528 



Fishery Bulletin 88(3). 1990 



Figure 7 



Yield per recruit (g) of croaker in Trinidad waters against fishing 

 mortality for natural mortality rates of 0.1. 0.2, 0.3. 0.4. and 0.5 per 

 year. 



also been recorded in Brazil (Vazzoler 1962, 1971; 

 Castello 1982; Isaac-Nahum and Vazzoler 1983), and 

 in Guyana (Lowe-McConnell 1966). 



Other workers have been successful in determining 

 the age of tropical and subtropical sciaenids using 

 otoliths (e.g., Bayagbona 1969, Villyamar 1972, Pan- 

 nella 1974, Barger 1985). Annulus formation coincides 

 with both the period of peak spawning and the wet 

 season when salinity decreases due to river run-off. A 

 similar pattern occurs in other species of tropical sciae- 

 nids (e.g., Rao 1966, Le Guen 1971). The annulus in 

 immature croaker is probably formed in the wet season 

 of the year following their year of birth. Use of the 

 Cassie method had two disadvantags: the subjectivity 

 in choosing the inflexion points and the crowding of 

 older age groups into the upper part of the cumulative 

 frequency curve. However, the close agreement of 

 results obtained by both methods validates the use of 

 otoliths for age determination of the croaker. 



The higher value of A' for males than for females sug- 

 gests a higher mortality rate for the former (Beverton 

 and Holt 1959). This could also account for the presence 

 of only six age groups for males and the predominance 

 of females in the population. 



The variation in growth between Trinidad and Brazil 

 indicates that the croaker does not present a uniform 

 population in this region. According to Vazzoler (1971) 

 geographical differences in growth characteristics of 

 this species are due to genotypical and ecological fac- 

 tors such as salinity and temperature. 



Estimation of mortality rates from length-converted 

 catch curves is subject to several assumptions, the most 

 important of which is that of constant recruitment 

 (Ricker 1975). Examination of length-frequency distri- 

 butions indicated that the modal length in the catch was 

 similar for all years of sampling (33.0-34.9 cm), and 

 no particularly strong or weak year class was apparent. 

 Another assumption is that of constant fishing effort, 

 which did not change appreciably during the period of 

 sampling, as indicated by the number of active vessels. 

 Since only fully recruited age groups were used in this 

 analysis, Z was assumed to be constant over these ages. 

 Trawling was considered to be a random method of 

 sampling for older, fully recruited age groups, and the 

 sample was assumed to be representative of that part 

 of the population which was used to compute Z. Vio- 

 lation of these assumptions often results in non-linear 

 right limbs of catch curves (Ricker 1975). However, the 

 linear right limb of the catch curve in this study and 

 the significant regression suggest that the estimate of 

 Z is acceptable. 



All of the conditions, except that of isometric growth, 

 accompanying the Beverton and Holt model are 

 assumed to exist for the croaker. The assumption of 

 isometric growth, when growth is in fact allometric, 

 leads to incorrect estimates of yield (Paulik and Gales 

 1964). However, the absolute levels of these estimates 

 are not of as much interest as the differences in yield 

 that result from varying t,. and/or F. The relative 

 error in such differences, when using an incorrect b, 

 tends to be much less than that in the absolute levels 

 (Ricker 1975). 



Locally, fishing is the major cause of mortality of the 

 croaker. At the present t,. and the level of F the max- 

 imum sustainable Y/R is already being obtained, and 

 any increase in F would result in overexploitation. 

 Increasing t,. to 3 years may not be economically 

 worthwhile since the resulting increase in Y/R is only 

 6 g. Such a move however, would protect 2-year-old 

 fish that are spawning for the first time. Because of 

 the flattening of the yield curves at high F values, the 

 effect of overexploitation on yield is small. However, 

 the effects of overexploitation on recruitment is un- 

 known, and requires future study. 



Acknowledgments 



The authors wish to thank the following: The Univer- 

 sity of the West Indies and the Institute of Marine 

 Affairs, through which this study was carried out; The 

 National Fisheries Company Ltd. and the Caribbean 

 Fisheries Training and Development Institute for as- 

 sistance with obtaining samples; the Seismic Research 

 Unit for assistance with preparation of otolith sections; 



