A dynamic model describing ecosystem-level changes in the Strait of Georgia from 1960 to 2010 Dave Preikshot ⇑ , Richard J. Beamish, Chrys M. Neville Fisheries and Oceans Canada, Pacific Biological Station, Nanaimo, B.C., Canada V9T 6N7 article info Article history: Available online 28 May 2013 abstract We developed an ecosystem model of the Strait of Georgia which emulates biomass and mortality changes between 1960 and 2009 to study ecosystem mechanisms governing dynamics in fished species and marine mammals. The model uses hindcast annual variation in bottom-up production, fisheries catches and predator-prey dynamics to simulate observed changes in fish, mammal and bird populations in the Strait of Georgia. This model emulates the timing and magnitude of historic changes in biomass and mortality of Coho and Chinook salmon as well as other major species like Pacific herring, orcas, har- bour seals, lingcod, spiny dogfish and marine birds. Simulated production trends indicate the Strait of Georgia had relatively high production from the mid-1970s to late 1980s and entered a lower production regime in the early 1990s that has persisted to 2009. The simulations also indicate that the mean trophic level of vertebrates declined over the period 1990 to 2009. This model provides a tool to evaluate poten- tial ecosystem changes in the Strait of Georgia. Ó 2013 Published by Elsevier Ltd. 1. Introduction The Strait of Georgia is a critical ecosystem on the Pacific coast of Canada. Its importance is related both to the valuable fisheries it supports and the rearing habitat it provides for the juveniles of those fished species like Pacific herring and Pacific salmon. The Strait of Georgia is also an arena within which higher trophic level predators like killer whales (Orcinus orca), harbour seals (Phoca vitulina), lingcod (Ophiodon elongatus) and adult Coho (Oncorhyn- chus kisutch) and Chinook (O. tshawytscha) salmon congregate and forage. Due to its importance to numerous species for rearing and seasonal feeding there is a need to identify ecosystem status to foster good environmental stewardship. Economically and cultur- ally important species in the Strait of Georgia such as Pacific her- ring, Coho salmon, Chinook salmon, lingcod and harbour seals have experienced large population changes between 1960 and the present. The fluctuations in Coho salmon, Chinook salmon, Pa- cific herring and lingcod populations are recognised to be from both fisheries and environmental changes (Beamish et al., 2004, Beamish et al., 1999; Ware and Schweigert, 2002; King and Surry, 2000). In addition, a large-scale Pacific salmon enhancement pro- gram was initiated in the late 1970s and continues to contribute millions of juvenile Pacific salmon to the ecosystem each year (La- belle, 2009; Beamish et al., 2008). During the period from 1960 to 2009 there were dramatic changes in the way certain species were fished and/or enhanced in the Strait of Georgia ecosystem. For example harbour seal boun- ties and harvests were curtailed in the early 1970s with a resultant dramatic increase in seal populations (Olesiuk, 2010). Approxi- mately 4 dozen orcas, largely from the southern resident popula- tion, were harvested in the 1960s and 1970s for the aquarium trade (Carretta et al., 2011). Large-scale Pacific salmon enhance- ment operations were started in the 1970s, which continue to con- tribute juvenile Coho and Chinook smolts to the Strait. Since the 1980s approximately 20–30 million Chinook salmon juveniles and 5–8 million Coho salmon juveniles have been introduced to the Strait every year via hatchery operations (Labelle, 2009). Com- mercial fisheries on species like Pacific cod and lingcod in the Strait of Georgia were closed in the early 1990s. Lastly, the Pacific herring fishery in the Strait changed dramatically after a collapse in the late 1960s. Harvest rates declined dramatically and the product targeted by the fishery shifted from the fish itself to its roe (DFO, 2002). The physical environment of the Strait of Georgia has also chan- ged. There have been decadal-scale changes in sea surface temper- ature in the Strait of Georgia (Beamish et al., 2007 and Gower, 2002) and a depth integrated temperature increase of 0.024 °C per year over the period from 1970 to 2005 (Masson and Cummins, 2007). Over a similar period, the Fraser River has experienced ear- lier peak flow and increased temperature (Morrison et al., 2002). Decadal-scale changes in wind are also evident locally (Tuller, 2004) and across the North Pacific basin (Peterson and Schwing, 2003). These changes are also nested within the waxing and wan- ing of North Pacific decadal-scale fish production regime shifts which have been reported by Hare and Mantua (2000) and Hol- lowed et al. (2002). 0079-6611/$ - see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.pocean.2013.05.020 ⇑ Corresponding author. Tel.: +1 250 756 7050. E-mail address: dave.preikshot@gmail.com (D. Preikshot). Progress in Oceanography 115 (2013) 28–40 Contents lists available at SciVerse ScienceDirect Progress in Oceanography journal homepage: www.elsevier.com/locate/pocean