536 
Fishery Bulletin 95(3), 1997 
Based on this growth equation, predicted absolute day for the first 150 days of life. Early growth was 
growth rate (predicted SL/age in days) was 2.4 mm/ characterized by relatively slow growth for the first 
23 days of life (1.9 mm/day) followed 
by a surge of rapid growth from 23 
to 40 days, during which growth 
rates approached 5.0 mm/day. Pre- 
dicted absolute growth of older ju- 
veniles (40-150 days) was 2.0 mm/ 
day. 
Discussion 
Validation of daily formation of the 
microstructural increment is a nec- 
essary prerequisite to using otoliths 
for ageing larval and juvenile fishes. 
Determination of the stage of com- 
pletion of the most recently formed 
increment over a daily light cycle 
does not directly validate daily in- 
crement formation but lends strong 
support to the hypothesis that in- 
crements are deposited daily. 
Several studies have shown that 
increment deposition is most likely 
controlled by an endogenous rhythm 
that can be modified by physical or 
behavioral parameters (or both), 
such as light and dark periodicity, 
temperature regimes, feeding frequency, food avail- 
ability, activity patterns (such as daily vertical mi- 
grations), or a combination of these and other fac- 
tors (Jones, 1986; Campana and Neilson, 1985, for 
review). There is presently no information available 
on the effects of changes in environmental factors on 
the periodicity or pattern of increment formation in 
larval or juvenile scombrids. However, work done 
with other teleosts (Taubert and Coble, 1977; Tanaka 
et al., 1981; Campana, 1984; Neilson and Geen, 1985; 
Jenkins and Davis, 1990) suggests that an internal 
diel clock alone is not responsible for daily increment 
formation but that it is entrained by some external en- 
vironmental cue that can vary between species of fishes. 
Observations on the seasonal occurrence and dis- 
tribution of larval Spanish mackerel in the northern 
Gulf of Mexico and the South Atlantic Bight suggest 
that they are restricted to middle and inner conti- 
nental shelf waters (Dwinell and Futch, 1973; 
MacEachran et al., 1980; Collins and Stender, 1987). 
Since daily fluctuations in salinity and turbidity are 
minimal in shelf waters outside estuarine influence, 
they are not likely to modify cyclic daily deposition 
of increments in larval Spanish mackerel. It seems 
more likely that feeding periodicity or diel vertical 
+ Right otolith • Lett otolith 
Figure 5 
Mean standardized marginal increment and standard deviation for left and 
right lapilli of Spanish mackerel larvae and juveniles collected throughout the 
day and night. Mean SMI for opaque bordered marginal increments are plotted 
separately from translucent bordered marginal increments in samples taken 
from 1613-2330 h. Capture time ranges (dashed lines) and sample sizes are 
indicated. 
