MISMATCH OF LABORATORY EGG SINKING RATES AND CROSS-SHELF VERTICAL DISTRIBUTIONS OF EARLY ONTOGENETIC STAGES OF TWO KRILL SPECIES OFF THE OREGON COAST

Egg sinking rates of the sympatric species Thysanoessa spinifera and Euphausia pacifica were investigated in the context of cross-shelf seawater conditions and vertical distributions of early ontogenetic stages (eggs, nauplii, calyptopes, and furciliae) along the Oregon coast during summer, 1977. The highest abundances of eggs, calyptopes and furciliae were near the surface (<30 m), and nauplii were in deeper layers (50–100 m), indicating that the eggs sank to <90 m depth and calyptopes migrated into shallower waters. Mean T. spinifera egg diameters were less (chorion 0.376; embryo 0.331 mm) than those of E. pacifica eggs (chorion 0.406; embryo 0.364 mm). Experimental mean egg sinking speeds of single cell stage eggs, measured at constant 11°C temperature, 33.436 salinity, and 25.686 sigma-t, were almost identical for T. spinifera (126 m d-1, range 60–193 m d-1) and E. pacifica (128 m d-1, range 103–175 m d-1). Instantaneous egg-sinking speeds calculated from the Navier-Stokes Law showed that egg densities of both species were always greater than the seawater density observed at Oregon coast. However, Navier-Stokes predicted egg sinking speeds were between 5–12 times faster than inferred from in situ vertical distributions of successive ontogenetic stages, theoretically suggesting that a proportion of the eggs may actually reach the sea floor. Upwelling, stratification, turbulence, horizontal currents and seafloor boundary layer effects do not explain the apparent egg sinking speed (<80 m d-1) inferred from egg and larvae distributions. However, a combination of them may. I infer that sinking of eggs and early ontogenetic daily migrations maintain most of the early stages close to the coast due to the predominant cross-shelf currents. Offshore transport near the sea surface and onshore transport in deeper layers keep the larvae over the continental shelf. Keywords: Navier-Stokes law, larval drift, ontogenetic segregation, Euphausia pacifica, Thysanoessa spinifera.