Young Chinook Salmon at the Mouth of the Yukon: Where the Decline May Begin

Yukon River Chinook salmon were once the mainstay of subsistence communities and commercial fisheries along the whole river, with runs close to 300,000 adults a year. Since the late 1990s, far fewer have returned to spawn. Commercial and sport fishing have closed, subsistence fishing is tightly restricted, and many of the hundreds of fish camps that once lined the river have been abandoned, from the river mouth to the headwaters in Canada.

Why the first weeks at sea

Nobody knows what is driving the decline, but there is strong evidence that the early marine period, when young salmon move from fresh water to the sea, is critical. Yukon Chinook spend one or two winters in fresh water, then pass through the estuary to the sea. In their first summer there they must grow and store energy before their first winter at sea, and young salmon that are too small by the end of that summer are less likely to escape predators and survive. So how the fish use their habitat, how much good food they find and how many predators they meet may decide how many come back.

Timing matters too. Models have linked the date the ice breaks up at Dawson, Yukon Territory, to the peak of young fish leaving the river. If they reach the sea when prey is plentiful, they arrive bigger and healthier.

Sampling a shifting delta

Near its end the Yukon splits into three main mouths, south, middle and north, each with its own sloughs and channels. The river is miles wide in places but very shallow, full of shifting bars hidden in silty water, and fishermen from Emmonak and Alakanuk helped the scientists set up and sample three stations on each mouth. Local knowledge, the team says, has been critical.

Sampling stations: red on the south mouth, yellow the middle, blue the north; lines are the transects on the delta front. NOAA Fisheries.

The study began in 2014, led by the Alaska Fisheries Science Center with the Alaska Department of Fish and Game and the Yukon Delta Fisheries Development Association. The only earlier study of young Chinook in the lower river, of the south mouth alone, dated from 1986.

In the river, a net towed between two skiffs caught the fish, three times a week at each station from mid-May, just after the ice went out, to the end of July in 2014 and 2015, with water temperature, depth and flow recorded for each tow.

Beyond the mouths lies the platform, a huge flat under water averaging less than 2 meters (6 feet) deep and reaching up to 30 kilometers (18 miles) offshore. Young salmon must cross it, but it is too shallow to trawl. At its seaward edge, the delta front, it drops into deeper water where the river's plume mixes with the Bering Sea. From the 42-foot chartered fishing vessel Anchor Point, out of Nome, the team sampled five transects across the front at depths of 7, 10 and 14 meters, using a surface trawl for the top 2 meters and, at the deeper stations, a midwater trawl for 2 to 5 meters down.

Setting the CTD instrument from the chartered fishing vessel Anchor Point. NOAA Fisheries.

A CTD (conductivity, temperature, depth) instrument with a turbidity meter profiled the water at every mile, and the cruises ran in June, July and August of both years. Every fish and invertebrate caught was identified and counted, and the salmon were brought back to the lab to measure their condition and see what was in their stomachs.

A catch from the two-boat tow net. NOAA Fisheries.

What they found

Catches varied a lot between 2014, 2015 and 1986, and were generally higher, over a longer season, in 1986, when salmon were plentiful. In the south mouth, 2015 and 1986 both showed two peaks of young fish leaving, about two weeks apart; in 2014 the pattern was much weaker there but clearer in the other two mouths. The peaks fell at different times in different mouths, which may mean fish from different parts of the Yukon use different channels. Genetic samples from every young Chinook caught should tell.

Catch per 10-minute tow of young Chinook in the south mouth, by year and week. NOAA Fisheries.

Temperature may play a part. The river averaged about 1.3°C warmer in 2015 than in 2014 or 1986, warmed faster and stayed warm longer, reaching 18.1°C on June 26 and staying above 18°C until July 17. In both 1986 and 2015 the first peak left the south mouth when the water was close to 14°C, which came two weeks earlier in 2015, and those early 2015 fish were much smaller than fish at the same point in the other years. Warm water may have sent them to sea before they had time to feed and grow. Fish at the end of the season were also smaller in 2015. In 2014 the young salmon ate mostly insects washed into the river and larval aquatic insects.

Average water temperatures by year. NOAA Fisheries.

On the delta front, most young Chinook turned up at the northern stations, few off the south mouth, even though the south mouth's catches were high. The Earth's rotation deflects the river's outflow northward (the Coriolis effect), so the fish may follow that current toward Norton Sound for food or safety. Nearly all were in the top 3 meters, most where river and sea water were actively mixing, and most were caught in June. Since young Chinook appear in Bering Sea surveys in late August and early September, they may spend only a short time in the plume.

The nets brought in much else besides: young whitefish and ciscos, burbot, shellfish and Arctic lamprey in the river, and herring, rainbow smelt, saffron cod and sticklebacks on the delta front.

So far

Young Chinook were scarcer in 2014 and 2015 than in 1986, but their migration patterns were similar, and they differed between the three mouths. One idea held that the run peaked before the ice went out on the lower river; this research puts the peak after ice-out, perhaps set by temperature, food or other factors. In 2016 the team planned to sample the insects drifting in the river as well, to see what food is there and how nutritious it is.

Sources

• NOAA Fisheries, "Early Marine Ecology of Juvenile Chinook Salmon on the Yukon Delta"; rewritten in hubnx's own words. Map, charts and photographs: NOAA Fisheries.