Estimating the energetic savings for green sturgeon moving by selective tidal stream transport

As fish move, they incur an energetic cost of transport (COT) from the use of aerobic muscles. Water currents are an integral component of the physical world of fishes, and if water currents are present, fish may pay higher costs fighting the currents, or may use the currents to facilitate movement and reduce COT. Some fish use selective tidal stream transport (STST) to move efficiently through tidal regions, swimming into the water column when the current is favorable, and returning to the bottom during opposing tides. This behavior has been reported in marine fish migrating through tidal habitats, but it is also likely of value in daily movements of fish residing in those habitats. It is extremely difficult to directly measure COT in wild fish; however, it is possible to combine analysis of field telemetry data of individual swimming efforts and measurements of metabolic costs of swimming from respirometry in order to estimate the COT in the wild, and to calculate the costs or savings of swimming with or against currents. We describe this novel analytical approach and demonstrate it using data from two green sturgeon tracked in San Francisco Bay, California. In this analysis, when moving at the surface and employing STST, the fish benefited from the current, swimming within 85.5% of optimal efficiency. When conducting non-STST movements near the bottom, swimming was less efficient, with a COT similar to swimming directly into the current. These results suggest that green sturgeon may opportunistically utilize stream transport in daily movements, swimming at the surface and orienting with currents to achieve substantial energetic savings.