Net Pens Help Acclimate Chinook Fingerlings to Great Lakes Waters
May 5, 2011
Most anglers understand that the Great Lakes salmon fishery is in part dependent on hatchery produced fish. Fisheries Division staffers take eggs and milt from returning salmon and raise fingerlings to stock rivers running into the Great Lakes.
Although the majority the fish are stocked directly into rivers from hatchery trucks, a portion of them are placed in holding pens near the mouths of the rivers where, for a period of a week to a month, they acclimate to their new surroundings. The imprinting net pens are operated by volunteer angler groups in cooperation with Fisheries Division. The groups are usually associated with the rivers where the pens are operated.
"The theory is that when fish are stocked directly from a hatchery environment to a natural river system, they will be disoriented for some period of time and may be more vulnerable to predation --- and maybe even disease outbreaks - because they're scared,' explained Jim Dexter, Fisheries Division's Lake Michigan Basin coordinator. "By putting them in imprinting net pens, you're giving them a significant time period to acclimate to a foreign environment while our partners continue to feed them.
"When they come out of the hatchery they're in good shape," Dexter continued. "The hope is they'll continue to feed, gain some additional weight, and maintain the good physical condition they had in the hatchery in their new environment. Any assistance we can give them that will result in better survival is just good business."
According to fisheries research biologist Dave Clapp, survival among net-penned Chinook salmon - the main species held in imprinting net pens - is up to two times greater than for those fish that are released directly into the river. The straying rate - fish that return as adults to streams other than the stream in which they were planted - is slightly decreased, too.
Imprinting net pens are always located at the lower end of the river, depending on the stream. Dexter said that at South Haven, for instance, the pens are just inside the pier heads. At Grand Haven they're up in town, a good mile or so upstream from the river mouth.
At Grand Haven, where imprinting net pens were first used, the project is conducted by a consortium of angling groups: Grand Haven Steelheaders, the Grand Rapids Steelheaders, the Grand Haven Charter Boat Association and Off-Shore Challenge Fishing Tournament.
Roger Belter, a retired school teacher and charter boat captain, is president of the Grand Haven Steelheaders. He said the group is responsible for building and maintaining the imprinting net pens and feeding the fish while they're in the pens.
"They're fed three times a day - breakfast, lunch and dinner," Belter said.
The pens are six feet by 18 feet with a 55-inch depth. They're constructed of wood, aluminum and PVC pipe with the net baskets manufactured in Washington. The Pens at Grand Haven, which is on its fourth sets of pens since the project began in the early 1990s, cost about $3,000.
"The mesh nets alone cost just under $2,700," Belter said.
Salmon fingerlings are kept in the pens until they are ready to smolt (head out to big water).
"The fish tell us when they want to leave," Belter said. "They'll be jumping out of the nets. The DNR will come and check the fish's health, then we just unzip the nets and the fish swim out on their own."
It takes a boom truck (donated by Rycenga Building Center) to put the nets in and take them out.
In Lake Michigan, net-pen projects are located at Grand Traverse Bay, Manistee, Ludington, Muskegon, Grand Haven, South Haven, Holland, Saugatuck and St. Joseph.
In Lake Superior, imprinting net pens are in Marquette and Ontonagon this year. It's unlikely the imprinting net pens at Black River Harbor will be used this year.
In Lake Huron, imprinting net pens are found at Harbor Beach, Harrisville and Oscoda.
"Our cooperators do an outstanding job," Dexter said. "They've been doing it for years and they have the procedures down pat. The cooperators do 95 percent of the work - they make the pens, they put them in, they feed the fish, measure the fish, weigh the fish and give us a report. Our fish production section will occasionally be called to look at fish that may look sick or are not feeding as expected, but not very often."
There are, of course, no guarantees.
"We make sure through our cooperating agreements with the groups that they understand that if something happens to the fish, there are no replacements," Dexter said. "They understand there is a risk in doing it. More than a few times, we have had fish in a net pen and then a seiche (an oscillating wave) rolls in, the nets go up and down, and it will rub the fish to death. We've had two events in the last 10 years when they've lost everything they've had in localized imprinting net pens."
So imprinting net pens are no panacea. But the projects have helped maintain Great Lakes salmon fishing.