A multi-agency search for Asian carp in the Chicago waterway system this week yielded no specimens of the invasive fish.

Shipping interests are declaring victory, saying the results vindicate their conviction that the fish has not entered Lake Michigan. However, opponents say a wealth of DNA evidence collected over three years shows their presence at least as conclusively as producing an actual fish.

In the past two months, 11 positive DNA samples have pointed to the presence of the invasive species, prompting the search. Since 2009, 85 samples have tested positive. To date, only one Asian carp has been found: a 19-pounder fished out of Lake Calumet, a small lake located 6 miles upstream from Lake Michigan, in 2010.

On Monday, the four-day search of Lake Calumet began, carried out by commercial fisherman and a team of biologists from the US Fish and Wildlife Service, the US Army Corps of Engineers, the Illinois Department of National Resources, and Southern Illinois University.

On one side of the debate, shipping interests want the waterways to flow freely, so barge traffic can cruise from Lake Michigan to the Mississippi River. On the other side, a coalition of biologists, sports fishermen, and governments of neighboring states all worry that the Asian carp threatens the fragile ecosystem of the Great Lakes and endangers commercial and recreational fishing and tourism.

A legal battle in federal court, spearheaded by Michigan but with support from Wisconsin, Minnesota, Ohio, and Pennsylvania, is fighting to permanently close two shipping locks in Illinois, between Lake Michigan and the Mississippi River. Their goal: Block any chance of an Asian carp swimming into the lake from the river, where they are known to live.

The DNA evidence was found in Lake Calumet - not yet in Lake Michigan, but past the electric barrier designed to keep the Asian carp out.

After the four-day search found no trespassing carp, opponents of the closures renewed their challenges to the eDNA sampling.

Mark Biel, chairman of UnLock Our Jobs, a coalition of barge interests in the region, says the failure to find an actual Asian carp “once again calls the practice into question.”

Sampling is “controversial and unreliable,” he said in a statement released late Thursday. “Unproductive rhetoric calling for extreme and virtually impossible action like [lock] closure and separation suggests that political carping is becoming a higher priority for some than stopping the advance of the species itself.”

Environmental groups say that because the fish primarily travels in deep water, DNA sampling of water columns can produce more dependable results than traditional methods such as netting or electro-shocking, which primarily work in shallow habitats. They point to testimony given last year at a federal hearing by Duane Chapman, an Asian carp expert with the US Geological Survey, who supported the reliability of DNA sampling and said that more-traditional methods are not as effective in detection.

The failure to find a fish in the Chicago waterway “was not a surprise,” because they used a second-rate method to detect the carp’s presence, wrote John Sellek, communications director for Michigan Attorney General Bill Schuette, in an email.

“[USGS] experts testified that the fishing methods they used this week are not an effective way to find the leading edge of an Asian carp invasion,” Mr. Sellek wrote. “Such methods only capture a small percentage of all fish present.”