Distribution and abundance of ctenophores, and their zooplankton food in the Black Sea. I. Pleurobrachia pileus

The distribution of Pleurobrachia pileus Muller, 1776 in the Black Sea was determined using plankton samples collected above the anoxic zone (maximum of 200 m) in the winter, spring, and summer of 1991 to 1995; The summer sam ples were collected in 1991 to 1993 (for a previous) and are included in th is paper for comparative purposes. High concentrations of P. pileus were fo und at the northern edges of anticyclonic eddies along the southern coastal regions. The biomass and abundance of P. pileus increased from winter thro ugh spring to a peak in summer. The highest mean wet weight during a sampli ng period was 250 g m(-2), while the maximum wet weight was 1429 g m(-2) P. pileus was mostly found in a layer extending from the lower parts of the t hermocline down to the anoxic zone, where the temperature was <8 degrees C. The vertical distribution of P. pileus biomass had two clear maxima at nig ht: an upper maximum at 20 to 40 m was less pronounced than the lower maxim um at 90 to 120 m depth. Mean body length of P, pileus did not exceed 12 mm . Smaller individuals (9 to 10 mm length) occurred in winter. P. pileus had two length classes in early spring (March 1995) and late summer (August 19 93), indicating the presence of both newly hatched and larger individuals. Overall, the stomach contents of P. pileus consisted mainly of Copepoda (90 %), Cladocera (1%), Mollusca (1%), fish eggs and larvae (1%), and other tax a (7%). The preferred food of P. pileus (frequency of occurrence) was: Cala mus euxinus (39%), Pseudocalanus elongatus (30%), Acartia clausi (28%), Oit hona similis (2%), and Paracalanus parvus (1%). The endoparasite Hysterothy lacium aduncum was commonly found in P. pileus. Abundances of Mnemiopsis le idyi and P. pileus were either negatively correlated (r = -0.5 to -0.7) or positively correlated at a low significance level (r = 0.25 to 0.3) with ab undance of A. clausi in different months of the year. Aurelia aurita abunda nce was correlated mainly with the abundance of C. euxinus from June 1991 t o March/April 1995. Over the same period the abundance of P. pileus was sig nificantly correlated with the abundance of P. elongatus, an important prey species.