The trade‐off between fecundity and egg size in a polymorphic population of Arctic charr (Salvelinus alpinus (L.)) in Skogsfjordvatn, subarctic Norway

Abstract

Reproductive traits differ between intralacustrine Arctic charr morphs. Here, we examine three sympatric lacustrine Arctic charr morphs with respect to fecundity, egg size and spawning time/site to assess reproductive investments and trade‐offs, and possible fitness consequences. The littoral omnivore morph (LO‐morph) utilizes the upper water for feeding and reproduction and spawn early in October. The large profundal piscivore morph (PP‐morph) and the small profundal benthivore morph (PB‐morph) utilize the profundal habitat for feeding and reproduction and spawn in December and November, respectively. Females from all morphs were sampled for fecundity and egg‐size analysis. There were large differences between the morphs. The PB‐morph had the lowest fecundity (mean = 45, SD = 13) and smallest egg size (mean = 3.2 mm, SD = 0.32 mm). In contrast, the PP‐morph had the highest fecundity (mean = 859.5, SD = 462) and the largest egg size (mean = 4.5 mm, SD = 0.46 mm), whereas the LO‐morph had intermediate fecundity (mean = 580, SD = 225) and egg size (mean = 4.3, SD = 0.24 mm). Fecundity increased with increasing body size within each morph. This was not the case for egg size, which was independent of body sizes within morph. Different adaptations to feeding and habitat utilization have apparently led to a difference in the trade‐off between fecundity and egg size among the three different morphs.

Morphological divergence between three Arctic charr morphs – the significance of the deep‐water environment

“…The two profundal morphs (PP and PB) were generally more similar to each other than to the littoral morph (LO), despite their large differences in size (Smalås et al. 2013, this study). These two deep-water morphs have evolved under identical abiotic conditions with constant low light and temperature and a uniform physical habitat and have an overall similarity in morphological adaptations in accordance with our expectation.…”

“…2013; Smalås et al. 2013; this study). Such omnivorous feeding behavior is also common for charr in monomorphic populations, especially when there is strong competition for benthic resources in the littoral zone (Jørgensen and Klemetsen 1995; Knudsen et al.…”

“…Low temperature generally leads to slow growth rate in fish, and the two profundal morphs have lower growth rates than the LO-morph in Skogsfjordvatn (Smalås et al. 2013). However, the two morphs have adopted strikingly different strategies in the investment trade-off between somatic growth and reproduction.…”

“…For more details of the fish sampling, see Smalås et al. (2013). During field sampling, the temperature in the littoral/pelagic zone (1 m depth) ranged from 3.5°C in May (full spring circulation) to 12.0 and 13.3°C in June and August, respectively.…”

“…The two profundal spawning morphs utilize different prey items, including a benthivore morph (the PB-morph) and a piscivore morph (the PP-morph), and have highly contrasting life-history traits (growth rates, adult sizes, and age and size at maturity) (Smalås et al. 2013). The profundal slow-growing PB-morphs have the appearance of a young charr with a pale yellow coloration and pale brown parr marks.…”