MacNair et al.: Age, growth, and mortality of Paralichthys californicus 
599 
females, which are generally larger, may be more capable 
of escaping a trawl net than males. Other factors that may 
have affected the growth parameters include the follow- 
ing: method of collection, time periods of sampling, and dif- 
ferent environmental conditions in the years prior to each 
study. 
Many factors can affect growth rates of fish including 
differences in the seasonality of spawning, environmental 
factors, amount and size of food, and genetics (Weatherly 
and Gill, 1987; Moyle and Cech, 1988). Southern and cen- 
tral California are biogeographically different. Differences 
between central and southern California coastal waters 
include temperature, water circulation patterns, bottom 
topography, and substrate. In particular, Jow (1990) noted 
differences in bottom topography and substrate between 
the trawl areas for California halibut in southern and 
northern (San Francisco Bay area) California. The bio- 
geographical differences among these regions could possi- 
bly cause growth rates of the same species to differ. Stud- 
ies of other fish species have shown differences in growth 
between geographical regions. For example, Parrish et 
al. (1985) found latitudinal differences in the growth of 
northern anchovy, Engj-aulis mordax. They found that the 
growth rate of juvenile northern anchovy in central Cali- 
fornia was greater than that of juvenile northern anchovy 
in southern California. Butler et al. (1996) found differ- 
ences in maturation and length at age of Pacific sardine 
between latitudinal regions. Sardine appeared to mature 
at a younger age off both southern and Baja California 
than off Monterey; one-, two-, and three-year old fish were 
smaller at age off Baja and larger off Monterey. Deriso et 
al. (1996) confirmed the findings of sardine growth differ- 
ences by Butler et al. ( 1996). The exact mechanism for lati- 
tudinal differences in growth rates of California halibut is 
still unclear and further research is needed to clarify this 
phenomenon. 
Acknowledgments 
We wish to express our appreciation to Larry Jacobson 
with the National Marine Fisheries Service, for his detailed 
review of an earlier draft of the paper. We appreciate the 
efforts of Steve Wertz, Sandra Owen, and Paul Gregory 
who helped with various aspects of this study. We also 
thank the reviewers for their comments and suggestions 
that improved the paper. 
Literature cited 
Beamish, R. J., and D. A. Fournier. 
1981. A method for comparing the precision of a set of age 
determinations. Can. J. Fish. Aquat. Sci. 38: 982-983. 
