Roubal: Seasonal occurrence of transversotrematid trematodes

Ectoparasitic Prototransversotrema steeri Angel infected Acanthopagrus australis (Günther) in western (W. Bay) and southern (S. Bay) Moreton Bay with greatest abundance in winter (June August), and none in summer (December February). This confirms previous observations on seasonality of P. steeri from a small estuary in New South Wales. Transversotrema licinum Manter, which is a new host and geographical record, infects A. australis in W. Bay but not S. Bay, with large abundances in summer and winter, lowest abundance in spring. Increased size of P. steeri in winter in S. Bay could be explained by increased size of fish sampled since fish length and size of each parasite species were correlated positively. Transversotrema licinum increased in size from summer to winter irrespective of fish size. Number of eggs in utero and parasite size were correlated for both species; worms in mid-size range had most eggs, and P. steeri had more eggs (average = 19.2, 0-80) than T. licinum (3.3, 0-21); eggs were similar in length. Number of eggs per parasite, adjusted for parasite size, was greatest in autumn (P. steeri) or summer (T. licinum), i.e. early in infection period. Spawning migration to eastern Moreton Bay did not influence number of eggs per parasite. There are few studies on the seasonal occurrence of ectoparasites on Australian marine fish. Roubal (1990) found that species of ectoparasitic Monogenea and Copepoda on the sparid fish Acanthopagrus australis (Günther) from a small estuary in northern New South Wales exhibited different patterns of seasonal occurrence: a) prevalent year-round, b) most prevalent in summer, c) non-seasonal but not random, and d) apparently random occurrence. The digenean Prototransversotrema steeri Angel was unique in that it occurred only in winter. It is not known if this parasite occurs only in winter on A. australis in other localities in eastern Australia. Transversotrematid digeneans infect a variety of fish in Moreton Bay, southern Queensland (Cribb et al. 1992), but it is not known which species infect the economically important A. australis in this locality. Prototransversotrema steeri was suggested as a cause of skin ulceration on mugilid mullets as well as A. australis from the Noosa river to the north of Moreton Bay (Rodgers and Burke 1988). The present study was done to determine which species of transversotrematid digenean infect A. australis in Moreton Bay, and if there are any significant effects of season, locality and host size on the occurrence of infection, size of parasite and the number of eggs. MATERIALS AND METHODS Individuals of Acanthopagrus australis were collected by handline and tunnel net from different parts of Moreton Bay (15310’E, 2720’S) (Fig. 1). Fish were sampled at Wynnum (20/2, 7/12/1990), Green Island (28/2/1990, 7/2/1991), Mud Island (12/4/1990, 25/9/1991) and Wellington Point (23/5, 12/7, 28/8, 17/10/1990, 2/5/1991) in western Moreton Bay (W. Bay), near the mouth of the Pimpama River in southern Moreton Bay (S. Bay), and the winter spawning grounds at Jumpinpin in eastern Moreton Bay (E. Bay) (Fig. 1, Table 1). Average monthly surface water temperature (C) and salinity (parts per thousand), respectively, at Toondah Harbour, near Wellington Pt. (W. Bay) during 1995 were: January (27, 37), February (25, 32), March (25, 34), April (20, 35.3), May (18, 35.4), June (17, 35.3), July (16, 36.5), August (18, 36.2), September (18, 36.8), October (23, 36), November (26, 35.8), December (28, 37.5) (Roubal and Graham, unpublished). Similar values were found at Toondah Harbour during 1990 by Shields and Wood (1993). Upon capture all fish were killed by spinal severance and preserved immediately in 10% formalin in sea water. Because fish were often caught by commercial fishermen, it was most expedient to preserve all fish from one sample in the same 30 l bin of formalin rather than individually. This meant that any detached parasites could not be ascribed to individual fish. Transversotrematid digeneans were identified from Cribb et al. (1992). The abundance (average number per infected and uninfected fish) (Margolis et al. 1982) was determined from total counts of attached and detached parasites in each sample. Those parasites that remained attached enabled an examination of the relationships between fish size and number and size of attached parasites. Attached parasites were collected by examining the skin and scrapings of the body surface with a dissecting microscope. All parasites were preserved in 70% ethanol. The standard length (SL) of each fish was measured. FOLIA PARASITOLOGICA 45: 205-210, 1998 Address for correspondence: F.R. Roubal, Department of Parasitology, University of Queensland, Brisbane, Queensland 4072, Australia. Phone: ++61 07 46394031; E-mail: [email protected]