Life history, growth and production of Mysis relicta in the large, fiord‐type Lake Mjøsa, Norway

Abstract

SUMMARY. Biomass, production and life history of Mysis relicta were studied in the large Lake Mjøsa during the years 1976–80. Biomass fluctuations were large, but biomass averaged about 1 g wet wt m−2 or 200 ind. m−2. Cumulative net production during summer ranged from 1.6 to 2.1 g wet wt m−2. Mean population P/B‐ratio was close to 2. Neither production or biomass of Mysis were correlated with the biomass of phytoplankton or the main Zooplankton prey species (Daphnia and Bosmina), suggesting a predominant ‘top down’ control of Mysis in this lake. The life cycle was well synchronized with development of food items, but predation pressure also affected life‐history patterns. The proportion of juveniles apparently increased in years with strong fish predation pressure, producing a predominantly 1‐year life‐cycle pattern the following year. Mysis had a predominant 2‐year life history, but a traction of the population may reproduce after 1 or 3 years. Reproduction occurred exclusively during winter. Even in such a large and stable system, where Mysis has coexisted with its main food items and predators for 8–10 000 years, a flexible life history is maintained and is probably an important buffer against year‐to‐year fluctuations in food and predator abundance.