Seasonal Variability in Abundance and Vertical Distribution of Parathemisto gaudichaudi (Amphipoda: Hyperiidea) in the North East Atlantic Ocean

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Abstract
Vertical distribution, seasonal and diurnal migrations and seasonal variability in abundance of P. gaudichaudi were investigated using the Longhurst Hardy plankton recorder (LHPR) between 1971 and 1974 at Ocean Weather Station (OWS) India (58.degree.00''N 19.degree.00''W) in the northeastern Atlantic Ocean. The results of this program were compared with those from the wider geographical coverage of the continuous plankton recorder (CPR) survey and placed in the context of an 18 yr series of data (1960-1977) from the northeastern Atlantic Ocean. The results from the CPR survey showed that P. gaudichaudi was widespread over the northern north Atlantic, being most abundant in the Labrador/Greenland current system and in the oceanic area to the west of the UK from June-Oct., while at OWS India they were most abundant between July and Sept. The young juveniles (less than 3 mm in length) accounted for 76% of the population at OWS India; they were found in the surface waters (0-50 m) during both day and night. There were considerable changes in the vertical distributions of adults by night and day with amplitudes of diurnal migration of 200 m. These migrations would not have been evident if the population had not been divided into size categories to identify the different behavior patterns exhibited with age. The population data suggest a 6-8 wk generation time from egg to adult (8 mm), allowing 2 wk from egg laying to liberation of young from the brood pouch and a further 4-6 wk to grow from 3 mm to an adult with a corresponding weight increase of 120-fold from 0.025 mg (3 mm) to 3.00 mg (8-14 mm).