Mark P. Ebener

Lake Trout Rehabilitation in Lake Huron

Efforts to restore lake trout (Salvelinus namaycush) in Lake Huron after their collapse in the 1940s were underway in the early 1970s with completion of the first round oflampricide applications in tributary streams and the stocking of several genotypes. We assess results of rehabilitation and establish a historical basis for comparison by quantifying the catch of spawning lake trout from Michigan waters in 1929-1932. Sixty-eight percent of this catch occurred in northern waters (MH-1) and most of the rest (15%) was from remote reefs in the middle of the main basin. Sea lampreys (Petromyzon mari-nus) increased in the early 1980s in the main basin and depressed spawning populations of lake trout. This increase was especially severe in northern waters and appeared to be associated with untreated populations in the St. Marys River. Excessive commercial fishing stemming from unresolved treaty rights also contributed to loss of spawning fish in northern Michigan waters. Seneca-strain lake trout did not appear to be attacked by sea lampreys until they reached a size > 532 mm. At sizes > 632 mm, Seneca trout were 40-fold more abundant than the Marquette strain in matched-planting experiments. Natural reproduction past the fry stage has occurred in Thunder Bay and South Bay, but prospects for self-sustaining populations of lake trout in the main basin are poor because sea lampreys are too abundant, only one side of the basin is stocked, and stocking is deferred to allow commercial gillnetting in areas where most of the spawning occurred historically. Backcross lake trout, a lake trout x splake (S. fontinalis x S. namaycush) hybrid, did not reproduce in Georgian Bay, but this genotype is being replaced with pure-strain lake trout, whose early performance appears promising.

Development and Implementation of an Integrated Program for Control of Sea Lampreys in the St. Marys River

Abstract The development and implementation of a strategy for control of sea lampreys ( Petromyzon marinus ) in the St. Marys River formed the basis for rehabilitation of lake trout ( Salvelinus namaycush ) and other fish in Lakes Huron and Michigan. The control strategy was implemented by the Great Lakes Fishery Commission (GLFC) upon recommendations by the interagency Sea Lamprey Integration Committee, and many managers and scientists from United States and Canada federal, state, provincial, tribal, and private institutions. Analyses of benefits vs. costs of control options and modeling of the cumulative effects on abundance of parasitic-phase sea lampreys and lake trout produced a strategy that involved an integration of control technologies that included long- and short-term measures. The longterm measures included interference with sea lamprey reproduction by the trapping and removal of spawning-phase sea lampreys from the river and the sterilization and release of the trapped male sea lampreys. The theoretical reduction of larvae produced in the river from these two combined techniques averaged almost 90% during 1997 to 1999. Lampricide treatment with granular Bayluscide of 880 ha of plots densely populated with larvae occurred during 1998, 1999, and 2001 because modeling showed the sooner parasitic-phase sea lamprey populations declined in Lake Huron the greater the improvement for restoration of lake trout during 1995 to 2015. Post-treatment assessments showed about 55% of the larvae had been removed from the river. An adaptive assessment plan predicted high probability of detection of control effects because of many available indicators. The GLFC will face several critical decisions beyond 2001, and initiated a decision analysis project to aid in those decisions.

Lake Trout Mortality and Abundance in Southern Lake Huron

Abstract Populations of lake trout Salvelinus namaycush in the main basin of Lake Huron collapsed in the 1940s because of predation by sea lampreys Petromyzon marinus and commercial fishing. Efforts to rehabilitate lake trout have emphasized reduction of mortality and the stocking of hatchery-reared lake trout to reestablish populations. We fit a statistical catch-at-age model for lake trout in the southern main basin of Lake Huron using a maximum likelihood approach to estimate mortality rates and abundance during 1984–1993. This represents the first such analysis for lake trout in the Great Lakes, and a flexible application of the approach proved useful for integrating diverse information and assessing population and mortality trends. Sea lamprey-induced mortality and recruitment of lake trout to age 1 were calculated external to model fitting. Recruitment was based on numbers of lake trout stocked because natural recruitment is negligible. Sea lamprey-induced mortality rates were based on observed woun...

Sea Lamprey (Petromyzon marinus) Parasite-host Interactions in the Great Lakes

Prediction of how host mortality responds to efforts to control sea lampreys (Petromyzon marinus) is central to the integrated management strategy for sea lamprey (IMSL) in the Great Lakes. A parasite-host submodel is used as part of this strategy, and this includes a type-2 multi-species functional response, a developmental response, but no numerical response. General patterns of host species and size selection are consistent with the model assumptions, but some observations appear to diverge. For example, some patterns in sea lamprey marking on hosts suggest increases in selectivity for less preferred hosts and lower host survival when preferred hosts are scarce. Nevertheless, many of the IMSL assumptions may be adequate under conditions targeted by fish community objectives. Of great concern is the possibility that the survival of young parasites (parasitic-phase sea lampreys) varies substantially among lakes or over time. Joint analysis of abundance estimates for parasites being produced in streams and returning spawners could address this. Data on sea lamprey marks is a critical source of information on sea lamprey activity and potential effects. Theory connecting observed marks to sea lamprey feeding activity and host mortality is reviewed. Uncertainties regarding healing and attachment times, the probability of hosts surviving attacks, and problems in consistent classification of marks have led to widely divergent estimates of damages caused by sea lamprey. Laboratory and field studies are recommended to provide a firmer linkage between host blood loss, host mortality, and observed marks on surviving hosts, so as to improve estimates of damage.

Spatial and Temporal Distributions of Lake Whitefish Spawning Stocks in Northern Lakes Michigan and Huron, 2003–2008

ABSTRACT Adult lake whitefish were tagged and released from the Big Bay de Noc (BBN) and Naubinway (NAB) stocks in northern Lake Michigan, and the Detour (DET) and Cheboygan (CHB) stocks in northern Lake Huron during 2003–2006 to describe their spatial and temporal distributions. The contemporary spatial distributions were compared with past distributions of the BBN and NAB stocks. Sixty-two percent of BBN tag recoveries occurred in Wisconsin waters during winter, spring and summer, but 83% of fall tag recoveries were made near the tagging site. Eighty-eight percent of the NAB tag recoveries were made in the management unit of tagging and 7% occurred into northern Lake Huron. Over 90% of the DET stock remained in the vicinity of the tagging sites regardless of the season, while 75% of the CHB tag recoveries were made in northwestern Lake Huron and 17% were made in Ontario. Based on regression tree analysis, there were strong stock, season, and year effects on movement distances, with weaker effects due to sex and length at tagging. Spatial distribution of the BBN stock changed from 1978–1982 to 2003–2008, but spatial distribution of the NAB stock did not. Substantial differences in movement and distribution existed among the four stocks, large seasonal differences in spatial distribution were found within some stocks, and lake whitefish exhibited strong spawning site fidelity. Present management unit boundaries are inappropriate for managing three of our four stocks, and agencies should consider developing single harvest limits for both northern Lake Huron and western Lake Michigan.

Diet and Prey Selection by Lake Superior Lake Trout during Spring, 1986–2001

ABSTRACT We describe the diet and prey selectivity of lean (Salvelinus namaycush namaycush) and siscowet lake trout (S. n. siscowet) collected during spring (April–June) from Lake Superior during 1986–2001. We estimated prey selectivity by comparing prey numerical abundance estimates from spring bottom trawl surveys and lake trout diet information in similar areas from spring gill net surveys conducted annually in Lake Superior. Rainbow smelt (Osmerus mordax) was the most common prey and was positively selected by both lean and siscowet lake trout throughout the study. Selection by lean lake trout for coregonine (Coregonus spp.) prey increased after 1991 and corresponded with a slight decrease in selection for rainbow smelt. Siscowet positively selected for rainbow smelt after 1998, a change that was coincident with the decrease in selection for this prey item by lean lake trout. However, diet overlap between lean and siscowet lake trout was not strong and did not change significantly over the study period. Rainbow smelt remains an important prey species for lake trout in Lake Superior despite declines in abundance.

Relative abundance, site fidelity, and survival of adult lake trout in Lake Michigan from 1999 to 2001: Implications for future restoration strategies

Abstract We compared the relative abundance of lake trout Salvelinus namaycush spawners in gill nets during fall 1999–2001 in Lake Michigan at 19 stocked spawning sites with that at 25 unstocked sites to evaluate how effective site-specific stocking was in recolonizing historically important spawning reefs. The abundance of adult fish was higher at stocked onshore and offshore sites than at unstocked sites. This suggests that site-specific stocking is more effective at establishing spawning aggregations than relying on the ability of hatchery-reared lake trout to find spawning reefs, especially those offshore. Spawner densities were generally too low and too young at most sites to expect significant natural reproduction. However, densities were sufficiently high at some sites for reproduction to occur and therefore the lack of recruitment was attributable to other factors. Less than 3% of all spawners could have been wild fish, which indicates that little natural reproduction occurred in past years. Wound...

A Synthesis of Cisco Recovery in Lake Superior: Implications for Native Fish Rehabilitation in the Laurentian Great Lakes

Abstract Populations of cisco Coregonus artedi in the Laurentian Great Lakes supported large-scale commercial fisheries and were the primary forage of piscivores during the first half of the 20th century. However, by 1970 populations had collapsed in all of the lakes. Since then, ciscoes have staged a recovery in Lake Superior. In this synthesis, we describe the status of ciscoes in Lake Superior during 1970–2006 and provide a comprehensive review of their ecology. Better understanding of age estimation techniques, application of hydroacoustic and midwater trawl sampling, and compilation of long-term data sets have advanced our understanding of the species. Management agencies contemplating rehabilitation of cisco populations should recognize that (1) knowledge of cisco ecology and population dynamics is increasing; (2) ciscoes are long-lived; (3) Great Lakes populations are probably composed of both shallow-water and deepwater spawning forms; (4) large year-classes can be produced from small adult stocks...

Lake Trout Status in the Main Basin of Lake Huron, 1973–2010

Abstract We developed indices of lake trout Salvelinus namaycush status in the main basin of Lake Huron (1973–2010) to understand increases in the relative abundance of wild year-classes during 1995–2010. Sea lamprey Petromyzon marinus wounds per 100 lake trout declined from 23.63 in 2000 to 5.86–10.64 in 2002–2010. The average age-7 lake trout catch per effort per recruitment (CPE/R; fish·305 m of gill net−1·million stocked yearlings−1) increased from 0.56 for the 1973–1990 year-classes to 0.92 for the 1991–2001 year-classes. Total CPE (fish/305 m of gill net) declined from 16.4 fish in 1996 to 4.1 fish in 2010, but the percentage of age-5 and younger lake trout steadily decreased from more than 70% before 1996 to less than 10% by 2009. The modal age in gill-net catches increased from age 5 before 1996 to age 7 by 2005. The average adult CPE increased from 2.8 fish/305 m of gill net during 1978–1995 to 5.34 fish/305 m of gill net during 1996–2010. The 1995–2010 year-classes of wild fish were more abundan...

Spatial and temporal variation of maturation schedules of lake whitefish (Coregonus clupeaformis) in the Great Lakes

Fish maturation schedules vary greatly among systems and over time, reflecting both plastic and adaptive responses to ecosystem structure, physical habitats, and mortality (natural and fishing). We...