Comparing the somatic growth and swimming kinematics of Atlantic cod (Gadus morhua, Linnaeus) and haddock (Melanogrammus aeglefinus, Linnaeus) larvae.

Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) are found on both sides of the North Atlantic and often their spawning overlaps in time and space. Yet, haddock has a much more variable recruitment than cod in areas where they are sympatric, a difference that is consistent across ecosystems. At the larval stages, differences in feeding behavior have consequences for growth and survival, potentially contributing to the differences in recruitment between these species. We tested whether haddock displayed different swimming behavior to cod under abundant food conditions, while also comparing their growth every 7 days until 38 days post hatching (dph). Swimming kinematics were quantified from the three-dimensional trajectories of larvae filmed in an aquarium at 11, 21, and 32 dph. Cod swam faster than haddock starting at 21 dph. Differences in swimming speed between species were consistent with body morphology. Haddock was heavier than cod at a given age and body length. At 11 dph, haddock exhibited longer pauses and wider horizontal and vertical turn angles than cod, indicating a moderately larger visual search field compared to cod. Longer pause duration indicates that haddock are searching for prey for longer periods than cod at 11 dph. The differences in prey search disappeared at 21 and 32 dph. The differences in feeding and swimming behavior reported here could play an important role for survival, with consequences for the recruitment of these species.