Bertram et al.: Growth and development during the early life stages of Pleuronectes americanus 
3 
the same day were treated as a discrete cohort. 
The creation of these cohorts was repeated at 
intervals of 3-8 d until all fish had metamor- 
phosed. Table 1 summarizes the rearing condi- 
tions for the 3 families used in the study. 
At weekly intervals, length data on individual 
larvae were recorded by using a dissecting mi- 
croscope linked to a video system at 6x magni- 
fication. Larvae were filmed without being re- 
moved from their rearing containers. Fish were 
not anesthetized at any time. Larval movement 
was restricted by confining larvae within a 6- 
cm diameter plexiglass ring placed within the 
rearing container. Length data were collected 
only when fish were in the horizontal plane. To 
account for variation in the position of the larvae in 
the vertical plane, we constructed a small set of 
“stairs” with a plastic ruler segment attached at each 
level. After filming each larva, we immediately cali- 
brated the image against the ruler segment that was 
in focus. For fish that were close to, or past, meta- 
morphosis, the process of filming was simplified be- 
cause these fish generally remained motionless on 
the bottom of the container. Following metamorpho- 
sis, individual juveniles were filmed weekly for up 
to 4 weeks, when rearing was terminated. Standard 
lengths of all fish were obtained by using an image 
analysis system (Optimus vers. 3.11, Bioscan Corpo- 
ration, Seattle, WA). We used the image analysis 
system to “capture” two images for each fish for esti- 
mating standard length at age and used the largest 
value in all analyses. 
Analysis 
We constructed individual growth trajectories for 
larvae that survived until metamorphosis, using 
spline functions fitted to repeated measures of size 
at age. Individual larval growth trajectories were 
based on between 3 and 9 weekly observations per 
larva. Individual growth trajectories were examined 
quantitatively by using four indexes: 1) larval size 
at 30 ± 1 d (roughly midway through the development 
period, an index of larval growth rate [e.g. Travis, 
1981]); 2) average larval growth rates, defined as the 
difference between the length at metamorphosis and 
the mean length at hatching for the family divided 
by the time elapsed between the two events; 3) la- 
tency period, defined as the time between the age at 
which maximum larval length was attained and the 
time of metamorphosis; and 4) larval-period dura- 
tion, defined as the age at metamorphosis. Correla- 
tion analyses (Pearson’s correlation coefficient) were 
used to examine the relationships among pairs of the 
above variables for individual larvae. Variables were 
Table 1 
Summary of rearing conditions and filming schedules for the 3 
families used in the study. 
Family 1 
Family 2 
Family 3 
Larval container size (L) 
0.4 
0.4 
38 
Number of larvae/container 
1 
1 
400 
Weekly measures of larvae 
Yes 
Yes 
No 
Juvenile container size (L) 
0.4 
0.4 
0.4 
Number of juveniles/container 
1 
1 
1 
Weekly measures of juveniles 
Yes 
Yes 
Yes 
tested for normality with normal probability plots 
(Wilkinson, 1990). When heteroscedasticity was de- 
tected with techniques outlined in Zar (1984), vari- 
ables were log-transformed. For comparison with the 
individual growth trajectories, we also constructed 
a composite size-at-age plot by using data for all lar- 
vae used in the study. 
We checked for size-dependent mortality during 
the first part of the larval period by comparing the 
length of those fish that lived until their next weekly 
measurement with those that died during that time, 
using Ctests for independent samples. Size-depen- 
dent analyses were conducted for larvae after hatch- 
ing (1-2 d); week 1 (8-9 d); week 2 (15-16 d); and 
week 3 (22-23 d). Group-reared larvae that were used 
as replacements for fish that died during the first 3 
weeks were not included in the analysis. 
Individual juvenile growth rates were estimated 
from the slope of a least squares fitted to weekly 
measures of individual size at age from metamor- 
phosis to week 3 of the juvenile period. Thus, growth 
estimates were based upon up to 4 size-at-age mea- 
surements. Growth parameters were not calculated 
when less than 3 size-at-age measurements were 
available. We examined the correlations between 
juvenile growth rates and both age at metamorpho- 
sis and length at 30 d. Juvenile growth rates were 
also examined in relation to Bertram et al.’s (1993) 
measure of average larval growth rate estimated as 
the difference between the mean length at metamor- 
phosis for fish that metamorphosed on the same day 
and the mean size at hatching for the family, divided 
by the number of days between the 2 events. 
For comparison with the work of Bertram et al. 
(1993), we restricted the analysis of juvenile growth 
to weeks 1 through 4 for fish that had been reared 
together as larvae. The relation between juvenile 
growth rates and age at metamorphosis was exam- 
ined by using regression and correlation analyses. 
Similar analyses were performed to examine the re- 
