Essington et al.: Shifts in the estuarine demersal fish community after a fishery closure in Puget Sound, Washington 
213 
California sea lion (, Zalophus californianus), but we are 
unaware of any abrupt changes in predator densities 
in Port Madison to explain these patterns. Jeffries et 
al. (2003) reported a monotonic increase in harbor seal 
abundance in Puget Sound from the late 1970s to the 
1980s that was followed by little change in abundance 
during the mid- to late 1990s. 
Fourth, groundfish densities can be sensitive to 
water quality, especially to oxygen concentrations at 
the seafloor that result in distributional shifts to nor- 
moxic conditions (Breitburg et al., 2009; Essington and 
Paulsen, 2010), and chronic hypoxia exposure could 
diminish productivity of groundfish prey (Diaz and 
Rosenberg, 1995). There is no consistent sampling for 
dissolved oxygen in Port Madison to evaluate this hy- 
pothesis, but the exposure of this area to strong tidal 
currents and subsequent high mixing likely mean 
that this area is not particularly prone to low dis- 
solved oxygen (Nichols, 2003). Moore et al. (2008) 
did not detect a change in water temperatures 
throughout Puget Sound from 1993 to 2002; there- 
fore, changes are unlikely a result of a shift in 
temperature on a scale larger than Puget Sound. 
Several important limitations of our data war- 
rant specific discussion. First, our bottom trawl 
sampling — although highly standardized in time, 
space, and method — may not be representative of 
the entire Puget Sound. Indeed, one of our main 
conclusions is that shifts in densities of demersal 
fish species more likely were indicative of distri- 
butional shifts than of population shifts. Also, the 
opening of the bottom trawl was small and, there- 
fore, likely had low selectivity for large-size ground- 
fishes (e.g., >50 cm). Additionally, because sampling 
was restricted to a standardized and limited time of 
year, the data cannot account for seasonal changes 
(Reum and Essington, 2011) and may not reflect 
trends apparent in different seasons. 
Our environmental data were collected from 
monitoring sites near the study area for bottom 
trawl surveys, and bottom temperature was not al- 
ways recorded. We used sub-mixed-layer tempera- 
ture as a proxy for bottom temperature, which ap- 
peared to be robust for years in which bottom data 
were available. Bottom water temperature may de- 
viate from temperature of the shallower sub-mixed 
layer because of water exchange between Admi- 
ralty Inlet, the Strait of Juan de Fuca, and the 
coastal Pacific Ocean. However, because deepwater 
dynamics reach equilibrium over time scales of 
months, they reflect local, seasonal environmental 
conditions (e.g., air temperature, freshwater runoff) 
(Ebbesmeyer and Barnes, 1980) and, therefore, are 
useful for interannual comparisons. Despite these 
limitations, this study presents the first long-term 
standardized assessment of the groundfish commu- 
nity in Puget Sound and, therefore, can provide a 
baseline for expanded sampling efforts. 
A large body of research on estuarine fishes fo- 
cuses on the roles of estuaries as nursery habitats, 
the value of protecting specific critical habitats, and de- 
scriptions of patterns of juvenile survival and growth. 
Estuaries are often viewed as critical habitats that 
support coastal fish populations (Beck et al., 2001), and 
nearshore habitat features, such as eelgrass beds, are 
commonly identified as key features of estuarine habi- 
tats (Levin and Stunz, 2005). Although loss of eelgrass 
beds has been identified as a threat in Puget Sound, 
their importance to the groundfish species examined 
here is unknown. In well-studied estuarine ecosys- 
tems, extensive time series of fish abundance indices 
have permitted exploration of the roles of density de- 
pendence, overwinter survival, predation, and growth- 
dependent mortality on year-class strength of fishes 
(Hurst and Conover, 1998; Buckel et al., 1999; Kim- 
