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The Case of the Croaking Fish

The victim lay belly up. Scott "Sherlock" Ogle noted the date of death as September 3, 1999, and scratched his head. In three months of weekly toxicity tests of water samples from the head of Mare Island Strait on the Napa River, this was the first, but not the last, stiff. Two weeks later more fish croaked.

"This really is a mystery," says Ogle, a scientist with Pacific EcoRisk studying toxicity in Delta smelt habitats for the Interagency Ecological Program, and runoff from Bay-Delta agricultural watersheds for the Regional Monitoring Program for Trace Substances (RMP). The test fish, in this case, were inland silversides, a non-native estuarine fish similar to smelt. "These results are pretty unusual in their extremity. In order to kill every one of the fish in 24 hours it has to be something pretty bad." The results also surprised Ogle because similar tests on shrimp produced no dead bodies.

Unlike Sherlock, Ogle couldn't just sniff or taste the river water and announce the identity of the poison. His first clue was the speed with which it acted: "My gut feeling is this was too fast for a bacteria, disease or fungus." His second clue came after he filtered the lethal samples (as part of an RMP TIE), and discovered that the toxicity disappeared. Thus, the mystery substance had to be attached to the particles in the water.

This result ruled out a few other likely suspects, says Rainer Hoenicke who runs the RMP program. The pervasive household pesticide diazinon, for example, is so soluble it would have stayed in the water after filtering, not to mention killing the shrimp. Likewise, old DDT derivatives lying around in the sediments wouldn't kill a fish, just maybe give it cancer or the like 10 years later, he says. After this basic testing, all the scientists know is it probably is a "synthetic organic" chemical coming from municipal sewage or agricultural runoff.

The next clue didn't turn up till this spring, when Dave "Watson" Schoellhamer of the U.S. Geological Survey heard about Ogle's results. Schoellhamer and associate John Warner checked hydrodynamics data from the Mare Island Strait for those two September dates and found just what the former had suspected: The dates coincided perfectly with two neap tides, in which tidal flushing was particularly low and sediment trapping particularly high in the Strait. They also noted that both samples were collected during the slack after an ebb tide when the marshes all the way down the Strait would have been filled with water from the Napa River.

"Basically, you've got two toxic samples during a tidal phase when we'd expect the poorest water quality and when the water is coming from upstream," says Schoellhamer. "Flow from the Napa River would have been small for several months, and residence time of a water or sediment particle greater than it is at any time of the year."

More clues may lie in archived samples collected as indicators of ecological health in San Pablo Bay by a U.S. EPA funded program called CISNET. If scientists can re-examine samples from before, after and during the September fish kill, then they should be able to discover more about its persistence. Geoff Schladow, who runs the program, also plans to keep the mystery in mind in planning his future sampling.

"This may happen five, ten or fifty times a year, but we've just never sampled at the right time to see it, or it may be much more sporadic," says Hoenicke.

Timing is a key clue, especially to the glamour side of the case. If the dead fish include front page material like the endangered Delta smelt, then the hunt is on. But if the smelt aren't anywhere near the scene of the crime, then interest in the case could flag.

U.C. Davis fish expert Bill Bennett says the smelt only hang out in the Napa River during the wetter months. Sifting through the few clues now at hand, Bennett suspects that any of several fish species (splittail and tule perch, among others) in the Napa River might be affected by the mystery toxicity, but that on a Bay-wide basis, overall populations would not be influenced.

What the scientists hope to discover next is: if the toxicity is only a dry season phenomena; if it persists for several weeks or just a short period; and what happens if samples are taken at a different time in the tidal cycle. As all the scientific sleuths try to decide exactly what should be done next to get more clues, and who should pay for it, one thing remains clear: the case of croaking fish is far from elementary.

Contact: Scott Ogle (925)313-8080, Dave Schoellhamer (916)278-3126 or Geoff Schladow (530)752-6932

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