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

The life history of listed salmonids plays a large role in determining exposure. Adult salmon and
steelhead spend weeks to several months in freshwater habitats during their migration and
spawning. Immediately after emerging from the gravel substrate, salmonid fry move to habitat
where they can swim freely and forage. Chinook salmon, coho salmon, and steelhead fry
typically select off-channel habitats associated with their natal rivers and streams. Off-channel
habitats provide fry with protection from the primary water flow within rivers and streams.
Diverse invertebrate communities also populate off-channel habitats, adding to juvenile
salmonids’ reliance on these areas. Coho salmon and steelhead rear in freshwater for more than
a year. Sockeye salmon fry most frequently distribute to shallow beach areas in the littoral zones
of lakes before moving into deeper water and taking on a more pelagic existence. In contrast,
chum salmon migrate downstream to estuaries near the mouth of their parent stream almost
immediately. Fry reside in estuaries for as little as one or two weeks before moving offshore or
into deeper habitats within the nearshore environment.
Based on general knowledge of field runoff and drift from pesticide applications, we expect
higher pesticide concentrations in edge-of-field, low-flow, and shallow aquatic systems.
Therefore, Chinook salmon, coho salmon, and steelhead have an increased risk of exposure due
to their preference for off-channel habitats. Coho salmon and steelhead are at greater risk of
exposure because of their extended residency periods.
We expect carbaryl, carbofuran, and methomyl found in surface water runoff and pesticide drift
will cause acute lethality and sublethal effects to juvenile salmonids. Based on exposure for four
days at the reported LC50s, a severe consequence to a salmonid population’s growth rate is
expected. The most pronounced effects are within off-channel habitats based on NMFS
exposure estimates. The likelihood of population-level effects as a result of mortalities of
juvenile salmonids increases as populations spend longer periods in freshwater adjacent to
agricultural or developed areas. Species at the greatest risk include steelhead, coho, stream-type
Chinook, and ocean-type Chinook salmon.
Methomyl concentrations required to result in population-level impacts are not expected to occur
in the majority of aquatic habitats used by juveniles. However, we expect that carbaryl and
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