Lopez-Rasgado and Herzka: Assessment of habitat quality for juvenile Paralichthys cahfornicus 
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Experimental design 
Density, recent growth rates, and environmental 
parameters (temperature, salinity, dissolved oxygen 
and gut fullness levels as a proxy for food availabil- 
ity) were measured approximately every other month 
between October 2004 and October 2005. To evaluate 
growth and abundance over a gradient of environmen- 
tal conditions, the inner, central and outer reaches 
of the estuary were chosen as sampling and caging 
locations (Fig. 1). Six 28-day caging experiments were 
performed throughout the year to evaluate recent oto- 
lith growth, somatic growth, and gut fullness levels 
(Table 1). Density of natural populations was evaluated 
at the beginning and end of each caging experiment. 
Juveniles collected at the beginning of each caging 
period were used for caging experiments, whereas 
those captured at the end were used for measuring 
otolith growth rates and evaluating gut fullness levels 
of natural populations. To minimize the range of sizes 
evaluated for growth and gut fullness levels and allow 
for direct comparison of otolith growth rates from 
natural populations and caged individuals, we limited 
the sizes of fish analyzed to between 50 and 160 mm 
standard length (SL). 
Density and environmental variables 
On each sampling date, five 10-minute tows were con- 
ducted in each section of the estuary with a 7.6-m wide 
otter trawl (headrope length 9.5 m, 2.0- and 0.5-cm 
mesh in the body and codend, respectively). The sam- 
pling gear was not efficient at catching fish <40 mm 
SL. For each section and sampling date, we sampled 
at a range of depths and areas to obtain a representa- 
tive index of density. Sampling was mostly limited to 
the mid-to-high tidal range of spring tides because we 
needed to gain access to the cages during low tides. All 
tows were performed with the prevailing current at a 
target tow speed of 3 km/hr. For each tow, initial and 
final coordinates, mean current speed and the distance 
covered by the trawl (approximately 500 m) were regis- 
tered by using a hand-held GPS. Temperature, salinity 
and dissolved oxygen (DO) were measured at the end of 
each tow with an YSI 85 dissolved oxygen and conduc- 
tivity meter (Yellow Springs Intruments, Concord, CA). 
Because previous studies have reported a relationship 
between depth and abundance of juvenile California 
halibut in shallow estuarine systems (Kramer, 1990; 
Fodrie and Mendoza, 2006), a consistent depth was 
maintained during each tow. The depth of each tow was 
monitored and recorded with an onboard fish finder. To 
obtain a density estimate representative of each sec- 
tion of the estuary, tows were performed at a variety 
of depths and locations. 
After capture, California halibut were immediately 
measured for standard length. Length-frequency distri- 
butions were constructed for each sampling period and 
section of the estuary by 10-mm size classes. Density 
on an area basis was calculated by assuming a 7.6-m 
Figure t 
Map of Punta Banda Estuary, Baja California, Mexico where 
density and growth rates of California halibut ( Paralichthys 
californicus) were assessed between October 2004 and October 
2005. Black asterisks indicate locations where experiments 
were performed. Sampling for natural abundance and growth 
estimates was conducted in the outer, central, and inner sec- 
tions of the estuary; the areas between these sections were 
not sampled. The gray solid line indicates the channel that 
runs along the main axis of the estuary. 
trawl width and standardizing the tow to 1000 m 2 (re- 
ported as fish/1000 m 2 ). Estuarine emigration has been 
reported to occur during summer at lengths ranging 
from 140 to 200 mm (Haaker, 1975; Kramer, 1990; 
Hammann and Ramirez-Gonzalez, 1990). Hence, mean 
density (±standard error [ S E ] ) was calculated for fish 
<200 and >200 mm SL for each section of the estuary 
and sampling time. 
Given that our primary interest was young juveniles, 
the densities of fish <200 mm SL were analyzed by 
two-way analysis of variance (ANOVA) with time and 
section of the estuary as fixed factors. Normality and 
homogeneity of variance were evaluated by using the 
Kolmogorov- Smirnov and Levene’s tests, respectively. 
The data were log (x+1) transformed before we conduct- 
ed the ANOVA to comply with the assumption of nor- 
mality. A Tukey HSD test for unequal sample sizes was 
used to identify homogenous groups. The size-frequency 
