Color profile: Disabled Composite Default screen 2290 Quantification of historic lake trout (Salvelinus namaycush) spawning aggregations in Lake Michigan Kristine A. Dawson, Randy L. Eshenroder, Mark E. Holey, and Camille Ward Abstract: We used commercial catch reports to determine site-specific characteristics of lake trout (Salvelinus namaycush) spawning aggregations in Michigan’s and Wisconsin’s waters of Lake Michigan before reproduction ceased in the 1950s. One hundred sites where annual catches exceeded 25 kg/year were identified. Two thirds of the catch was made in offshore waters and nearly half was made in the northeast sector of the lake. Catch was a better descriptor of the size of spawning aggregations than catch per unit effort (CPUE). CPUEs were not significantly different among onshore, offshore, and southern deepwater reefs. Spawning activity as measured by CPUE had peaked by the week beginning 29 October at all locations. Spawning aggregations were strongly clustered in the northeast section of the lake where Devonian rocks were subject to brecciation, i.e., fracturing and recementing following slumping. Areas zoned as refuges in the current lake trout rehabilitation plan account for 36% of the historical catch targeted at spawning aggregations. We recommend a refocusing of rehabilitation efforts in northern waters from sites where historical catches were modest to sites that produced the largest catches of lake trout aggregated for spawning. Résumé : Nous avons utilisé des rapports sur les captures commerciales pour déterminer les caractéristiques locales propres aux concentrations de géniteurs de touladi (Salvelinus namaycush) dans les eaux du lac Michigan (Michigan et Wisconsin) avant que n’y cesse la reproduction dans les années 1950. Nous avons repéré 100 sites pour lesquels les captures annuelles dépassaient 25 kg/an. Deux tiers des prises ont été faites au large et près de la moitié dans la partie nord-est du lac. Les captures constituaient un meilleur descripteur de la taille des concentrations de géniteurs que l’indice des captures par unité d’effort (CPUE). En effet, les CPUE des régions littorales, du milieu du lac ou des récifs en eaux profondes dans la partie sud ne présentaient pas de différences significatives. La fraye, mesurée d’après les CPUE, avait atteint son sommet la semaine du 29 octobre à tous les emplacements. Les concentrations de géniteurs étaient regroupées en grosses grappes dans la section nord-est du lac, là où des roches dévoniennes ont été soumises à la bréchification, c.-à-d. qu’elles ont été fracturées, ont glissé puis ont été recimentées. Les secteurs choisis comme refuges dans le plan actuel de rétablissement du touladi fournissaient 36% des captures antérieures qui visaient les concentrations de géniteurs. Nous recommandons de réorienter les efforts de rétablissement dans le secteur nord en délaissant les sites où les captures historiques étaient modestes au profit de ceux qui produisaient les plus importantes captures de touladi pendant les rassemblements de fraye. [Traduit par la Rédaction] Introduction it represented the single largest extirpation of a char in recorded history. Attempts to rehabilitate stocks began in the 1960s, Lake Michigan is the second largest of the Great Lakes in when programs to suppress sea lamprey and reintroduce lake volume and the third largest in surface area, but among the five trout were initiated. Reestablishing wild stocks has proven to lakes, it had the highest yields of lake trout (Salvelinus be more difficult than originally envisioned — a review of the namaycush) until stocks began collapsing in the 1940s. By the rehabilitation program by Holey et al. (1995) indicated that 1950s, lake trout were virtually extirpated from Lake Michigan, wild fish are scarce. a result of over a century of commercial fishing and of preda- The difficulty of rehabilitating a salmonine extirpated from tion by the sea lamprey (Petromyzon marinus), which was first a large system, like Lake Michigan, is a function of the diver- seen in these waters in 1936 (Wells and McLain 1973). The sity of such populations which are understood to be composed loss of the lake trout from Lake Michigan was catastrophic — of many small, locally adapted stocks, each imprinted to a specific breeding site (see Goodier 1981). Lake trout spawn in the fall on rocky reefs, and their eggs incubate overwinter in Received March 5, 1996. Accepted February 28, 1997. interstices that protect them from predators and from being J13335 washed away. Lake trout rehabilitation in Lake Michigan is K.A. Dawson, R.L. Eshenroder,1 and C. Ward. Great Lakes based on the assumption that hatchery-reared fish will seek out Fishery Commission, 2100 Commonwealth Blvd., Suite 209, and reproduce on numerous spawning areas formerly used by Ann Arbor, MI 48105, U.S.A. the native stocks (Foster 1984). The near lack of wild juveniles M.E. Holey. United States Fish and Wildlife Service, Fishery in Lake Michigan indicates that a serious life history bottle- Resources Office, 1015 Challenger Court, Green Bay, neck occurs. Holey et al. (1995) have speculated that hatchery- WI 54311, U.S.A. origin lake trout may not be spawning on the formerly used 1 Author to whom all correspondence should be addressed. reefs, but their hypothesis cannot be tested without compara- e-mail: [email protected] tive information on the historical distribution of spawners and Can. J. Fish. Aquat. Sci. 54: 2290–2302 (1997) © 1997 NRC Canada F97-136.CHP Thu Dec 04 10:47:44 1997 Color profile: Disabled Composite Default screen Dawson et al. 2291 a better understanding of the reproductive biology of this spe- closed (spawning) season for the purpose of collecting spawn for cies in large systems. hatcheries. The closed season for lake trout fishing in Michigan’s Because research programs on Lake Michigan were small waters of Lake Michigan during 1929–1932 extended from 20 at the time stocks were collapsing, information on the histori- October to 22 November (Brege and Kevern 1978). Without the per- cal distribution of spawning lake trout is limited. Efforts have mit system, no catch records would be available for 34 days during the middle of the lake trout spawning period. been made since the 1970s to document the locations of the We would have preferred to evaluate simultaneous years in former spawning sites. Peck (1979) identified spawning sites Wisconsin and Michigan, but because the daily catch reporting sys- in Michigan’s waters using information from biologists who tem did not begin in Wisconsin until 2 years after the permit fishery had interviewed commercial fishers, and Coberly and Horrall was banned in Michigan, this approach was not possible. To mini- (1980) used other interview data to map spawning sites in mize the time separation with the Michigan data, we compiled catch Wisconsin’s waters of Lake Michigan. Goodyear et al. (1982) reports for the nearest, comparable 4-year period in Wisconsin, which and Thibodeau and Kelso (1990) compiled these and other began in 1934. Wisconsin also had a permit system for fishing spawn- reports for the entire Great Lakes. Side-scan sonar and other ing lake trout; it allowed fishing during the 15 October to 20 November methods have been used to physically map substrates on a few closed season. We did not include commercial catch data from Illinois well-known spawning sites in Lake Michigan (Edsall et al. and Indiana because the catch reporting system in these states did not begin until 1950 (Hile 1962), when the collapse of the lake trout 1989; Edsall et al. 1995). None of these studies quantified the population in Lake Michigan was too far advanced for our analysis. historical distribution of spawners among sites, even though We used a combination of criteria to identify gillnet hauls that such information should reveal the relative importance of in- were targeted at spawning lake trout because catch report forms com- dividual sites and should allow for comparison with the present pleted by commercial fishers did not require such information. Some distributions of hatchery-reared lake trout. A comparison lake trout were caught as bycatch in gill nets set for lake whitefish, could be used, for instance, to determine whether current man- and immature lake trout large enough to be marketable were also agement zones for lake trout rehabilitation are appropriate. targeted. The location of each haul was plotted on a National Oceano- Although recollections of commercial fishers have been graphic and Atmospheric Administration lake chart, and fishing tar- valuable in identifying qualitative characteristics of lake trout geted at spawners was inferred if (i) the depths fished were less than spawning sites in Lake Michigan, systematically collected re- 40 m (except on southern reefs in the middle of the lake) or a shallow- cords of their catch and effort have the potential to quantify water reef was nearby and (ii) lake whitefish were not present or were rare in the haul. These criteria are based on the idea that the spawner such differences. In this study, we use records from Wisconsin fishery started when mature lake trout began to aggregate in shallow and Michigan commercial fishers to establish (1) the distribu- water near spawning reefs and that lake whitefish, which begin tion during the 1920s and 1930s of lake trout yields from spawning later, and immature lake trout remained in deeper water. In spawning aggregations, (2) the utility of catch per unit effort using the 40-m cutoff, we acknowledge uncertainty about just how (CPUE) as an alternative measure of spawning aggregations, deep lean-type lake trout spawned in Lake Michigan. The best spawn- (3) the temporal pattern of spawner aggregations, and (4) the ing habitat observed on Julian’s Reef (Lake Michigan) was as deep as distribution of spawning aggregations in relation to bedrock 34 m (Edsall et al.