Swimming capacity and pleopod beat rate as a function of sex, size and moult stage in Northern krillMeganyctiphanes norvegica

An experimental method to estimate the relative swimming capacity of pelagic crustaceans is proposed. Swimming capacity measured as propulsive force can be estimated by attaching the animal to a virtually friction free rotational displacement transducer with an arm allowing only forward and backward movements. The method also allows determination of beat rates of the pleopod cycle. Results from the use of the method on different sizes, sexes and moult stages of Northern krillMeganyctiphanes norvegicaindicate that the swimming capacity of krill increases with the size of the animal, although pleopod beat frequency decreases with increasing size of the animal. We found no difference in swimming capacity between females and males, but female krill had a higher pleopod beat frequency than males. The moult stage of the animal affects the swimming capacity, with newly moulted krill being weaker swimmers. It also affects pleopod beat frequency, with newly moulted krill having the lowest frequency. We argue that results of this kind may be useful in attempts to interpret observed demographic distribution patterns in migration studies as well as in studies of krill population dynamics.