Journal of Fish Biology (1997) 50, 341–357 Habitat, reproductive biology and size composition of Parequula melbournensis, a gerreid with a temperate distribution G. A. S, G. A. H   I. C. P School of Biological and Environmental Sciences, M urdoch University, M urdoch, W estern A ustralia 6150 (R eceived 18 M arch 1996, A ccepted 10 A ugust 1996) Trawling was carried out over sandy substrates in the shallow (5–15 m) and deeper (20–35 m) waters of four regions along 250 km of the lower west coast of Australia, during seven consecutive seasons. This yielded 32 752 individuals of the gerreid Parequula melbournensis, which constituted c. 42% of the total number of fish. Densities of P. melbournensis were greatest in the most southern region, reaching a seasonal maximum of 835 fish ha 1 at one site in that region. Since P. melbournensis is restricted largely to the southern coastline of Australia, it has a temperate rather than subtropical or tropical distribution and thus is not a typical gerreid. Furthermore, unlike most other gerreids, it does not spend part of its life cycle in either estuaries or nearshore marine waters. The maximum total length of P. melbournensis was 175 mm, with the length at maturity (L 50 ) being 115 mm in females and 121 mm in males. No clear monthly trends were exhibited by gonadosomatic indices, the prevalence of mature ovaries and the oocyte size–frequency distributions of female P. melbournensis, and no clear and consistent modes were observed in length–frequency data for this species. These strong indications that spawning occurs throughout the year were substantiated by the occurrence of post-ovulatory follicles in the ovaries of large fish in all months but August, and by the presence in that month of advanced yolk granule oocytes in some ovaries, which implies that spawning was imminent. The spawning of P. melbournensis throughout the year contrasts with the far more restricted spawning periods recorded for other teleosts in the same temperate Australian waters. In this respect, P. melbournensis exhibits the characteristics of the essentially tropical family to which it belongs. Annuli, which could be detected on the otoliths of c. 40% of fish, suggest that the majority of P. melbournensis were <3 years old.  1997 The Fisheries Society of the British Isles Key words: Gerreidae; water depth; oocytes; post-ovulatory follicles; spawning; size composition. INTRODUCTION The Gerreidae, which comprises eight genera and approximately 40 species, commonly known as mojarras, silver biddies or purse mouths, is found in most warm seas, with several species spending part of their life cycle in estuaries (Cyrus & Blaber, 1982; Kerschner et al., 1985; Blaber et al., 1989; Nelson, 1994). Despite their very high numbers in many areas and commercial importance in some regions (El-Agamy, 1988; Kurup & Samuel, 1991), there have been relatively few detailed studies on the biology of species comprising this family. Indeed, the only study on the reproductive biology of gerreids, which has incorporated the type of histological examination of gonads that is usually required to determine the precise duration of the spawning period and whether spawning occurs more than once in a breeding season, is that carried out by Cyrus & Blaber (1984a) on three species of Gerres in estuaries in tropical southern Africa. However, the absence of Tel.: + 61 9 360 2579/2524; fax: + 61 9 310 3505. 341 0022–1112/97/020341+ 17 $25.00/0/jb960305  1997 The Fisheries Society of the British Isles