The foraging sorties hypothesis : evaluating the effect of gut dynamics on copepod foraging behavior

While the diel vertical migration of copepods has been known for over a century, the details of this migration remain elusive. Field studies in Dabob Bay, a fjord in Puget Sound, Washington State (U. S. A.) show that copepods found in food- poor layers had gut contents similar to copepods found in food-rich layers. Two hypotheses have been proposed to explain these observations; the in situ feeding hypothesis and the foraging sorties hypothesis. The in-situ feeding hypothesis states that copepods with food in their guts were feeding in the layers where they were captured. The foraging sorties hypothesis predicts that copepods migrate into food-rich layers, where they feed to saturation, in brief bouts that lasts several tens of minutes. They then sink out as they digest. Once their gut is cleared the return to the shallow layer and repeat the cycle. In both cases feedback from the gut during feeding is critical. The present work has two components. The first consists of a series of laboratory experiments where I use a planar laser imaging fluorometer to quantify the dynamics of food movement through the guts to determine the time scales that are likely to dictate feeding behavior of individual copepods and their resulting vertical distribution. The second component of this work consists in deploying a profiling multi-spectral imaging system. This system is used to record individual copepod location in the water column and to quantify their corresponding gut content. Used in conjunction with an IBM, partially based on the laboratory work, I show that both in situ feeding and foraging sorties hypotheses are feasible strategies. Furthermore I show that dominance of one strategy over the other will depend greatly on the interaction between the copepods' physiology and the environment they experience. Thus, prediction of the general foraging pattern may be possible from knowledge of the hydrographic structur