By Jim Diana
The Au Sable River has interested both anglers and the research community for a long period of time. Most of you have familiarity with it through your fishing experiences, and many researchers have begun working on the river because of the importance of that fishing experience. My name is Jim Diana and I am a faculty member at the University of Michigan. During the last fifteen years, I have been involved in several studies on the Au Sable River system, mostly focusing on trout. In this article and in the next several newsletters, I will describe the studies conducted from the perspective of what they mean to you as an angler and as a conservationist.
The studies I will describe have been targeted on a variety of fishery management needs on the Au Sable River. They began about 1985 with an interest in catch-and-release fishing, and asked the question: "How large of an area must be protected if no-kill regulations will be effective in producing trophy trout." For some time, Michigan DNR had done research on the Au Sable River and had established a no-kill section on the South Branch. However, they had no knowledge of whether this section was large enough to encompass the seasonal movements of most trophy brown trout that were the targets of the regulations.
I should explain that trout are believed to be very residential. That is, young trout establish social hierarchies in riffles and pools of a river and spend much of their life within that limited home range (sometimes less than 100 yards of stream). During that time, they feed on drift and establish stations in the stream where they can hide behind obstructions, but dart out to capture drifting invertebrates. Because of this site fidelity, one would most commonly find the same trout in the same pool from year to year.
Large brown trout, and probably large trout of other species, demonstrate changes in both movement and foraging behavior. These changes were first described in the 1970s when scientists, visually observing the behavior of residential trout, noted large nomadic trout moving through their study reaches. Gaylord Alexander, from Michigan DNR, conducted studies on the food habits of trout in the Au Sable River system and found that large trout shifted in behavior so that they became predators. This shift in behavior toward increased predation on fishes, coincided with an abandonment of strong territoriality and the exhibition of larger range movements by the fish. Thus, it appeared that adult brown trout behaved differently than young brown trout, and might not be protected by a small area with catch-and-release regulations.
In the mid 1980s, a UM graduate student, Dave Clapp (currently a Michigan DNR biologist at the Lake Charleviox Research Station), began a telemetry study of trophy brown trout in the South Branch of the Au Sable River. This study was funded by a variety of groups, including the Michigan DNR and the Michigan Fly Fishing Club, based in Livonia. We implanted transmitters in the body cavities of eight brown trout. These trout ranged in size from 17-25 inches and were the trophy trout that were the target of the catch-and-release regulations. We then followed the behavior of these trout over the next two years, particularly focusing on diel behavior during the summertime period. Radio telemetry functions by sending a signal through the air, which can be picked up on a modified radio system. It is fairly simple to locate the fish on a river system such as the Au Sable, as one can drift downstream with a canoe and always pick up a signal from somewhere within the width of the river itself. Once a fish has been located, one can often walk into the stream area and find the fish, especially after behavior patterns have been identified.
When we began this study, it was our belief that large brown trout were probably more wide-ranging than juvenile fish. However, we had no idea how wide-ranging or what the details of their behavior would be. Studies to date had shown that brown trout did not regularly occupy limited locations. Since most studies were done by visual observation, the fish could not be visually observed if they moved widely and, therefore, were not the subject of many trout behavior studies. Clapp made an unusual discovery almost immediately: large brown trout often held locations in cover during the day, but in the evening became mobile and dispersed. At night, these movements might occur where brown trout would be displaced as much as three or four miles in the river system, but then would return to their same home site by the next morning. Tagging observations until that time had shown brown trout to be fairly residential under home cover, especially when recaptured during the day using angling techniques. This coincided with our observation that fish regularly returned to the same home structure. However, it did not describe this large-ranging behavior that occurred at night.
When brown trout were in their home locations, they chose deep, slow areas, usually with heavy log cover. When they moved, they ranged widely over large areas of stream. We believe that this movement occurred because they were foraging on mostly residential trout and other fish species that remained in a limited area; therefore, they moved a larger distance to encounter more prey. We believe they move upstream or downstream as they are foraging, and continue to move until they capture prey. Thus, their movements are sporadic. On days when they have very good prey capture success, they may only have to move a short distance before they are full. On days when they have limited capture success, they might move a large distance. Unfortunately, we do not know the true motivational status of these fish. That is, we only know that they moved, but not when or how much they fed at any particular time.
The use of regular home sites by brown trout led us to believe that cover sites may be limited in many areas. This first study was conducted on a stretch of the South Branch of the Au Sable near Smith Bridge, which actually has relatively good cover. Yet the fish commonly used the same pieces of cover and, in fact, by electroshocking, we often found several large fish under one piece of cover. Possibly, additional cover would expand the number of home sites for adult brown trout and might increase their population. However, almost all observations of brown trout in the South Branch showed that they sought home sites under natural cover, rather than under manmade shelters.
In addition to the regular behavior of brown trout during the summer, we also quantified their movements over the fall and winter periods. There was significantly larger long-range movement in fall and winter than in summer. Brown trout in the study reach moved extensive distances to what appeared to be spawning sites in the fall, then even further distances to overwintering sites. Most of the tracked brown trout overwintered in areas near Roscommon in very still and untrout-like waters. They spawned in limited upstream reaches of the river below Roscommon, and after overwintering returned to roughly the same home site the following year. Many of them were found under the same piece of cover in subsequent years, indicating familiarity with territory, but a large total range of movement. Total ranges of movement varied for individual fish from approximately 400 yards to about 20 miles. The average range of movement for a fish in summer was about four miles, while in winter it was approximately nine miles.
As originally intended, this study shed much light on the use of catch-and-release regulations. Clearly, trout that were found in a given stretch of stream could cover as much as 20 miles of stream during their normal activity. Thus, they might regularly move out of restricted areas and be vulnerable to capture. This is especially true when one looks at the total annual movements of fish, where in this case on the South Branch, they moved an extensive distance to overwintering and spawning sites as well. This led the DNR to believe that they should protect large areas with special regulations to actually protect adult brown trout populations.
The results of this study were somewhat surprising, given the history of studies indicating residential behavior by trout. It led to a number of questions as well, including: 1) Was this behavior simply an adaptation to this specific river system or was it commonly held? 2) Does this behavior change with size or is it behavior of all brown trout in the Au Sable River system? 3) Is there some factor that affects this ranging behavior, such as the current velocity, presence of hiding spaces, or other characteristics? This led to much interest on the movements of large brown trout and a series of additional studies. In subsequent articles, I intend to address the regularity of brown trout behavior, as well as current studies on brown trout reproduction and interactions with walleye in the lower Au Sable River system.
If you are interested in more details on this study, it was published as a Michigan DNR Fisheries Research Report (Number 1907, published 1988) as well as in the Transactions of the American Fisheries Society (1990, Volume 119, pages 1022-1034). Michigan DNR Fisheries Publications are documented on a website (http://www.dnr.state.mi.us/); most publications are available there in abstracts, some in full text. RWOL
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