Synchronization in the molting and spawning activity of northern krill (Meganyctiphanes norvegica) and its effect on recruitment

The molting, spawning, and recruitment of northern krill (Meganyctiphanes norvegica) were analyzed over an annual cycle (1999–2000) in the Clyde Sea (west coast of Scotland). Results supported the hypothesis of a functional relationship between egg production and molt development for the duration of the reproductive season (March to August), with one reproductive cycle being made up of two molt cycles. Females remained in reproductive condition throughout the reproductive season, and the timing of their spawning and molting was synchronized at the population level throughout this period. A semiempirical model predicted that the krill population produced an egg pulse every 20 to 26 d (depending on temperature), and three cohorts were evident in net samples taken later in the year. The likely date on which the first cohort was spawned was around 26 d after the main phytoplankton bloom, suggesting that the bloom triggered egg development in all adult females. Such a synchronized spawning period was observed directly in adult females 26 d after a bloom in March 2000. A total of three cohorts over the 6-month reproductive season is less than the maximum of seven that would be possible if spawning occurred at a periodicity of between 20 and 26 d, suggesting that larval recruitment was not always successful. Analysis showed that successful recruitment was only achieved when chlorophyll a levels were adequate during both the period of egg maturation in the ovary and the subsequent development of larvae, especially the furcilia stages. The reproductive cycles of euphausiids are known to be adapted to local conditions. The length and timing of the spawning season varies interspecifically, from the apparently continuous breeding of Euphausia hanseni in the subtropical Benguela upwelling system to the seasonally distinct breeding episodes of Euphausia superba in the Antarctic (Siegel 2000). Intraspecific variation is also evident in widely distributed species, such as northern krill (Meganyctiphanes norvegica), that have short reproductive seasons in Mediterranean waters (e.g., the Ligurian Sea) but longer reproductive seasons in temperate boreal regions (e.g., the Clyde Sea and the Kattegat). A common pattern throughout the range of this species is the coincidence of reproductive scheduling with periods of optimal trophic conditions (Cuzin-Roudy and Buchholz 1999). The abundance of food items in boreal North Atlantic areas is generally high from early spring to late summer, facilitating a reproductive season that lasts from March to October. In the Mediterranean, the large blooms of 1 Present address: British Antarctic Survey, Madingley Road, Cambridge, CB3 0ET, U.K. ([email protected]).