| Summary: | Population structure and life cycle of Pseudocalanus minutus and P. newmani in Toyama Bay, southern Japan Sea, were investigated based on seasonal samples obtained by vertical hauls (0-500 m depth) of twin-type Norpac nets (0.33-mm and 0.10-mm mesh) over one full year from February 1990 through January 1991. Closing PCP nets (0.06-mm mesh) were also towed to evaluate vertical distribution patterns in September 1990, November 1991 and February 1997. P. minutus was present throughout the year. The population structure was characterized by numerous early copepodite stages in February-April, largely copepodite V (CV) in May-November, and a rapid increase of adults in November to January. As the exclusive component of the population, CVs were distributed below 300 m in September and November both day and night. These CVs were considered to be in diapause. In February most of the Cl to CIV stages were concentrated in the top 100 m. All copepodite stages of P. newmani were collected for only 7 months of the year, disappearing from the water column in Toyama Bay from mid-June onward and their very small population recovered in November. Vertical sampling in September failed to collect them down to 1000-m depth. The period of their absence corresponded to the period of seasonal warming of surface waters. Unlike P. minutus, P. newmani appears to lack the ability to sink to cooler water during the warm summer season in Toyama Bay. Vertical sampling in February revealed that all copepodite stages of P. newmani were distributed largely in the top 100 m. From these results, combined with the temperature-generation length relationship established by McLaren et al. (1989a), it is estimated that P. minutus and P. newmani have two and three generations, respectively, per year. Further, the present results for both Pseudocalanus species are compared with those reported from coastal waters in the northwestern Pacific and other regions.
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