Ferreira and Russ. Population structure of Plectropomus leopardus 



631 



the size, age, and sex structure of leopard coral- 

 grouper populations are investigated. 



Materials and methods 



Four mid-shelf reefs off Townsville, Central Great 

 Barrier Reef (Fig. 1), were chosen as the sample reefs 

 for this experiment. Two reefs, Grub and Hopkinson, 

 were located in the General Use Zones and were open 

 to spear-fishing, whereas the other two, Glow and 

 Yankee, were located in National Park Zones, and 

 had been closed to line fishing since September 1987. 

 The four reefs were sampled twice a year, during 

 June— July and September-October, in 1990 and 1991 





$ 



i 





Palm 



4, Islands 



Gi 0% y 



Yankee #*"«! 



Hopkinson 

 <£* GnibCi' 



<s> 



3> 



9 ^ 



& 





Magnetic Island 



25 Nautical miles 



Figure 1 



Map showing the location of the sampled reefs: Glow and Yankee 

 (closed to fishing) and Grub and Hopkinson (open to fishing). 



(Table 1). During each sampling trip, a crew of four 

 line fishermen fished one reef per day (during the 

 daylight hours) for a period of approximately four 

 hours. The same vessel was used for each trip. The 

 fishing crew was relatively consistent in composition, 

 and overall fishing ability was presumably consis- 

 tent between trips. 



Laboratory analysis 



All fish were measured and weighed, and their 

 otoliths and gonads were removed. The gonads were 

 preserved in FAAC (formaldehyde 4%, acetic acid 5%, 

 calcium chloride 1.3%) on board, sectioned, and 

 stained by using the standard techniques described 

 in Ferreira (1993). Each gonad was classified 

 into one of the following gonadal developmen- 

 tal stages, following Ferreira (1993, 1995): 



Immature female: no evidence of prior 

 spawning. 



Mature female: evidence of prior spawning 

 or active vitellogenesis. 



Transitional-stage: gonads with proliferat- 

 ing testicular tissue in the presence of degener- 

 ating ovarian tissue. Dorsal sperm sinuses absent. 



Young male: post-transitional, newly trans- 

 formed testis. Dorsal sperm sinuses formed. 

 Ovarian tissue dominating the lamellae. 



Mature male: developed testes, presenting 

 typical lobular form and presence of intra- 

 lobular or "central" sperm sinuses. 



To determine the age of each fish, the otoliths 

 were read whole and sectioned by following the 

 method described by Ferreira and Russ (1992). 

 The number of opaque zones or rings were 

 counted from the center to the margin of each 

 otolith. Because leopard coralgrouper recruit- 

 ment occurs in the first months of the year 

 (Doherty et al., 1994), the birth date was as- 

 signed as 1 January. Opaque zones are formed 

 once a year, from July to November (Ferreira 

 and Russ, 1994); therefore, they were counted 

 only when there was further deposition of a 

 translucent zone, i.e. from December onwards. 

 In this way, the number of rings corresponded 

 to the real age of the fishes. 



Statistical analysis 



Nested analyses of variance (reefs nested within 

 fishing status) were used to compare mean age 

 and size of leopard coralgrouper between closed 

 and open reefs (=fishing status). Factorial 

 analyses of variance and Kruskal- Wallis tests 



