(CN) — The endangered Southern Resident killer whales in the waters near Washington and British Columbia have stalled in their population recovery, and, according to new research, a major factor limiting their growth is their preference for preying on Chinook salmon.
In a study published Wednesday in the journal PLOS One, researchers present new data on environmental stressors facing the orcas and propose investment in the conservation of Chinook salmon to aid in the recovery of the population.
Killer whales are some of the most recognizable mammals in our seas with their distinct black and white markings. While they can be found in every ocean, they have broken off into small populations, creating different sub-species known as transient, offshore, and resident. The three groups are unique to one another, with different physical attributes as well as social structures and behavioral habits.
There are multiple populations of resident killer whales, but the authors of this study looked specifically at Southern Resident killer whales. These orcas mostly inhabit the waters around Oregon, Washington and British Columbia, and make up the smallest of the resident populations.
While killer whales are apex predators, meaning they have no natural predators, they face many threats including hunting, capture for captivity, limited resources, and deteriorating ocean health. They are protected under the Marine Mammal Protection Act, but some populations, including the Southern Residents, are so depleted they receive special protections. After a 20% population decline in 2001, they received protection under the Endangered Species Act. To date, there are only 74 Southern Resident killer whales left in the ocean, grouped into three pods referred to as the J, K, and L pods.
One of the known factors inhibiting their growth is a lack of prey, since their main source of food is the protected Chinook salmon. Previous studies have shown that the killer whales mostly consume Chinook salmon during the summer when they are further inland, but outside of that, very little is known about the whales’ feeding needs.
This is the gap that wildlife biologist Brad Hanson with the NOAA Northwest Fisheries Science Center aimed to fill, investigating the whales’ diet patterns throughout other times of the year in the outer coast.
Hanson and his colleagues set out on the Salish Sea and outer waters to obtain field samples of the killer whales’ feces and prey remains, yielding 152 samples of fish scales and discarded tissue and 81 samples of feces. They collected this data from 2004–2017 during the months of October to May, following the pods through Puget Sound, the Strait of Juan de Fuca, the San Juan Islands, the northern Georgia Strait, and in open waters along the Pacific Northwest.
After visually examining and performing genomic DNA analysis on the samples, the authors pieced together the Southern Resident killer whales’ diet throughout the seasons. According to their findings, the Chinook salmon are a year-round delicacy and account for about 50% of their food intake in the fall, 70%-80% in the winter, and almost exclusively in the spring. The researchers did, however, see more of a variety of prey in the late fall to early winter, including other species of salmon such as chum, coho, and steelhead.
This is likely due to the killer whales traveling to a region where Chinook salmon are less abundant. While some regions have healthy populations of these fish, along the West Coast of the U.S., almost all are threatened or endangered. Despite this, they continue to be harvested by fisheries presenting even more of a challenge for the endangered orcas.
The samples of Chinook salmon obtained during the winter were predominantly from the Columbia River, the population of which is threatened or endangered according to NOAA. Because of this, the authors called for salmon conservation efforts around the winter migration regions in order to begin reviving the Southern Resident killer whale population.
This would be a relatively large undertaking, as many of the frequented feeding grounds have already been undergoing hatchery programs. And climate change has and will continue to have an effect on the survivability of both predator and prey, as some salmon have shown vulnerability and some killer whale pods have spent less time in the summer around their usual inland waters. But the authors say it will be increasingly important to invest in conserving these species as time goes on.
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