Fly-fishermen likely cause of New Zealand mud snail spread


Fly-fishermen are the most-likely cause in the spread of the invasive New Zealand mudsnail (NZMZ) into two more Southern California waters. But that is speculation based on solid circumstantial evidence. While the Department of Fish and Wildlife is not saying it is the fault of fly-anglers, the evidence clearly points to that group as inadvertently spreading the destructive nuisance.

This month the DFW announced that NZMZ had been discovered in the upper Santa Ana River and Bear Creek. Bear Creek is a designated Wild Trout stream and popular with Southern California fly anglers – and while the upper Santa Ana does not have any special designations, it is also a popular trout fishing destination. NZMZ had been previously found on the lower reaches of the Santa Ana River in Orange County.

If you look at the list of water infected with the tiny (one to six millimeters in length) snail species, it reads like a favorites list of fly-fishing trout waters: Hot Creek in the Eastern Sierra, Crowley Lake, both the upper and lower Owens River, Rush Creek below Grant Lake, and Bishop Creek. Overall, a total of over 40 locations in the Eastern Sierra and Southern California have been identified as having the mud snails.

The snails are not good news for native or wild trout fisheries. First, the snails provide little or no food value to the trout in the waters where they now occur. In studies, rainbow trout fed an exclusive diet of unlimited NZMS passed 54 percent of mud snails through the digestive tract alive, and subsequently lost up to half-a-percent of their initial body weight each day, which is nearly equal to the impact of starvation. Second, the populations can become very dense, very quickly. A single snail can lead to a population of 40 million in a single year, and there can be up to a million per square yard of space. With so many snails in a water, they displace and outcompete most native aquatic species, consuming up to half of the food resources in a stream. The snails are especially hard on trout foods such as mayflies, caddisflies, midges, and stoneflies, reducing a water’s productivity. The tiny snails, juveniles as tiny as a grain of sand, hitch rides to new places on the boats, float tubes, nets, and wading gear of anglers, able to survive in a dry environment for a short period of time. Damp areas in waders or wading shoes, float tubes, or other fishing gear will allow them to survive for much greater periods of time.

Anglers can take precautions to prevent the spread of the mudsnails, but most do not. Gear, float tubes, and boats must be completely drained and dried out for at least 48 hours. Waders and nets can be frozen for 24 hours. Both treatments will completely eliminate the possibility that the tiny hitchhikers will survive to be transported to another watershed.

Frankly, the agencies charged with preventing the spread of mudsnails are doing mediocre jobs are getting the word out to anglers. The DFW, USFWS, and U.S. Geological Survey have wonderful web sites and handouts explaining the problem, but the outreach in fishing publications, websites, and in the field is woefully inadequate. In less than 25 years, the mudsnails have spread from a single location on the Snake River in Idaho in 1995 (likely arriving there with trout eggs or fry) to every Western state except New Mexico. They now occur on most of California’s major trout and steelhead/salmon rivers.

Fly-fishermen are some of the most environmental conscious anglers, and yet, they are the likely the inadvertent spreaders of the mudsnails. This is because outreach to this and other fishing groups has been poor and the impacts the snails can wreak on a fishery is not well explained.

Once established, the snails are currently impossible to eradicate, and they are now throughout the Santa Ana River system in Southern California, from the headwaters in Bear Creek and the upper Santa Ana all the way to the ocean. What impacts the snails will have on the wild trout fisheries in this watershed is unknown.

NO KEEPING ROADKILL FOR DINNER: In spite of Internet posts to the contrary, you can’t pull over if you see a freshly road-killed deer and butcher it to utilize the meat. Yes, legislation did pass that past year (SB 395) that takes effect Jan. 1 that allows for the development of a government-run program to salvage wild big game killed on roadways to be utilized. It does not permit the general public to collect and utilize road kill.

DFW IVORY DNA DATABASE GROWS: The DFW’s forensic scientists have been sampling mammoth and mastodon tusks from state collections to add to their DNA database of ivory. It is now illegal in California even to sell recovered ivory from extinct mammoths and mastadons. African ivory, walrus ivory, or hippopotamus.

The DFW says AB 96 closed a loophole in the law that allowed the continued sale of tusks from mammoths and mastodons, because those animals were extinct. The exemption gave traffickers the chance to fraudulently claim that they were trafficking in legal material, when it was poached recently. Now that all forms of ivory trafficking are illegal – regardless of whether the species of origin is extinct – CDFW officials hope that an expanded DNA database will be the key to shutting down the ivory trafficking trade in California.


Jim Matthews is a syndicated Southern California-based outdoor reporter and columnist. He can be reached via e-mail at or by phone at 909-887-3444.

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