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This study describes the long-term seasonal and interannual variations in krill spawning using abundance of krill eggs collected during an on-going long-term monitoring program at an anchor station in the lower St Lawrence Estuary from 1992 to 2009 and data collected in the same region in 1979 to 1980. The long-term seasonal semi-monthly climatology in egg abundance revealed that krill generally reproduced during two periods, i.e. in late spring (mid-May to late June) and in late summer (August to mid-September), when phytoplankton biomass in the upper 50 m was greater than 75 mg chlorophyll  a  m −2 . The identification of krill eggs to the species level in 2007 revealed that  Meganyctiphanes norvegica  egg abundance was related to the biomass of phytoplankton averaged over the month prior to sampling, corresponding to the duration of one spawning cycle (two intermolt periods) known for this species. Overall krill egg abundance varied significantly between years, showing high abundance every 3–5 years with no long-term interannual trend. The annual mean egg abundance normalized for the duration of krill spawning showed the same interannual long-term pattern. Both egg abundance indices were independent of the annual phytoplankton biomass, indicating that interannual variations in krill spawning biomass would be the most likely candidate to explain interannual variability in egg abundance. We propose that such normalized annual egg abundance based on high-resolution seasonal sampling could be a useful index of interannual variations in krill spawning biomass which is otherwise difficult to sample.

Long-term seasonal and interannual variations of krill spawning in the lower St Lawrence estuary, Canada, 1979–2009