218 



Fishery Bulletin 100(2) 



depths of 27 to 33 m. The depth sounder 

 indicated that these small G. hebratcum 

 were most consistently caught over hard 

 substrate that lay adjacent to reefs — a con- 

 clusion later confirmed by video footage. 

 Although a considerable amount of effort 

 and a variety of techniques were employed 

 in attempts to catch fish with lengths of 

 150-300 mm, only a small number offish of 

 this size were collected. However, a few G. 

 hebraiciim of this length class were caught 

 by an experienced spearfisher while diving 

 over low-lying reefs with rock ledges <30 cm 

 high. Large numbers of dhufish >300 mm 

 in length were obtained from rod and hand- 

 line fishermen who were fishing in waters 

 that were shown by video camera and com- 

 mercial echo sounders to be located over 

 limestone and coral reef formations and, in 

 particular, where the "drop-offs"( reef edges) 

 were two or more metres in height. 



Comparisons between number of 

 opaque zones visible in whole and 

 sectioned otoliths 



Sectioned otoliths 



Numbef ol otolil 

 Number of 

 opaque zones 



§5 





The number of opaque zones observed in 

 each sectioned otolith, in which up to six 

 such zones could be seen, was the same 

 as those visible on the same otolith prior 

 to sectioning (Fig. 1). However, this fre- 

 quently did not apply when a greater 

 number of opaque zones were present. Fur- 

 thermore, where such discrepancies occurred, the differ- 

 ences between the number of opaque zones detected prior 

 to and after sectioning rose as the number of opaque 

 zones increased. In all cases where there were discrepan- 

 cies, the number of opaque zones detected after section- 

 ing was greater than prior to sectioning. Underestimates 

 of the number of growth zones with whole otoliths, based 

 on comparisons with those detected in sectioned otoliths, 

 rose from one in whole otoliths with seven to nine opaque 

 zones to between one and seven in those with 10-21 

 opaque zones (Fig. 1). In otoliths with a large number of 

 opaque zones, the differences sometimes exceeded eight 

 and for one such otolith was as high as twelve. These com- 

 parisons demonstrated that, for validation that opaque 

 zones are formed annually and that these zones can thus 

 be used for aging G. hehraicum. experiments should be 

 conducted on sectioned otoliths. 



Validation that opaque zones are formed annually 



The mean monthly marginal mcrements on sectioned oto- 

 liths with 2 to 16 or more opaque zones rose from a 

 low level in January to a maximum in September, before 

 declining precipitously to a minimum in October and then 

 rising slightly in December (Fig. 2). They thus reached 

 high levels in early spring, before declining markedly in 

 mid-spring, as the outermost opaque zone became delin- 



Figure 1 



Comparisons between the number of opaque zones obsei-ved on the otoliths 

 oi Glaucosoma hebraicum prior to and after the sectioning of those otoliths. 

 The numbers above enclosed circles represent the percentage number of 

 underestimates of the number of opaque zones when using whole rather 

 than sectioned otoliths. 



eated through the formation of a new translucent zone, 

 and then increased progressively in the ensuing inonths 

 as the translucent region increased in width. Although 

 fish possessing otoliths with one opaque zone were not 

 caught in all months, the trends exhibited by the mean 

 monthly marginal increments for those months when such 

 fish were caught were consistent with those exhibited by 

 otoliths with a larger number of opaque zones. 



Because the mean monthly marginal increment rose 

 and declined only once during the year, irrespective of the 

 number of opaque zones in the otolith, a single opaque 

 zone IS laid down in the otoliths of G. hebraicum each year. 

 The number of opaque zones in sectioned otoliths can thus 

 be used, in conjunction with the birth date of G. hebrai- 

 cum and the month when the opaque zone(s) become de- 

 lineated, to age this species. 



Growth of Glaucosoma hebraicum 



Because the trends exhibited by the GSIs and stages in 

 gonadal maturation and oocyte development demonstrated 

 that the spawning of G. hebraicum peaked from late Janu- 

 ary through early February, this species was assigned a 

 birth date of 1 Februai'v. Age O-i- G. hebraicum were first 

 caught by trawling over hard substrate in April and May, 

 when their lengths ranged from 57 to 81 mm (Fig. 3). How- 

 ever, substantial numbers of the O-i- age class were not 



