Reum and Essington: Season- and depth-dependent variability of a demersal fish assemblage in a fjord estuary 
195 
communities by dampening oscillations in prey popula- 
tions through behavioral mechanisms such as switching 
prey (McCann et al., 2005). Moreover, understanding 
how large predators use different habitats is impor- 
tant for estimating prey mortality rates and system 
energy flows (Bax, 1998). Information on spiny dogfish 
movement patterns in Puget Sound is limited to tag- 
ging studies from the 1940s and 1970s. Results from 
these studies indicate that approximately 70% of the 
spiny dogfish population resides in the Sound and the 
adjoining Strait of Georgia for multiple years, while the 
remaining dogfish are transient (McFarlane and King, 
2003). Spiny dogfish on the outer Washington and Brit- 
ish Columbia coasts are known to exhibit seasonal lati- 
tudinal migrations, but seasonal movements of dogfish 
tagged within Puget Sound and the Strait of Georgia 
are less clear (McFarlane and King, 2009; Taylor and 
Gallucci, 2009) and remain the subject of ongoing re- 
search. It is unknown whether spiny dogfish migrate in 
the winter to other subbasins within Puget Sound, to 
habitats shallower than 20 m, or whether they simply 
feed higher in the water column, disassociating with 
the benthos. There is evidence that catch rates of spiny 
dogfish in monitoring surveys have declined since the 
mid 1980s (Palsson, 2009) and that growth and size-at- 
maturity have also undergone substantial shifts (Taylor 
and Galluci, 2009). Characterizing movement patterns 
and population dynamics will clarify the impact of spiny 
dogfish on nearshore food webs. 
As demonstrated in Puget Sound, fjordal fish as- 
semblages vary markedly in space and time, and this 
variation has practical implications for modeling energy 
flows and interspecific interactions such as predation 
and competition. Food web models can foster ecosystem 
based management because they offer a framework for 
summarizing system knowledge and permit the simu- 
lation of alternative management scenarios. However, 
food web models for Puget Sound (as well as any other 
temperate system) parameterized by using fish abun- 
dances from one season alone may misrepresent the 
importance of different feeding modes and mischarac- 
terize patterns of trophic links in the fish assemblage 
(Greenstreet et al., 1997). In addition, stark differences 
between deep and shallow assemblages in terms of 
total biomass and species composition indicate strong 
spatial structuring in the likelihood and intensity of 
interactions among species, which may be a more gen- 
eral feature of fjord estuaries with similar deep basin 
bathymetrics. The data presented here, when coupled 
with diet information, provide a basis for determining 
the parameters of trophodynamic models that account 
for spatial and temporal variability in the fish assem- 
blage (e.g., Pauly et al., 2000). 
The use of three separate methods for analyzing dif- 
ferences in the demersal fish assemblage allowed us to 
investigate whether our interpretation of assemblage 
variation differed depending on the method. All three 
methods indicated significant differences between shal- 
low (20, 40, and 80 m) and deep waters (160 m) across 
seasons, with the exception of N, where values from 20 
and 40 m waters did not differ from values in 160 m 
waters. We note however, that variation in N may have 
been artificially reduced by our exclusion of rare spe- 
cies from the analysis. In contrast, species diversity, H\ 
which takes into account the relative biomass of each 
species, should be more robust to the exclusion of rare 
species. Results from taxon-based CCA complemented 
results from the diversity metrics by highlighting those 
species that co-varied with depth and simultaneously 
depicted site similarity in ordination space. Lastly, 
size-based analyses revealed differences in assemblage 
structure based on biomass spectra, a macroecological 
descriptor of assemblage structure (Jennings, 2005). 
Because body size is correlated with trophic level in 
aquatic systems (Kerr 1974; Jennings et al., 2001), dif- 
ferences in biomass spectra among assemblages may re- 
veal fundamental differences in trophic structure (Rice, 
2000; Sweeting et al., 2009). Significant depth related 
differences in the biomass spectra paralleled results 
from our taxon-based analyses and offer evidence that 
food web structure likely varies with depth. Combined, 
these approaches offer alternative prisms through which 
to view the demersal fish assemblage and mutually 
confirm important differences in assemblage structure. 
As with other marine fish surveys, our results are 
partly contingent on the effectiveness of the sampling 
gear and are premised on the assumption that catch- 
ability varies little among species. To maintain compa- 
rability with past demersal fish studies in Puget Sound 
we intentionally sampled using trawl equipment with 
specifications nearly identical to those used in previ- 
ous agency surveys in the region. The use of bottom 
trawls, however, also meant that species associated 
with rocky reef habitats would be excluded from our 
survey because the gear was suitable only for trawling 
in soft-bottom habitats. Moreover, large species such as 
six gill shark, which typically exceed 2 m in length in 
Puget Sound, were missed from the survey altogether. 
For future comparisons with other fjord systems, an 
effort should be made to sample regions with similar 
soft bottom habitats. We note that the data presented 
here reflect daytime distribution patterns, and diel 
movements of species may potentially connect deep and 
shallow communities. Another limitation of the present 
study is that our results span only a single time period 
and cover only a single basin in Puget Sound. Thus, we 
do not presume that the patterns described here will 
necessarily hold for all regions and be stable across 
time. Indeed, ample evidence from marine ecosystems 
points to the dynamic nature of community structure 
(Anderson and Piatt, 1999). Future research may very 
well improve our estimates of diversity and increase our 
understanding of temporal shifts in the Puget Sound 
fish assemblage. 
Although depth and season were clearly important 
in explaining community structure and species abun- 
dances, roughly one-half of the variation in these met- 
rics was not related to depth or season. Differences in 
temperature and salinity between the northern and 
southern CTD stations were relatively slight and differ- 
