This DFO summary is a little dated, and does not include some of the more recent research - but its a good overall summary of what was becoming a broader understanding of the growing predation problem:
Simulation modelling indicates that mortality rates of Chinook Salmon from marine mammal predation increased in the 1990s relative to levels during the preceding 30 years (Figure 26). These results may be partially explained because predators were eating a higher proportion of fish due to declining Chinook Salmon populations (Bailey pers. comm. 2018), although other models suggest this interpretation may not be supported by the evidence (Preikshot et al. 2013). Nevertheless, populations of Sea Lions, Harbour Seals, White-sided Dolphins, and Humpback Whales dramatically increased since the 1970s and may have led to higher consumption (Riddell et al. 2013; Chasco et al. 2017). Between 1970 and 2015, for example, the annual biomass of Chinook Salmon consumed by pinnipeds in Puget Sound (Washington State, USA) rose from 68 to 625 tons (Chasco et al. 2017).
Northern and southern resident killer whales – which in 2013 totalled approximately 350 animals in BC waters – are considered salmonid specialists (Brown et al. 2013a). These whales congregate in groups during summer and fall in specific areas to intercept salmon migrating to natal spawning rivers. Extensive field studies of foraging behaviour indicate that resident killer whales forage selectively for Chinook Salmon and, to a lesser extent, Chum Salmon (Ford and Ellis 2006; Hanson et al. 2010). The whales appear to target large fish, with most being four years of age or older. Riddell et al. (2013) discuss workshop findings that identified the South Thompson Chinook Salmon population (DU12, DU13 and DU14) as the dominant stock in the diet of southern resident killer whales. Other Fraser River stocks of Chinook Salmon, some of which are declining, also figure prominently in the diet of resident killer whales. While only assessed during a single year and not considered in relation to relative DU abundance for that year, Hanson et al. (2010) ranked each DU in terms of inferred importance as follows: Upper Fraser (DU11), Middle Fraser (DU7, DU8, DU9, DU10), South Thompson River (DU12, DU13, DU14), and Lower Fraser stocks (DU2, DU3, DU4, DU5, DU6).
Harbour Seal abundance along the Pacific coast has increased dramatically since harvests ended in the late 1960s (Brown et al. 2013a). Consistent with trends south of the border, Harbour Seal abundance increased in the Strait of Georgia at a rate of 11.5 percent per year after the mid-1970s before stabilizing in the mid-1990s at about 40,000 animals. This trend is typical of the BC coast generally, with current total abundance estimated at 105,000 animals (Olesiuk 2010). Extensive scat collections during the 1980s indicated that Harbour Seals in the Strait of Georgia consumed a wide variety of prey species, but their diet was dominated by Herring and Hake. Overall, salmonids represented only about 4-7 percent of their diet, with salmonid consumption concentrated on pre-spawning adult salmon in estuaries and rivers (Olesiuk 1993; Thomas et al. 2016). Such predation can potentially be a major source of mortality for returning adult Chinook Salmon in cases where run size is small and habitat modification increases vulnerability to predation (for example, channelization of lower Puntledge River). Juvenile salmon, including Chinook Salmon, are also preyed upon by Harbour Seals (Thomas et al. 2016). Predation of juveniles can occur in marine areas as well as in rivers. Predation rates of downstream migrating juveniles can be significant in areas that are artificially illuminated at night such as bridge crossings (for example, Puntledge River, Olesiuk et al. 1996). The constrained morphology of a river can increase vulnerability to highly mobile and agile predators such as seals. The extent of predation on juvenile Chinook Salmon by Harbour Seals in natural settings is currently unknown. Chasco et al. (2017) estimate that between 1970 and 2015 the annual biomass of Chinook Salmon consumed by pinnipeds (Harbour Seals [Phoca vitulina], California Sea Lions [Zalophus Californianus], Steller Sea Lions [Eumetopias jubatus]) in Puget Sound increased from 68 to 625 metric tons. By 2015, pinnipeds consumed double that of resident killer whales and six times the combined commercial and recreational catches.
Steller Sea Lion abundance in British Columbia has increased approximately three-fold in BC since harvesting ended in the late 1960s (Brown et al. 2013a). Prior to 2013, abundance was increasing at 5 percent per annum and, based on pup production, current abundance in BC and adjacent waters of Southeast Alaska is approximately 60,000 animals, which is considerably greater than the estimated abundance for the early 1900s. Steller Sea Lions range widely in coastal waters, but during summer the majority congregate at traditional breeding rookeries, the largest of which are found in the Scott Islands, off the north end of Vancouver Island, and at Forrester Island, Alaska just north of the Haida Gwaii (Queen Charlotte Islands). Diet studies using prey remains found in scats collected at these rookeries and other haul-out sites indicate that Steller Sea Lions feed on a variety of fish and cephalopods, and that salmon constitutes a significant portion of their diet particularly in summer and fall. Salmonids have been estimated to represent about 10 percent of their overall diet (Olesiuk et al. 2010). On average, Steller Sea Lions eat about 18 kg of prey per capita per day, which may translate to about 17,200 tonnes per year for the population that uses Canadian waters. Preliminary studies on the salmonid species composition of Steller Sea Lion diets indica