Carbaryl, Carbofuran, and Methomyl - National Marine Fisheries ...

alcoves, channel edge sloughs, overflow channels, backwaters, terrace tributaries, offchannel
dredge ponds, off-channel ponds, and braids (Swift III 1979; Anderson 1999).
Diverse, abundant communities of invertebrates (many of which are salmonid prey items)
also populate these habitats and, in part, are responsible for juvenile salmonids reliance
on off-channel habitats. Juvenile coho salmon, stream-type Chinook salmon, and
steelhead use off-channel habitats for extended durations (several months). Although
these habitats typically vary in surface area, volume, and flow, they are frequently
shallow, low to no-flow systems protected from a river’s or a stream’s primary flow.
Thus, rearing and migrating juvenile salmonids use these habitats extensively (Beechie T.
and Bolton 1999, Beechie T. J. et al. 2005, Caffrey 1996, Henning 2006, Montgomery
1999, Morley S. A. et al. 2005, Opperman and Merenlender 2004, Roni 2002).
Small streams and some off-channel habitats represent examples of habitats used by
salmonids that can have a lower capacity to dilute pesticide inputs than the farm pond.
The PRZM-EXAM estimates assume that a 10-hectare (approximately 25 acres) drainage
area is treated and the aquatic habitat is assumed to be static (no inflow or outflow).
Pesticide treatment areas of 10-hectares and larger occur frequently in agricultural crops,
particularly under pest eradication programs. Additionally, aquatic habitats used by
salmon vary in volume and recharge rates and consequently have different dilution
capacities to spray drift and runoff events. The assumed drainage area to water volume
ratio (100,000 m 2 :20,000 m 3 ) is easily exceeded for small water bodies. For example, a
one acre pond with an average depth of 1 m would exceed this ratio for treated drainage
areas of approximately five acres in size and larger. The assumed aquatic habitat and size
of the treated area for the PRZM-EXAMS scenarios suggest that exposure is
underestimated for listed salmonids that utilize relatively small aquatic habitats with low
dilution capacities.
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