Income and capital breeding— <i>Calanus</i> reproduction in a subarctic fjord

Abstract Copepods of the genus Calanus are key species in the energy transfer from primary producers to higher trophic levels in Arctic and sub‐Arctic seas. The timing of reproduction can differ between species, depending on whether they rely on external energy sources to initiate reproduction (income breeders) or use internal energy sources (capital breeders), or both. The high morphological similarity among species, especially when co‐occurring in sympatry, has limited the reliability of species‐specific conclusions on life history traits in previous studies relying on morphological identification only. Here, we applied molecular species identification to reliably compare breeding strategies among co‐occurring Calanus hyperboreus , Calanus glacialis , and Calanus finmarchicus in a northern Norwegian fjord. Lipid content of adult females was measured with high temporal resolution over an entire annual cycle, and the presence of nauplii was determined for each species. Calanus hyperboreus and C. glacialis were found to be strict capital breeders, whereas C. finmarchicus acted as an income breeder.