(D—‘CD III III II 31293 IIII IIIIIIIIIII i L 00850 0773 L3” “:3 2.51.12; 1."; . 2 . , ‘5 I! 4 ‘51? ‘\-,-I‘\ u-n-‘u-U‘v,“- “Cu-” EFL. Léri‘n." ‘1" —-b ‘w-Ua‘! This is to certify that the thesis entitled An Evaluation of Lake Trout (Salveiinus namaycush) Hooking Mortaiit) in the Upper Great Lakes presented by Andrew J. Loftus has been accepted towards fulfillment of the requirements for Master of Soience degmmin Fisheries and Niidiife flfl %? Major professor/ Date June 4, 1986 0-7639 MS U u an Affirmative Action/Equal Opponunity Institution hASLI RETURNING MATERIALS: Place in book drop to remove this checkout from LIBRARIES , “ your record. FINES W111 be charged if book is returned after the date stamped below. ..031~* 4% . 9. - \ 'Q‘ I§I§4e AN EVALUATION OF LAKE TROUT (SALVELINUS NAMAYCUSH) HOOKING MORTALITY IN THE UPPER GREAT LAKES BY ANDREW JOHN LOFTUS A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Fisheries and Wildlife 1986 ABSTRACT AN EVALUATION OF LAKE TROUT (SALVELINUS NAMAYCUSH) HOOKING MORTALITY IN THE UPPER GREAT LAKES BY Andrew John Loftus In order to assess the effectiveness of restrictive fishing regulations on Great Lakes lake trout stocks, investigation of lake trout hooking mortality was conducted in Lakes Michigan, Huron and Superior during 1984 and 1985. Fish were collected with charterboat operators and sport fishermen. Evaluation of short term mortality was accomplished using an aerated, chilled tank system on board the boat. Long term mortality was estimated by returning hooked fish back into the water, tethered to a line- buoy system. Overall hooking mortality was determined to be 14.9% (95% C.I. 7.4-25.7). Significantly higher mortalities were noted in fish which were not immediately discovered to have been hooked. In addition, lake trout which were hooked in internal regions produced a mortality of 71.4 % while those hooked in the upper or lower jaw exhibited a mortality of’ 6.9%. A significant difference was also seen in the mortality between length classes of fish, with smaller size classes producing higher mortalities. The depth from which the fish was angled, the temperature differential from this depth to the surface, gear type and handling times had no significant effect on survival. These results appear to support the use of size limits, creel limits and season restrictions as effective methods to reduce sport fishing mortality of lake trout. ACKNOWLEDGMENTS This study was funded by contributions from Dingell- Johnson project F-35-R, the Michigan Department of Natural Resources and funds provided by Michigan State University. Equipment and time were donated by the Michigan Steelhead and Salmon Fisherman's Association, Heddon Corporation, S and K Products, Penn Corporation and Plano. I would like to express my sincere appreciation to the members of my guidance committee, Dr. Richard Hill, Dr. W.C. Latta and Dr. Howard Tanner for the support, encouragement and education which they have provided me with over the years. A special note of gratitude goes to my major professor, Dr.‘William Taylor, for the opportunities and challenges which he posed to me. I cannot begin to adequately express my thanks to him for his guidance, patience and friendship in this endeavor. A special note of gratitude is given to Bob Day. Had it not been for his assistance and close friendship, this project would have suffered greatly. I can only say, thank you. Dan Hayes, Mike Nurse and Robin Ziegler also deserve a special mention for the assistance and very special friendships which they all gave. Thank you all for sharing your ideas and skills in all aspects of this project and my life. Thanks also to Stu Kogge and.Jill DuFour'for their superb art work. iii All of the personnel at the Michigan Department of Natural Resources Great Lakes Fishery Station provided invaluable assistance and advice. Myrl Keller not only provided equipment and lodging, but his expert advice and personal guidance through every phase of this project was greatly appreciated. Thanks also to Gaylord Alexander for his input to the design of this research. Had it not been for all of the charterboat operators and sportfishermen around Michigan, this project would not have been possible. To all of you who helped, I say thank you for your time and effort. Although many people were involved, a special note of appreciation is extended to Andy Pelt, Bob Hamilton, Clarence White» Gary Somers and Chuck Pistis for the extra assistance which they provided. The deepest note of gratitude belongs to my family. Thanks Mom and Dad for your support and particularly your love which you gave me. Although you may not have always understood my motives, you stood behind me all the way. iv TABLE OF CONTENTS Page LIST OF TABLES............................................vi LIST OF FIGURES..........................................vii INTRODUCTION.............................................. 1 METHODS....................................................3 Study Area............................................4 Tank Method...........................................4 Line-buoy Method......................................6 STATISTICAL ANALYSIS.......................................9 RESULTS AND DISCUSSION....................................10 Overall Mortality....................................lO Size.................................................12 Handling Times.......................................l4 Gear Type............................................l6 Anatomical Hooking Site..............................18 Temperature..........................................20 Depth................................................20 MANAGEMENT IMPLICATIONS...................................22 CONCLUSION................................................24 LIST OFREFERENCES...OO0.0.0.0....0.0...0.0.0.00000000000027 Table Table Table Table LIST OF TABLES Page Hooking mortality of lake trout (Salvelinus namaycush) returned to natural waters in the upper Great Lakes, 1984-1985. Confidence intervals from Mainland et al. (l956)..........11 Mortality by length of lake trout caught in the upper Great lakes, 1984-19850 0 e o e o o e o o e o 13 Number of lake trout caught at individual water temperature differentials in the upper Great Lakes, 1984-1985. Numbers in parenthesis indicate the number of mortalities....OOOOOOOOOOOOOIOOOOOOOOO0.00.0.0021 Parameters incorporated into the lake trout population model of Clark and Huang (1985)00......00......0.0.00000000000000025 vi Figure Figure Figure Figure Figure LIST OF FIGURES Page Sampling stations for lake trout hooking mortality in 1984 and 1985.....................5 Placement of clip in the mandible of lake trout for the line-buoy method.................7 Schematic diagram of the line-buoy method used to assess lake trout hooking mortality....8 Hooking mortality associated with different hock types.OOOOOOOOOOOOOOOOOOO...0.0.0000...00.17 Classification of hook placement areas (modified from Mongillo 1984.).................l9 vii INTRODUCTION Lake trout (Salvelinus namaycush) were once abundant in the Great Lakes and constituted one of the major predators in this system (Christie 1974). Until the 1950's, naturally reproducing stocks of this fish helped to support a thriving commercial fishery in the Great Lakes. Historically, lakes Michigan and Huron each consistently produced in excess of 500,000 pounds of lake trout annually while Lake Superior reached over 400,000 pounds per year (Baldwin et a1. 1979). Intensive over fishing, environmental degradation and the invasion of the sea lamprey (Petromyzon marinus) through the Welland Canal have often been cited as the causes of the total collapse of the lake trout stocks in Lakes Michigan and Huron and the near collapse of the stocks in Lake Superior (Christie 1974). With the development of a chemical lampricide (trifluoromethylnitrophenol) and subsequent reductions of the sea lamprey populations along with extensive limitations on commercial fishing it was felt that a suitable environment for the re-esablishment of lake trout stocks had been created (Peck 1984). Stocking of fingerlings began in Lake Michigan in 1965 with the intention of establishing self sustaining stocks of this fish. However, to date, only Lake Superior shows signs of significant natural recruitment (Rybicki 1983). Many theories have been advanced regarding the failure of reproduction in the planted lake trout stocks. Degradation of spawning habitat through changing land use practices, increased contaminants in the Great Lakes, lack of abundant spawners, genetic inviability in introduced strains and failure of lake trout to locate and deposit eggs successfully on suitable reefs have all been cited as possible reasons for this failure.(Rybicki and Keller 1978, Peck 1979). Currently though, attention has shifted to achieving the necessary critical mass of reproductive age fish in the population. To achieve this necessary critical mass, the Great Lakes Fishery Commission Lake Michigan Lake Trout Technical Committee (1983) advised that total annual mortality not exceed 40 %. Currently, mortality exceeds this in most statistical management districts of Lake Michigan (Rybicki 1983). Records available from the Michigan Department of Natural Resources indicate that the majority of lake trout do not reach.maturity until the age of six years in Lake Michigan and as a result the biomass of fish reaching a mature age constitutes a relatively small percentage of the total biomass of the stocks. In an attempt to increase spawner abundance, the Michigan Department of Natural Resources established regulations aimed at reducing the mortality of adult fish. The creel limit for individual anglers was reduced from five to three lake trout per day. Additionally, Lakes Michigan and Huron received further reductions to the current 2 lake trout per angler per day. In another attempt to decrease the adult mortality and increase the number of spawners, the Michigan Department of Natural Resources in 1984 imposed a season limit of May 1 to August 15 on the lower two lakes. The ultimate success of these regulations depends upon the degree of mortality experienced by lake trout that have been caught and released by anglers. Previously, an estimate of this hooking mortality was not available. Controversies arose among managers and between managers and anglers as to the effectiveness of regulations in reducing lake trout mortality. Prior hooking mortality studies on salmonid species indicated highly variable results ranging from 3.3 8 for Atlantic salmon (Salmo salar) (Warner 1976) to 90 t for rainbow trout (S3139 gairdneri) (Mason and Hunt 1976). Hooking mortality of lake trout was reported to be 6.98 % in Great Slave Lake, Canada (Falk et al. 1974). The purpose of this study was to evaluate the degree of hooking mortality of lake trout.in.Lakes Michigan, Huron, and Superior. In addition, relationships between mortality and various physical and biological factors were investigated. With these estimates, an evalauation of harvest regulations which are designed to insure lake trout rehabilitation, will be possible. METHODS Investigation of lake trout hooking mortality was conducted primarily with the cooperation of Great Lakes charter boat operators and sport fishermen. In addition, research personnel augmented angler fishing time in 1985. Three of the upper Great Lakes were sampled in 1984 and two in 1985. Eight ports were visited along the Eastern shore of Lake Michigan and one port each on Lakes Huron and Superior (figure 1). In 1985, Lake Superior was excluded due to inclement weather at the scheduled time of sampling. Different ports were visited in order to obtain samples from different fish stocks and also to sample a cross section of the "typical" Great Lakes angler and fishing techniques. Lake trout were caught by both fishermen and research personnel using traditional Great Lakes fishing methods. The majority of fish were caught by trolling artificial lures attached to downriggers. However, in rare instances, alternate methods such as wire line fishing were used by fishermen. At the time that the fish was brought on board, information was recorded on depth of capture, water temperature at the depth of capture, anatomical site of hooking, lure type, time required to land the fish (playing time), time the fish was on board (deck time), and surface water temperature. Two different methods were utilized to assess hooking mortality. The tank method involved immediate placement of the landed fish into a 2 ft x 2 ft x 3 ft tank filled with lake water. Dissolved oxygen levels were maintained at saturation with compressed oxygen while temperatures were kept between 10°C and 13°C with non-chlorinated ice. The lake trout were observed for the duration of the fishing trip, usually 2 to 6 hours. This method was used only in 6’ Lake Superior a; lll!’ Pete-key g; 1* Charlevoix ° “*9. 0 fl Barrisville 9, 5 OD .3 3 . A, .udingtan O :3 Flatware: .4 Grand Haven Holland I South Haven ‘11:: .___ Figure 1. Sampling stations for lake trout hooking mortality in 1984 and 1985. 1984 and represented mortality under ideal conditions. The buoy and line method involved placing a metal clip through the center lower mandible of the lake trout (figure 2). This was connected via a two way swivel to 75 yards of 15 pound test monofilament line which was in turn attached to a crab pot buoy. The fish, line and buoy were returned to the water where the lake trout could seek out its preferred temperature and depth strata. Observation times for this method ranged from 1 to 7 hours in 1984. Since unanchored buoys drifted with the current, the line-buoy method was modified in 1985 to allow for longer observation times.. The lake trout, after having the metal clip placed through its mandible, was tethered via a two way swivel to 50 yards of 30 pound test monofilament line. The fish was then released back into the water’and.all of the line was played out. The opposite end of this line was attached via another two way swivel to the anchor rope of the buoy. The fish was thus tethered to the anchor line (figure 3) but was free to move to its preferred depth strata. Horizontal movement was restricted to a 50 yard radius of the buoy. Ideally, fish were released for a minimum of 48 hours since 90-95% of hooking mortality in salmonids had been shown to occur within 48 hours of capture (Mongillo 1984). However, fishermen who came too close to this system often became entangled in the line. For this reason, release times were often shortened during times of heavy fishing pressure. .ooauoa >osnuocfia may you usouu axed uo sameness on» ca mfiao no unoEoomam .m madman .mufiasOHOfi msaxoon usouu oxma mmommc on com: tonnes mosaimcfia on» no Emuocwc caumEmsum .m ousofim unadzwéx «on ole m u .. . . .. .... } \Vdoffw’m/A: 5. irJ/f. 1.4... .o; a .. ._,e§s 2 dis... Tom 1 \ R After fish were retrieved from the buoy system or the fishing trip was completed (1984) lake trout were classified as alive or dead. Total length, weight, sex, and evidence of lamprey scars were recorded for each fish. Age was determined with Michigan Department of Natural Resources fin clip records and verified with scale aging techniques. STATISTICAL ANALYSIS .All factors or classifications of factors were compared to determine their impact on observed mortality. Since data were distributed binomially, non-parametric chi square contingency tables were used in all cases. Some data were categorized into groups for analysis. In these cases, the group intervals were the smallest which incorporated a sufficient number of observations in each cell that allowed for accurate testing using chi-square. In all tests, a 95 % level of probability was used to determine significance. 10 RESULTS AND DISCUSSION Over the two years of sampling a total of 90 fish were observed. Of these, 23 fish were placed in the aereated tank and the remaining 67 fish were returned to the water on the line-buoy system. In 1984 the use of the aerated tank system resulted in a mortality estimate of 4.3 it. This estimate is believed to be below the actual mortality experienced by angler caught and released lake trout. Fish that were returned to the water often experienced surface water temperatures that were ll°-14°C higher than the temperature from which.they were caught. However, when fish were placed in the tank system, the water temperature was approximately 10°C and the oxygen levels were at saturation. In addition, released fish were subject to seagull predation, as was witnessed in two cases, whereas fish held in the tank were not. The tank system thus provided an estimate of lake trout survival under optimal conditions and may not reflect the actual angler induced mortality. For these reasons along with the small number of mortalities, analysis on the possible factors related to mortality (size, depth, lure type eth was not conducted on these data. In 1984, 13.6% of the lake trout caught and released using the buoy system died. This estimate is comparable to 1985, when 15.6% of the fish died (table 1). There was no significant difference in mortality between the two years (xz- 0.0429, df-l). For the two year period, overall 11 Table 1. Hooking mortality of lake trout (Salvelinus namaycush) returned to natural waters in the Upper Great Lakes, 1984-1985. Confidence intervals from Mainland et al. (1956). Year Number Number of Percent Confidence caught mortal ities mortality intervals (95%) 1985 45 7 15.6 6.5-29.5 Combined 67 10 14.9 7.4-25.7 12 hooking mortality was 14.9% (table 1). Although the sample size was restrictive, it was felt that the concurance of the estimates between the two years provides strong support for the overall mortality estimate. This rate is higher than that found by Falk et a1. (1974). They determined that 6.98% of the caught and released lake trout died. However, several differences exist between Falkfls work and this study. The work done by Falk was carried out in Great Slave Lake, Canada, where fish were taken under different conditions and using different angling methods. In addition, severe temperature changes as experienced by fish caught from the Great Lakes did not occur in Falkfls study. An evaluation of the relationships of selected physical and biological factors to observed mortality was conducted. These parameters were thought to have an influence on mortality or had been shown by previous researchers to affect mortality. .As no significant difference in mortality was observed between the years of sampling, subsequent analysis of the factors possibly related to hooking mortality was carried out on the combined data set. Size Fish ranged in size from 461 mm to 801 mm with the average being 617 mm. Mortalities were noted over the entire size range. When fish lengths were placed into 51 mm (2 in) size categories (table 2) there was found to be a significant difference in mortality between various size classes (x2-7.359, df-6). The smallest size category of 13 Table 2. Mortality by length of lake trout caught in the upper Great Lakes, 1984-1985. Total Number Number of Length (mm) Caught mortalities 457-508 7 4 508-559 8 0 559-610 13 1 610-660 14 1 660-711 15 2 711-762 6 1 762-813 1 0 14 fish experienced the greatest mortality. Contrary to these results, Wydoksi et. a1 (1976) concluded that larger rainbow trout (Salmo gairdneri), when caught and released, were stressed to a greater degree than smaller ones under the same conditions. However, Pelzman (1978) found no correlation between the size of largemouth bass (Micropterus salmoides) and mortality. Handling Times The playing time, or the approximate time elapsed from the moment of initial hooking and the moment of landing the fish, ranged from 53 seconds to 5 minutes and three seconds. Grouping these times into one minute intervals, no significant difference was seen in mortality among the times (xz- 0.7440 ,df-3). Marnell and Hunsaker (1970) working with cutthroat trout (Salmo clarki) reported that mortality related to playing times of up to 10 minutes were not significantly different from their control group. However, Bouck and Ball (1966) found that 87% of rainbow trout that were angled to exhaustion died. Due to the relatively short playing times of lake trout in this study, it is doubtful that many were physically exhausted when landed. Several fish were not immediately discovered after they were hooked. These fish were omitted from the analysis of playing times (but not overall mortality estimates) since accurate times could not be established. When the mortality estimates of these fish were compared to the estimates of other angled fish, a significant difference in mortality was 15 observed (x2-6.3849,df-1). The fish which were not discovered immediately upon hooking produced.a:mortality estimate of 50% compared to 11.4% mortality for all other fish. This conclusion seems reasonable since fish that are not discovered immediately may be pulled behind the boat for some time, incrasing mortality through a combination of factors. Fish fighting the drag of the boat may accumulate a lethal level of the by-products of anaerobic muscular activity (i.e. lactic acid). In addition, reduced respiratory efficiency occurs due to a reduced ability to pass water over the gills and may result in suffocation. This could be a result of an inability to manipulate the buccal or opercular pumps effectively due to the increased drag on the opercular surface. Also, the exposed surface area of the lamellae in the gills may be reduced by the force of water crushing individual lamellae together or water may take a non-respiratory path through the gill sieve. Joans and Randall (1978) described these possibilities as increased diffusion dead space and increased anatomical dead space. The typical response to these impediments to oxygen uptake is to redistribute blood flow through secondary lamellae so as to expose a greater surface area of blood vessels of each lamellae to water for a longer period of time. However, lake trout, being a sedentary fish, may not have evolved a sufficient system to cope with ram ventilation. Any reactions from secondary lamellae to compensate for reduced oxygen availability may 16 be insufficient to provide oxygen at the rate required. Thus, these fish may be stressed from oxygen deficiency by the time that they are brought on board and the results would therefore coincide with those reported by Bouck.and Ball. This is assuming that the fish are actively'trying to pass water over their gills and have not ceased all attempts to respire. The amount of time that the fish spent out of the water (deck time) did not significantly influence the survival of hooked lake trout (x2-5.508, df-3). Deck times ranged from 1 minute and 10 seconds to 4 minutes and 30 seconds and were categorized into 1 minute intervals for analysis. This was apparently not long enough toIcause severe physiological damage to the fish. Gear Type There were three main lure types used by anglers. These consisted of small plug-like lures with small treble hooks (known as p-nuts), spoons with large treble hooks,and spoons with large single hooks.IAlthough higher mortalities were observed with p-nuts and large treble hook spoons (figure 4), these differences were not found to be significant (xz-l.905, 2 df). Smaller lures such as the p- nuts were often ingested more deeply by the lake trout. Thus, the potential for hooks to penetrate vital structures (esophogous, arteries eth or become entangled in the gills might have been greater for smaller lures, although this was not statistically substantiated. These findings agree with 17 40 35 - N so - 25 .. zo . II'"I'"'FI'" "III-“III 15 - to j Z ' IIIIIII l“ SHELL TREBLE LRROE TREBLE SINGLE HOOK TYPE «qum-«xoz Ole-030°: Figure 4. Hooking mortality associated with different hook types. 18 Falk et al. (1974) and Dotson.(1982). However, in a summary of hooking mortalities of salmonids, Mongillo (1984) concluded that the practice of using single hooks on lures causes slightly higher mortalities than treble hooks. Anatomical Hooking Site For statistical analysis, hooking sites were classified as being internal or external (figure 5). Approximately 72 % of the lake trout in this study were hooked in the upper or lower jaw (external). These fish experienced a mortality of 6.9%Iwhile fish which were hooked in one or more internal sites exhibited a mortality of 71.4 %. A significant difference therefore exists in mortality between the two classifications (x2-21.805, 1 df). This difference is a function of damage to internal structures and subsequent stress of the fish. Other investigators have similarly found hooking site to be important in determining mortality of hooked fish. Falk et a1. (1974) concluded that the primary cause of death in hooked lake trout was attributable to hook placement and bleeding. Hook placement in the gill arches produced excessive bleeding and increased mortality. Pelzman (1978) found that significant mortality occured among largemouth bass which had been hooked in the esophogeal region. In addition, Warner (1979) indicated that hooking mortality of landlocked Atlantic salmon.(§glmg £31.33) was more a function of hooking site (swallowing the hook) than gear type. 19 > UPPER JAW (EXTERNAL) p ROOF OF MOUTH (INTERNAL) + GULLET (INTERNAL) GILL ARCHES (INTERNAL) 4’ FLOOR OF MOUTH (INTERNAL) > LOWER JAW (EXTERNAL) OTHER: EYE, CHEEK, SNAG (EXTERNAL) Figure 5. Classification of anatomical hooking sites (modified from Mongillo 1984). 20 Temperature The difference between the temperature at the depth at which the lake trout was hooked and the surface water temperature did not appear to have an effect on mortality (table 3). When fish observations were placed into categories of temperature differential (0-5°C, 6-10°C or 10- 15°C), no significant differences were observed between the three categories (xz- 1.347, 2 df). This conclusion is consistent with that of Marnell and Hunsaker (1970) who found that the survival of cutthroat trout after hooking and releasing was not affected by surface water temperatures between 37°F and 62°F. IHowever, relationships have been established which suggest an increasing mortality with increasing water temperatures (Benson and Buckley 1963, Klein 1965, Dotson 1982). 22232 Lake trout were caught from depths ranging from.7.5m to 49 m. No significant difference was Observed.among depth ranges in the probability of death (x2-4.327, df-M).‘When fish are raised from great depths, a decrease in pressure at the rate of 1 atmosphere for every 10 m allows gas in the gas bladder to expand. Since lake trout are physostomous, they possess the capability Of releasing pressure from their expanding gas bladder via the esophogeal connection. Often, when lake trout were brought to the surface, they could be heard releasing gas from the gas bladder. Several fish which ultimately survived were Observed floating on the 21 TI.-.III..I'IIII|VIII 9 t lI',' Y 13...! t 1 II. _ - aIIIvai:-ii -- 4 he .e. m v~ mm - _~ =~ o— a— N_ o— m— e— n— m. __ =_ a g N “Dov Ousueuemaou have: ooeuusm .mmflufiacquE no Hones: on» success“ mamosusoucm ca mnmnasz .mmma can same .mmxmq pawns Lamas are :e maneucwumueae musumummsou nouns Hcscd>apca us unmsmo usouu axed no nonesz .m manna seven uosdoueusuo on use»; g3 V 22 surface for periods of up to 10 minutes, possibly as a result of the extended gas bladder. Management Implications The primary Objective of this study was to determine the degree of hooking mortality experienced by lake trout that are caught and released by Great Lakes anglers. This was in a effort to determine the effect of various fishing regulations upon the overall mortality of lake trout stocks and the ultimate plans for achieving their natural reproduction in the Great Lakes. Currently, two types of restrictions are imposed on lake trout fishermen. A creel limit of 2 lake trout per angler per day is imposed on fishermen in Lakes Michigan and Huron while the limit is raised to 3 in Lake Superior. Based on the results of this study, the relatively low overall mortality appears to support the use of this limit as a method of reducing sport fishing mortality. Great Lakes fishermen are Often able to target for alternate species (such as salmon) and therefore the incidental catch of lake trout after the daily limit has been reached.may not.be significantm However, lake trout that are incidental to the catch will exhibit a high survival when released, according to these results. The recently imposed season limit by the Michigan Department of Natural Resources may also be effective in reducing fishing mortality. Although the current season closes on August 15th when surface water temperatures are often high, we found no evidence which indicated an 23 increasing mortality with increasing differential in water temperatures. Lake trout still have a substantial chance for survival in the warmer surface waters. The basis for establishing opening and closing dates for lake trout fishing therefore would depend more on the evaluation of fishing pressure during different times of the year and socio-economic considerations. Tourism connected with Great Lakes fishing contributes substantially to the economy of Michigan. Many businesses would prefer to extend the lake trout season to Labor day, the traditional end of the tourist season. Imposing a size limit for lake trout has also been under consideration by management agencies. As was noted earlier, mortalities occurred over the entire size range of Observed fish, but the smallest size classes of fish showed the highest mortality. Generally, significant maturity in lake trout is not reached until the age of 6 years, at which time the fish are approximately 660 mm in length (Rybicki and Keller 1978). If this was imposed as the minimum size for lake trout harvest, approximately 66% (42/64) Of the fish caught in this study would.have.been returned to the water. Of these returned fish, 14.3 % would have died. It should be noted that this estimate is actually below the estimate for overall mortality. This is due to the fact that the sample size for calculations on length was reduced to 64 fish due to missing length data caused by gull predation in one case and loss of live fish upon retrieval 24 in two cases. Thus, a size limit designed to allow a greater number of fish to reach maturity may actually be an effective method to increase spawner biomass. Clark and Huang (1985) provide a lake trout population model which incorporates varying fishing levels and mortality ratest ‘With this model, they simulated population changes in one statistical management district of Lake Michigan (Frankfort-Good Harbor Bay, statistical district :MM5). Given that stocking remains at historical levels and that mortality and maturity schedules are accurate (table 4), the model predicts that rehabilitation will not be achieved in 25 years, as was noted by Clark and Huang. Rehabilitation was defined by the authors as the natural production of 25,000 four year olds, the approximate number now achieved by stocking; After 25 years, with the size limit of 660 mm and the hooking mortality rate of 14.9 % , approximately 13,000 four year Olds will be produced. CONCLUSION The objective of this study was to estimate the mortality that is experienced by lake trout that are caught and released by anglers in the Great Lakes. Overall.hooking mortality was found to be 14.9 %. Fish hooked in one or more internal areas experienced significantly higher mortality (71.4%) than those hooked in the upper or lower jaw (6.9%). In addition, fish which were not immediately discovered upon hooking exhibited a mortality of 50.0% while those that were immediately landed exhibited 11.4% 25 Table 4. Parameters incorporated into lake trout population model of Clark and Huang (1985). Age Number in Natural Percent original mortality Mature population 1 0 .46 0 2 63128 .46 0 3 39851 .46 0 4 25157 .46 6 5 13711 .36 17 6 7677 .36 92 7 4298 .36 97 8 2406 .36 100 9 1576 .36 100 10 762 .36 100 11 439 .36 100 12 215 .36 100 13 147 .36 100 14 63 .36 100 15 44 .36 100 Size of vulnerability to gear - 540 mm Instantaneous fishing mortality - .22 First year survival - .005 Eggs per Kg per female - 1850 26 mortality, creating a significant difference between the two classifications. The smallest size classes of fish died at a slightly higher rate than larger ones, although this mortality is still relatively low. These conclusions support the use of season limits, size restrictions and creel limits as a means Of reducing sport fishing mortality of lake trout stocks in the Great Lakes. However, even with these restrictions, the rehabilitation of lake trout stocks in the Great Lakes may not be readily achieved. LIST OF REFERENCES 27 LIST 9: REFERENCES Baldwin, N.S., R.W. Saalfeld, M.A. Ross and H.S. Buettner. 1979. Commercial fish production in the Great Lakes 1867-1977. Great Lakes Fishery Commission Rep. # 3. Ann Arbor, MI. Benson, N.G. and R.V. Bulkley.l963. Equilibrium yield and management of cutthroat trout in Yellowstone Lake. U.S. Fish and Wildlife Serv. Res. Rep. NO. 62. 44 pp. Bouck,G.R. and R.C. Ball. 1966. Influence of capture methods on blood characteristics and mortality in rainbow trout (Salmo gairdneri). Trans. Am. Fish. Soc. 95(2):l70-l76. 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