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Relative attraction of spawning lake trout to reef structure, conspecifics, and reef odor
Marsden, J. E.1 and Johnson, J.2
1308 D Aiken Center, Rubenstein School of Environment and Natural Resources, University of Vermont, Burlington VT 05405
2 Michigan DNR, 160 E. Fletcher, Alpena MI 49707
Restoration of self-sustaining populations of lake trout in the Great Lakes and Lake Champlain has been an elusive goal. Lake trout stocked as yearlings survive well to maturity, and good spawning substrate appears to be available at multiple sites lakes-wide. Spawning has been documented on numerous natural sites, but also on constructed ‘reefs’, both those purpose-built to enhance spawning, and structures built for other functions such as breakwalls and water intake lines. However, some apparently good spawning sites are not visited by spawners. Constructed spawning reefs have either been used by spawners within months of construction, or have remained little-used for several years. The goal of this project was to test competing hypotheses about factors that attract lake trout to spawning areas. We hypothesized that lake trout may locate suitable spawning sites based on attraction to physical reef characteristics alone (H1), to behavioral and/or pheromone cues from other spawners in fall (H2), or to fry ‘odors’ remaining in the substrate (H3). To address these hypotheses, we manipulated chemical cues on a set of artificial reefs constructed in Thunder Bay, Lake Huron, in an effort to attract spawners from a nearby natural reef where spawning activity is focused. Chemical cues comprised fry fecal material collected from a hatchery plus substrate moved from the natural spawning reef, and fry fecal material saturated in a slow-release polymer. Each cue was added to two replicate constructed reefs, with an additional pair of reefs with the same configuration serving as controls. An effort to add a third cue, adult lake trout in spawning condition held in cages on two reefs, failed due to severe weather that damaged the holding cages. Lake trout response to the cues was assessed by gillnetting near the treatment and control sites and the natural reef, and sampling for lake trout eggs at each site. We also implanted acoustic telemetry tags in 25 male and 15 female lake trout and tracked their movements using VEMCO receivers installed in an array encompassing the entire study area. Spawning lake trout were gillnetted on all of the reefs on which nets were set; the catch per unit effort was highest on the natural reef (96 lake trout per 30 m gillnet), lowest on two other two natural reefs (2 – 4 lake trout per 30 m), and intermediate on an unbaited reef and CKD-10, which was baited with fry feces (65 lake trout per 30 m; Table 1)). Eggs were found only on the natural reef and CKD-10. Of 16 tagged lake trout that remained in the study area for most of the study period, all focused most of their time on a small section of the natural reef; 12 encountered the constructed reef arrays and eight spent a few hours exploring a large unbaited reef (Lafarge) and the outer end of the CDK array, but there was no difference in time spent between treatment and control reefs. A single male spent several long periods on each of the constructed and natural reefs. There was no evidence from any of our data that lake trout responded to the odor cues. No lake trout visited the four smallest constructed reefs; however, five of the nine lake trout that encountered the constructed reefs focused their time on the only large reef, Lafarge, suggesting that reef size may be an important attractant. Use of the constructed reefs by spawning lake trout may be a slow process; given that most lake trout aggregate at East Reef, there may be little stimulus to use alternative sites with less mate choice. Future research could focus on the factors that make the small area of East Reef so attractive to spawning lake trout.