Offshore pelagic fish community

Abstract

Lake Ontario’s offshore zone, as defined by Stewart et al. (2013), comprises all waters of the lake where the bottom depth is greater than 15 m excluding those in embayments. When the lake is thermally stratified during June-October, the offshore pelagic zone includes the upper-warm and middle-cool layers of water which serve as important habitat for Alewife and other prey fishes, and for predators like salmon and trout. Early changes in the fish community of the offshore pelagic zone are well documented elsewhere (e.g., Smith 1972; Christie 1973) as are more recent changes (e.g., Owens et al. 2003; Mills et al. 2003). Currently the offshore fish community consists of a mix of native and non-native species. Native species are those that were present prior to European colonization and for the offshore pelagic zone, include predators like Atlantic Salmon and prey fish like Cisco, Emerald Shiner, and Threespine Stickleback. Non-native species are those that were introduced unintentionally like Alewife and Rainbow Smelt, or that were introduced intentionally like Chinook Salmon, Coho Salmon, Rainbow Trout, and Brown Trout. Non-native salmon and trout were introduced originally by fisheries managers to provide fishing opportunities and later to reduce an overabundance of Alewife. Alewife is the most abundant prey fish in the offshore pelagic zone and it dominates the diets of native and introduced predators (Brandt 1986; Lantry 2001). Alewife can have direct and indirect negative effects on other fishes through competition for food and/or predation on their larvae (Madenjian et al. 2008). Alewife also contain thiaminase, an enzyme that catalyzes the breakdown of thiamine, and fish that eat mainly Alewife can become thiamine deficient which impairs their reproduction (Honeyfield et al. 2005). Except for that of the Alewife, prey fish populations in the offshore pelagic zone are depressed, and not large enough to sustain the zone’s predators. Alewife remain necessary for a functional ecosystem that is required to sustain a highly-valued, trophy sport fishery (Stewart et al. 2013). Wild production of trout and salmon occurs in Lake Ontario tributaries, contributing to in-lake populations (Rand et al. 1993; Connerton et al. 2009; Connerton et al. 2014c). Stocking hatchery-reared fish (Fig. 1), however, remains an essential tool for managing Lake Ontario’s diverse trout and salmon fisheries and achieving the Offshore Pelagic Zone Goal (Stewart et al. 2013): Maintain the offshore pelagic fish community, that is characterized by a diversity of trout and salmon species including Chinook Salmon, Coho Salmon, Rainbow Trout, Brown Trout, and Atlantic Salmon, in balance with prey-fish populations and lower trophic levels. Here we review the fish-community objectives (FCOs) for Lake Ontario’s offshore pelagic zone (Stewart et al. 2013) and evaluate whether those objectives were met during this reporting period (2008-2013) by assessing the status of the objectives’ indicators. We also compare the status of indicators in this reporting period with those in the previous reporting period (2003-2007) (Connerton et al. 2014b). Specific objectives are in italics at the start of each major section and associated indicators of progress are given in Progress and Outlook subsections.