Scott M. Gallager, Jeff Van Keuren
and Phillip Alatalo
Historically, it has been thought that planktotrophic teleost larvae begin and continue to feed following yolk-sac absorption, a vitally critical period requiring prey to be available of optimal size, chemical composition and concentration. Copepod nauplii are usually considered the primary 'first food' for young cod larvae because they are the first hard-bodied organisms observed in the larval guts. Using a novel technique for quantifying ingestion of protozoa by larval fish (Fig. 1), we have shown that cod larvae Gadus morhua feed extensively on soft-bodied protozoans well before the yolk-sac is absorbed. This circumvents the classical food chain paradigm leading to a direct link between members of the microbial loop and higher trophic levels (Fig. 2). Our results show that newly-hatched cod larvae require soft-bodied microzooplankton in the size range of 40 to 80 m at a nominal concentration of about 2 cells ml-1 in order to maximize survival through yolk-sac absorption (~10-15 days post-hatch). Furthermore, the down-welling light field, mixing intensity (turbulence), and the motion of the prey organism can have a dramatic effect on successful foraging by young cod larvae. Optimal light wavelength and light intensity for foraging in young cod larvae appears to center around 520 nm and 10 to 80 W m-2s-1, respectively (Fig. 3). This combination of light quality and intensity may occur at depths of 30 m to near surface depending on time of day, cloud cover and the water's attenuation coefficient. As the larvae move vertically in the water column following optimal light conditions, they are exposed to a variety of different prey species and physical mixing conditions.
Under calm conditions, some protozoans can form dense layers or patches near the pycnocline where larval fish are exposed to an enhanced feeding environment (Fig. 4). As turbulence increases, through either wind mixing from the surface or by passage of an internal wave, the layers of prey are dispersed thereby reducing prey concentrations available to the larvae and consequently their feeding rates. If turbulence increases even further, ingestion rates increase again as the effective prey concentration increases as a function of the increased contact rate between larva and prey as provided by small-scale eddies. Although not shown in Fig. 4, very high turbulence, such as would be found near the surface during a storm (i.e., >10-5 W Kg-1), results in complete disruption of the foraging process leading to larval starvation.
While some protozoan prey swim slowly, predictably, and tend to form patches, others swim quickly and do not aggregate in response to density or light gradients. When larvae are exposed to such prey, an increase in turbulence only tends to decrease feeding rates (Fig. 5). Presumably, this is due to the combination of the prey's fast swim speed and the elevated turbulent velocity which leads to disruption of the foraging process. Note that in both Figures 4 and 5, ingestion by larval cod in the dark was virtually independent of turbulence suggesting that vision is crucial for detecting prey under elevated contact rates.
In conjunction with John Quinlan and Cisco Werner at the University of North Carolina, we have been modeling the effects of the light field and turbulence on foraging in early larval cod in relation to the microzooplankton prey field. During the months of January through March on the Northeast Peak of Georges Bank, the abundance of microzooplankton prey typically exceeds minimum levels for good growth in early cod larvae. Model results show, however, that foraging depends strongly on the ambient light field which may be optimal for only a few short hours each day under the best of conditions, and perhaps not at all under cloudy and stormy conditions (Fig. 6). These and further results are being incorporated into a 3-D advection-diffusion model to determine growth and mortality rates of young cod as they are transported around the bank.