NOTE Laurenson et al.: Growth curves of two estuarine populations of Cnidoglanis macrocephalus 



881 



Materials and methods 



Collection of fish 



Juvenile and adult Cnidoglanis macrocephalus were 

 collected by seining at eight sites, gillnetting at nine 

 sites, and otter trawling at six sites located through- 

 out the basin of Wilson Inlet between September 1987 

 and April 1989 (see Fig. 1 in Potter et al. [1993] for 

 location of this estuary and the sampling sites). Some 

 of the sampling by each method was carried out 

 monthly, while the rest was undertaken bimonthly 

 (see Potter et al., 1993). The seine was 41.5 m long 

 (stretched mesh = 51 mm in wings and 9.5 mm in 

 pocket), while the gill net consisted of six 30-m con- 

 tiguous panels, each with a different stretched mesh 

 size, i.e. 38, 51, 63, 76, 89, or 102 mm. The stretched 

 mesh in the wings and codend of the otter trawl were 

 51 and 25 mm, respectively. Seine netting and otter 

 trawling were carried out during the day, while gill- 

 netting was undertaken overnight. A small number 

 of larval and post-larval C. macrocephalus were also 

 collected in night-time plankton tows (Neira and 

 Potter, 1992) and from their nests by dip net (Lauren- 

 son et al., 1993a). 



Sampling in the Swan Estuary employed winged 

 funnel traps between August 1982 and April 1983 

 (see Nel et al., 1985). Fish were also taken in a seine 

 and otter trawl similar to those used in Wilson Inlet 

 and with gill nets containing panels with the same 

 mesh sizes as those employed in Wilson Inlet, but 

 with additional panels of 13- and 25-mm mesh. 



Validation of translucent zones as annuli 

 and otolith measurements 



The first 10 males and 10 females of C. macro- 

 cephalus caught in each panel of the gill nets at each 

 site in Wilson Inlet on each sampling occasion, to- 

 gether with all fish caught in seine nets, were kept 

 for ageing. All fish caught in otter trawls, except for 

 a small number that were retained for tagging ex- 

 periments, were also used for ageing. The total length 

 and wet weight of each fish were recorded to the near- 

 est 1 mm and 0.1 g, respectively. Each C. macro- 

 cephalus was sexed, except in the case of smaller fish 

 (<ca. 100 mm) where the gonad could not be distin- 

 guished as either ovary or testis. The small, round 

 asterisci and the larger, elongate lapilli otoliths were 

 removed from fish and stored dry in envelopes. 



Because preliminary examination showed that 

 translucent zones were more clearly detectable in the 

 lapilli than asterisci of C. macrocephalus from Wil- 

 son Inlet, lapilli were used for ageing this species in 

 Wilson Inlet. The lapilli were placed in a bath of 

 methyl salicylate and viewed under reflected light 



against a dark background with a dissecting micro- 

 scope. The number of translucent zones on each 

 otolith was recorded. Because the outermost opaque 

 region of the otolith was not sharply defined, it was 

 difficult to obtain consistent measurements of the 

 distance between the outer translucent zone and the 

 edge of the otolith. The mean monthly trends shown 

 by the width of this marginal increment did not fol- 

 low a very consistent pattern and therefore could not 

 be used to establish that the outer zones on these 

 otoliths were formed annually (see Maceina et al., 

 1987; Hyndes et al., 1992). To provide an alternative 

 method for validating that each of the translucent 

 zones corresponded to an annulus, the percentage of 

 lapilli with a clearly defined translucent zone at the 

 periphery of the otolith in each month was calculated 

 separately for otoliths with one, two, three, four, and 

 five or more inner translucent zones (e.g. Crozier, 

 1989; Beckman et al., 1990). 



The difficulty in obtaining consistent measure- 

 ments of the marginal increments in the lapilli of C. 

 macrocephalus from Wilson Inlet was not encoun- 

 tered with the asterisci of this species from the Swan 

 Estuary (Nel et al., 19851. The measurements of 

 asterisci and total lengths of C. macrocephalus from 

 the Swan Estuary were obtained from the raw data 

 used by Nel et al. ( 1985). When two or more translu- 

 cent zones were present, the relative values for the 

 marginal increment were obtained by expressing the 

 distance between the outer edge of the outermost 

 translucent zone and the edge of the otolith as a per- 

 centage of the distance between the outer edges of 

 the two outermost translucent zones. When only one 

 translucent zone was present, the relative values 

 were expressed as a percentage of the distance be- 

 tween the outer edge of the outermost translucent 

 zone and the nucleus. The mean relative marginal 

 increments are subsequently referred to as mean 

 marginal increments. The distances between the 

 nucleus of the otolith and the outer edge of each 

 translucent zone and the outer edge of the otolith 

 were measured to the nearest 0.05 mm along the long 

 axis of lapilli and asterisci otoliths from Wilson Inlet 

 and Swan Estuary, respectively. 



Calculation of growth curves 



The relationships g(x) between the natural loga- 

 rithms of total length (y) and lapillus radius (.r) of C. 

 macrocephalus in Wilson Inlet, and between the 

 natural logarithms of total length and asteriscus 

 radius of this species in the Swan Estuary, were de- 

 scribed by a third order polynomial of the formy = a 

 + b.r + ex 2 + dx 3 , where a, b, c, and d are constants. 

 Back calculations of fish length at the formation of 



