Seasonal variation of growth and reproduction of the subarctic krill species, Thysanoessa raschii, driven by environmental conditions in the Estuary and Gulf of St. Lawrence

Abstract

The aim of this study was to quantify somatic growth and reproduction of Thysanoessa raschii in response to environmental conditions in the St. Lawrence Estuary and Gulf of St. Lawrence, Canada. We sampled between 2010 and 2016 from spring to late summer and incubated individuals. Fresh molts were collected daily and measured to calculate the growth increment following the instantaneous growth rate method while eggs were counted daily. Our results showed a seasonal pattern of somatic growth and reproduction driven by temperature and chl. a concentration with a decrease in somatic growth in August when egg production was maximal, suggesting a trade-off. Functional relationship analyses revealed a narrow optimal temperature window for somatic growth with maximum temperatures observed between 1.2 and 2.0°C in the cold intermediate layer (50–150 m). Maximum egg production was observed at temperatures between 3.8 and 5.7°C in the surface layer (0–50 m). A required minimum concentration of chl. a of 9 mg.m−3 for somatic growth was observed. For egg production, the minimum observed was integrated chl. a (0–50 m) of 80 mg.m−2. We also observed the importance of optimal conditions lasting for one to 3 weeks to support biological processes in T. raschii.