| At all scales, from small ponds to oceanic gyres, fish must contend with the fact that the medium in which they move is itself in motion. Water currents can both impede and assist travel, either requiring compensatory orientation and movement or facilitating transport. The ability to detect, orient, and react to currents is particularly important for diadromous species that routinely pass through multiple complex environments as they inhabit oceans, estuaries, and rivers. It is expected that fish will have evolved behaviors to maximize the efficiency of movement, permitting available energy to be allocated instead to growth and reproduction. Here, I analyze the movements of anadromous green sturgeon (Acipenser medirostris), manually tracked during their seasonal residence in the tidally-complex San Francisco Bay Estuary, California. In Chapter One, I describe the general movement patterns of the species in relation to parameters such as depth, temperature, salinity, dissolved oxygen, and time of day. Movements were categorized as either non-directional, erratic movement near the bottom consistent with resting or foraging activity, or directional, extended periods of linear travel conducted in close proximity to either the surface or the substrate. The fish typically utilized the shallower regions of the bay, only passing through the deep channel areas during directional surface movement. Movement was independent of the other measured parameters. In Chapter Two, I further analyze the directional movements of the fish in relation to current and position in the water column. I show that green sturgeon respond to different current conditions with flow-vector kinesis, moving into the current near the bottom in shallow, slow-flowing regions of the bay, but orienting with the flow at the surface in the deep, swift-flowing channels. In Chapter Three, I calculate the metabolic cost of transport for fish exhibiting positive and negative flow-vector kinesis and demonstrate that the fish are maximizing the efficiency of their daily movements by utilizing local currents. While previous research has considered the role of positive flow-vector kinesis in fishes migrating in tidal currents, this study is the first consider these behaviors in a broader context and describe their role in non-migratory, daily behavior. |
