Spatial variation in right whale food, Calanus finmarchicus, in the Bay of Fundy

摘要

We quantified small- and basin-scale variation in the energy available in right whale (Eubalaena glacialis) food, Calanus finmarchicus Copepodite-5 (C5), as a function of depth, tidal variation and circulation during the late-summer feeding period in the Grand Manan Basin of the Bay of Fundy. Food energy density (kJ m~(-3)) is a function of C5 concentration (m~(-3)) and individual C5 lipid energy content (J) that increases by 2 orders of magnitude from near the surface to ~18 kJ m~(-3) at ~160 m depth; this observation helps explain why right whales concentrate their foraging efforts at depth. Food energy density can vary by a factor of 4 over a tidal cycle and 3.5 over a horizontal distance of ~2 km in the deeper strata of the Basin. By tracing water mass trajectories, we show that it is the warmer, saltier and denser water that contains the highest food energy density that is tidally advected within the Basin. At the scale of the Basin (10s of kilometres) we observed maximum food energy densities of ~25 kJ m~(-3) in the 140 to 160 m depth stratum where the highest energy densities spatially coincide with the highest historical right whale concentrations. We concluded that the tidal variation and tidally driven circulation represents the simplest explanation for the distribution of food energy density and for the distribution of right whales in the Grand Manan Basin region and that Basin topography and water mass excursions associated with tidal currents facilitate the persistence of C5 aggregations in the dense deep water.