FEEDTNG HABITS 0F IMMATURE LAKE TROUT (SALVELTNUS NAMAYCUSH) IN THE MTCHIGAN WATERS OF LAKE MTCHTGAN Thesis for the Degree 4)? M. S. MlCHTGAN STATE UNWERSITY KENNETH j. WRIGHT 1968 [Bk L I ‘ 1 Michigan 5‘: t ‘ Univer 2055 Tu I21! 181m mum: 1m: L T NIH! Tully!” T L" T til. W/ MAY 0 ti my matures? Wm" "B '\/ ‘ AP“ U 4 1989 ABSTRACT FEEDING HABITS 0F IMMATURE LAKE TROUT @ALVELINUS NAHAYCUSH) IN THE MICHIGAN WATERS OF LAKE MICHIGAN by Kenneth J. Wright Stomach contents of 1581 immature lake trout collected from the Michigan waters of Lake Michigan during 1966 and 1967 were examined to determine their feeding habits. Of this total, 1477 (93.4 percent) contained food and 104 (6.6 percent) were empty. Fish were found in 80.4 percent of the stomachs containing food and comprised 97.4 percent of the total volume of food found in the stomachs. The most frequently occurring fish were sculpins, alewives, and smelt. Invertebrates were found in 34.1 percent of the stomachs containing food, but, because of their small size, com- prised only 2.6 percent of the total volume. Pontoporeia affinis was found most often followed by Tendipedidae and El£$2.22; Differences in frequency of occurrence of food items with area, size of trout, season, and depth of water are discussed. Special reference is made to feeding habits in relation to alewives and sport or commercially important species. FEEDING HABITS OF IHMNTURE LAKE TROUT (SALVELINUS NAHAYCUSH) IN THE MICHIGAN WATERS OF LAKE MICHIGAN BY 7ij Kenneth JT‘Wright 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 1968 ACKNOWLEDGEMENTS I wish to extend sincere appreciation to the Hichigan Department of Conservation for the financial assistance and equipment which made this study possible. I am indebted to Hyrl Keller, Walter Crowe, and Lee Hoffit for their guidance and assistance in collecting data. I want to thank the members of my graduate committee, Dr. Eugene Roelofs, Dr. Peter Tack, and Dr. Hilton Steinmueller, for their en- couragement and guidance. I am indebted to Willard Cross for his assistance in the identi- fication of fish specimens. I especially wish to thank my wife, Diane, for her courage and patience during the study. 11 TABLE OF CONTENTS PAGE INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . l METHODOLOGY 6' Collection of Fish Specimens. . . . . . . . . . . . . . 6 Laboratory Analysis . . . . . . . . . . . . . . . . . . 6 Analysis of Data. . . . . . . . . . . . . . . . . . . . .7 RESULTS AND DISCUSSION . . . . . . . . . . . . . . . . . . 8 General Feeding Habits. . . . . . . . . . . . . . . . . 8 Feeding Habits by Size, Feeding Habits by Area. . . . . . . . . . . . . . . . . 19 ' Feeding Habits by Season. . . . . . . . . . . . . . . . 23 Feeding Habits by Depth . . . . . . . . . . . . . . . . 29 Feeding Habits with Special Reference to Alewives . . . 35 Predation Upon Sport or Comercially Important Species. 38 SUMMARY AND CONCLUSIONS. . . . . . . . . . . . . . . . . . 39 LIT ENTIRE c ITED O O O O O O O O O O O O O O O O O O O O 0 42 iii LIST OF TABLES TABLE PAGE 1. Frequency of occurrence of all food items found in the stomachs of 1477 immature lake trout taken from the'Michigan waters of Lake Michigan . . . . . . . . . . . . . . . . . . . . 10 2. Frequency of occurrence of various food items by year and by lake trout size. . . . . . . . . . . . 12 3. Percent total volume of various food items by lake trout size. . . . . . . . . . . . . . . . . . 15 4. Length frequency of measurable slewives found in the stomachs of the four size groups of imature lake trout O O O O O O 0 0 O O O O 0 O O O O 18 5. Frequency of occurrence of various food items by area and lake trout size . . . . . . . . . . . . . 22 6. Seasonal frequency of occurrence of various food items found in the stomachs of 1477 immature lake trout. . . . . . . . . . . . . . . . . . . . . . 25 7. Frequency of occurrence of various food items by depth. 0 O O O O O O O O O O O O O O O O O O O O O 31 8. Frequency of occurrence of various food items by depth and year O O O O O O I O O O O O O O O O O O 33 iv LIST OF FIGURES PAGE Statistical districts of Lake Michigan . . . . . 5 Monthly frequency of occurrence of slewives and sculpins found in the stomachs of immature lake trout, 1966 and 1967 . . . . . . . 27 Length frequency of 311 measurable alewives found in imature lake trout stomachs. . . . . . 37 INTRODUCTION No significant research on lake trout feeding habits in Lake 'Michigan has been pursued during the past thirty years, and only one study of major importance was undertaken prior to that time. Van Oosten and Deason (1938) conducted extensive research on this subject in 1930, 1931, and 1932. As described by Smith (1968), a rapid sequence of catastrOphic changes started in the early 1940's and led to a major alteration of the fish stocks in the Great Lakes. The sea lamprey became established in the deepwater population during the 1940's and its effect on the lake trout population was swift and drastic. The native lake trout stocks in Lake Michigan, abundant in the early 1930's, became virtually extinct by the mid-1950's from sea lamprey predation and an exploitive commercial fishery. With the lake trout nearly eliminated from.Lake Michigan, commercial fishing pressure and sea lamprey predation shifted to remaining species, reducing the numbers of all species vulnerable to the gill net or lamprey. This near absence of predators set the stage for another invasion. The alewife, a small pelagic plankton feeder closely resembling the shad or herring, was first reported in Lake Michigan in 1949, and soon reached staggering numbers. Smith (1968), states that the alewife has been the only fish in the Great Lakes that has occupied all sections of the lakes, their bays, estuaries, and tribu- taries in great numbers. It has become the most abundant and widely distributed species in Lake Michigan where it has displaced all major planktivorous species (Smith, 1968). An ambitious program is now underway to create a predator-prey balance in Lake Michigan. Sea lamprey control in Lake Michigan was initiated in 1960 and by the summer of 1966 all 98 of its lamprey streams had been treated once. An increase in whitefish production tram 266,000 pounds in 1962 to 1,422,000 pounds in 1966, a marked improvement in rainbow trout fishing, and the unprecedented 40 percent return of planted coho salmon provide evidence of the effectiveness of lamprey control in Lake Michigan. Lake trout stocking by states and the Federal Government started in 1965, has reached nearly two million fingerlings annually, and will be continued until natural reproduction is well established (Smith, 1968). Lake trout planted in the Michigan.waters of Lake Michigan numbered approxi- mately 600,000 fingerlings in 1965, 950,000 in 1966, 830,000 in 1967, and 800,000 in 1968. In addition, Michigan is now stocking coho and chinook salmon as well as steelhead in Lake Michigan on a yearly basis. Following the reintroduction of lake trout into Lake Michigan, research was needed to learn something of their habits and to appraise the success of the stocking program. The objective of this study was to gain a knowledge of the food organisms utilized by this new stock of lake trout. Other studies in progress include growth and migration. The Michigan waters of Lake Michigan have recently been divided into eight statistical districts and designated as MMrl through MM-8 (Smith, Buettner, and Hile, 1961) (Figure 1). For this study it was decided to conform to statistical district boundaries in recording and analyzing data. Figure 1. Statistical districts of Lake Michigan. Gd 1 ive? Lake Naubinway Manistique - ° T I Point 3 a 90 MM-l letour [4137\31-2 ”Q MM-3 Menominee r/ >-—JL-______ Charlevoix I ~ / Q “'3 / MM-4 WM-l / ~ / m—s } 7/ Traverse City / "‘ __ Arcadia / 104-6 :13 Sable Pt. r—---_- Little sable Pt- , White Lake H01 land -------- District boundary Mackinac Bridge -—----—- Interstate boundary METHODOLOGY Collection of Fish Specimens Lake trout for this study were collected by commercial gear as well as Michigan Department of Conservation gear. Commercial gear consisted of gill nets with stretched-mesh sizes of 2.0 to 5.0 inches set for the Great Lakes bloater (Coregonus 221;) and the Great Lakes whitefish (Coregonus clupeaformis). A few samples were collected in 82-foot otter trawls used fer taking alewives commercially. Department gear included gill nets with stretched-mesh sizes of 2.0 to 5.0 inches set specifically to take lake trout, and a 39-foot otter trawl. Mbst of the collections with Department gear were made in the areas of Lake Michigan where the commercial fishery did not' ordinarily Operate. Upon collection the lake trout were immediately weighed, measured (total length), sexed, and checked for fin clips. The portion of the stomach between the esophagus and the pyloric valve was then removed from the trout. The stomachs were then put into individual cloth bags, labeled, and preserved in a 10 percent formalin solution for later analysis. No attempt was made to identify food remains in the intestines. Laboratory'Anslysis Identification, enumeration, and volumetric determination of food items was facilitated by allowing the stomach contents to remain in distilled water for a period of one hour prior to examination. The food itemm were then identified and blotted on paper toweling to remove excess mbisture. Food volume was determined by measuring in milliliters the displacement of distilled water. Total length was also recorded for all whole fish found in the stomachs. Analysis of Data The items comprising the stomach contents are reported in two methods; percent frequency of occurrence and percent total volume in fish containing food. The percent frequency of occurrence is obtained by dividing the number of trout containing a specific food item by the number of trout containing food. The percent total volume is determined by dividing the volume of a specific food item by the total volume of stomach contents. Each Of these methods has certain limitations and individually may present misleading results (Lagler, 1956; Rounsefell and Everhart, 1953; Larimore, 1957). Percent frequency of occurrence merely expresses the presence or absence of a food type and thus magnifies the importance of smaller and less numerous items. Reporting items by percent total volume ignores their frequency of occurrence and therefore does not reflect the food habits of individuals. 0n the other hand, it does emphasize the importance of larger, more bulky food items. Both methods are affected by differ- ential digestion rates as are the other procedures for analyzing stomach contents. Therefore, these two methods are utilized in combina- tion to obtain a clearer perspective of lake trout feeding habits. RESULTS AND DISCUSSION General Feeding Habits During this study 1581 immature lake trout were collected for stomach analysis. Of this total 1477 (93.4 percent) contained food and 104 (6.6 percent) were empty. These statistics vary greatly from those obtained by Van Oosten and Deason (1938), who found that only 55.7 percent of 4,979 lake trout collected from Lake Michigan contained food. This difference is undoubtedly related to the length of time the fish were in the nets after capture. The present study made use of trawls and short-period sets (12-48 hrs.) with gill nets while the earlier study made use of gill nets set for periods of five to ten days. The major food item of immature lake trout in Lake Michigan is fish, primarily sculpins, alewives, and smelt (Table l). Invertebrates play a major role in the diet of lake trout under 20.0 centimeters (total length). The most frequently occurring invertebrates are Pontoporeia affinis, Mygig_s ., and Tendipedidae. These food items, with the exception of alewives were reported to be utilized by lake trout in Lake Michigan by Van Oosten and Deason (1938). Alewives, however, were reported by Dymond (1928) in the stomachs of lake trbut taken in Lake Ontario with commercial gill nets. Fish.were found in 80.4 percent of the 1477 stomachs containing food (Table 2) and comprised 97.4 percent of the total volume of food .cmwwnuax axed mo muouma damage“: may Eoum momma usouu mama assumes“ mesa mo «somEOOO on» em venom mama“ moom Ham «0 monouusooo mo homosvoum .H magma 10 m.o~ N O O O O O O O O O O H H O O O O O O O 0 I H HHHHOMHQQHMHMI—INHBNONN O Anoom quzHflH maauaou seam moauquaovqsb ammo seam imam noumszmoum mousse maouoan humane Hwouuoaw some; ozone Houmoan momma umouu nomanuo>sam xooum mono muosuuoa axed xuonoaxowun oawamoaaz mousse haanoh mammamu sumaaon mommauaomwma mamasom noumsnoon saafisou moosaoomm sandman mosuuox camuaom hawam magmaaom madam omahoa< swam ZHHH 11 .onwn usouu oxoa an new mama he «Emma moon uaoaum> mo mononuSOOo mo homosvoum E . . 4 .Whhhm! q? . LIII.‘\ .N sense Total 2543.0 cm 33.0-42.9 cm 20.0-32.9 cm 000-0909 cm 1967 725 88.0 45.8 12.0 1966 752 73.0 10.9 6.0 1967 121 52.9 25.6 1966 50.0 100.0 100.0 50.0 1967 353 94.9 49.0 12.7 1966 125 Percent 99.2 17.6 14.4 1967 236 75.8 1966 351 1967 15 1966 272 Frequency of occurrence of various food items 12 0030-! O MINI-i H I-lNln O O O 0‘61") #61 [NOW 0 so Inmu H GOO GOO le-l \va-O H 12.4 47.2 41.8 39.6 3.3 23.0 2.5 0.8 0.0 0.8 0.8 0.0 0.0 0.0 0.0 0.0 13.0 4.0 0.6 3.4 10.5 19.2 16.0 12.8 3.2 3.2 36.0 26.3 18.2 19.1 14.0 27.6 21.1 19.1 2.3 93.3 86.7 66.7 33.3 86.0 80.9 79.0 Hfi'd’ coo H mm O9 oo~o OH O O 0°65 90° MNN 0 0 O NHH O00 GOO GOO GOO COM 0 O 0 GOO GOO GOO GOO GOO F‘HH s so HI-lI-i GOO GOO Lake trout size (total length) ten I I: -H :0. viri or: .4 0 5m ~o av :33 'H u Dru-Cu 33?: 0 an Alewife Smelt Number of lake.trout containing,food Year Fish Pontoporeia affinis M sis sp. Insects Invertebrates Crustacea O u 'U q-Io 00 O- 83 dud 0H [-10 Q ‘< Mbllusca Clams Snails Leeches 4.0 0.8 28.5 5.6 1.4 0.0 7.6 11.4 33.3 61.4 Debris 13 found in the stomachs (Table 3). Invertebrates were found in 34.1 percent of the stomachs containing food but, because of their small size, comprised only 2.6 percent of the total volume. Feeding Habits by Size It has often been demonstrated that feeding habits of fish vary with size of the fish. Van Oosten and Deason (1938), for instance, reported changes in the feeding habits of Lake Michigan lake trout with size of trout. The total length of lake trout used in this study ranged from 12.0 to 64.5 centimeters. Following a preliminary examination of 100 lake trout stomachs the remaining lake trout were arbitrarily divided into four size groups, less than 20.0, 20.0-32.9, 33.0-42.9, and 43.0 or more centimeters. That invertebrates are extremely important in the diet of the smallest size group of lake trout is shown by their presence in 86.4 percent of those stomachs containing food and their contribu-1 tion of 53.6 percent of the total volume eaten by lake trout of this size (Tables 2 and 3). The most frequently occurring invertebrate was PontOporeia affinis followed by Tendipedidae, !!2$£,3 ., and Mollusca (Table 2). Fish occurred in slightly over one-fourth of these lake trout but consisted of nearly one-half of the total volume. Sculpins were the most frequently found fish followed by alewivés and’smelt. In the size group 20.0-32.9 cm, fish constituted the major portion of the diet. The most frequently occurring fish were sculpins followed by alewives and smelt as in the smaller lake trout. Inverte- l4 .onam usouu ome an maoua moom mflOfiHmmr HO U§HO> HQUOU UflQUHON gulf I. AI. . .1. I UH Pam .m manna 15 «o. oo. oo. Ha. as. .«uson so. No. He. no. no. «season no. co. co. Na. we. . sausage: so. me. no. so. as. oaeaeoaaeeoe mo. no. ma. mo. on.” nausea 3. 8. 3. an; 8.“ 4m 3am». eo.~ co. «m. on.~ ne.om unnamed sauuosouaom he.“ co. we. om.~ we.~n noouuaspo sn.~ me. a“. ~m.~ sn.mn nouuuaouuosua as.e mo.e oo.m mm.e Ho.m «saunas seem easesuaoeaea as.n ae.u nN.H as. oo. . ease guano No. we. so. as. as. soasoasusuu «cannons: on. ma. AN. mm. HA.H saunas season wm.na so.o om.ma ao.- em.aa senses“ unease» nonmauaoesap ~e.~ «N. a~.n mm.~ an. essence posse mm.~ ca.~ mm.~ wa.~ «a. unease. euauuo: mn.na a~.~ so.nH AH.m~ so.m~ unease» magma He.om «s.oa uq.a~ sm.on no.h~ «essence ~m.m~ aw.mm o~.aa ae.om mo. .ufioam me.am ma.ee om.me oa.w~ em.~a «masma< sn.~a as.ms ok.am ao.aa ha.me Ir sans unsouom aouH mama“ moon opaqus> km a: omofl «same» no uaoouom Ha N.amaa Ha o.amaa Ha n.~mam Ha_m.nmaa Ha w.-~ nauseous :a sauna vOOH HO UgHO> Hduou. sauce a0 o.neum so m.~s-c.mn so o.~n-o.o~ so m.oa-o.o nesmeud Huuouv ones uaouu axed l6 brates were found in nearly one-third of these stomachs but constituted a negligible portion of the total volume. The more important inverte- brates to this group were Pontoporeia affinis, H12$§.§2;n and Tendi- pedidae. Fish occurred in 96.0 percent of the stomachs of the third size group (33.0-42.9 cm) and cOntributed 99.8 percent of the total volume. Alewives were the most frequently occurring fish followed by sculpins and smelt. The fourth size group (43.0 cm and larger) ate fish almost exclusively. Fish were found in 100 percent of the stomachs contain- ing fecd and constituted virtually all of the total volume. The *most frequently occurring fish were alewives, smelt, and sculpins. Frequency of occurrence of alewives increased proportionally ‘with an increase in the size of lake trout. The average size of the alewives consumed also increased as the size of the lake trout in- creased; however, small alewives (under 9.0 cm) still made up a significant portion of the food items consumed by the larger trOut (Table 4). Smelt consumption also increased in direct proportion to the increasing size of the lake trout. If the trend indicated by this study continues, smelt may become as important as alewives in the diet of large lake trout. Dryer, Erkkila, and Tetzloff (1965) report smelt occurring in 68.1 percent of the stomachs of lake trout over 43.0 cm in Lake Superior during 1963. 17 .uaouu oan spouses“ mo museum ones uaom on» we anomaoum ecu ea venom no>wseam omnmuaeooa mo mucosvouw summed .e oases 18 E m In. E e. :3... m m ¢.wH n H.0H o m m o.oH n H.¢H MH a m o.¢H a H.NH m H a o.NH a H.oH N H H o.oH a o.a o a N m.m a H.m HN N m CH o.m : o.m an m HH mH m.w a H.w an oH 0N NH O.» q o.h we mH mH 0H H m.N u H.N 0N 5 NH m H o.N a 0.0 mH H 9 HH m.o 1 H.@ SN m 0H m o.m a o.m NN H NH N N m.m a H.m NH H a a m o.n : 0.: NH m w H m.¢ a H.¢ N H H o.¢ n o.m H H m.m a H.m onouw ONHn some eH mo>HamHm OHAmuanmoa mo panama AmuouoaaumooV uo>Hono mo aumaoH Huuoa Huuoa tau o.n¢M as m.N¢no.mn Bo m.Nnuo.ON as m.oH u 0.0 oan uaouu smug 19 The two middle size groups (20.0-32.9 and 33.0e42.9 cm) utilize sculpins as food to a greater extent than do the smaller or larger groups of lake trout. Van Oosten and Deason (1938) found sculpins to be the mOst frequently occurring fish in the stomachs of lake trout under 38.0 cm in Lake Michigan. It appears that sculpins may become unimpOrtant to the diet of lake trout larger than 65.0 cmTwith the increasing importance of alewives and smelt. uFrequency of occurrence of invertebrates decreases as the size of the lake trout increases. No lake trout over 45.0 centimeters in length contained invertebrates in its stomach. It appears that invertebrates are important as food items only to the smaller lake trout (under 20.0 cm). The most important invertebrate in frequency and volume was Pontoporeia affinis. Tendipedidae were present in significant numbers in lake trout of 0.0-32.9 centimeters in length but failed to constitute a significant portion of the total volume of food eaten by these fish. M213. 92; was the only other invertebrate found to be of real importance in the diet of immature lake trout in Lake Michigan. Feeding Habits by Area Previous studies have shown that feeding habits of lake trout may vary from one area to another. Van Oosten and Deason (1938) found that feeding habits of lake trout in northern Lake Michigan differ from.those in southern.Lake‘Michigan. They divided Lake Michigan into southern and northern portions by means of an imaginary line drawn from Frankfort, Michigan, to Algoma, Wisconsin. The northern boundary 20 of statistical district MM-6 was chosen as the dividing line between southern and northern Lake Michigan for the present study (Figure 1). This line is within ten miles of that used by Van Oosten and Deason (1938) to divide the lake into southern and northern portions. Feeding habits were analyzed by size as well as by area since the average size of lake trout taken from southern Lake Michigan was considerably less than those taken from northern Lake Michigan. The northern portion of the lake was first stocked with lake trout in 1965; however, the southern portion did not receive lake trout until 1966. Some fish captured in northern Lake Michigan therefore would be one year older than those captured in the southern portion of the lake. Smelt were found more frequently in the stomachs of lake trout taken from northern Lake Michigan regardless of lake trout size (Table 5). This is to be expected as Lake Michigan commercial fishing records for smelt indicated that they are more abundant in the northern portion of the lake. Sculpins in general were found more often in the stomachs of northern Lake Michigan lake trout than in those from southern Lake Michigan. The crustacean Pontoporeia affinis was extremely important in the diet of small lake trout (0.0-19.9 cm) in southern Lake Michigan. Pontoporeia affinis was found in 83.0 percent of the small lake trout taken from the southern portion of the lake while it occurred in 35.7 percent of those from northern Lake Michigan. Mzsis £2, was found more 21 .ouHm usouu ome new menu an mEUUH meow n=OHuo> mo monouuaooo mo moaoavouh ,m sense 243.0 33. 0-42 09 71 Percent 65 Southern Lake Michigan 20.0-32.9 an-1909 259 22 OOOOOO OOOOOOOOO O O O 0. 680699 OOOOOOOOO ”SDI-n O‘N‘i‘ OfiQQGOOOO moan NH NMNNJOOOO 0‘"! MN 8.0 \TNSOQO‘O lfifiNHOfl‘QOO MB¢M00 ”NGC';¢U;€OO 05¢ 03 IANONBO OQOOMOQOO‘ O O O Ind'OOl‘O QQNnélfir-lOI-T N H com ('3 243.0 120 33.0-42.9 407 Percent 522 Northern Lake Michigan 20.0-32.9 28 Frequency of occurrence of various food items 0. 0-19 0 9 cmnooow neocooooooow O O O O O O O O I 0.. O .0 ONhtnInO NOOOOOOOO C>vsoap4 pa HnOmON OQQQ’HOONN O O O O O 0 Odd‘wMI-O whéawOOOO O‘QHNH V1F1F10\O\O\ V\V\O\¢>O\ O O 0 O O O cnud\o¢n ON MQ‘O‘O‘NOOHO ”N Q‘N MN \TOVDBMO €€h¢r1°°~€° 666n¢n H6nhN66m6 d' ml". “4’55!" 1".” Item Lake trout total length (centimeters) Number of lake trout containing food Area m 0H a a a H m an a HH a: m Ho H a :m m mom . o o u v mm umomw m 0 sad 6 m a :4 H o o m m m m m H u amuwm mam Hug u magma HHfi gvdodud o u m o '6 'Hvfl 0 «mm: uammo mom nHao o madam 34mm :0 Hom mo monouusooo mo accosuoum ammomoom .9 «same 25 H.6 6.6 6.Hm 669666 n. 6.6 6.~ assumed 6.6 6.6 64 a 2.3066 6.6 6.6 6.n magnum 6.6 «.6 6.~ sumac 6.6 6.6 6.6 sausage: a.» o.o H.m~ unvavoaauaoa 6.6H 6.H 6.6m uuooodH 6.6 6.6 6.6 dum.mmmwm 6.6 6.5 6.66 awaammu «Houquuaom 6.~H 6.6a 6.66 noouuusuo N.H~ «.6H 6.66 adudhb¢uh0>un N.H 6.6 6.H Mounoaxowuo magnumaaz 6.H 6.H 6. Manama haanH ~.~ 6.6 H.6 swassou voauuox 6.5 6.6 6.6N :«qasom maaam 6.6a. «.6fi 6.66 aawnasum 6.NH «6 n6 3.28 6.66 6.66 m.~ omwsmfid 6.56 «.66 6.66 sown unsouom 53H mama“ voom unequu> mo sonouunooo mo hoaoavoum n66 66H 666 voom 6aaawsudoo uaouu onus mo nobauz 566a seesaw 566~n666H nous“: 666a uoalbm condom 26 .aomfi can 66¢H .usouu oxen unwanoax 036A ousumaaa mo «souaoua ago 66 venom acwafisua 6am uo>gkofiu mo ousouuaouo mo 5oaoswouw Manama: .N suswwm 27 4’ 566A «b D 666a h \D «plfi "'d «382 E. In flflQfl—dom l Percent frequency of occurrence 28 high throughout the remainder of 1966 and slowly decreased in 1967 from a January high frequency of 70.3 percent to an August low of 29.7 percent. During this extremely high alewife consumption period from October 1966 through January 1967, sculpin consumption drapped off to 18.2 percent frequency of occurrence from its previous summer figure of 54.8 percent. Sculpins were found in only 15.4 percent of the stomachs examined during the summer of 1967. The lake trout apparently started feeding heavily upon the alewives as the alewives concentrated in deeper water following the fall turnover of Lake Michigan in October, 1966. Why they continued to feed heavily upon alewives the following summer instead of return- ing to their previous summer diet of sculpins has not been determined. It may be that the presence of the alewife has directly or indirectly reduced the number of sculpins available to the lake trout. The lake trout are thereby forced to feed upon what is available; the alewife. It may also be that heavy winter feeding upon alewives occurred because of the extensive availability and the lake trout continued to feed upon a them the following summer because of some learning or habit process, or again because of their extreme availability. Eschmeyer (1964) suggests that the fishes eaten by lake trout are of whatever kind" happen to be available in the immediate environment. Some of the increased summer frequency of occurrence of alewives during 1967 can be explained by the fact that the average size of lake trout sampled during 1967 was somewhat larger than those sampled during 1966 and it has already been pointed out that frequency of 29 occurrence of alewives in the stomachs increases proportionately with lake trout size. This, however, is not nearly as important as it may first appear. Frequency of occurrence of alewives in the stomachs of lake trout was higher in 1967 than in 1966 in every lake trout size except the smallest group (Table 2). Alewives were the dominant food item in the two medium size groups in 1967 both by frequency of occurrence and by total volume. These two size groups of lake trout had the sculpin as their dominant food item in 1966. In general, invertebrates tended to be less important as food items for the lake trout during the winter than in summer. Mbst invertebrates tend to be less available during the winter months and are therefore consumed less frequently. The only invertebrates that were found frequently in lake trout sampled during winter were Pontoporeia affinis and 11191.5 22.: The frequency of occurrence of both these organisms did not vary significantly from.their summer occurrence. Feeding Habits by Depth It has been often observed that depth may influence lake trout feeding habits. Dryer, Erkkila, and Tetzloff (1965) reported changes in lake trout feeding habits with depth in Lake Superior as did Van Oosten and Deason (1938) in Lake Michigan. Lake trout feeding habits were therefore analyzed on the basis of depth. For the purpose of this study it seemed reasonable to choose four depth classifica- tions; 0-9, 10-19, 20-29, and 30 or over fathoms (Tables 7 and 8). The number of lake trout sampled in waters of over 30 fathoms was very small so there is no significance attached to breaking down the 30 .nuaov an mama“ voom unauum> mo oomouusooo mo moooavoum .5 manna 31 6.6 6.66 5.66 6.6 «69606 6.6 6.6 6.H 6.6 assumed 6.6 6.6 6.6 6.6 .mawdfim 6.6 6.6 6.H 6.6 mafiao 6.6 6.6 6.6 6.6 6065666: 9c 3. ma .3 46.: 33.2 6.6H 6.66 6.66 6.6 ouvwvoawvcoa 6.66 6.56 6.56 6.6 mucouaH 6.6 «.66 6.6 6.6 .dum.mwmmm 6.6 5.6g 6.56 5.6 awaamua «Houoaouaom o.o 6.6N H.6N 5.6 noouuuauo 6.6H 6.66 «.66 6.6H nouuubouuoan 6.6 6. 6.6 6.66 damfiaom.6oauuoz 92 m; «.2 6.8 538. 53 6.66 6.5H 6.66 6.66 dwaumom 6.~H 6.6 6.6 6.6a uaoam 6.56 6.66 6.66 6.66 0663666 6.66 6.65 6.66 6.66 5666 uaoouom amuH mamua voom msoquu> mo monouusuoo mo hoaoavouh 6 656 6666 66 ween magmwouaoo anon» mama mo noaauz 8N 2-8 3-3 a-.. $36.3 fiaon 32 .uao5 6am 66606 66 namua voou msowua> mo mucouusuoo mo homosaouh .6 manna 33 {3 (D OInInOOO InOOOanOOOOO O O. . .0. . .0 .00. .. O. 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H 34 depths over 30 fathoms into further categories. There was no significant difference in the frequency of occurrence of either alewives or smelt between the four depth cate- gories. Dryer, Erkkila, and Tetzloff (1965) report smelt occurring much more frequently in the stomachs of Lake Superior lake trout taken in depths of 35 fathoms or less than in greater depths; however, their data were not adequate to show changes by small depth intervals. Sculpins tended to be consumed more frequently at depths between 0-9 and 10-19 fathoms than at the greater depths. With few exceptions the sculpins identified in this study were of the genus go_t_£g_s_. Van Oosten and Deason (1938) found that the genus 925323 was more widely used as food by the shallow water lake trout than by the deep water trout in Lake Michigan; however, any depth under 45 fathoms was con- sidered shallow water in their study. Invertebrates with the exception of Tendipedidae, were absent from fish sampled in water of 30 fathoms or more. Pontoporeia affinis was most frequently observed in fish taken at lO-l9 fathoms while 513;! EL: was most often present in lake trout taken at 20-29 fathoms. Snails and leeches were present only in fish sampled at depths between 0-19 fathoms while clams were found only in fish sampled at depths of 10-29 fathoms. Van Oosten and Deason (1938) found no definite correlation between the percentages of stomachs that contained invertebrates and the depth of water from which the trout were taken in Lake Michigan. Dryer, Erkkila, Tetzloff (1965) stated that Mysis £10. 35 and insects were less frequent in stomachs of lake trout from deep water than in those from shallow water in Lake Superior. Feeding Habits with Special Reference to Alewives Due to its extreme availability and significant importance to the future of all salmonids present in Lake Michigan, the alewife as food of the lake trout was analyzed in some detail. As stated previously, alewives become increasingly important to the lake trout diet as the lake trout size increases. The average size of the alewife found in the stomachs also increases as the size of the lake trout increases; however, over three-fourths of the alewives consumed by each lake trout size group were under 9.0 cm. The average size of the alewives consumed by all lake trout was 7.4 cm, although the size ranged from 3.3 to 18.4 cm. Eighty-eight percent of the alewives consumed were 9.0 cm or less. Alewives between 6.1 and 8.4 cm.made up 64.6 percent of the total consumed (Figure 3). It appears then that small alewives are much more important as , food for the immature lake trout than are the larger alewives. I observed that although an occasional large alewife was found in several of the larger lake trout sampled, it was common to find from four to sixteen small alewives in a single stomach. This indicates that small alewives will be an important food item in the diet of large trout. By feeding predominately upon small alewives the lake trout are in effect more efficiently helping to control alewife numbers. Not only are they consuming greater numbers of alewives by this practice, but are in effect removing large numbers before they have become sexually mature. «L. aura-11‘ 36 .mnooaoum uoouu oxwa assumes“ a« venom ao>wao~o manwuomooa 666 no hoaoovoum summon .6 ousmum 37 .t 16.1 - 18.4 1. 14.1 - 16.0 4b 12.1 - 14.0 s- 10.1 - 12.0 ‘L 9.6 " 10.0 Amuouosaucoov summed deuce 9.1 - 9.5 uh- ‘* 8.6 - 9.0 8.1 - 8.5 «P 706 - 8.0 ‘r 7.1 - 7.5 606 " 700 6.1 - 6.5 5.6 - 6.0 ‘L 5.1 - 505 «L 4.6 - 5.0 4.1 - 4.5 306 - 400 1’ 301 - 305 1v6~ 416m ATOM 1 I M M é r66 .r. Number of alewives 38 Predation Upon Sport or Commercially Important Fish I found no evidence of predation upon any sport species other than smelt. No salmonids or other game species were found in the stomachs of lake trout in this study. This correlates with the findings of Van Oosten and Deason (1938). They found no evidence of larger lake trout feeding upon smaller ones in Lake Michigan. Eschmeyer (1964) noted that small lake trout have been found only infrequently in the stomachs of larger lake trout, probably because of the greater relative abundance of other forage species rather than because of any scruples against cannibalism. I found only one example of predation upon a commercially important species other than smelt or alewives. One chub (Coregonus 222;) was found in a lake trout 58.5 centimeters in length. Van Oosten and Deason (1938) found fish belonging to the genus Coregonus in 15.0 percent of lake trout stomachs examined from.Lake Michigan. At the time of their study, however, alewives were absent from Lake Michigan and smelt were extremely scarce. Both species are now abundant in the lake and have apparently replaced the chub in diet of lake trout. SUMMARY AND CONCLUSIONS The study of imature lake trout feeding habits in the Michigan waters of Lake Michigan has shown that: l. The major food item of imature lake trout in Lake Michigan is fish, primarily sculpins, alewives, and smelt. 2. Invertebrates play a major role only in the diet of lake trout under 20.0 centimeters. The most frequently occurring inverte- brates are Pontpporeia affinis, 11131.3 32., and Tendipedidae. 3. Smelt were found more frequently in the stomachs of lake trout taken from northern Lake Michigan than those sampled from the southern portion regardless of lake trout size. 4. Sculpins in general occurred more often in the stomachs of northern Lake Michigan lake trout than in those from southern Lake Michigan. 5. The crustacean Pontoporeia affinis is extremely important to the diet of small lake trout (0.0-19.9 cm) in southern Lake Michigan, occurring in 83.0 percent of those fish sampled. It is not nearly as important in the northern portion of the lake where it occurred in 35.7 percent of the small lake trout containing food. 6. we. _s£._ was found more frequently in the stomachs of lake trout from northern Lake Michigan regardless of lake trout size. 39 1 It“ wh- dish‘ru. 931:1? 1 _ J 40 7. Sculpins were the most frequently occurring fish in lake trout 0.0-32.9 centimeters in length while alewives were the most frequently found fish in lake trout over 33.0 centimeters. 8. Frequency of occurrence of alewives increased proportionally with the increase in lake trout size. The average size of the alewives consumed also increased as the size of the lake trout increased; however, small alewives (under 9.0 cm) still made up a significant portion of the food iteme consumed by the larger trout. 9. Smelt consumption increased in direct pr0portion to the increasing size of the lake trout. If the trend indicated by this study continues, smelt may become as important as alewives in the. diet of large lake trout. 10. Frequency of occurrence of invertebrates decreases as the size of the lake trout increases. No lake trout larger than 45.0 cm contained invertebrates. ll. Alewives were of little importance in the diet of lake trout until October, 1966, when they became extremely important and remained so throughout the remainder of the study period. 12. In general, invertebrates are less important as food for the lake trout during the winter than in summer. ‘Most invertebrates tend to be less available during the winter months and are therefore con- sumed less frequently. 13. There was no significant difference in the frequency of occurrence of either alewives or smelt with depth. _ 4 1.. i. 41 14. Sculpins occurred more frequently at depths of 19 fathoms or less. 15. Invertebrates with the exception of Tendipedidae, were absent from fish sampled in depths of 30 fathoms or more. 16. Pontoporeia affinis was most frequently observed in fish taken at depths of 10-19 fathoms while Mz§i£_gp;ywas'most often present in lake trout taken at 20-29 fathoms. 17. Small alewives (under 9.0 cm) are more important as food for the immature lake trout than are the larger alewives. The lake trout are efficiently helping to control alewives by removing large numbers before they have become sexually mature. 18. Predation by immature lake trout upon sport or commercially important fish species other than smelt and alewives, is insignificant in Lake Michigan . LITERATURE CITED Dryer, William H., Leo F. Erkkila, and Clifford L. Tetzloff. 1965. Food of Lake Trout in Lake Superior. Trans. Amer. Fish. Soc. 94 (2): 169-176 Dymond, John R. 1928. Some Factors Affecting the Production of . Lake Trout (gristivomer namaycush) in Lake Ontario. Univ. Toronto Studies, Biol. Ser., Publ. Ontario Fish. Res. Lab. No. 33 (1928): 29-41 Eschmeyer, Paul H. 1956. The Early Life History of the Lake Trout in Lake Superior. Mich. Dept. Cons. Inst. Fish. Res., Misc. Publ. No. 10, 31 pp. Eschmeyer, Paul H. 1964. The Lake Trout (Salvelinus namaycush). U.S. Dept. Int., Fish. Leafl. 555, 8 pp. Lagler, Karl F. 1956. Freshwater Fishery Biology. Second Edition, Hm. C. Brown Co., Dubuque, Iowa 421 pp. Larimore, R.W. 1957. Ecological Life History of the Narmouth (Centrarchidae). Ill. Nat. Hist. Surv. Bull., 27 (l): 83 pp. Rounsefell, George A. and H. Harry Everhart. 1953. Fishery Science: Its Methods and Applications. John.Niley and Sons, Inc., New York, 444 pp. Smith, Stanford H., Howard J. Buettern, and Ralph Hile. 1961. 'Fishery Statistical Districts of the Great Lakes. Great Lakes Fish. Comm. Tech. Rep. No. 2, 24 pp. Smith, Stanford H. 1968. Species Succession and Fishery Exploitation in the Great Lakes. J. Fish. Res. Ed. Canada, 25 (4):667-693 Van Oosten, John, and Hilary J. Deason. 1938. The Food of the Lake Trout (Cristivomer namaycush namaycush) and of the Lawyer (Leta maculosa) of Lake Michigan. Trans. Amer. Fish. Soc. 67: 155-177 42 MICHIGAN STQTE UNIV. LIBRQRIES 31293101282055