Seasonal vertical distribution and population dynamics of the chaetognath Parasagitta elegans in the water column and hyperbenthic zone of ConceptionBay, Newfoundland

The vertical distribution and population dynamics of the chaetognath Parasa gitta elegans Verrill were determined in the water column and hyperbenthic zone of Conception Bay, Newfoundland from April 1997 to June 1998. The wate r column depth at the study site (47 degrees 32.2'N; 53 degrees 07.9'W) was 235 m. The temperature below the thermocline was < 0 <degrees>C the year r ound. Chaetognath samples from the water column were collected with a Tucke r Trawl. Those from the hyperbenthic zone, were collected with an epibenthi c sledge. Depending upon whether the hyperbenthic zone was assumed to exten d either 1 m or 10 m above bottom, the grand mean, areal abundance of chaet ognaths in the hyperbenthic zone ranged from 6% to 40% of the total abundan ce in the water column (including the hyperbenthic zone), and the grand mea n, areal biomass ranged from 25% to 77%. Large, mature individuals were col lected only in the hyperbenthic zone, whereas small, immature individuals w ere collected primarily in the water column. According to body length and o vary maturity data, three cohorts were identified in the hyperbenthic zone during the study period. Within each cohort, the length frequency of reprod uctively mature individuals was bimodal, with groups of mean length 33 mm a nd 41 mm reproducing from May to October. The recruitment period of juvenil e chaetognaths extended from July to February, coinciding with the recruitm ent period of copepods. The estimated individual growth rate of P. elegans was 1.0 mg C year(-1). The approximate generation time of the two groups of individuals with mean length at maturity of 33 mm and 41 mm was 450 and 78 0 days, respectively. This study demonstrates that a failure to sample the large, mature P. elegans living in the hyperbenthic zone leads to serious u nderestimates of the total abundance and biomass of chaetognaths and an ina ccurate picture of seasonal population dynamics.