* Fish kill netted one carp, but others may have sunk

* State of Michigan to demand watersheds be separated

* Electric barrier does not solve, environmentalists say

CHICAGO, Dec 7 (Reuters) - The waterway that allows invasive Asian carp into the Great Lakes should be closed, Michigan’s attorney general said on Monday as the state prepared a lawsuit to protect its fishing and tourism industries.

“Asian carp must be stopped now because we will not have a second chance once they enter Lake Michigan,” Attorney General Mike Cox said.

His office said it would file suit soon against the U.S. Army Corps of Engineers, the state of Illinois and Chicago’s sewer authority. It would ask a judge to order the separation of the Mississippi River’s watershed from that of the Great Lakes, at least temporarily, to protect Michigan’s $7 billion lake-related leisure and tourism industry.

Authorities fear that if the carp swim up to the Great Lakes, which supply water for tens of millions of people, they would ruin the lakes’ $7 billion fishery and disturb the environment by consuming the bottom of the food chain and crowding out native species.

Shippers have said closing the canal leading to the lakes could be disastrous for them, and would require 15 million tonnes of commodities that travel by barge annually to move by rail or truck instead. Leisure boaters could also be affected.

The Army Corps of Engineers completed maintenance work on Friday on a custom-made electric barrier in a canal designed to keep the prolific and voracious Bighead and Silver carp out of Lake Michigan, and the barrier was turned back on.

A toxin was poured into the canal last week to kill all the fish for 6 miles (10 km) below the barrier to keep the carp at bay while the electricity was off.

So far, only one Bighead carp was found among the thousands of fish scooped up in the huge fish kill, but authorities said carp tend to sink when poisoned so that was no indication of how close the fish have gotten to the barrier or to Lake Michigan.

Bighead carp DNA was detected recently on the lake side of the barrier, and environmentalists warned it may already be too late to keep the carp out.

“The electrical barrier has never been intended as a permanent solution. We certainly can’t rely on (the poison) rotenone as the solution. How do we solve this problem once and for all?” Joel Brammeier of the Alliance for the Great Lakes said.

A spokesman for the Army Corps of Engineers had no comment on the pending lawsuit. The Army Corps is supposed to be analyzing the feasibility of permanently closing the link, which was created more than a century ago in a massive engineering project that included reversing the direction of the Chicago River.