GUEST COMMENTARY: Don’t be too quick to write off Delta smelt

Guest commentary by Bill Bennett

The situation for the endangered Delta smelt is pretty dire, to say the least. Monitoring results show that the population of this tiny, silvery fish – which only lives for one year, used to be one of the most abundant fish in the Sacramento-San Joaquin Delta, and is at the heart of California’s water wars – has declined to record low levels in recent years.

This population decline has led numerous commentators to conclude that the species is “virtually” or “effectively” or “functionally” extinct.

Not so fast. To quote the famous “bring out your dead” scene in Monty Python and the Holy Grail, Delta smelt are “not dead yet.”

There are two main reasons behind the false impression that the fish is extinct.

First, some argue that the fish must be extinct because no Delta smelt were caught at all in the two main monitoring efforts, the summer tow-net survey and the fall mid-water trawl survey. But these surveys were designed to sample striped bass, and otolith (fish ear-bone) analysis suggests some fish are either too small or too large to be caught in the mesh size of the nets used. Also, most sampling takes place mid-channel and not in all the other places where smelt occur. This doesn’t mean the surveys are bad or unreliable, just that they only sample part of the population and were never intended to sample the entire Delta smelt population. Even with that limitation, Delta smelt were once abundant in the sampling surveys. Their disappearance from those surveys means that the species has seriously declined, to the point where it is “below the radar” of sampling – but that is different from being extinct.

A second misunderstanding is that the fish are doomed to extinction because there are so few Delta smelt left that they cannot find each other for spawning(known as the “Allee effect”). But the species naturally occurs in loosely aggregated schools, most likely an adaptation to the turbid, low visibility waters of the Delta, and detect each other by the characteristic cucumbery smell of Delta smelt. Extreme low abundance does not necessarily prevent Delta smelt from reproducing – but it does make the species highly vulnerable to random or local environmental changes and to harmful management practices.

Endangered species regulations implemented over the past fifteen years have succeeded in preventing the historically high levels of entrainment of Delta smelt that once occurred at the giant export pumps in the south Delta and in ensuring that some water is released in the fall of wet years to prevent the fish’s habitat from becoming completely unusable.

A lot of energy is spent fighting over these regulations. But instead of begrudging the relatively small amount of water now used to prevent Delta smelt extinction, we need to augment these efforts and provide additional tools to support the recovery of the species, like increasing access to high quality habitat in places like Suisun Marsh and combating the effect of invasive aquatic plants and animals like Brazilian waterweed and Mississippi silversides on Delta smelt habitat quality.

Most Delta smelt only live one year, which means the species has evolved to take rapid advantage of good conditions when they occur. Unfortunately, high levels of water diversion, large-scale habitat conversion, and exotic species introductions have meant that good conditions almost never occur anymore. On the rare occasions they do occur the species has rebounded, and it can rebound again –if we give it the chance by changing the conditions resulting from decades of diverting too much water from the ecosystem and other environmental insults. Don’t count Delta smelt out just yet.

Bill Bennett is an Associate Research Ecologist (retired) at the UC Davis Center for Watershed Sciences and a Senior Ecologist at the Bay Institute. He is one of the state’s leading experts on Delta smelt and other fishes of the San Francisco Bay estuary.

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