The macrohabitat preferences of 1+ fish (10 cm FL) and the microhabitat use of 0+ juvenile fishes in the River Lee catchment (UK) were examined from data collected over a 3‐year period between late autumn and early spring using depletion sampling and point abundance sampling, respectively. Canonical correspondence analysis and habitat profiles revealed preferences in the more rheophilous fish species (e.g. brown trout, Salmo trutta L., barbel, Barbus barbus (L.)) for features characteristic of upstream natural channels and meander sections of the by‐passed old river, with the more ubiquitous (e.g. perch, Perca fluviatilis L.) and limnophilous fishes [e.g. pike, Esox lucius L., tench, Tinca tinca (L.)] preferring habitats mainly in downstream channelised stretches. The microhabitat of 0+ juvenile fishes in the River Lee was similar to that reported elsewhere for the same species, influenced mainly by channel width, depth, and distance from the bank, though microhabitat overlap in 0+ roach, Rutilus rutilus (L.), and gudgeon, Gobio gobio (L.), was greater in the Lee than observed in larger, more open river systems.

Quantitative population dynamical information derived from laboratory-and field-based experiments is provided for the fish-parasitic copepod, Lernaeocera branchialis, infecting flounder (Platichthysflesus) and whiting (Merlangius merlangus). Adult, post-metamorphosis females from whiting can produce more than one set of egg-strings. The mean number of eggs in each egg-string pair was 1445. At 10 C these eggs took about 12.7 d after extrusion before hatching of NI nauplii began. Hatching took up to 12 days to be completed with an exponentially declining pattern of output over this period. In the laboratory about 44% of the egg-string egg population successfully passed through the NI to NII nauplius moult and the NII to copepodid moult to produce infective copepodids, a process lasting about 2 d. The non-feeding copepodids had a maximum survival time at 10 C of 18 d, with a time to 50% survival of 7.5 d. In laboratory infection experiments at 10 C, copepodids infected flounder and passed through all their developmental stages to adulthood and copulation in a minimum of 25 d. Field experiments on the seabed off Lowestoft in June 1987 with a sea temperature of about 16 °C suggested that the developmental period in those conditions could be as short as 11 d. Previously uninfected flounder in the field experiments became naturally infected with copepodids at a mean rate of not less than 30 parasites per fish d-.

Whiting collected from 36 stations in the North Sea in August 1985 and February 1986 were examined with reference to the prevalence, intensity and microhabitat utilization of three ectoparasites, namely hrnueocera brunchialis (Copepoda: Pennellidae), Clavella adunca (Copepoda: Lernaeopodidae) and Diclidophoru merlangi (Monogenea: Polyopisthocotylea). Maximum prevalences and mean intensities, at any one station, for the three parasites were 74,45.8,60% and 1.07,0.56 and 1.30, respectively.At the stations sampled, infestation levels with L. branchiulis and C. udunca were significantly higher in the winter than in the summer, with winter mean parasitic intensities being between five and eight times higher than those in summer. A converse situation applied with D. merlangi, with summer mean intensities being over four times larger than those in winter.Pooled winter and summer parasitic infestation data, from the 36 stations, enabled large-scale zoogeographical parasitization patterns to be recognized. Both L. branchialis and C. udunca are rarely seen on whiting in the North Sea north of latitude 56" N. South of this latitude, prevalences and intensities of L. branchialis are higher on the eastern than on the western side of the sampling zone, while those of C. adunca are similar on both sides of the North Sea. D. merlungi's infestation exhibits a striking latitudinal cline in the study area, with prevalences and intensities increasing steadily from south to north.All three ectoparasites were non-randomly distributed within the whiting gill and/or buccal cavities and showed marked microhabitat preferences. Possible explanations for the seasonal and geographical variations in the parasitic infestations are discussed.