Assemblages of ectoparasites of a pelagic fish, slimy mackerel (Scomber australasicus), from south-eastern Australia.

Four-hundred and fifty-three Scomber australasicus, ranging in length from 14.1 to 46.5 cm and taken in 12 samples over 5 years from a single locality in south-eastern Australia, were examined. Ten species of ectoparasites were recorded: six monogeneans, three copepods and one isopod. The maximum number of parasite species in any one fish reached five, and the maximum total parasite intensity reached 39. Host size is an important determinant of the structure of ectoparasite assemblages of slimy mackerel: the average number of species per host peaked at 2.2 in fishes between 20 and 25 cm in length, then declined in larger fish; the abundance of all parasites on each fish similarly peaked in fishes 20-25 cm (mean of 9.9). The monogenean Pseudokuhnia minor had the highest prevalence and abundance of all parasites, infecting almost 80% of fish < 25 cm long. Over half of the total number of parasites belonged to this species, and it was dominant in intensity in just over half of the fish in which it occurred. When approximate volume was considered, assemblages were dominated by Kuhnia scombercolias in fish < 20 cm, by Kuhnia scombri in fish 20-34.9 cm, and by P. minor in fish larger than 35 cm long. The intensities of these three monogeneans were very strongly and positively correlated with each other, as were the intensities of P. minor and Grubea australis; the intensities of only one pair of species were associated negatively (K. scombercolias and the isopod Ceratothoa imbricata). A nested subset analysis indicates that the ectoparasite assemblages are random. This and the low prevalences and abundances of infection, as well as low species richness per fish, indicate that infra-assemblages are isolationist.