FITCHBURG, Wis. - Alert trout anglers' reports to state fish biologists have brought to light a potential threat to Wisconsin's native brook trout, and all trout anglers are now being asked to help track that threat.
Populations of a small parasitic crustacean -- called a copepod by scientists but known commonly as gill lice -- appear to be increasing in some southwestern Wisconsin trout streams.
Brook trout and gill lice have always lived together in Wisconsin streams, according to Department of Natural Resources fisheries biologists, but recently the balance appears to be tipping toward higher gill lice numbers in some streams. The creature attaches to a brook trout's gills making it difficult for the fish to breath and slowing normal growth and development. This increase in gill lice in some streams may be reducing trout numbers.
DNR fish scientists will be taking a closer look at gill lice in select Wisconsin trout streams during the 2013 field season seeking a better understanding of why gill lice populations may be increasing and where they are increasing. In addition to streamside research, a website has been set up to make it easy for anglers across the state to report the appearance or absence of gill lice in the streams they fish.
"Gill lice are not new in Wisconsin streams," said Matthew Mitro, research scientist with the DNR Bureau of Science Services, "they've always lived in balance with our native brook trout with neither having a significant negative impact on the other. Many trout anglers are familiar with them and the lice pose no threat to human health. What is new to us was an increasing number of anglers telling our fish biologists about increases in gill lice in waters where they'd seen few or none previously. That's what got our attention.
"Reports from anglers of a growing gill lice population in some streams first started coming in 2010 and 2011. Our early survey work in 2012 showed a dramatic increase in infections between April and October 2012 in one stream where in April, 42 percent of fish surveyed had the lice. By October we found 95 percent infected. This is far ahead of anything we'd expect to find. Many of the infected fish had high numbers of the gill lice. With so many fish infected so heavily the end result may be lower growth rates, smaller fish, a higher death rate and a smaller brook trout population in the stream."
While researchers know that gill lice have been present all along they don't have good knowledge of how widespread they are in Wisconsin's trout streams or at what level. There are historic anecdotal reports of severe infections in Seas Branch, a small creek in Vernon County, and Duncan Creek in Chippewa County, but nowhere else. Wisconsin fisheries scientists know that gill lice are present in other states with native brook trout populations, such as Minnesota, but little hard data are available.
"This is where Wisconsin trout anglers can really help us document the concern," said Mitro. "It's through a process called citizen-based monitoring and doesn't require any science training to participate.
"All trout anglers are asked to do is go fishing, as they would otherwise, and for each location they fish, fill out a simple report on the species of trout caught and if they observed any gill lice on brook trout. It's equally important to report when they didn't observe any gill lice where they fished. Location information is general so favorite fishing spots are not disclosed."
Survey information will go into a master database and will be available to the public as the information is entered.
A website has been set up for the angler reports with the help of Wisconsin Trout Unlimited which maintains the website and the River Alliance of Wisconsin (all links exit DNR). The effort is funded by a Citizen Based Monitoring grant.
Mitro says it is too early in this investigation into gill lice populations to identify trends but suggests the lice are appearing in younger trout more frequently than before, potentially affecting population growth.
During the 2012 field season researchers found trout less than one year of age with the parasites. Slower growth and development in young-of-the-year fish means surviving their first winter is more of a challenge. This in turn may negatively affect brook trout population growth rates.
"There are a number of factors we hope to evaluate as we look into this more deeply," says Mitro. "We just do not know at this time how things like water temperature, fish population density, the presence or absence of other trout species and the physical characteristics of trout stream habitat may contribute to an increase in gill lice, if at all. These are some of the questions we hope to answer."
FOR MORE INFORMATION CONTACT: Matt Mitro, 608-221-6366