Changes In Predation Pressure Research Articles

Gills as an Attachment Site for Salmincola edwardsii (Copepoda: Lernaeopodidae)

in length (Fig. 1). The increasing density observed in the P. acutus infrapopulations as related to length of definitive host, was not as stable as in the case of prevalence. In contrast, no noticeable influence of the host's sex was detected in size of infrapopulations of P. acutus, because neither prevalence (chi-square = 1.31; P > 0.05) nor mean intensity of infections (t = 0.30; P > 0.05) were significantly different between sexes of the host. Eighty-one percent and 67% of 21 males and 33 females examined were infected, respectively, harboring mean log(n + 1) densities of 0.55 (s2 = 0.35) for males and 0.56 (s2 = 0.17) for females. The prey of S. chilensis were determined from the analysis of 24 stomach contents. The items found (measured as frequency of appearance in the stomachs (%) with respect to the total of stomachs with some food content) were as follows: Cancer plebejus, 80%; polychaetes, 30%; Urechis chilensis, 3%; and others with less frequency of appearance and scarce biomass. It was also noticed that C. plebejus occured more frequently in stomachs of larger fishes as compared to the smaller ones (chi-square = 4.3; P < 0.05); therefore, there was a high correlation between prevalence of infections with P. acutus and frequency of appearance of C. plebejus (rs = 0.85; P < 0.05). It has been reported that crustacean decapods are the intermediate hosts in the life cycle of these nematodes. This is suggested by the presence of larval Proleptus sp. in Carcinus maenas (McIntosh, 1865, J. Microsc. Sci. 5: 1-4). Eupagurus bernhardus is suggested as another possible intermediate host for P. obtusus in Scyliorhinus canicola, although not confirmed to date (Lloyd, 1920, Proc. Zool. Soc. Lond. 1920: 449-456). We observed that all crabs found in stomachs of dogfishes were recently molted. No measurements were made on carapace lengths of crabs, but selective predation could be a factor similar to that reported for Micropterus dolomieu preying upon the crayfish Orconectes propinquus (Stein, 1977, Ecology 55: 1237-1253). We suggest that length and diet of S. chilensis play a relevant role in the magnitude of infrapopulations of P. acutus. These results are interpreted as associated with changes in predation pressure which, through time, S. chilensis would exert upon C. plebejus. This study was supported by Grant Dirc. 185/82 of the Direccion de Investigaci6n, Universidad Cat6olica, Chile. cy of ap earance of C. pleb jus (rs =

Early composition and biomass of the crustacean Zooplankton in Bough Beech Reservoir, south‐east England

SUMMARY. The crustacean Zooplankton community in Bough Beech, a flooded valley reservoir in south‐east England, was investigated in the early years of impoundment. The nutrient status and phytoplankton were typical of a lowland eutrophic lake, and this was reflected in the composition of the Zooplankton. At any given time, the latter comprised one or two cladoceran and two or three copepod species, of which the cladocerans were generally numerically predominant. Marked alterations in the Zooplankton community occurred in the first 2 years of the study and were related to changes in predation pressure by fishes. By the end of the third year, the composition of the Zooplankton community had stabilized.The cladoceran biomass was normally in excess of that of the copepods, with maxima in spring and autumn. Peaks in copepod biomass occurred within 2 months of those of the Cladocera.