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Fishery Bulletin 114(2) 
tor habitat manipulation require estimates of pretreat- 
ment predation rates and of how predation rates fluc- 
tuate with changes in predator abundance and habitat 
condition. These predation rates may be estimated in 
various ways. One method involves conducting coor- 
dinated studies of predator and prey distribution and 
abundance, in combination with predator diet studies 
(Rieman et al., 1991). However, this approach is labor 
intensive and time consuming, making it difficult to 
replicate in several different areas at once. Consid- 
ering that the Sacramento-San Joaquin Delta has a 
complex suite of hydrological processes and geography 
(the south delta is especially affected by municipal and 
freshwater export processes), there is potential for sub- 
stantial spatial heterogeneity in fish predation rates, 
thus requiring replication of predation rate studies in 
many different areas and habitats. 
Diet-based predation studies lack statistical power 
when prey species of concern are rare and make up 
only a small part of a predator’s diet. Predation-event 
recorders (PER) provide an alternative that has the ad- 
vantage of being relatively inexpensive and also capa- 
ble of being implemented over a broad spatial and tem- 
poral scale. The intent of this method is not to quantify 
absolute predation rates, but rather to provide a rela- 
tive comparison of predation rates among study areas 
to substantiate predator density and environmental 
covariate hypotheses. Additionally, the identification of 
predation “hotspots” gives us insight into the underly- 
ing physical and biological mechanisms that contribute 
to observed mortality. By simultaneously collecting en- 
vironmental data, we were able to construct and select 
appropriate statistical models to describe the contrib- 
uting factors that affect predation. 
Fish ecologists have used stationary tethers to study 
predation on fishes (Linehan et al., 2001; Adams et al., 
2004; Chittaro et al., 2005). However, our free drifting 
PERs have features that are more useful in free-flow- 
ing rivers. Drifting PERs may be the preferred design 
under conditions where movements through a habitat 
feature are favored, resulting in a more natural pre- 
sentation and larger area sampled. This is especially 
important when assessing interactive effects of vari- 
ables such as water exports, flow rate, total discharge 
and tidal mechanics on the movement and survival of 
young fish. Alternatively, anchored PERs enable target- 
ed sampling around specific habitat features like the 
lower water column in deeper river sections, littoral 
habitats, or around fixed structures. The PER design 
has the advantage that it may be altered (size, shape 
etc.) for sampling in different conditions. Owing to the 
tidal nature and relatively low current velocities in our 
study site, we designed our PERs to maximize the ef- 
fect of subsurface current forces in order to counteract 
the effect of wind. PERs may also be adapted to study 
predation on other fish species, such as delta smelt 
( Hypomesus transpacificus ) or steelhead (Oncorhynchus 
my kiss) that also occur in the Sacramento-San Joaquin 
Delta, and it may be modified for use in rivers, lakes, 
estuaries and coastal ocean environments. 
Acknowledgments 
We would like to thank the many people who assisted 
with fieldwork, particularly M. Sabal, V. Lo, T. Brown, 
B. Lehman, and M. Miller. In addition, M. Henderson 
and F. Cordoleani provided insightful comments on 
early drafts and three anonymous reviewers provided 
helpful suggestions during the review process. The 
California Department of Water Resources funded this 
work under agreement no. 46-10100 with the National 
Marine Fisheries Service. Material and logistical sup- 
port was also provided by The National Marine Fisher- 
ies Service Southwest Fisheries Science Center, Santa 
Cruz, CA. 
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