Summary
Annual reproductive surveys monitored nesting location, reproductive success and the age and size of individually tagged male smallmouth bass Micropterus dolomieu that reproduced in Millers Lake, a 45 ha widening of the Mississippi River, Ontario, and in a 1·5 km pool and riffle section of the river directly upstream. The vast majority of males displayed fidelity to either the river or the lake as reproductive habitat throughout their lifetimes. Nearly, half of the males that reproduced in successive years exhibited strong nest‐site fidelity by nesting within 20 m of their previous year’s nest site. In most years, when compared to those in the lake, reproductive males in the river differed significantly in reproductive characteristics including age and size at maturation and nesting success rates. A 3 year telemetry project identified two distinct habitat use patterns: lake‐resident fish remained in the lake throughout the year and potamodromous individuals migrated from the lake to upriver spawning habitat in the spring and then returned to the lake prior to the onset of winter. Integration of habitat use and reproductive data suggests that there are significant differences in the life‐history strategies of fish that reproduce in the river v. the lake.
Methodology
Study site
The Mississippi River is a shallow, warm‐water tributary of the Ottawa River (Frontenac County, Ontario, Canada; 44°56′ N; 76°42′ W), and this study was conducted on a section of river that encompasses two distinctly different habitats (Fig. 1). The upstream 1·5 km is riverine habitat, with eight pools separated by riffles and rapids of varying magnitudes. All pools have a maximum depth >6 m and surface areas ranging from 0·49 to 2·14 ha (mean ±s .d . surface area 1·00 ± 0·57 ha). At the downstream end of this section, the river flows over a set of rapids into Millers Lake. None of the rapids between the uppermost pool and the lake are barriers to M. dolomieu movement. The lake is 1·75 km long and 0·25 km wide (45 ha), with a maximum depth of nearly 25 m. The study site is bordered upstream by c. 2 km of white‐water environment (extremely marginal spawning habitat for M. dolomieu ) that includes a natural waterfall with a 2 m vertical drop. This waterfall serves as a complete impediment to M. dolomieu emigration and probably limits immigration greatly as well. There is another series of rapids and waterfalls downstream of the study site with c. 2 m vertical declinations but each with somewhat of a more gradual grade suggesting that they might serve as less than a total obstruction to M. dolomieu movement than the upstream falls. Telemetry studies, however, reported here did not detect fish movements out of the study site in either direction. The riverine section of the study site is only accessible by canoe portage and receives negligible angling pressure. There is public access to the lake, which does receive more angling pressure than the river but does not, however, attract a substantial number of anglers targeting M. dolomieu (Kubacki et al., 2002). In this part of Ontario, the M. dolomieu angling season is closed until the last weekend in June, thereby protecting nesting M. dolomieu through the reproductive period.
Telemetry study
A 3 year telemetry study was initiated in May of 2000, when radio transmitters (MBFT‐6A, 36 month battery life, 3·7 g in water; Lotek Wireless Inc., Newmarket, Ontario, Canada) were surgically implanted into the peritoneal cavity of 16 male and 13 female adult M. dolomieu using methods described in Cooke et al., (2001). Individuals were captured by hook and line angling: nine females and two males were captured at the rapids between the river and lake habitats, two females and six males were captured near known nesting areas in the river and two females and eight males were captured near known nesting areas in the lake. To ensure transmitter size did not exceed 2% body mass and interfere with natural behaviour, larger individuals were selected to be part of the study (mean ±s .d . total length, L T, = 372 ± 49 mm, L T range = 285–486 mm). Fish were located with a four‐element Yagi antenna and SRX_400 receiver (Lotek Wireless Inc.) from either a boat or canoe during most of the year, but from shore or the lake surface when there was ice cover. Fish locations were triangulated using shore‐based landmarks, while moving towards the estimated location and simultaneously reducing gain to ensure accurate locations. Radio‐tagged fish were tracked three times a week during the reproductive period and less frequently during the non‐reproductive periods.