Seals and sea lions may be slowing salmon recovery, hurting orcas
Increased consumption of Chinook salmon by seals and sea lions in the Salish Sea “could be masking the success of coastwide salmon recovery efforts,” according to a new study published in the journal Scientific Reports. Endangered resident orcas are said to be declining in part due to a lack of available Chinook, the orcas' preferred prey.
Puget Sound’s endangered killer whales are waiting at the end of a long food line for a meal of Chinook salmon — basically the only food they really want to eat.
Ahead of them in the line are hundreds of salmon-craving killer whales in Alaska and British Columbia. Even farther ahead are thousands of seals and sea lions that eat young Chinook before the fish have a chance to grow to a suitable size for orcas.
The major problem facing the Southern Resident orcas of Puget Sound may not be the total number of Chinook salmon being produced in streams and hatcheries throughout the region, according to a new study in the journal Scientific Reports. Instead, the study points to the number of marine mammals that eat Chinook before the Southern Residents get their turn.
A shortage of Chinook salmon has been identified as a limiting factor in the stability of the Southern Resident orca population, which has fluctuated through the years but now stands at 76 animals — essentially the same number observed 30 years ago.
From 1975 to 2015, the number of Chinook salmon (mostly small, juveniles) consumed each year by West Coast seals, sea lions and killer whales has increased more than six-fold.
“Why aren’t the Southern Residents getting enough food?” asks Oregon State University scientist Brandon Chasco, lead author of the new study. He answers the question this way: “There are a lot of predators that eat before the Southern Residents do.”
The Southern Resident killer whales range mainly in and around the Salish Sea — which includes Puget Sound and the Strait of Georgia in British Columbia. In the Pacific Ocean, they are known to travel as far north as Canada’s Vancouver Island and south to the Columbia River, sometimes venturing as far south as California. Northern Resident and Alaskan resident killer whales occupy habitat that is mostly to the north of the Southern Residents' range.
The new study follows similar findings in The Canadian Journal of Fisheries and Aquatic Sciences and generates provocative implications for managing both Chinook salmon and the animals that feed on them. Both state and federal officials are examining the issue of marine mammal predation closer than ever before.
The Puget Sound Partnership, the state agency designated to coordinate ecosystem restoration in Puget Sound, is calling for studies and pilot projects to clarify the predation issue and identify ways to reduce overall consumption of Puget Sound Chinook, which are listed as threatened under the Endangered Species Act.
Harbor seals, protected by the Marine Mammal Protection Act of 1972, have nearly doubled in number in inland waterways and along the West Coast over the past 40 years — going from 210,000 to 355,000. Adult California sea lions grew from 5,900 animals in 1975 to 47,000 in 2015. Steller sea lions increased as well, from 74,400 to 78,500.
Except for the 76 Southern Residents, killer whales have been doing well in northern British Columbia, Southeast Alaska and the Gulf of Alaska. The total West Coast population of fish-eating resident orcas has more than doubled, from an estimated 292 to 644 over the 40-year time period.
A gauntlet of marine mammals
For Chinook salmon to reach the feeding grounds of Southern Residents, young fish must swim a gauntlet of marine mammals that begins at their doorstep in the estuaries and extends into the North Pacific. If they make it that far, the fish must then survive the return trip as full-size adults before they become a meal for the Southern Residents.
From 1975 to 2015, the number of Chinook salmon (mostly small, juveniles) consumed each year by West Coast seals, sea lions and killer whales has increased more than six-fold — from 5 million to 31.5 million fish — according to the new report. By weight, the total consumption went from 6,700 to 16,800 tons of Chinook salmon.
That roughly 10,000-ton increase in consumption by marine mammals basically negates a cutback in sport and commercial fishing for Chinook salmon over that same 40-year period, the report says. The reduction in fishing during this time was from 3.6 million to 2.1 million Chinook salmon caught per year, or an estimated 7.500 tons less in 2015 than in 1975.
That increased consumption by marine mammals “could be masking the success of coastwide salmon recovery efforts,” according to the report. In other words, the millions of dollars spent on salmon recovery may be paying off with increased salmon productivity, but those increases may not be noticed because of the increasing predation.
Estimates of Chinook consumption in the new study are derived from the number of marine mammals of various kinds, their individual energy needs and the proportion of Chinook of various sizes in their diet. This approach, called a bioenergetics model, depends on numerous assumptions when firm data are not available.
The study did not consider any changes in Chinook predation by fish, birds and other animals. Assuming that consumption by those other predators has stayed the same or declined, the total biomass of Chinook appears to have increased over the past 40 years with marine mammals being the primary beneficiaries.
Impacts on young salmon
According to the new report, the total number of young migratory salmon, known as smolts, increased coastwide from the 1970s to the 1990s while staying relatively stable since then. Accounting for both hatchery and wild chinook, about 406 million juveniles were produced in 2015 — nearly twice the 225 million estimated for 1975.
Much of that growth in smolt production came from hatcheries during the 1970s and 80s. Later declines in hatchery production were offset by increases in wild runs, including those coming from the Columbia River system, according to the report.
The Salish Sea stands out as a growing hot spot for Chinook predation by seals and sea lions. Of an estimated 27.4 million Chinook consumed by harbor seals along the west coast of North America in 2015, 23.2 million were smolts taken in the Salish Sea.
Harbor seals grew from a Salish Sea population of 8,600 in 1975 to 77,800 in 2015, and their diet includes a greater percentage of Chinook smolts than seals in other areas. Consequently, harbor seals in the Salish Sea consume about 86 percent of all the Chinook eaten as smolts along the West Coast.
Killer whales, however, remain the largest consumers of Chinook salmon biomass on the West Coast. They take about 12,000 of the 16,200 tons of Chinook consumed by all the marine mammals.
All populations of killer whales have faced a decline in the availability of Chinook salmon, but none are in worse shape than the Southern Residents, which depend largely on Chinook stocks originating from the Salish Sea, according to the report.
North versus south
The Southern Resident killer whales feed primarily on Chinook who spawn in Salish Sea rivers, while whales from other areas hunt for Chinook from a wider geographic range. Since the Chinook from many areas along the West Coast migrate to the north while growing to adult size, the fish-eating orcas that live in the north appear to have a competitive advantage over the Southern Residents, who must wait for the fish to return to the south.
Whether humans can or should adjust predator populations to help the Southern Residents or to increase the number of spawning salmon is a question likely to be debated increasingly over the coming months.
Southern Residents were listed as endangered in 2005. A 2008 recovery plan developed by the National Marine Fisheries Service listed prey availability as a major threat, along with toxic chemicals, noise, disturbance from vessels and other concerns. Food competition with other marine mammals was briefly mentioned as a potential issue, but no actions were proposed.
Addressing the impacts
Actions to address marine mammals was not part of the original recovery plan for Puget Sound Chinook salmon, which were listed as threatened in 1999. Now the Puget Sound Partnership is taking steps to understand how marine mammal predation may be impeding Chinook recovery and what might be done about it. The next Action Agenda, to be adopted in 2018, is proposed with studies and pilot projects to be funded over the next four years.
One study would consider possible ways to control the seal and sea lion population, what overall effects those actions would have, and what changes in regulations would be needed to implement them. One major question is whether a reduction in smolt predation in Puget Sound would actually result in more Chinook coming back or if other predators would consume those fish along the way.
Under the Marine Mammal Protection Act, control of marine mammal populations through lethal or nonlethal means is allowed in limited ways, but only after extensive review of the impacts, according to Lynne Barre, who manages the Seattle branch of NOAA’s Protected Resources Division.
California sea lions were removed from the Columbia River near Bonneville Dam, but only after problem animals were individually identified and when other means of control — such as hazing — were proven not to work on those particular animals.
Another study proposed by the Puget Sound Partnership is to better understand the food web and predation by birds, fish and other animals. Indirect actions for addressing predation on Chinook could include reducing haul-out areas where harbor seals congregate, changing the timing of hatchery releases and increasing the numbers of other small fish to provide an alternative food supply.
“We have a lot of local and regional management prescriptions for restoring salmon,” Chasco told Salish Sea Currents, “but you need to remember that not only are salmon highly migratory but so are their predators. The impacts (of management actions) may not be where you expect; they may occur thousands of miles away.”
Authors contributing to the article in Scientific Reports: Brandon Chasco, contractor to NOAA’s Northwest Fisheries Science Center and a graduate student at Oregon State University; Isaac C. Kaplan, Dawn P. Norem, Michael J. Ford, M. Bradley Hanson, Andrew O. Shelton, Eric J. Ward, Kristin N. Marshall and Brian Burke, all with the Northwest Fisheries Science Center; Austen C. Thomas, Smith-Root Research Division; Alejandro Acevedo-Gutiérrez, Western Washington University; Jonathan Scordino, Makah Fisheries Management; Steve Jeffries, Washington Department of Fish and Wildlife; and Craig Matkin, North Gulf Oceanic Society.