Killer whales (Orcinus orca) of the North East Pacific region have been well documented in recent years, and are known to occur from the Californian waters to the Western Aleutian Islands chain of the U.S. state of Alaska (Mayne 1862). Long-term studies have identified three killer whale ecotypes in this area: fish-eating residents,mammal-eating transients, and offshores. Although residentsand transientsoverlap geographically, studies have shown that they should be considered as two distinct forms of the species, as they differ in dietary specialization (Ford et al. 1998), genetics (Barrett-Lennard 2000, Barrett-Lennard and Ellis 2001), vocalization patterns (Barrett-Lennard et al. 1996), social organization (Bigg et al. 1990), and morphology (Bigg et al. 1987, Baird and Spacey 1988).
Similarly, the resident population encompasses two sub-populations: the southern resident killer whales (SRKW) and the northern resident killer whales (NRKW).. The SRKW and NRKW populations were first identified as distinct assemblages that did not interact socially with others in the early 1970s, and genetic analysis later provided strong evidence that the SRKW and the NRKW populations were reproductively isolated (Barrett-Lennard and Ellis 2001, Bigg et al. 1987). Today, the historical size of both populations is unknown. The SRKW population was on the order of 100 in 1965, and has since declined to 75 individuals, and is listed as endangered in both the US and Canada (NMFS 2008, NOAA 2017). The historical range of this population is thought to extend from California to Haida Gwaii at the north coast of British Columbia (Scammons and Cope 1869, Bigg et al. 1987). Today, sightings occur year-round in British Columbia and are closely tied with salmon runs in the southern end of Vancouver Island during summer months (Baird 2001). Conversely, the NRKW population geographical range extends from the waters off northern Vancouver Island to Southeast Alaska (Olesiuk et al. 2005, Fisheries and Oceans Canada 2008). This population is thought to have been growing exponentially from the 1970’s to the early 1990’s, reaching about 200 individuals before dropping slightly in the mid-1990’s (Olesiuk et al. 2005). Today, the NRKW population comprises about 300 individuals and is listed as threatened in Canada (Fisheries and Oceans Canada, 2018). Both populations are at risk, mainly because of their small population sizes (Bigg et al. 1987), low levels of genetic variation (Barrett-Lennard and Ellis 2001), vulnerability to water contaminants (Ross et al. 2000) and underwater noise and physical disturbance (Erbe 2002, Bain et al. 2006).
Both the SRKW and NRKWpopulationsare salmonid predators with a strong selectivity for Chinook salmon (Oncorhynchus tshawytscha) Chum (Oncorhynchus keta) and Coho (Oncorhynchus kisutch) salmon are secondary prey (Ford and Ellis 2006, Hanson et al. 2010). Chinook salmon represent up to 90% of their diet during the summer months (Ford et al. 1998, Ford and Ellis 2006, Hanson et al. 2010). Chinook is the largest of the five Pacific salmon species, and is historically the most abundant and generally distributed species of the Northwestern American Pacific coast (Smith 1895). Yet, as the salmon industry started to take a prominent place in the commercial activity of the area in the late 19th century, Chinook salmon stocks dropped substantially (DeLoach 1967). For instance, there has been a 60% reduction in the Chinook population in the Salish Sea between 1984 and today (EPA 2018).
The decline in salmon population has been suggested as a risk factor impacting both residentkiller whale populations. Ford et al. (1998, 2010) and Ward et al. (2009) showed a correlation between Chinook abundance and resident killer whales mortality and fecundity from the period 1973-2007. However, with the exception of the papers referred to above, bottom-up processes limiting killer whale numbers have received little attention. Killer whales are apex predators, and as such the top-down effects of killer whale predation on prey populations are expected to be of considerable magnitude (Heise et al. 2003, Williams et al. 2004). Because of the species’ high degree of prey specialization, Chinook salmon abundance is expected in turn to limit resident killer whale numbers through bottom-up processes (Ford et al. 2010). Assessing the historical levels of salmon populations could thus highlight the critical importance of prey availability for resident killer whales.
Analytical methods became an important tool for scientists and ecologists to understand fish population dynamics in the 1930s (Haddon 2010). Today, population modeling is widely used in fisheries sciences, as it provides an understanding of how specific perturbations could affect marine ecosystems while predicting future population trends (Haddon 2010). Most recent models built on salmon populations focus on the impact of anthropogenic activities and climate change on Atlantic salmon (Piou et al. 2015, Sundt-Hansen et al. 2018), but little attention has been given to Pacific salmon species (Shelton et al. 2018). In this project, we will use ecological modeling approaches to reconstruct Chinook, Chum, and Coho populations in the north East Pacific region, as it will allow us to synergistically incorporate salmon life-history characteristics, fisheries, canneries and spawning escapement data, as well as spatial and temporal variations within salmon populations over time (Shelton et al. 2018). Most of the recent studies on salmon populations status and variations focused on Chinook salmon (Myers et al. 1998, Weitkamp et al. 2010, Burke et al. 2013, Sheldon et al. 2018), and used Coded Tag Wire (CTW) recoveries and fisheries data to estimate the spatial and temporal distribution of salmon stocks. No studies yet compiled life-history characteristics, fisheries, and oceanographic data to reconstruct salmon populations over time. Moreover, the goal of our study is to expand the time frame of research beyond the 1970s, which has not yet been done. Finally, a study by Weitkamp and Neely (2002) focused on migration patterns of Coho salmon along the North American coast between 1979 and 1993, but no studies ever focused on chum salmon populations.
These models will have implications for the management and conservation of both salmon and killer whales, and we expect that they will also provide an extendable framework for understanding the dynamics of other specialist predators and their preys. The use of ecological modeling will give an innovative insight on how the ecosystem might evolve as food-web interactions fluctuate (Sheldon et al. 2018).
Aside from food limitation, inter-predator competition is another major factor in the regulation of populations (Matsuda et al. 1993). In this project, it is thus important to consider the interactions between resident killer whales and other species that prey on salmon. Three species of pinnipeds with a diet that includes salmon occur in the coastal waters of western US and Canada: Harbor seal (Phoca vitulina), California sea lion (Zalophus californianus), and Steller sea lion (Eumetopias jubatus). The successful recovery of those pinniped populations over the last 30 years is today associated with a dramatic decline in Chinook (O.tschawytscha) and Coho (O. kisutch) salmon populations within this region (Chasco et al. 2017, Browne et al. 2001, Laake et al. 2002, Thomas et al. 2017, Nelson et al. 2018). Here, the question of the impact of interactions between top predators sharing common prey species needs to be addressed. Similarly to pinnipeds, the question of the geographical overlap between the two sympatric SRKW and NRKW populations may be of critical importance. Matsuda et al. (1993) showed that the predatory behavior of one species could affect the fitness of another predatory species when sharing the same prey species. Similarly, it today remains unclear whether indirect predatory interactions occur between resident and transient killer whales, as the primary food source of transients (i.e. pinnipeds) are competitors for salmon (Baird et al. 1992). Previous studies suggested that transientsappear to usually avoid residents, and that mixed groups are never observed (Baird and Dill 1995). The killer whale researcher G. Ellis also mentioned personal observation of a presumable attack and chasing of transientsby southern residentkiller whales (unpublished data). The possibility of developing a model considering inter-predator competition is innovative and needed, as top predators are still very poorly represented in marine ecosystem models (Goedegebuure et al. 2017). Incorporating top predators interactions in ecosystem models is essential to develop more realistic ecosystem-based management and conservation plans regarding those populations (Goedegebuure et al. 2017).
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