Observations on larval fish transport and retention on the Scotian Shelf in relation to geostrophic circulation

Abstract

We draw inferences about the dynamic processes responsible for the dispersal of ichthyoplankton on small marine banks using physical and biological data derived from static point-estimates of water-mass characteristics and ichthyoplankton collected concomitantly on the central Scotian Shelf. Where the density ®eld evolves slowly and ageostrophic forcing is weak, the near-surface geostrophic¯ow can be derived from hydrographic data using the dynamic height method modi®ed for shallow seas. We assess our interpretations of larval distributions using simple particle tracking. The hydrography of the Scotian Shelf during November of 1997 was typical of late autumn, when density is determined by surface variation in salinity. Surface isopycnals generally paralleled isobaths, and there was no evidence of strong surface fronts. Sizes of larvae of pelagic origin (e.g. cod and hake) on Western Bank (sole spawning source) increased and became skewed towards larger animals (cod, 3±10 mm; hake, 5±15 mm) in water-mass (Temperature and Salinity) space along isopycnals, consistent with gradual mixing and limited transport in the geostrophic¯ow (i.e. retention). Conversely, larvae of benthic origin (e.g. herring, 6±25 mm) were distributed across water-mass space, consistent with multiple origins and substantial transport. Our results indicate that dispersal from small, low-energy marine banks results from the interaction of spawning location, geostrophic currents and bathymetric steering, and requires neither convergence nor larval behaviour.

Movements of Marine Fish and Decapod Crustaceans: Process, Theory and Application

“…Although hydrodynamics can explain distributions without provision for larval behaviour (Reiss et al, 2000), lack of consideration of behaviour (and morphology) that may enable larvae to move differently from the motion of the surrounding water will limit their application. Models assuming passive transport are limited by the absence of any behavioural responses (e.g.…”

Fisheries oceanography and the ecology of early life histories of fishes: a perspective over fifty years

“…Fish propagules can be wafted away from local populations by physical processes within or around eddies, or they can be retained by these same processes, depending upon their vertical position in the water column and the horizontal spatial scale of the eddy (e.g. Hare et al, 1999;Reiss et al, 2000;Allain et al, 2001;Loger-well and Smith, 2001). The surface of an understanding of the influence of eddies and fronts, and their inherent physical processes, on population recruitment has only recently been scratched.…”

Genetic Differentiation, Temporal Stability, and the Absence of Isolation by Distance among Atlantic Herring Populations

“…The residual circulation in the Gulf of Maine area (which indicates a recirculation with potential con- nections to the interior Bay of Fundy, as modeled by Greenberg 1983) and the discontinuity in larval abundance between southwestern Nova Scotia and Scot's Bay (as reported by Sinclair and Iles 1985) are consistent with such a suggestion. The association of the Western Bank herring collections with this southwestern Nova Scotia complex could also be explained by the dispersal pattern of herring larvae and the attendant circulation on the Scotian Shelf, where circulation models and larval length distributions have demonstrated a connection between the offshore banks and the coastal current (Reiss et al 2000). The relative distinctiveness of herring collected from Eastern Passage is not easily explained.…”