The importance of the cost of swimming to the foraging behavior and ecology of larval cod (Gadus morhua) on Georges Bank

Energy expenditure by larval cod, Gadus morhua, during foraging was quantified based upon laboratory observations of search behavior and measurements of the cost of swimming. A large-volume (250-L) observation system employing stereo-paired video cameras was developed to record foraging behavior in three dimensions, and a respirometry system was developed to measure the cost of swimming of individual larvae.Application of the derived cost of swimming model to activity levels observed within the large observation tank showed that activity was a substantial and variable component of a larval cod's total bioenergetic budget. The estimated routine metabolic rate of the freely swimming larvae was 3.8–5 times greater than the measured basal rate depending upon their activity level. This is greater than the range of routine factorial scopes previously reported for larval fish in general. Future trophodynamic models developed for small marine fish larvae should consider these greater scaling factors when estimating the active metabolic rate of larvae foraging in the ocean.A trophodynamic model for larval cod on Georges Bank was developed incorporating observed foraging behavior, measured swimming costs, and the theoretical effect of turbulence on predator–prey contact rates. This model was used to estimate the prey density required to meet the estimated minimum daily metabolic demand. The estimated nauplius and copepodite concentrations required for the survival of small larvae were within the range of mean homogeneous springtime concentrations observed on Georges Bank. However, for the smallest post-yolk-sac larvae (5 mm), favorable low-turbulence foraging conditions, encountering patches of high prey density, or exploitation of alternative prey sources such as protozoans may be necessary.