Study looks at juvenile salmon mortality in Columbia's plume

By The Columbia Basin Bulletin

As juvenile salmon and steelhead enter the ocean, the common murre and sooty shearwater, offshore avian predators that feed on forage fish such as salmon, throw up a “predator gauntlet” while the fish are still in the Columbia River plume, according to a recent study.

Plume size makes a difference with larger plumes of brackish water in May and June each year that can spread out north and south from the river’s mouth hundreds of kilometers and reach a volume of as much as 8,738 square kilometers (3,374 square miles), as occurred in May 2011, providing a less deadly transit to the north for chinook and coho salmon than a smaller plume that concentrates both the seabirds and the juvenile fish.

By the time the juveniles have left the Columba River, they already have run an avian gauntlet along their freshwater migration route by Caspian terns and double-crested cormorants, but the number of seabirds that feed on the juveniles as they enter the ocean is far greater, the study says.

“Given that there is at least an order of magnitude more murres (about 131,000) and shearwaters (about 2.2 million) than terns (12,500) and cormorants (27,000), and that murres and shearwaters consume salmon in similar proportions to terns and cormorants, juvenile salmon mortality by seabirds is certainly higher than currently estimated,” the study says.

The study found that when the surface area of the Columbia River plume is large, as it is in May and June during high runoff years, the density of the sea birds in the plume is lower than in years when the plume is smaller.

“When plume surface area is small, seabirds concentrate in the plume,” said researcher Elizabeth Phillips: PhD candidate, School of Aquatic and Fishery Sciences, University of Washington. “As freshwater flow increases plume surface area, densities of seabirds in the plume decline. Spatial overlap between foraging seabirds and their juvenile salmon prey (including coho and chinook salmon) was highest when plume surface area was smallest.”

As for the two seabirds studied, she said that “murres aggregated in lower-salinity plume waters near the river mouth, compared to shearwaters that consistently aggregated near Willapa Bay and Grays Harbor (to the north of the Columbia River mouth, but still in the plume), where we found forage fish and yearling coho and chinook salmon abundances were highest.”

The study concludes that predation pressure on the juveniles is greatest early after their entry into the ocean and that predation is related to variations in the plume’s size. That means, Phillips said, that seabird predation on the juveniles may be highest when the river’s discharge is the lowest and near the northern edge of the river plume.

“Predator-prey interactions influenced by a dynamic river plume” was published online in the Canadian Journal of Fisheries and Aquatic Sciences.

Phillips co-authors are John Horne, professor, School of Aquatic and Fishery Sciences, University of Washington, and Jeannette Zamon: research fisheries biologist, NOAA Fisheries, Northwest Fisheries Science Center.

Although other studies have shown that the size of Columbia River flows affect the size of the plume, this study shows that the plume’s size and shape also affects the distribution of both predators, like murres and shearwaters, and prey, the forage fish that includes juvenile salmon. That, Phillips said, provides “a possible link to salmon mortality during early marine residence.”

Fisheries managers interested in improving juvenile salmon survival during early marine residence could use river discharge as a potential indicator of predator densities that may impact salmonid survival, she said.

Abundances of forage fish, which are alternate prey for seabirds, are also influenced by river discharge and therefore may also be an important component of the ecosystem to monitor, she added.

Other forage fish found in the plume are northern anchovy, Pacific herring, Pacific sardine and smelt. Yearling and sub-yearling salmon are considered a forage fish as they enter the ocean because their length and appearance are similar to other forage species, the study says.

Of the 17,357 seabirds counted during the three year study in May and June of 2010 through 2012, some 51 percent of all sitting or feeding birds were sooty shearwaters, a bird that nests in New Zealand, and 31 percent were common murres, which nest nearby. Murres foraged within 20 km (about 20 miles) of the plume’s center, while on average shearwaters can forage as far as 100 km north (62 miles).

The size and location of the plume shifts from month to month and year to year. In May 2010, for example, the plume was oriented to the north of the river’s mouth, which is typical of downwelling conditions, the study says. In May 2012, the plume was bidirectional (north and south), typical of when winds shift from a downwelling to an upwelling-favorable condition in June. In May 2011, when the plume’s volume was at its largest of the three years of the study, the plume was oriented to the south and offshore.

Both seabird species consume a wide range of fish species, including juvenile salmon, Phillips concluded.

“To fully understand how seabirds are impacting juvenile salmon survival during early marine residence, and the role of plume dynamics, we need to know what fish these salmon predators are actually eating while they are foraging near the Columbia River plume,” she said. “This approach will allow researchers to better understand what, if any, effects these seabird species have on juvenile salmon survival, and how abundances and distributions of forage fish affect consumption of juvenile salmon.”