those that spawn in even years.

Historically, many more pink salmon spawn in odd years than in even years. This sets up a natural experiment because biologists can look at how things change when there’s a large number of spawning pink salmon versus years when there’s a much smaller number.

Basically, Shaul said that there are some interesting interactions between pink salmon, squid and coho salmon in the ocean. Large numbers of pink salmon might result in smaller coho salmon as pink salmon put a lot of pressure on squid prey.

Shaul and his colleagues actually developed a predictive model for coho salmon size based on pink salmon abundance. The model has been fairly successful in recent years, even through the recent warm water “blob” that skewed a lot of fisheries data.

“Needless to say, it took some time, extensive review of marine salmon literature and a lot of head scratching to fit together pieces of this puzzle,” Shaul said.

Their model predicted a small size of cohos and Chinooks returning in 2018, which they believe might be linked to the two largest recorded North American pink salmon returns in 2013 and 2015. He called this a “lagged relationship”: large runs of pink salmon (again, two-year fish) in 2013 and 2015 might have meant less prey for growing coho (one to two years at sea) and Chinook (two to seven years at sea) that we are just now seeing because the affected salmon are just now returning to spawn and be counted.

Hatchery facilities in Southeast, Shaul noted, account for less than 10 percent of the statewide releases of pink salmon.

So, long story short: local chum salmon releases likely are having only a minimal competitive effect on Chinook and coho compared with pink salmon. While we don’t release many hatchery pink salmon in Southeast, large releases across the state could be having a negative effect on coho.

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