Pick Our Brain - February 2011

With decreased pumping of Delta water and more outflow to the Pacific Ocean, how have populations of the [delta] smelt reacted? --Phil Nichols

It’s too soon to tell. Due to sampling difficulties, there isn’t information on actual delta smelt populations, but there is an index to help explain what’s going on. So far there has only been an insignificant uptick in smelt indices, which is consistent with scientists' expectations. There will not be instantaneous recovery of an almost extinct species with a one-or-two-year effort. Recovery requires a sustained long-term response that may require continued human intervention in some aspects of the recovery program. Delta smelt population indices have been at or near record low levels over the past decade. Other open water fish species in the Delta have shown a similar trend. Investigations about the causes for the low indices have shown that they are likely a response to multiple and often interacting stressors, including pumping. Moreover, the Delta ecosystem may have shifted to a new state that supports non-native and nuisance species, but not the native smelt. Smelt numbers are now so low that the small population size itself may impede recovery. This is known as “Allee effect.” Because of the multitude of stressors, the severity of the ecosystem alterations, and possible Allee effects, stressor reduction or ecosystem restoration is not expected to lead to immediate smelt recovery.

Steelhead Range in the North Pacific
Delta smelt
(Image courtesy of U.S. Bureau of Reclamation)