Davis and West Reproductive biology of Lutjanus vittus from North West Shelf of Australia 



225 



ing. Samples were also obtained at 4h intervals dur- 

 ing 6-9 October 1988 to further investigate diel peri- 

 odicity of spawning. 



In the laboratory, ovaries were blotted dry and then 

 weighed to the nearest 0.1 g. Whole oocytes were ob- 

 tained by cutting a complete cross-section 1-2 mm thick 

 from the middle of the ovary, and teasing the oocytes 

 apart with needles. Oocytes were examined in water 

 under a stereomicroscope using transmitted light and 

 brightfield illumination. We measured maximum oo- 

 cyte diameter (MOD) as the diameter of the largest 

 oocyte (to the nearest 20 urn) in the sample. Because 

 oocytes were basically spherical, the orientation of 

 oocytes with respect to the line of measurement was 

 random. 



The size-frequency distribution of oocytes within ova- 

 ries at different stages of maturity was determined. 

 Sections 1-2 mm thick were cut from the middle of one 

 ovary, the oocytes teased apart with needles, and fur- 

 ther separated by immersion in an ultrasonic bath for 

 ~5 min. Oocytes were pipetted into grooves on a perspex 

 microscope slide and examined on a moving stage at- 

 tachment on a stereomicroscope at 50 x, using trans- 

 mitted light. Orientation was random. As well as mea- 

 suring each oocyte, we also noted its appearance so 

 that the stage of development could be assessed. 



We used whole-oocyte staging to assess ovarian ma- 

 turity (Hilge 1977, Forberg 1983, West 1990). Our 



three-stage scheme (Table 1) differed from Hilge 's 

 ( 1977) in that we distinguished late-migratory nucleus 

 oocytes (Fig. lb) from yolked oocytes (Fig. la) by the 

 presence of some translucence at the periphery of 

 the vitelline area (West 1990). Ovaries with late- 

 migratory nucleus oocytes or hydrated oocytes were 

 considered ripe (Fig. lc). 



The relative gonadal index (RGI) was used to quan- 

 tify the reproductive cycle (Erickson et al. 1985). This 

 index is independent of body size and is based on the 

 model, W=a,S 11 ', where W is gonad weight (g), S is 

 ovary-free body weight (kg), and a, and p, are param- 

 eters estimated for maturity stage i. As (3, did not dif- 

 fer significantly between the three maturity stages 

 (ANCOVA, F= 1.304, df 2,1046, p=0.27), a common (5 

 was used. 



To study short-term spawning rhythms and to 

 verify whole-oocyte staging, we histologically examined 

 477 ovaries from one cruise during the height of the 

 spawning season (November-December 1982). Histo- 

 logical sections were 10 pm thick and stained with 

 haematoxylin and eosin. The sequence of oocyte matu- 

 ration is similar in most teleost species (Wallace & 

 Selman 1981). We used the histological criteria of 

 Yamamoto ( 1956) for assigning oocytes to developmen- 

 tal stages. 



The presence of postovulatory follicles was noted in 

 the histological material. These follicles are readily 



