Cercariae of the trematode Cryptocotyle concavum, which encyst in skin and/or kidney of sticklebacks and gobies, were studied in the Schlei Fjord (western Baltic Sea). Mean incidence of dermal cysts was 48 % in Gasterosteus aculeatus and 37 % in Pungitius pungitius. No cysts were found in the kidneys of sticklebacks. While 97 % of Pomatoschistus microps had encysted metacercariae in the kidneys, only 2 % had cysts in the skin. Pomatoschistus minutus, however, showed hardly any cyst infestation of either skin or kidney. In P. microps the intensity of infestation by metacercariae was frequently more than 50 cysts; in contrast, sticklebacks rarely exhibited more than 5 dermal cysts. Infested fish were larger than 10 mm in total length, the incidence rate increasing with growth. Parasitic infestation depends on ambient salinity: C. concavum was not found at salinities below 4 %o. In contrast to the high incidence in fish, the first hosts -the snails Hydrobia stagnalis and H. neglecta -showed remarkably low infection rates (3 to 5 %). The findings reported are related to the distribution of C. concavum, the mode of life of infested fish, the feeding habits of the final hosts and the infestation of P. micropsby other parasites. Evidently, P. microps represents an optimal second host for C. concavum.
The daily food intake and feeding activities of the common goby Pomatoschistus microps (Kroyer) and the nine-spined stickleback Pungitius pungitius (L.) were investigated in the brackish Schlei fjord. At the investigation site of Olpenitz, salinities varied between 11 and 15 %o, and water temperatures between 5 and 18 ~ during the period of in-situ experiments in 1981 and 1982. Common gobies sometimes attained a density of more than 100 individuals per square metre, nine-spined sticklebacks as much as 18 individuals per square meter. Their food changed depending on the supply of plankton or benthos. Regarding numbers, their food consisted mainly of harpacticoids, in springtimes of calanoids; with regard to weight, amphipods, polychaetes or chironomid larvae often prevailed. The total food ingestion, measured by means of its relation to fish weights (fullness index), was highest in spring and summer: 2.3 % in P. microps and 2.6 % in P. pungitius. Low fullness indices of 0.8 % in P. microps and 0.3 % in P. pungitius were found during times of low water temperatures. 24-h field investigations revealed that the adult P. microps presented clear diurnal rhythms with highest fullness indices after dawn and a further maximum at dusk. Only young gobies ingested some benthos at night. P. pungitius displayed similar rhythms in which maximum fullness indices varied during the day. This species seems to forage every four hours. At dusk, both species preferred benthos to plankton. The results are discussed with respect to the implications of diurnal and seasonal fluctuations in prey choice by small-sized fish populations.9 Biologische Anstalt Helgoland, Hamburg
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