Should the striped bass be a suspect in the decline of the Delta smelt?
Writing in the March issue of San Francisco Estuary and Watershed Science, US Fish and Wildlife Service biologists Matt Nobriga and Will Smith suggest that the smelt’s baseline might have shifted long before anyone was paying attention
, and striped bass predation may have constrained its numbers before recent water diversions and food web changes added their effects. The smelt’s historic abundance is unknown; systematic surveys didn’t begin until 1959. But it’s thought to have evolved its boom-and-bust life history in response to predation pressure from native fish or birds. Was the bass, introduced in the late 19th century, one predator too many, or too effective? Nobriga notes that Delta smelt don’t respond predictably to changing flows: “From what we know about its basic biology, it should have good responses in wet years and poor responses in dry years, like longfin smelt and young striped bass. The data haven’t shown that. Something is getting in the way of its response.” That something could well be the voracious bass, which feeds on crustaceans, other invertebrates, and multiple fish species, including younger bass. “In order to get through the year, one striped bass has to eat a hundred times its own body weight,” says Nobriga. “A tiny change in the fraction of Delta smelt in its diet would account for a big change in the abundance of Delta smelt.” This effect could account for apparently chaotic patterns of smelt abundance. Nobriga and Smith don’t advocate bass suppression as a smelt management tool (which would be controversial; like the eucalyptus, the bass is an exotic with a strong fan base). Such an effort might have unanticipated consequences, such as increased competition for the smelt from other bass prey species, like Mississippi silversides. But they suggest any attempt to supplement the dwindling smelt population with captive-reared fish should consider the bass factor. “There’s nowhere Delta smelt can go to get away from striped bass,” Nobriga observes. A smelt can dive to evade birds or swim out of a pikeminnow’s salinity comfort zone, but that wouldn’t deter a bass. “What seems to help is turbid water.” Smelt tolerate turbidity, but striped bass have trouble locating prey in turbid conditions.