526 



Fishery Bulletin 88(3). 1990 



Figure 3 



Mean monthly marginal increment ( ± 95% CL) of otoliths of the 

 croaker in Trinidad waters. Number of fish is given for each month. 



distribution of combined sexes showed six age groups 

 (Fig. 4). There was close correspondence in lengths at 

 respective ages found by the two methods (Table 3). 

 Based on the above, use of otoliths for age determina- 

 tion of the croaker is considered to be valid. A pre- 

 liminary plot of the observed length at age revealed 

 that growth of the croaker could be adequately 

 described by the von Bertalanffy equation. Growth 

 parameters showed that females achieved a greater 

 asymptotic length, but grew at a slower rate than 

 males. In Table 4 these parameters are compared with 



those found for different populations of croaker in 

 Brazil. 



Mortality 



Total mortality rate (Z) which was the slope of the 

 descending part of the length-converted catch curve 

 in Figure 5 was 1.2/year (r- = 0.96, P<0.05). The 95% 

 confidence limits for Z were 0.91 and 1.49. Natural 

 mortality rate (M) was 0.4/year, and fishing mortality 

 rate (F) was 0.8/year. 



Yield per recruit 



At the present level of F and t,., Y/R is already at the 

 maximum of 175 g (Fig. 6). Increasing F results in a 

 decrease in Y/R. Increasing t,. to 3 years results in a 

 Y/R of 181 g at the same level of F. At values of M 

 of 0.1, 0.2, and 0.3 the croaker is overexploited at the 

 corresponding levels of F (Fig. 7), while at M of 0.4 

 and 0.5 it is fully exploited. At low M values the yield 

 curves are domed with peaks at low F values, while 

 at higher M values they are relatively flat. 



Discussion 



Several similarities exist in the reproductive biology 

 of the whitemouth croaker throughout its geographical 

 range. Early maturity and year-round spawning have 



Figure 4 



( 'umulative ]X'rcent;ige length- 

 frequency curve (dotted line) 

 and separation into six age 

 groups (straight lines) for the 

 croaker in Trinidad waters. 

 Arrows indicate inflexion 

 points chosen by eye. 



