Habitat Switch and Niche Overlap in Coregonid Fishes: Effects of Zooplankton Abundance

Vendace (Coregortus albula) in Lake Mjøsa lived in the pelagic zone and gillnet catches were correlated with water temperature (r2 = 0.457, p < 0.005). Whitefish (Coregonus lavaretus) exploited both epibenthic and pelagic areas. Whitefish with body lengths between 25 and 35 cm moved from the epibenthic to pelagic zone during summer; other length groups remained in epibenthic habitats. The pelagic fish returned to epibenthic areas in the autumn. Cladocerans were important food items for both fish species in the pelagic zone. The large copepod Limnocalanus macrurus was eaten only by vendace whereas surface insects were more important prey for whitefish. This suggests that vendace is the more specialized zooplanktivore. The pelagic gillnet catch of whitefish and the pelagic habitat overlap between whitefish and vendace increased with increasing zooplankton densities at depths between 0 and 50 m (r2 = 0.609 and 0.494, respectively, p < 0.01). During spring and autumn we observed a time lag between the change in zooplankton abundance and habitat switches of whitefish. The fish moved to the more profitable habitat several days after food availability had changed, indicating an element of learning in choice of habitat.

Planktivorous Feeding Ecology of Arctic Grayling (Thymallus arcticus)

A component analysis approach was applied to studies of the zooplankton feeding of Arctic grayling (Thymallus arcticus) of 3–20 cm standard length using all arctic zooplankton species common in the area. Unlike other fish species, larger grayling have greater reactive distance. As expected, the ability of grayling to locate prey was found to increase with size of prey and with light intensity. Grayling vision was found to increase dramatically at light intensities higher than 100 lx. Generally, grayling located and attacked any species within the visual field, locating all by core body size. One exception was the copepod Diaptomus pribilofensis, which was of appropriate size and pigmented a bright red, but was never attacked and eaten by any grayling. The large copepod Heterocope septentrionalis was able to evade grayling feeding attack, but Heterocope evasion was dependent on grayling size and water temperature. Analysis of inter-gill-raker spacing showed that spacing increases linearly with fish length up to 13 cm, at which length no further increase was observed. Such change in inter-gill-raker spacing suggests the feeding niche of grayling may be broader than that of similar sized, temperate centrarchid planktivores.Key words: Arctic grayling, Thymallus arcticus; planktivorous fish, predator–prey interaction, prey evasion, zooplankton

Comparison of the Feeding Behaviour of Calanus and Pseudocalanus in two Experimentally Manipulated Enclosed Ecosystems

INTRODUCTIONImplicit in some recent models of the dynamics of planktonic systems, for example that of Steele & Frost (1977) is the concept that filter-feeding copepods may be scaled by size when considering feeding in relation to the size composition of their paniculate food. For a particular organism ingestion of phytoplankton has been considered to be a function of its own weight and the size composition of the phytoplankton available to it. Such scaling has been assumed to occur both in an intra- and interspecific sense, with model development involving a ‘large’ copepod {Calanus) able to feed and grow more efficiently on ‘large’ cells (diatoms) and a ‘small’ copepod {Pseudocalanus) better adapted to feed on ‘small’ cells (flagellates). However, although the general relationship between growth and metabolic rate and body size is widely accepted (Banse, 1976; Fenchel, 1974) this relationship between dietary particle-size composition and size of organism has remained a working hypothesis with little experimental support..

Copepods from the Interstitial Fauna of a Sandy Beach

Attention was first drawn by Wilson (1932) to the large copepod fauna which can be found in an ordinary sandy beach. In his introduction he describes his method of collecting these copepods (p. 7) and in the course of the paper describes a number of new genera and species thus obtained.Sand-dwelling animals, particularly Crustacea and worms (excluding sessile forms), are usually regarded as burrowers, since in their migrations they displace the particles of their environment. The fauna opened up by Wilson's discovery is of a quite different type. In contrast to true sand-burrowing animals, these copepods do not displace the particles of the sand through which they move but crawl over the surface of the grains, which, by capillarity, always hold more or less water, even high up on the beach at low tide. Such copepods, together with nematodes, rotifers, protozoa and other animals sufficiently small, may be regarded as part of an “interstitial” fauna.