532 
Fishery Bulletin 95(3), 1997 
1988. In addition, 25 larval and juvenile Spanish 
mackerel were collected in ichthyoplankton samples 
from coastal waters off Charleston, SC, during May- 
October 1988 and 1989 (Fig. 1). During 1988, samples 
were obtained with a 0.5 m x 1 m (0.505-mm mesh) 
side-towing neuston net. The 1989 samples were 
taken with a 1 m x 2 m (0.947 mm mesh) neuston 
net. In addition, larger juvenile Spanish mackerel 
(> 60 mm SL) were obtained during 1983-89 along 
the coast of North Carolina, South Carolina, and 
Georgia from SCDNR research cruises aboard the 
RV Oregon and RV Lady Lisa with trawls, gill nets, 
seines, and from commercial shrimp trawling bycatch 
(Collins et al., 1988; Beatty et al. 1 ). 
Larvae and juveniles (<100 mm SL) were preserved 
in 95% ethanol and measured (standard length [SL], 
fork length [FL], and total length [TL] ) to the near- 
est 0.1 mm with dial calipers or ocular micrometer 
(Wild, M5 dissecting scope). Owing to the poor con- 
dition of the caudal fin on many of the smaller fish, 
standard length was used in the age and growth 
analysis. A factor of 3% was added to the length of 
each fish to account for shrinkage in ethanol 
(Schmidt, unpubl. data, 1988). All fish were identi- 
fied following Wollam (1970) and Richardson and 
MacEachran ( 1981). Sagittae and lapilli were excised 
from larvae and small juveniles by immersing the 
head region in 5% sodium hypochlorite solution for 
no more than 30 minutes (Brothers, 1987). Otoliths 
were separated from undissolved tissue and bone 
under a dissecting microscope with transmitted 
crossed polarized light. Otoliths from larger juveniles 
were removed by dissecting out the entire otic cap- 
sule and by separating the otoliths from their respec- 
tive ampullae. Excess tissue was dissolved in sodium 
hypochlorite solution. Otoliths were then rinsed in 
water, mounted whole (concave side down, unpolished) 
in immersion oil on a microscope slide and examined 
on a video-enhanced (Hitachi, MOS) compound micro- 
scope (Nikon, Labophot). Lapilli were used to estimate 
age in Spanish mackerel larvae and juveniles because 
increments were more discernible in the lapilli than in 
the sagittae. Young-of-year juveniles (>100 mm SL) 
were treated according to the same procedures used 
for YOY king mackerel by Collins et al., 1988. 
Marginal increment analysis 
To confirm the hypothesis of daily increment deposi- 
tion, a marginal increment analysis was performed. 
1 Beatty, H. R., J. W. Hall, and E. L. Wenner. 1988. Results 
of trawling efforts in the coastal habitat of the South Atlantic 
Bight 1987-1988. South Carolina Division of Natural Re- 
sources, P.O. Box 12559, Charleston, SC 29422. SEAMAP 
Report, 94 p. 
In this analysis, the stage of completion of the mar- 
ginal increment was compared with the adjacent fully 
formed increment on the lapilli from fish captured 
over a daily cycle (Fig. 2). Because Breach Inlet speci- 
mens were captured over an entire flood tide, it was 
impossible to know their precise time of capture. 
Therefore, the mean stage of completion of the mar- 
ginal increment of several specimens, captured over 
5-6 hour periods that progressed throughout the day 
and night, was compared. Large collections were 
subsampled by selecting as many as 35 individuals 
representing the size range of fish captured in the 
sample. Additional mackerel taken in SCDNR trawls 
and nearshore ichthyoplankton samples were also 
used. The time of capture of these specimens was 
known to within 30 minutes. A total of 165 larval 
and juvenile Spanish mackerel (7-97 mm SL) were 
examined. Attempts to find evidence for the daily 
nature of otolith rings in larger juveniles by measur- 
ing diel variation in marginal increments with SEM 
were not successful. 
Measurements of the marginal increment and the 
adjacent increment were made along each of three 
separate axes on each otolith. These axes were cho- 
sen because their optical properties allowed accept- 
able ring resolution. Occasionally, it was not possible 
to measure all three axes owing to opacity or dam- 
age to the otolith. Increments were displayed on a 
video monitor at l,000x and measured to the near- 
est 0.1 mm with dial calipers. Care was taken in ob- 
serving the opaque and transparent zones because 
different focal planes may invert their appearance. 
Consistent counts and marginal increment measure- 
ments were obtained at a “high” focal point (the dis- 
tance [with the highest lens power] to object that will 
produce a well-defined image). We were unaware of 
time of capture while performing the measurements. 
A standardized marginal increment (SMI) for each 
axis of measurement was calculated as 
W 
SMI = — 
W 
VV (n-l) 
where W n = width of marginal increment; and 
W (n _ 1) = width of complete adjacent increment. 
The SMI’s for each of the axes were averaged to ob- 
tain a mean SMI for each otolith. Two independent 
mean SMI’s were calculated for each otolith from 
separate measurements. Although there was no sig- 
nificant difference between the two measurements 
(paired Atest, P=0.153), the second measurement was 
used in the analysis because we were more experienced 
at locating and measuring the marginal increment. 
