Reum and Essington: Season- and depth-dependent variability of a demersal fish assemblage in a fjord estuary 
187 
Here we evaluated depth- and season-related 
changes in the demersal fish assemblage struc- 
ture over a 10-month period in the Central 
Basin — the largest and deepest of four sub- 
basins within Puget Sound. We anticipated 
strong spatial gradients in community com- 
position on the basis of typical patterns of de- 
mersal fish assemblages in other ecosystems 
(e.g., Mueter and Norcross, 1999) and also 
hypothesized that species might exhibit sea- 
sonal patterns of habitat use that would be 
reflected in community structure. First, we 
evaluated differences in assemblage diversity 
metrics across depths and seasons. Second, we 
performed taxon-based multivariate analyses 
on the fish assemblage and explicitly tested 
whether assemblage structure was related to 
season and depth. Finally, we performed a 
size-based analysis, examining variability in 
the distribution of biomass across body size 
classes (biomass spectra) to identify whether 
the prevalence of small-body or large-body in- 
dividuals also changed with depth and season. 
The value of examining biomass spectra is 
grounded in the observation that trophic level 
generally increases with body size in aquatic 
systems (Kerr, 1974; Jennings et al., 2001), 
and the relative importance of different body 
size classes to the overall flow of energy in the 
food web can be revealed by biomass spectra 
(Haedrich and Merrett, 1992). The patterns 
revealed by each of these approaches were sub- 
sequently compared. 
Materials and methods 
48°0'0"N 
47°00'0"N 
Straight of Georgia 
Possession 
Sound 
North CTD 
rl, 
Olympic 
Peninsula 
Kitsap 
Peninsula 
South CTO 
_Tacoma 
Tacoma Narrows 
Southern Basin 
Puget 
Sound 
WA 
1 23°0'0"W 
1 22°00'0"W 
Data collection 
The demersal fish assemblage was sampled by bottom 
trawl during 18-22 October 2004, 10-14 March, and 
7-11 July 2005 along the eastern coastline of the larg- 
est subbasin within Puget Sound, the Central Basin 
(Fig. 1). The Central Basin is separated from the Strait 
of Georgia and Whidbey Basin by a sill (-60 m depth) 
at Admiralty Inlet and by Possession Sound to the 
north, respectively, and from Southern Basin by a sill 
at Tacoma Narrows (Fig. 1). We identified six sampling 
stations located in low-relief regions and amenable to 
bottom trawls spaced 5-10 km apart (high relief, hard 
bottom habitats are not common in this region of Puget 
Sound). At each station, we sampled four sites at depths 
of 20, 40, 80, and 160 m. In October we sampled stations 
1-4, sampling all four depths. To increase the spatial 
coverage of the survey in March, we visited stations 1, 
and 3—6, but only sampled at 40 and 160 m. Lastly, in 
July we visited stations 1, 3, 4, 5, and 6, and sampled all 
four depths. In this manner we were able to increase the 
spatial coverage of the survey while maintaining overlap 
with the previous season. We modified our survey when 
Figure 1 
Demersal fish assemblages were sampled with bottom trawls at 
six stations (indicated by the circled numbers) in the Central 
Basin of Puget Sound, WA. At each station, depths at 20, 40, 
80, and 160 m were sampled. Sampling occurred in October 
2004 and March and July 2005. Isobaths at 40 and 160 m 
are indicated. Sampling sites where conductivity-tempera- 
ture-density instruments (CTDs) were deployed by the King 
County Puget Sound Marine Monitoring Program to determine 
temperature and salinity are denoted by square symbols. 
necessary because of limitations in boat and crew time 
and because of the absence of prior information on which 
we could base our survey. 
Sampling was performed during daylight hours with a 
400-mesh Eastern otter bottom trawl lined with 3.2-cm 
mesh in the codend. The net had a head rope and foot 
rope of 21.4 m and 28.7 m, respectively, and the gear 
was towed across the seafloor for 400 — 500 m at 2.5 
knots. Catch was sorted to species, weighed, enumerat- 
ed, and subsampled for length composition of fish. Catch 
was standardized to biomass density (g/m 2 ) by using 
measurements of the area swept by the net. The area 
