Reum and Essington: Season- and depth-dependent variability of a demersal fish assemblage in a fjord estuary 
189 
partial CCA models was tested by comparing a pseudo 
F statistic to a null distribution generated by permut- 
ing the species-site matrix 5000 times (Legendre and 
Legendre, 1998). 
The analysis of biomass spectra paralleled our analy- 
ses of the biomass of assemblage species. Length-fre- 
quency information for each species was used to divide 
species biomass into log 2 body size classes. The total 
sum of biomass (all species) in each size class was then 
normalized by dividing the biomass by the antilog body 
size interval of each respective size class, as is com- 
monly done in analyses of biomass spectra (Kerr, 1974; 
Sprules et al., 1983). In subsequent partial CCA, the 
size class biomassxsite matrix was treated as the re- 
sponse variable. 
To determine whether the analyses should account 
for spatial autocorrelation, we tested residuals result- 
ing from the global species biomass and biomass spec- 
tra CCA models for spatial dependence, using a multi- 
scale direct ordination technique. The method entails 
performing a constrained ordination on the global mod- 
el CCA residuals with an explanatory matrix that is 
coded for geographic distance (Wagner, 2004). We used 
a grain-size equivalent to 0.1° latitude that resuled in 
four distance classes. Results were nonsignificant for 
species biomass and biomass spectra models, indicating 
that spatial structure at the assemblage level was not 
detectable. We therefore disregarded spatial dimensions 
and pooled samples by depth and season in subsequent 
analyses. During the exploratory stage of our analysis, 
univariate and multivariate tests that were performed 
with biomass densities resulted in conclusions similar 
to those obtained with numerical densities. We chose 
to limit our analyses to biomass densities to avoid re- 
dundancy and because the importance of a species to 
energy flow in a food web is more readily (although not 
perfectly) approximated by information on its biomass. 
We considered all statistical tests significant at the 
P=0.05 level. 
Results 
We captured 23,100 individual fish in our survey that 
represented 62 species from 23 different families. Of 
these, 32 species occurred in more than 10% of the 
trawls. In general, small size classes (<30% of maxi- 
mum recorded length) for individual species were rare 
and only English sole ( Parophrys vetulus), and spiny 
dogfish ( Squalus acanthias ) were abundant enough for 
inclusion in the analysis as separate size classes. Water 
conditions were relatively homogenous throughout the 
area surveyed. Temperature and salinity values dif- 
fered by less than 1°C and 0.2, respectively, between 
the northern and southern CTD stations for each depth 
and month combination (Table 1). Differences in tem- 
perature and salinity among depths were small in Octo- 
ber and March but were more evident in July; waters 
20 m deep were warmer and fresher by 1.3° and 0.8°C, 
respectively (Table 1). 
Table 1 
Temperature (°C) and salinity measurements obtained 
with conductivity-temperature-density (CTD) casts from 
the northern (N) and southern (S) regions of the area sur- 
veyed in Central Basin, Puget Sound. Data were obtained 
within ten days of trawl sampling. Although stations at 
20 and 80 m were not sampled in March, water properties 
are provided to aid comparisons among sampling months. 
Temperature Salinity 
Month 
Depth (m) 
N 
S 
N 
S 
October 
20 
12.1 
12.1 
30.3 
30.5 
40 
11.9 
12.0 
30.5 
30.6 
80 
11.7 
11.8 
30.6 
30.7 
160 
11.4 
11.5 
30.7 
30.8 
March 
20 
8.7 
8.5 
29.6 
29.5 
40 
8.7 
8.5 
29.7 
29.6 
80 
8.7 
8.4 
29.9 
29.9 
160 
OO 
bo 
8.3 
30.1 
29.8 
July 
20 
12.8 
13.0 
29.6 
29.6 
40 
12.1 
12.1 
29.8 
29.8 
80 
11.5 
11.4 
30.0 
30.0 
160 
11.5 
11.4 
30.4 
30.4 
Overall, spotted ratfish ( Hydrolagus colliei ), spiny 
dogfish, and flatfish were the dominant taxonomic 
groups in the survey. Biomass patterns observed at 
each depth and month combination are depicted in Fig- 
ure 2. Shallow waters (20 and 40 m) were dominated 
by flatfishes, which composed between 64% and 83% 
of the fish assemblage biomass in all three months. In 
deep water, assemblage biomass was nearly double that 
found in shallow waters (80 and 160 m; Fig. 2). In to- 
tal, spotted ratfish composed approximately 80% of the 
fish assemblage at 160 m in all three months (Fig. 2). 
Spiny dogfish were found primarily at depths of 80 and 
160 in October, were nearly absent from the survey in 
March (two individuals were captured), and present at 
all depths in July, with the highest biomasses occurring 
at depths of 80 and 160 m (Fig. 2). 
Diversity metrics 
Variation in species richness (N) was observed across 
depths in both October+July (ANOVA, F| 3 32 j = 3. 9, 
P=0.01) and all-months tests (ANOVA, F (1 26] = 103.7, 
P<0.001) where N at 40 m was higher than at 160 m 
(Fig. 3). Post hoc analyses of the October+July test indi- 
cated that N at 160 m was significantly lower than at 80 
m, but that both of these depths did not differ from N 
at 20 and 40 m (Fig. 3). Similar patterns were observed 
for species diversity (H'), with significant differences 
across depth for both the October+July and all-months 
tests (ANOVA, P [132] =18.4, P<0.001 and P (1 26] =137.3, 
P<0.001, respectively; Fig. 3). In both all-months and 
