REIS: AGE AND GROWTH OF MARINE CATFISH 



VS. 



a.e. 



s.e. 



d.s. 



Figure 2.— A lapillus otolith oiNetuma barba showing opaque ( + ) 

 and hyaline (b) zones, the nucleus (N), the axes where back- 

 calculation was made (Co = distance between the nucleus and the 

 otolith's edge; ci = distance from the nucleus to "i" hyaline zone) 

 and the position of otolith on fish head (a.e. = antisulcal end; s.e. 

 = sulcal end; d.s. = dorsal surface; v.s. = ventral surface; and 

 hyaline zones = I-IX). 



cient, and t a correction on the time axis. The 

 parameters of Equation (1) were estimated by deter- 

 mining the predictive regression of ln(Loo - Lt) 

 against t (Beverton 1954): 



lnCZ^ - Lt) = \nL„ + K(t - t) 



(2) 



where K is the slope of the regression line and the 

 ^/-intercept of Equation (2) can be equated to In L m 

 + Kt providing the value of t (Ricker 1975). Trial 

 plots, including values of L m first derived by the 

 methods of Walford (1946) and Gulland (1964), 

 yielded the L^ which gives the straightest line. The 

 agreement between observed and calculated lengths 

 for year class was determined by residual variance 

 (S 2 y) expressed by 



„,„ Z. (observed Lt - calculated Lt) 2 



o y = (o) 



N - 1 



where N is the number of age classes. 



Length-weight relationship was determined for 

 males and females 



Wt = t*Lt v 



(4) 



where Wt is the weight at time t, and \x and v the 

 coefficients of the functional regression between Wt 

 and Lt (Ricker 1973). The condition factor was 

 calculated for each sex as follows: 



K = 



Wt 

 Lt v 



(5) 



Wt = W*, [1 - e -K«-y]" 



(6) 



expressed growth in weight, where W„ is the max- 

 imum attainable weight obtained by solving for L^ 

 in Equation (4). 

 The life span was estimated: 



^•0.95 - ^0 



ln(l - P) 

 K 



(Taylor 1960) (7) 



where A 95 is the time required to attain 95% of 

 Loo, P = 0.95 and t and K are derived from the 

 growth equation. The natural mortality coefficient 

 (M) was estimated according to Taylor (1960) 



M = 



ln(l - P) 



A . 



(8) 



95 



and 



Statistical analyses were done when necessary 

 (Snedecor and Cochran 1970; Sokal and Rohlf 1981). 



RESULTS AND DISCUSSION 



Age Determination 



The lapillus otolith used for the determination of 

 age oiNetuma barba is the most developed ear bone 

 in the Ariidae (Stinton 1975), its length attaining 

 3% of fish fork length (Reis 1982b). Growth zones 

 can be observed on a sectioned otolith from the 

 sulcal to the antisulcal end and from the nucleus on 

 the dorsal face to the ventral one (Fig. 2). The hya- 

 line and opaque zones are clearly evident even in 

 otoliths of old specimens. Under transmitted light 

 the opaque zones, or fast-growth zones, are white 

 (broad) and hyaline zones, or slow-growth zones, are 

 dark (narrow) (Fig. 2). Warburton (1978) counted 

 growth checks on whole otoliths of Galeichthys 

 caerulescens (Giinther), and Dmitrenko (1975) 

 studied Arius thalassinus (Riippel) by viewing the 

 otoliths the same way as in the present paper. The 

 number of hyaline zones on sectioned otoliths and 

 of growth checks observed on whole otoliths (War- 

 burton 1978) were compared. A smaller number of 

 growth checks was encountered in all cases when 

 using whole otoliths. 



In the present study only 2.4% of the otoliths were 

 considered illegible. About 60% agreement was ob- 

 tained when otoliths were read on two different oc- 

 casions separated by a month. Disagreement was 

 due to the inability to distinguish the first hyaline 

 zone and those near the otolith's edge. When the 

 same otoliths were analyzed for the third time, the 



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