Distribution and some biological features of skate Bathyraja parmifera (Rajidae, Rajiformes) in the northwestern part of the Sea of Japan
In the northwestern part of the Sea of Japan, skate Bathyraja parmifera in catches of the bottom trawl occurs at depths of 40 to 830 m at the near-bottom temperature of 0.4–4.9°C. Throughout the year, the skate performs migrations within the continental slope and, in the feeding period, inhabits mainly the depth range of 300 to 500 m. Maximal average density of aggregations of B. parmifera is observed in waters of northern Primorye; minimal average density is observed in Peter the Great Bay and on the Yamato Bank. In catches, skates with a length of 17–130 cm and a weight of 0.5–18.8 kg are recorded; however, fish with a length of 80–115 cm dominate. Individuals with a length of up to 40 cm feed mainly on amphipods, and larger immature and adult individuals feed on decapods, cephalopods, and fish. The value of the daily ration of B. parmifera with its growth decreases from 2.4 to 0.9% of the body weight. Specific ecological features of B. parmifera are a cause of its considerable underfishing by the bottom trawl; therefore, the total resource of this species in the considered region may be at the level of maximal estimates obtained during surveys and comprise no less than 20000 t.
Breaking with tradition: redefining measures for diet description with a case study of the Aleutian skate Bathyraja aleutica (Gilbert 1896)
Characterization of fish diets from stomach content analysis commonly involves the calculation of multiple relative measures of prey quantity (%N,%W,%FO), and their combination in the standardized Index of Relative Importance (%IRI). Examining the underlying structure of dietary data matrices reveals interdependencies among diet measures, and obviates the advantageous use of underused prey-specific measures to diet characterization. With these interdependencies clearly realized as formal mathematical expressions, we proceed to isolate algebraically, the inherent bias in %IRI, and provide a correction for it by substituting traditional measures with prey-specific measures. The resultant new index, the Prey-Specific Index of Relative Importance (%PSIRI), is introduced and recommended to replace %IRI for its demonstrated more balanced treatment of the relative measures of prey quantity, and less erroneous behavior across taxonomic levels of identified prey. As a case study, %PSIRI was used to examine the diet of the Aleutian skate Bathyraja aleutica from specimens collected from three ecoregions of the northern Gulf of Alaska (GOA) continental shelf during June-September 2005–2007. Aleutian skate were found to primarily consume the commonly abundant benthic crustaceans, northern pink shrimp Pandalus eous and Tanner crab Chionoecetes bairdi, and secondarily consume various teleost fishes. Multivariate variance partitioning by Redundancy Analysis revealed spatially driven differences in the diet to be as influential as skate size, sex, and depth of capture. Euphausiids and other mid-water prey in the diet were strongly associated with the Shelikof Strait region during 2007 that may be explained by atypical marine climate conditions during that year.
Die Populationsdynamik Von Canthocampt Us Staphylinus (Jurine) (Copepoda, Harpacticoida) Im Litoral Des Bodensees
The population dynamics of Canthocamptus staphylinus in the littoral zone of Lake Constance are described (fig. 2). Adult animals aestivate as cysts. The main part of the population emerges from the cysts in autumn. During the resting stage and the first spell after emerging, mortality is very low (fig. 3). Reproduction, expressed as numbers of ovigerous females, begins in November and reaches its maximum in January. The new generation becomes adult in May/June and encysts without previous reproduction. According to Sarvala (1979) it may be possible, that in littoral populations dormancy is induced by long day periods and temperatures of over 12° C (figs. 1, 2). In the profundal zone C. staphylinus also forms cysts, however, both light and temperature are not likely to induce this stage in depths of over 60 m.