A net sampling survey was carried out for krill in a standard station grid around Elephant Island during 27 January to 4 February 2001. In comparison with recent years the station grid was extended south, where a large proportion of small size classes, one-year-old juvenile krill was found. Results show a spatial separation of the juvenile krill and the spawning stock, Krill density was significantly higher than during the past years (229 krill 1000 m−3 or 13.0 g m−2). The proportional recruitment index for the entire survey area for the 1999/2000 year class was R1 = 0.573, which is among the highest values recorded during the past 20 years. The maturation index (based on the proportion of gravid stages) was G = 0.99, indicating an early initiation of the spawning season. The results indicate a turning point after a succession of years with poor recruitment success and low stock biomass. This is thought to be the first step for a successful spawning event and a later potential recruitment success of the 2000/01 year-class. The spatial extent of the station grid is discussed in the light of a representative coverage of the stock and the estimated recruitment index.

During their diel vertical migration in the Gullmarsfjord (Swedish west coast) Nordic krill Meganyctiphanes norvegica made excursions into regions of severe hypoxia, >7O m depth, dunng the day. Consequently, we investigated the capacity for anaerobic metabolism by M. norvegica and the extent to which they utilize this capacity in the field. L-lactate was the main end-product of anaerobic metabolism. The concentration of L-lactate in the haemolymph ([la~tate],,~) under conditions of acutely declining p 0 2 only increased below 4 to 6 kPa. Dunng anoxia, no krill survived > 1 h and accumulation of L-lactate was at its most pronounced. Handling stress had little effect on [lactateJHL. Field observations (January 1998) showed that during the day krill resided at a depth of 65 to 85 m (PO, = 3 to 10 kPa). Net cages were stocked with krill, trawled from 60 to ?O m depth at dusk, and then kept overnight at 40, 70 and 90 m depth ( p 0 2 >14, 6.1 and 1.8 kPa respectively). This resulted in krill mortality of 7, 70, and 100% respectively. While individuals caged at 40 m showed [1actatelHL no greater than that of normoxic laboratory individiials (3.04 i 1.05 mmol 1-I). survivors at 70 m showed elevated concentrations (9.91 I 1.68 rnmol 1"). Furthermore, newly trawled knll (at dusk) had [ l a~t a t e ]~, = 7.18 t 2.72 mmol 1-', indicating that, like the caged individuals at that depth, they too had resorted to anaerobic metabolism. This study has shown that while anaerobic metabolism is not well developed in M.norvegica, individuals do enter bodies of water where the p 0 2 is below that required to maintain aerobic metabolism. Utilization of anaerobiosis seems to be cntical to the ability to enter such O2 poor water, although the krill are close to the limits of their physiological capacity at such times.

No abstract

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 krill Meganyctiphanes norvegica indicate 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.

Most krill species undergo diel vertical migration (DVM) which is to some extent influenced by light intensity. During a solar eclipse (11 August 1999) the upward and downward movement of krill, Meganyctiphanes norvegica, inhabiting a Swedish fjord followed closely changes in light intensity. Here the eclipse was partial (77%) and the weather overcast and yet krill at 70–90 m depth were able to detect, and respond to very small changes in light. This observation of an event during mid-day which is not pre-programmed confirms light as an important triggering mechanism for krill DVM.