The White Sea threespine stickleback population: spawning habitats, mortality, and abundance T.S. Ivanova, M.V. Ivanov, P.V. Golovin, N.V. Polyakova and D.L. Lajus St. Petersburg State University, St. Petersburg, Russia ABSTRACT Hypothesis: Stickleback abundance in the White Sea is limited by availability of spawning habitats. Organisms: Threespine stickleback, Gasterosteus aculeatus; eelgrass, Zostera marina. Times and places: June (spawning period of stickleback) 2009–2011 and 2014; 60 locations along the White Sea coast. Methods: We sampled with a beach seine (length 7.5 m, height 1.5 m, mesh size 5 mm in wings and 1 mm in purse) in coastal zones within 30 m of the shore. Results: Around 60% of the entire stickleback population occurs in the northwestern part of the White Sea (Kandalaksha Bay). This region has favourable spawning habitats, i.e. protected inlets with a high density of eelgrass and other macrophytes. Other parts of the White Sea are more exposed to waves and have less vegetation. We estimated that the White Sea currently supports about 740 million stickleback at the beginning of the spawning season, with a total biomass of about 1600 metric tonnes. Keywords: abundance, distribution, Gasterosteus aculeatus, habitats, mortality, threespine stickleback, White Sea. INTRODUCTION The threespine stickleback (Gasterosteus aculeatus) is one of the most widely studied fishes, being a popular model in evolutionary and behavioural research (Wootton, 1976, 1984, 2009; Bell and Foster, 1994; Östlund-Nilsson et al., 2007; Huntingford and Ruiz-Gomez, 2009; Barber and Nettleship 2010; Hendry et al., 2013). However, some aspects of stickleback biology have attracted little attention. This is especially true for marine populations, even though stickleback often link different trophic levels (Lemmetyinen and Mankki, 1975), thus potentially shaping entire ecosystems (Harmon et al., 2009; Matthews et al., 2016). Because they feed on plankton, stickleback compete with important commercial fish such as herring (Jurvelius et al., 1996; Peltonen et al., 2004). Stickleback have been shown to cause a decline in coastal predatory fish such as perch and pike (Ljunggren et al., 2010; Bergström et al., 2015; Byström et al., 2015) and promote recruitment of macroalgae by feeding on invertebrate grazers (Sieben et al., 2011). This species represents an important prey item for Correspondence: D.L. Lajus, St. Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg 199034, Russia. email: dlajus@gmail.com Consult the copyright statement on the inside front cover for non-commercial copying policies. Evolutionary Ecology Research, 2016, 17: 301–315 © 2016 Dmitry Lajus