Species and stock identification of prey consumed by endangered southern resident killer whales in their summer range

Abstract

Recovery plans for endangered southern resident killer whales Orcinus orca have identified reduced prey availability as a risk to the population. In order to better assess this risk, we studied prey selection from 2004 to 2008 in 2 regions of the whales' summer range: San Juan Islands, Washington and the western Strait of Juan de Fuca, British Columbia. Following the whales in a small boat, we collected fish scales and tissue remains from predation events, and feces, using a fine mesh net. Visual fish scale analysis and molecular genetic methods were used to identify the species consumed. Chinook salmon, a relatively rare species, was by far the most frequent prey item, confirming previous studies. For Chinook salmon prey, we used genetic identification methods to estimate the spawning region of origin. Of the Chinook salmon sampled, 80 to 90% were inferred to have originated from the Fraser River, and only 6 to 14% were inferred to have originated from Puget Sound area rivers. Within the Fraser River, the Upper Fraser, Middle Fraser, South Thompson River and Lower Fraser stocks were inferred to currently be sequentially important sources of Chinook salmon prey through the summer. This information will be of significant value in guiding management actions to recover the southern resident killer whale population.

Ecological Knowledge, Leadership, and the Evolution of Menopause in Killer Whales

“…In resident killer whales neither sex disperses from the maternal group and both adult males and females remain with their mother [22]. Resident killer whales forage in groups and feed predominately on salmon, with Chinook salmon, Oncorhynchus tshawytscha, making up more than 90% of their diet during the summer salmon migration [23,24]. Salmon abundance varies considerably in space and time [25,26], fluctuating annually due to anthropogenic impacts and El Niño events [25,26], and is an important driver of both reproductive success and mortality in resident killer whales [9,10].…”

Assessing the coastal occurrence of endangered killer whales using autonomous passive acoustic recorders

“…Zamon et al (2007) had suggested that the timing of their sighting coincided with the return of spring Chinook to the Columbia River. Chinook salmon are known to be a primary prey item of SRKW in their summer range (Hanson et al, 2010b). The first direct evidence that Columbia River spring Chinook were consumed by SRKW, albeit limited, was provided by Hanson et al (2010a).…”

Interpreting vertical movement behavior with holistic examination of depth distribution: a novel method reveals cryptic diel activity patterns of Chinook salmon in the Salish Sea

“…Consistent with our DAP findings, Candy and Quinn [27] found that during the summer in Johnstone Strait, British Columbia, Canada, Chinook salmon swam deeper and had greater rates of vertical movement during the night than day. The nighttime bounce diving by Chinook salmon in summer and fall may result from them ascending to feed on prey that undergo nighttime DVM toward the surface [28,29] and then descending to avoid marine mammal predators including harbor seals [30,31] and southern resident killer whales [32,33]. However, we did not test any behavioral hypotheses to explain the DAPs, and their ecological purpose requires further examination.…”