Variation in salinity tolerance between and within anadromous subpopulations of pike (Esox lucius)

Abstract

Environmental heterogeneity is a key determinant of genetic and phenotypic diversity. Stable and homogenous environments tends to result in evolution of specialism and local adaptations, while temporally unpredictable environments may maintain a diversity of specialists, promote generalist strategies, or favour diversified bet hedging strategies. We compared salinity tolerance between two anadromous subpopulations of pike (Esox Lucius) that utilize freshwater spawning sites with different salinity regimes. Eggs from each population were artificially fertilized and incubated in a salinity gradient (0, 3, 5, 7, and 9 psu) using a split-brood design. Effects on embryonic development, hatching success, survival of larvae, and fry body length were compared between populations and families. The population naturally spawning in the stable freshwater habitat showed signs of specialization for freshwater spawning. The population exposed to fluctuating selective pressure in a spawning area with occasional brackish water intrusions tolerated higher salinities and displayed considerable variation in reaction norms. Genetic differences and plasticity of salinity tolerance may enable populations to cope with changes in salinity regimes associated with future climate change. That geographically adjacent subpopulations can constitute separate units with different genetic characteristics must be considered in management and conservation efforts to avoid potentially negative effects of genetic admixture on population fitness and persistence.

Predator–prey role reversal may impair the recovery of declining pike populations

“…Furthermore, in population O (Table ) we sampled across five years to examine temporal variation in stickleback predation effects across breeding seasons. Previous research has shown that these populations display homing behaviour (Tibblin, Forsman et al., ), fine‐spatial‐scale genetic population differentiation (Larsson et al., ) and local adaptations to specific streams/wetlands (Berggren et al., ; Sunde, Tamario, Tibblin, Larsson, & Forsman, ; Tibblin, Berggren, Nordahl, Larsson, & Forsman, ; Tibblin et al., ). Moreover, our study area experienced the most extreme decline of pike in the Baltic Sea during the 1990s and abundances are still very low despite that the pike fishery is partially closed and strictly regulated through bag and size limits (Figure b) (Bergström et al., ; Ljunggren et al., ; Nilsson et al., ).…”

Sex‐specific effects of outbreeding on offspring quality in pike (Esox lucius)

“…Adult pikes from each subpopulation were caught during 4 days (March 31, April 1, April 4, and April 5, 2016) using fyke nets that were placed in the inlet of the streams leading to the spawning grounds (Sunde et al., ; Tibblin, Berggren, et al., ), and ripe individuals (Harfjärden: 11 females, 12 males; Lervik: 10 females, 12 males; Okne: 11 females, 10 males) were stripped of gametes. Stripping was synchronized between the localities to avoid systematic error due to different storing times of gametes before artificial fertilization was conducted.…”

“…The artificial fertilization and the entire experiment was carried out in a constant room with a temperature of 12°C, using tap water that had been aerated for a minimum of four hours. The design was chosen such that the temperature was within the natural range (Frost & Kipling, ; Nilsson, Engstedt, & Larsson, ), and because other studies have successfully used similar approaches (Sunde et al., ; Tibblin, Koch‐Schmidt, Larsson, & Stenroth, ; Tibblin et al., ).…”

“…Offspring quality was estimated as (a) hatching success of eggs, (b) fry survival during 15 days following hatching, and (c) fry body length at 15 days post hatch (Figure b–d). We know from a previous study (Sunde et al., ) that there is a time period during which unfertilized eggs are not visibly discernible as dead, such that unsuccessful fertilization cannot be reliably distinguished from postfertilization embryonic mortality. We therefore estimate hatching success as a composite measure from eggs to successful hatchlings.…”

“…In this study, pike from three subpopulations (for sampling locations see Figure a) that display different local adaptations (e.g., larval traits (Berggren, Nordahl, Tibblin, Larsson, & Forsman, ), body size and growth rate (Tibblin et al., ), vertebral number (Tibblin, Berggren, Nordahl, Larsson, & Forsman, ), and salinity tolerance (Sunde, Tamario, Tibblin, Larsson, & Forsman, )) were used in a common garden admixture experiment. Three offspring quality traits (hatching success, fry survival, and fry length) were measured and compared between (a) population combinations (“population combination comparisons”) and (b) the two treatments within each population combination, based on the origin of the male and female, respectively (“sex‐separated comparisons”).…”