OOCYTE GROWTH AND DEVELOPMENT IN THE STRIPED MULLE'f, MUGIL CEPHALUS, DURING SEASONAL OVARIAN RECRUDESCENCE: RELATIONSHIP TO FECUNDITY AND SIZE AT MATURITY

Oocyte growth and development in the striped mullet, Mugil cephalus, were examined during the period of rapid ovarian recrudescence that precedes spawning in coastal waters of northeast Florida. Based on the de novo appearance of yolk proteins detected by polyacrylamide gel electrophoresis, the oocyte si2e corresponding to the onset of vitellogenic growth was determined to be 0.18 mm (diameter). Through in vitro studies of oocyte responsiveness to steroid stimulation of meiotic maturation, the minimum prematuration oocyte size was determined to be 0.60 mm; largest prematuration oocytes collected during the study were 0.72 mm. Females with vitellogenic oocytes were first collected in Matanzas Inlet in late September. Females with prematurational oocytes were first observed in mid-October. Minimum size at sexual maturity for female striped mullet in northeast Florida ranged from 23 to 27 cm SL. Oocyte size-frequency profiles led to the development of a staging system for striped mullet ovaries that can be related to simpler measurements of reproductive condition such as the gonadosomatic index and the largest oocyte diameter. According to this system, females with prespawning ovaries first appeared in Matanzas Inlet during mid-October, then disappeared from the Inlet in either mid-December (1985-86 season) or mid-January (1984-85 season). Females with ovaries in spawning condition were not observed in the Inlet during the 2 years of this study, supporting the commonly held assumption of offshore spawning. A few females with postspawn ovaries were collected as early as late November. The potential fecundity of the striped mullet in northeast Florida was calculated from the size of the single clutch of developing oocytes, and related to both body weight and standard length. The striped mullet, Mugil cephalus, is a euryhaline teleost with a nearly worldwide distribution in marine and estuarine waters. Economically important as a food and bait fish in many areas, the world commercial catch of mullet from 1979 to 1983 averaged nearly 185,000 t (metric tons) per year (FAO 1984). During this time, a yearly average of almost 14.000 t (nearly 8% of the world catch) were caught in the United States, more than in any other single country. Most were caught in the state of Florida, for an average of almost 12,000 t yearly in 1981 and 1982 (National Marine Fisheries Service preliminary data, in Comp and Seaman 1985), equivalent to 87% of the United States and 6% of the worldwide catch. In addition to being the basis of large natural fisheries, M. cephalus has been reared for aquacultural purposes in brackish and freshwater ponds (Bromhall 1954; Thomson 1966; Pien and Liao 1975) and !Whitney Laboratory, The University of Florida, Route 1, Box 121, St. Augustine, FL 32086. "Whitney Laboratory, The University of Florida, Route 1, Box 121, St. Augustine, FL 32086, and Department of Anatomy and Cell Biology. College of Medicine, University of Florida, Gainesville, FL 32610. [Direct all reprint requests to Robin A. Wallace at the Whitney Laboratory.] Manuscript accepted February 1987. FISHERY BULLETIN: VOL. 85. NO.2. 1987. has been the subject of induced breeding in the laboratory (Kuo et al. 1973, 1974a, b; Kuo 1982). General information on the biology of M. cephalu8 and related species of mullet can be found in Anderson (1958), Stenger (1959), and Thomson (1966). Striped mullet have one breeding cycle per year lasting from 2 to 5 months depending on the location (Jacot 1920; Breder 1940; Bromhall 1954; Anderson 1958; Arnold and Thompson 1958; Stenger 1959; Tang 1964; Zhitenev et al. 1974; Pien and Liao 1975; Timoshek and Shilenkova 1975; Finucane et al. 1978; Apekin and Vilenskaya 1979; Azoury and Eckstein 1980; Chubb et al. 1981; Dindo and MacGregor 1981). In coastal waters of the southeast United States, spawning has been reported to occur from October through February as determined from the time of appearance and size of larvae and fry (Anderson 1958; Arnold and Thompson 1958), from the presence of migrating mullet with "developing" ovaries (Breder 1940; Arnold and Thompson 1958; Stenger 1959), and by monthly gonadosomatic index (GSI) changes (Dindo and MacGregor 1981). In view of the extensive interest in the mullet, it