Study looks at salmon ocean entry

Columbia River basin salmon and steelhead, which are from the same species, but of differing stocks, enter the river’s estuary and the ocean at different times and at different sizes, a variation that contributes to the resilience of the fish.

Because these fish begin to grow rapidly when entering the ocean, the timing differences between early-arriving fish and those fish that arrive later give the early migrating fish a growth advantage, according to a recent study.

That timing advantage allows early-migrating fish to reach larger sizes sooner than the late-migrating fish, said Laurie Weitcamp, research fisheries biologist at NOAA Fisheries Northwest Fisheries Science Center, Newport Field Station.

“These timing differences likely also influence marine survival because larger fish generally survive better than smaller fish,” she said, although investigating that issue was beyond the scope of the study.

“Our results serve as a good reminder that salmon have complex life histories and spend most of their life outside of their natal basin,” she said. “For example, although many hatchery programs are segregated from wild stocks, hatchery and wild fish from many stocks end up together in the estuary, limiting the effectiveness of the segregation.”

The authors saw this large overlap as hatchery and wild fish arrived in the lower Columbia River estuary and occupied the estuary at the same time, providing opportunities for behavioral interactions, including competition, if resources are limited.

“Wild fish were consistently smaller than hatchery fish, giving wild fish a disadvantage if size matters in behavioral interactions,” Weitcamp said. “Our results also emphasize how hatchery-dominated populations of chinook and steelhead in the Columbia really are; the vast majority of the fish we caught in both the estuary and ocean were hatchery fish.”

This is a reminder that the different phases of the salmon life cycle are intertwined, she said, so things occurring at one stage may influence future stages. “For example, the size and timing of different stocks of salmon we observed largely reflect the size and timing of hatchery release, so hatcheries effectively influence early ocean growth.”

The Columbia River basin is home to five evolutionary significant units (ESU) of chinook salmon listed under the federal Endangered Species Act and five ESUs of steelhead, also listed. Some 140 million hatchery juvenile salmon are released into the Columbia River basin each year, the report says, citing the Fish Passage Center at http://www.fpc.org.

The study, “Stock-specific Size and Timing at Ocean Entry of Columbia River Juvenile Chinook Salmon and Steelhead: Implications for Early Ocean Growth,” was published online http://www.tandfonline.com/doi/full/10.1080/19425120.2015.1047476#.Vusbp_krLIU, in the journal Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science.

Weitcamp’s co-authors are David Teel and Donald Van Doornik, both geneticists at NOAA Fisheries NW Fisheries Science Center, Manchester Field Station; Martin Liermann, statistician, NOAA Fisheries NW Fisheries Science Center, Seattle; and Susan Hinton and Paul Bentley, both research fisheries biologist at NOAA Fisheries NW Fisheries Science Center, Point Adams Field Station.

The study looked at analysis of both wild and hatchery juvenile chinook and steelhead in the Columbia River basin by NOAA Fisheries NW Fisheries Science Center and by Oregon State University over the years 2007 – 2011. It included five different stocks of steelhead and 11 of chinook salmon.

Juvenile chinook enter marine waters as subyearling (age 0) or as yearling (age 1) smolts. In the Columbia River spring chinook typically migrate as yearling smolts, the fall run is subyearlings smolts and the summer run of chinook can have either, the study says. Hatcheries can and do release fall chinook smolts as yearlings.

Hatchery steelhead are typically released as yearling smolts. However, wild steelhead will grow in freshwater from one to three years prior to migrating.

The authors found a considerable variation in the size and timing of juvenile salmon entering the river’s estuary. Lower river steelhead arrived several days earlier than other stocks and were more than 10 millimeters (0.4 inches) shorter than the upriver stocks. Hatchery steelhead were 25 mm (about 1 inch) longer.

Peak arrival in the estuary of Willamette River spring yearling chinook was about 30 days earlier than the upper river summer and fall stocks. Snake River spring and mid- and upper-Columbia spring yearling chinook were 10 – 20 mm (0.4 inches to 0.8 inches) shorter than other stocks. Hatchery fish were about 11 mm (0.43 inches) longer than wild fish.

The earliest arriving subyearling chinook was from the Spring Creek Hatchery, arriving some 89 days earlier than the latest subyearlings, the west of Cascade stocks. Peak arrival in the estuary for hatchery fish averaged 22 days earlier than wild stocks. Generally, stocks of smaller fish arrived in the estuary sooner.

"It has long been recognized that different stocks of salmon – including those originating from the same river – often differ in life history traits that are easily measured while the fish are in freshwater,” the study says. “Our study adds to an increasing body of literature demonstrating that stock-specific variation in marine life history traits rivals that of freshwater traits.”

That variation begins with stock-specific size and timing at ocean entry and continues through the first summer or two in the ocean.

Growth rates in the ocean, as measured in May, were highest for the earliest yearling chinook stocks migrating into the ocean – Willamette River and west Cascade spring chinook – with the growth rate as high as 1.6 mm per day (0.6 inches) and about 3.6 percent of body weight per day. The growth rate for later migrating stocks was about 1.1 mm per day (about 2.9 percent per day). However, by June, growth rates had equalized among early-migrating and later-migrating stocks.

Growth rates for subyearling chinook generally followed the same pattern. The earlier Spring Creek group had higher growth rates (0.1 mm per day, 2.6 percent body weight) than did the Snake River fall chinook. By September, growth rates among the subyearling groups were similar (0.8 – 1.1 mm, 2.6 percent), despite the fact the ocean entry date varied as much as 90 days.

The size and timing differences in initial growth opportunities “likely interact with a suite of other factors,” the study says. Those factors include prey availability and predator abundance.

“(T)his stock-specific variation likely contributes to the overall resilience of salmon populations, allowing species to persist despite unpredictable environmental variation that may favor some strategies over others in given time periods,” the study says.

“On a positive note,” Weitcamp concluded, “our results show that despite all we've done to salmon in the Columbia over the last 1 1/2 centuries, there is still considerable life history diversity among salmon stocks, as shown by variation in timing. This variation is essential for the resilience of salmon in the future.”