Metabolic costs of pursuit and attack in early larval Atlantic cod

Metabolic costs of pursuit and attack for early larval Atlantic cod (Gadus morhua) feeding on 2 different prey types, a protozoan (Balanion sp.) and a copepod nauplius (Pseudodiaptomis sp.) were estimated from 5 to 20 d post-hatch. Parameters describing feeding patterns were determined from 3-dimensional observations using 2 orthogonal cameras, Feeding parameters were calculated using a digital image-analysis system for larvae in 3 size classes, and included pursuit duration, distance and speed for both burst and glide components, and attack duration, distance and speed. Metabolic rates were estimated for each component of pursuit and attack from the relationship between specific metabolic rate and swimming speed from previous respirometry studies on cod larvae. Estimated specific metabolic rates were used in a new model to determine specific and total energy expenditures, which incorporated each of the components of pursuit and attack. Values of total energy expenditure were used in a model to determine relative net energy gain during pursuit and attack of a larva on a protozoan and a copepod nauplius in 3 larval size classes. In Size Class 1 (5 to 6 mm total length, TL) first-feeding larval cod were small and poorly developed, had long pursuit and attack times and distances, and slow swimming speeds. In Size Classes 2 (6 to 7 mm TL) and 3 (7 to 8 mm TL) larger larvae were better developed, had shorter pursuit and attack times and distances, and faster swimming speeds. Specific metabolic rates during pursuit of a protozoan increased with increasing larval size, while specific energy expenditure decreased due to shorter pursuit times. Results from net energy gain equations showed that a cod larva gained 3 orders of magnitude more energy from consumption of a nauplius than a protozoan even though its energy expenditure during pursuit of a nauplius was 2 to 3 times that during pursuit of a protozoan.