REIS: AGE AND GROWTH OF MARINE CATFISH 



remaining months, when Netuma barba is at sea 

 actively feeding and the gonads are resting (Reis in 

 press), the opaque zones appear to be laid down due 

 to fast somatic growth. According to Pannella 

 (1974), fishes of temperate environments tend to 

 form opaque zones or fast-growth zones during 

 warm months but the synonymity of the terms sum- 

 mer and opaque, winter and hyaline has to be 

 demonstrated in each instance rather than accepted 

 as a general fact. For Netuma barba slow-growth 

 zones are formed during warm months and may be 

 related to the maturation of the gonads and a pause 

 in feeding activity. Gonad maturation may be one 

 of the causative factors of hyaline formation in 

 adults; however, a plausible cause still needs to be 

 established for immature specimens. 



GROWTH 



Growth in Length 



Sectioned otolith lengths (measured as shown in 

 Figure 2), and fish lengths were best fitted to the 

 power curve: 



Lt = 1.89 Co 1047 



r = 0.960; 



n = 689, 



and the equation for back-calculation was 



, 1.047 



Lt i = Lt 



~Co 



where Lt % is the length of fish when zone "i" was 

 formed. Observed and back-calculated mean lengths 

 for year class for each sex increase as one opaque 

 and one hyaline zones are formed in the otolith each 

 year (Table 1). Up to age 11, the mean lengths are 

 similar; older females had mean lengths greater than 

 males. The same was true for mean weight although 

 a small number of specimens were analyzed from 

 age 11 onward. Observed lengths are usually higher 

 than back-calculated lengths except in ages that few 

 specimens were analyzed. 



Lengths corresponding to ages 8 to 12 are most 

 frequent in the samples since they are most affected 

 by the mesh size of the fishing gear used in the 

 estuary. Mean observed lengths at these ages agree 

 closely with mean back-calculated lengths (Fig. 5) 

 for both sexes. Gill nets are highly size selective and 

 retain fish at lengths of 370-520 mm (Reis 1982a). 

 The analysis of variance (Sokal and Rohlf 1981) 

 showed that observed lengths at ages 5, 6, and 7 

 are significantly higher than back-calculated lengths 

 (P < 0.05) which could be due to the capture of the 



largest specimens of these ages since the minimum 

 size of fish held by gill nets depends on the maxi- 

 mum body girth (opercle). Mean back-calculated 

 lengths showed no definitive tendencies for any 

 age class (Fig. 6) indicating no growth changes. Fur- 



Table 1 .—Mean observed and back-calculated lengths of males 

 and females of Netuma barba for each age class (sample size in 

 parentheses). 



Estimated 

 age 



Mean observed 

 length 



Male 



Female 



Mean back-calculated 

 length 



Male 



Female 



683 



