Three-dimensional zooplankton distributions seen through the lens of fish in the Barents Sea

• Main Author: Aarflot, Johanna Myrseth
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: The University of Bergen 2019
• Subjects: Barents Sea; Calanus finmarchicus; Copepods
• Online Access: http://hdl.handle.net/1956/19660

My aim with this dissertation is to enhance our knowledge of food availability and foraging opportunities for planktivorous fish over the Barents Sea bathymetry. The Barents Sea is a highly productive sub-Arcic shelf sea supporting some of the largest fish stocks in the world. Lipid-rich calanoid copepods are key prey for fish and other planktivore predators in northern latitude ecosystems. Visual detection of individual prey is the dominant foraging mode in planktivorous fish, and prey detection is arguably the most limiting phase in the predation process. Light is a prerequisite for visual foraging and decreases exponentially with depth in aquatic systems. Furthermore, vertical movement has become a widespread strategy among Calanus and other zooplankton for avoiding visual predation. The bathymetry may, however, constrain vertical distributions and force zooplankton into more illuminated parts of the water column. Environmental constraints on distributions may be important for fish searching for zooplankton and affect predator-prey dynamics in pelagic ecosystems. In Paper I, we focus on Calanus spp. and use an extensive (30-year period) dataset on zooplankton biomass and species-specific abundance to show that calanoid copepods are a major driver of variation in zooplankton biomass in this ecosystem. Calanus finmarchicus, C. glacialis and C. hyperboreus constitute on average 80 % of the total biomass, and older copepodites (stages CIV and CV) make the largest contribution. The Calanus species co-occur in all areas, and interspecific variation in spatial biomass distribution may be related to the distribution of water masses. Biomass of Calanus spp. increases with bottom depth, but does this mean that prey is more available to fish in deeper parts of the Barents Sea? In Paper II, we utilize a unique dataset on vertical zooplankton distributions to determine the weighted mean depth (WMD) of zooplankton in three size fractions and assess the effect of topography and light on the depth distributions. The vertical ...