. ,c1·.-.·~ .,.~

ep~ Annual Production by the Slimy Sculpin Population in a Small Minnesota Trout Stream1

2 and CHARLES E. PETROSKY AND THOMAS F. WATERS !-0, Department of Entomology, Fisheries, and Wildlife 1me University of Minnesota, Sc. Paul, Minnesota 55108 ·an­ and ABSTRACT :SS., Annual production by the slimy sculpin, cognatus Richardson, was estimated at 59.4 kg/hectare wet weight for the period July, 1970, to July, 1971, in Valley Creek, Minnesota. ind Mark-and-recapture population estimates made by electrofishing and aging with otoliths were -D, the hues for determining mean biomass and instantaneous growth rates. Annual turnover ratio, me i.e., ratio of annual production to mean standing crop, wai; 1.2. Unusual silt and turbidity con­ ditions, occurring in 1971, reduced the 1971 year class drastically, but had less serious effects lies on total production by all age groups. Food of the sculpins was similar in most respects to that 11). of the inhabiting the same stream. Young sculpins fed heavily on small dipterans, mainly chironomids; older sculpins fed mainly on Gammarus and Trichoptera larvae. Compari­ :la. sons with annual production estimates for brook (Salvelinus foniinalis) and rainbow trout (Salmo gairdneri), made previously in Valley Creek for comparable time periods, suggested a normal he total annual production for all fishes of abour 200 kg/hectare, with sculpins contributing about IS. IA and trout about %.

10. ns in Freshwater sculpins (Family ) so that comparisons could be made with 4): often constitute an important component of the trout data. stream ecosystems, particularly in streams In 1965 and 1966, damaging floods oc­ of c. ecologically suited to salmonids. Little ob­ curred in Valley Creek that severely re­ served by anglers, they nevertheless are duced the trout population and trout pro­ ry often abundant as cohabitants with stream duction rates (Elwood and Waters 1969); o­ trout. They may serve both as food and a. the trout PQpulation recovered in post-flood competitor to the trout. The relative years and reached normal levels of abun­ dynamic importance of each species may dance and production by 1970 (Hanson and ·): best be understood jn a comparison of Waters 1974). The present study, therefore, td production rates among all species; included a period of time when the trout whereas many estimates of production population, and presumably the sculpins as >r have been obtained for stream salmonids, well, had recovered from the unstable I. considerably less information exists on conditions caused by the earlier floods. production rates for sculpins in natural en­ In 1971, however, Valley Creek suffered vironments. from another catastrophe in the form of The present study was conducted on a heavy siltation and turbidity, the result of population of the slimy sculpin, Cottus erosion at the site of a new housing de­ cognatus Richardson, in Valley Creek, a velopment in the upper watershed. Silt and small trout stream in east-central Min­ clay filled the substrate gravels and appar­ nesota, from June, 1970, to February, 1972. ently caused another serious decline in the The reach of stream used as a study sec­ trout population, and perhaps in inverte­ tion was the same as that used in recent brate populations as well. Consequently, an­ studies of trout production {Elwood and nual data for the sculpin are presented in Waters 1969; Hanson and Waters 1974), this paper in two time frames, each of an­ covering in part the same time and area, nual duration: July 23, 1970, to July 20, 1971, a period in which the sculpins 1 Paper No. 8546, Scientific Journal Series, Min­ through the 1970 year class were probably nesota Agricultural Experiment Station, St. Paul, little affected by the silt; and from January Minnesota 55108. 2 Present address: lchthyological Associates, Fricks 21, 1971, to February 14, 1972, during Lock Road, R.D. l, Pottsrown, Pennsylvania 19464. which time the silt had the greatest effect, 237 238 TRANS. AMER. FISH. SOC., 1975, NO. 2

TABLE 1. -Percentage of marked slimy sculpins in suond sample of population estimate (capture efficiency) the Date ter ea• Siz.e Jun Jul Sep Jan Apr Jul Sep Feb group(mm) 1970 1970 1970 1971 1971 1971 1971 1972 WE frc <50 14.2 27.0 7.6 13.4 IS.l 7.1 15.6 21 .0 50-59 11.6 22.4 21.0 20.7 15.9 20.3 24.7 36.8 st1 60- 69 17.0 21.7 21.7 26.3 18.9 21.7 23.4 30.7 ta >69 22.9 26.9 22.2 29.7 21.7 23.8 39.9 34.7 p mainly on the 1971 year class. No physical Diptera, and the gastropod, Physa. The t< data were obtained on suspended solids in brook trout, Salvelinus fontinalis, was the e the water or fine sediment content of the predominant salmonid, with the brown, 1 substrate. Salmo trutta, and rainbow, Salmo gaird­ ( The principal objective of the present neri, present in varying numbers; the study was to estimate annual production American brook lamprey, Lampetra lamot­ ' by the slimy sculpin population for com­ tei, was also present. Except for the scul­ parison with concurrent trout production pin, Cottus cognatus, no other fishes were rates and with fish production data of found in Valley Creek. other authors from different streams. Cog­ nate objectives were to contribute to life METHODS history knowledge of the slimy sculpin in Population estimates were made in regard to food, growth, and fecundity. The different seasons of the year from June, occurrence of the siltation conditions in 1970, to February, 1972; using the Petersen 1971 also permitted some assessment of mark-and-recapture method, following the the effect of this factor on the sculpin procedure described by Shetter (1957). population dynamics. Sculpins were captured by electrofishing with a 220-volt DC, 2500-watt engine­ STUDY AREA driven generator. All fish were measured Valley Creek rises as a series of springs to the nearest 2.5 mm. Population esti­ and flows approximately 6 km through a mates were made separately for size classes, deeply incised valley to the St. Croix since there were usually higher rates of River, the boundary at that point between recapture with larger fish; recapture rates Minnesota and Wisconsin. The study area ranged from about 7% for young of the consisted of a section about 0.4 km long year to 40% for the largest size classes with an area of 0.18 hectare, located ab-Out (Table 1). midway along the course of the stream. Additional sculpins were taken from The substrate was composed of rubble, short sections of the stream immediately gravel, and sand, in alternating pools and below and above the study section at the riffles. The water is characterized by high time of each estimate to obtain age com­ alkalinity (220 ppm), pH of about 8, and position and length-weight data; a total of normally great clarity. Maximum tempera­ 446 sculpins were collected for this pur­ tures in the summer rarely exceeded 15 C, pose. The assumption was made that such with winter temperatures usually about 5 C sampling did not affect the population in and occasionally reaching 0 C in excep­ the study section. These fish were kept on tionally cold weather. Watercress, moss, ice in the field and taken to the laboratory and diatoms on the rocks as periphyton where they were frozen. Otoliths were re­ were the principal aquatic plants. The moved and stored in a 1:1 glycerine and major invertebrates were the amphipod, water mixture until they were read. Age Gammarus pseudolimnaeus, the composition was determined by fitting Baetis vagans, several species of Trichop­ symmetric curves to length frequency dis­ tera, the black fly genus Simulium, tributions of aged fish with the aid of Chironomidae, and a few other burrowing otolith samples. The relative heights of PETROSKY AND WATERS-SLIMY SCULPIN PRODUCTION 239 these curves at each 2.5-mm interval de­ termined the percentage composition of each age group in that interval. Mean wet weights of age groups were determined from length-weight relationship curves con­ structed at each estimate from the samples taken for aging. Ii.I a: Production rates were calculated as the ....c product of mean biomass and the instan· u taneous growth rate for each age group in Lii :i:..... each period between estimates (Ricker Ill 0 1946; Allen 1949). Mean biomass was cal· ~20 Ill culated in the manner of Chapman (1968) ::> with use of tables from Ricker (1958, Ap­ ~ pendix II). Cohort production was com­ .... puted using the gTaphical method of Allen (1951), with a composite curve of the sev­ eral incomplete year classes (cf. Cooper and Scherer 1967). Stomach contents of most sculpins used for age determination, as well as several young of the year collected from outside the study section, were examined. Stom­ ach contents were separated into major in­ vertebrate taxa. Data were summarized as percentage occurrence and percentage composition by volume of the total stomach content. Fecundity was estimated on the basis of egg counts made on 34 gravid females col­ JJASONOJFMAMJJASONOJF lected in the spring of 1971 and the winter 1970 1971 1972 of 1972 with the use of a simple linear re­ gression on length. FIGURE 1. -Standing crop of the slimy sculpin in numbers (upper) and biomass (lower) in Valley Creek, Sources of Error from Jun e, 1970, to February, 1972. Vertical lines rep­ resent 95% confidence intervals, numbers only. Accuracy of mark-and-recapture popula­ tion estimates depends on several assump­ tions, among which is that there is not differential mortality or movement between tissue elaboration present after about three marked and unmarked fish. Handling of months of life, and none of that previously fish was kept to a minimum, and mortality elaborated and lost to mortality, production of marked fish appeared to be negligible rate estimates of this age group were min· by visual observation. Sculpins generally imal. are stationary benthic fish, and movement Production rates were assumed to have of unmarked fish from outside the study been zero in periods with negative growth. section appeared unlikely. The recapture Weight losses were observed in the older rates were high enough to suggest good fish in the winter and in one age group at precision in the calculated estimates (Table spawning. There may have been elabora­ 1). tion of somatic tissue during these periods, However, si nce initial estimates of however, and in this case, production was young-of-year sculpins represented only underestimated. 240 TRANS. AMER. FISH. SOC., 1975, NO. 2

24 22 20 ~18 < t; 16 llJ ::c..... 14 z~12 ~10 :> ~ 8 I- 6 4 1970 JJASONOJFMAMJJASONDJF 2 1970 1971 197Z FIGURE 3.- Mean weigh! of slimy sc ulpins by year JJASONOJFMAMJ JASONOJF class. 1970 1971 1972 FIGURE 2. -Standing crop of the slimy sculpin in numbers, by year class. in numbers was only about half of that in the previous winter. RESULTS Total biomass remained relatively stable throughout the study, even with seasonally Standing Crop, Growth, and flu ctuating numbers, ranging from a low Production Rate of 42.3 kg/hectare to a high of 52.8 During the study period, the mean kg/hectare (Fig. 1). This relative stability standing crop of sculpins in numbers was in biomass remained, despite the lower about 20,000/hectare, with a range of numbers, because of higher growth rates 11,254/hectare on- July 29, 1970, to and lower mortality of the larger fish, ap­ 29,580/hectare on September 16, 1970 (Fig. parently compensations for the lower nu­ 1). This increase in number was due to re­ merical density. cruitment of age group 0 (1970 year class) Population levels of each year class in into the estimable portion of the popula­ numbers/hectare were relatively stable for tion. A similar increase, but of much lower comparable points in the life cycle, with degree, was noted from July 20, 1971, to the exception of the 1971 year class, which September 14, 1971. There was a numeri­ was less than half as numerous as the 1970 cal decline near the end of the study, and year class the previous year (Fig. 2). by February, 1972, the total standing crop Growth of sculpins was greatest in spring and early summer (Figs. 3, 4). Growth in length ceased in the fall, as did TABLE 2. -Production contributed by each year class of also the growth in weight of non-spawners. slimy .sculpitu from}uly, 1970, to j uly, 1971 But there was considerable growth in Annual weight among the spawners during the Year production winter, due to gonadal development. De­ cl as! kg/hectan: Percentage creases in mean length of some cohorts 1970 38.7 65.l were occasionally observed (Fig. 4), due 1969 11.9 20.0 1968 7.2 12.2 probably to differential mortality of larger 1967 1.5 2.5 individuals. Annual instantaneous growth 1966 0.1 0.2 rates were 1.40, 0.88, 0.46, and 0.35 for Total 59.4 100.0 age groups I through IV, respectively. PETROSKY AND WATERS-SLIMY SCULPIN PRODUCTION 241

25 12 1970 IH7 10 20 ~ 0 ,... . 8 / w j: a: ~ z 6 '/" :! llJ u 15 ...J w z ....:z: 4 UI "' c "'2 'Z 2 UI"" 10 ::J 0 JJASONOJFMAMJJASONDJF :z: I- 1970 1971 1972 5 FtCURt 4.-Mean length of slimy sculpins by year l c:lass.

1966 2 4 6 8 10 12 14 16 18 20 Mortality rate of sculpins was lowest in MEAN WEIGHT- GRAMS the winter and highest in either spring or FJCURE 5.-Composite Allen curve used to estimate summer, depending on the age group (Fig. cohort production of the slimy sculpin. Data for the 2}. Annual instantaneous coefficients of 1971 year class were excluded in the planimetric me.a­ mortality were 1.47, 0.49, 0. 77, and 2. 72 for surement. age groups I through IV, respectively. Annual production during July, 1970, to analyzed by the percentage contribution by July, 1971, was 59.4 kg/hectare, including year classes, in the July-to-July time frame the . young-of-year production that was unaffected by silt, there appeared a de­ unaffected by siltation. Annual production creasing contribution by older year classes. during January, 1971, to February, 1972, Approximately 65% of annual production was 52.4 kg/hectare, a lower estimate due was accounted for by age group 0 and 85% probably to the lower survival, and conse­ by age groups 0 and 1 combined (Table 2). quently lower production, of the young of Production of a cohort, estimated the year affected by the heavy silt load in planimetrically from a composite Allen 1971. The highest production rate for all curve, was 64.3 kg/hectare (Fig. 5). The age groups occurred in spring and early 1971 year class was excluded from the summer, at the time when growth rate was computation because it was abnormally highest. When annual production was low.

TABLF. 3. -Items in stomachs ofslim y sculpins expreucd as percentage occurrence and a.s average percentage con­ tent by volume for the youngest age group (13-53 mm) and the combined older age groups (41 - 111 mm). Num­ bers of stomachs in parentheses

Average percentage content by volume

Percentage Youngest Older Food item occurrence (116) (326)

Gamma nu 54 29 42 Diptera larvae 48 SI 13 Trichoptera larvae 36 13 22 PhJ'la lS I 7 Baeti.s 4 2 l Miscellaneous 8 4 IS (Empry stomachs) (5) (18) 242 TRANS. AMER. FISH. SOC., 1975, NO. 2

Food of Sculpins Spawning and Fecundity Feeding by sculpins in Valley Creek, as Sculpins in Valley Creek generally he· indicated by stomach contents, continued came sexually mature at age II, although throughout all seasons, with one exception: one female was mature at age I and a few adult females fed very little during ­ of both sexes were still immature at age ing. A total of 442 sculpin stomachs was II. Spawning occurred in late April; fry examined (Table 3). were first observed in June. An estimated The most important food group found in 2,958 gravid females/hectare were present sculpin stomachs was Gammarus, followed on April 18, 1971, and by April 21, over by Diptera larvae, Trichoptera larvae, and half had spawned. The larger females ap­ snails. Feeding on Gammarus, Trichoptera peared to have spawned first. The fecun­ larvae, and snails increased with the age dity observations, from egg counts of the of fish, whereas feeding on Oiptera larvae 34 gravid females taken outside the study decreased. Andreasson (1971) found similar section, ranged from 59 eggs in a fish 43 results for Cottus gobio in S.weden. mm in length and one year old, to 645 Gammarus were found in 54% of all eggs in a 111-mm, 5-year-old fish. The stomachs. They were important to all age length-fecundity relationship was computed groups except newly hatched fry, and be­ to be E = (10.l)L - 460.0, where E = came increasingly important to the older number of eggs and L= total fish length in fish. mm. By combining the length-fecundity re­ Diptera larvae were found in 48% of all lationship with estimated numbers of stomachs. Chironomidae larvae were espe­ gravid females, an estimated 510,000 cially important to young fish and became eggs/hectare were available for spawning decreasingly important to the older fish; in 1971. Andreasson (1971), again, reported similar results for Cottus gobw. Simuliidae larvae DISCUSSlON were eaten occasionally by all ages, and It is helpful to compare sculpin produc­ small Tipulidae and Tabanidae larvae were tion with total fish production rates in eaten infrequently by all age groups, ex­ different salmonid streams; sculpins are cept young of year. Chironomidae larvae frequent cohabitants with stream trout and, comprised 78% of all dipterans found in from the fishery manager's point of view, the stomachs, compared to 20%, 1 %, and may constitute a substantial competitive 1% for Simuliidae, Tipulidae and Taba­ element with the more economically impor­ nidae, respectively. tant salmonids. In an analysis of fish pro­ Trichoptera larvae, primarily Clcissosoma, duction in ten salmonid streams in En­ were found in 36% of all stomachs. These gland, Le Cren (1969) found a maximum larvae were eaten by all age groups but annual salmonid production of about 120 were more important to the older sculpins. kg/hectare, with sculpin production rates The gastropod, Physa, constituted the severalfold higher in some streams. Since fourth most important food, occurring in then, a number of reports have indicated 15% of the stomachs. The snails were im­ somewhat higher stream salmonid produc· portant mainly to the older age groups III tion rates (Egglishaw 1970; Mann 1971; through V. Cooper and Scherer 1967). Annual produc­ The remaining foods included the mayfly tion rates for trout in Valley Creek for the Baetis, isopods, beetle adults and larvae, calendar year 1970 were estimated at 132 annelids, ostracods and nematodes. Scul­ pin eggs were found in the stomachs of kg/hectare (brook trout 124, rainbow 8) by two males, age IV, during .the spawning Hanson and Waters (1974). The year 1970 season; Bailey (1952) suggested that male was without any catastrophic event, such sculpins attend incubating eggs and re­ as floods or excessive siltation. If the July, move dead ones. No trout eggs or fry were 1970, to July, 1971, annual production of ever found in any of the sculpin stomachs. sculpins at 59.4 kg/hectare was represen- PETROSKY AND WATERS-SLIMY SCULPIN PRODUCTION 243

tative, it appears that normally the total strength of the 1971 year class was so low be­ annual fish production would be about 200 relative to the previous year class: 9000/ gh kg/hectare, with sculpins contributing hectare compared to nearly 23,000/hect­ ew about Y3 and trout about %. are for the 1970 year class (Fig. 2). ge Sculpin production rates reported by However, the increase in growth rate and :ry other authors ranged from 8.5 kg/hectare decrease in mortality of older fish appar­ ed to 431 kg/hectare, while percentage con­ ently compensated largely for the lower nt tribution by sculpins to total annual fish numbers, and, consequently, mean biomass er production has ranged from 7.3% to 83% and production rates were not reduced p­ (fable 4). The main generalizations that nearly as much, relative to the loss in n- can be postulated at this time are that numbers. Increases in total numbers were 1e salrnonid production rates appear to have a observed in the autumn as the new year ly maximum of about 150-200 kg/ hectare, class was recruited into the estimable b3 and that maximum sculpin production population; in 1971, this increase was ~5 rates are higher and more variable. much smaller than in 1970, and the total 1e Turnover ratio, defined as the ratio of biomass decreased at this time, the result d production rate to mean standing crop, of natural mortality among the older year may provide a means of obtaining an ap­ classes without the usual increase in the · = new year class. a proximation of production rate from stand­ The difference in production rates be­ :- ing crop data. The annual turnover ratio for the slimy sculpin, for the July tween the two time frames-59.4 kg/ f hectare and 52.4 kg/hectare-was not '.) 1970-July 1971 period, was 1.2; the cohort turnover ratio (calculated with the compos­ great and may not have been significant. It ite Allen curve, Fig. 5), was 6.3. These would be difficult to predict the ultimate effect on production had the silt problem ratios are similar to, but somewhat higher continued repeatedly, but probably a suc­ than, those reported for brook trout in cession of weak year classes would even­ Valley Creek for comparable times: 1.1 and tually result in lower production rates. 4.5, respectively (Hanson and Waters While the possibility of interspecific 1974). competition exists in Valley Creek between There seems to be little question that brook trout and the sculpins, it is not ap­ the unusual silt load and turbidity in 1971 parent from this study. In small productive affected the sculpin population, since the streams such as Valley Creek, the reg-

T ABLE 4. -Annual production of3culpins in. various id $/reams, compared to total annual production for all &pecie5, kg/hectare

Percen1a1:e Production Produc1ion of contribution Stream Species of seulpin all species by &culpin Auihority Valley Creek, Coitus cognatus S9.4 191. 32 Minnesota Big Springs Couus sp. 8.5 118 7.2 Goodnight and Creek, Idaho Bjornn 1971 Lemhi River, Coitus sp. 9.9 136 7.3 Goodnight and Ide.ho Bjomn 1971 Deoet Creek, Cottus perple:

"Includes data from Hanson and Waters (1974). bPreliminary dat1 for these same •Iream s, essentially the same, were given by Le Cren (1969). 244 TRANS. AMER. FISH. SOC., 1975, NO. 2 ulator of trout density may be a space­ BAILEY, J. E. 1952. Life history and ecology of the sculpin Coitus boirdi punciulatus in southwest­ shelter one, such as was suggested by ern Montana. Copeia 1952: 243-255. Saunders and Smith {1955), and not related BROCKSEN, R. W., G. E. DAVIS, AND C. E. W• .\RREN. to food at all. 1968. Competition, food consumption, and pro­ Both the slimy sculpin and the brook duction of sculpins and trout in laboratory stream communities. J. Wildlife Mgt. 32: 51-75. trout in Valley Creek utilize a common CHAPMAN. D. W. 1965. Net production of juvenile food resource. Gammarus is the main food coho salmon in three Oregon streams. Trans. item for the brook trout (Hanson 1972), as Amer. Fish. Soc. 94(1): 40-52. ---. 1968. Production. In: Ricker, W. E. (ed.), well as for the sculpin. Chironomidae and Methods for Assessment of Fish Production in Trichoptera larvae are important to both, Fresh Waters. IBP Handbook No. 3, Blackwell, but of the trichopterans, brook trout fed Oxford and Edinburgh, p. 182-196. primarily on Limnephilus and sculpins fed CLARY, J. R. 1972. Predation on the brown trout by the slimy sculpin. Prog. Fish-Cult. 34: 91-95. primarily on Glossosoma. COOPER, E. L., AND R. C. SCHERER. 1967. Annual The modes of feeding of these two production of brook trout (Salvelinus fontinalis) fishes may be quite different. The sculpin in fertile and infertile streams of Pennsylvania. Proc. Pa. Acad. Sci. 41: 65-70. apparently feeds totally by foraging on the ECCLISHAW, H. J. 1970. Production of salmon and bottom, while the brook trout feeds mostly trout in a stream in Scotland. J. Fish. Biol. 2: on drift (Keenleyside 1962; White 1967). 117-136. ELWOOD, J. W., ANO T. F. WATERS. 1969. Effects of Brocksen, Davis, and Warren (1968) have floods on food consumption and production rates shown that in a laboratory stream, sculpins of a stream brook trout population. Trans. can influence the food consumption and Amer. Fish. Soc. 98(2): 253-262. production of trout through cropping the GOOONICHT, W. H., ANO T. C. BJORNN. 1971. Fish production in two Idaho streams. Trans. Amer. benthic food organisms, thus reducing the Fish. Soc. 100(4): 769-780. numbers of drifting organisms. The trout, HANSON, D. L. 1972. Abundance and production rate however, may affect the production of of a stream brook trout population. M.S. thesis, sculpins very little through food competi­ University of Minnesota, 91 p. ___ , AND T. F. WATERS. 1974. Recovery of tion, because their consumption of drifting standing crop and production rate of a brook organisms usually does not reduce the trout population in a flood-damaged stream. benthic populations. Trans. Amer. fish. Soc. 103(3): 431-439. Clary (1972) reported predation on hatch­ KEENLEYSIOE, M. H. A. 1962. Skin-diving observa­ ing brown trout eggs and sac fry by the tions of Atlantic salmon and brook trout in the Miramichi River, New Brunswick. J. Fish. Res. slimy sculpin, but only at the time of Board Can. 19: 625-634. hatching. He found predation on brown LE CHEN, E. D. 1969. Estimates of fish populations trout eggs and fry to be a function of size and production in smaU streams in England. In: Northcote, T. G. (ed.), Symposium on Salmon of the predator, and only those sculpins and Trout in Streams. H. R. MacMillan Lec­ about 5 cm or longer fed on eggs or fry. In tures in Fisheries, Univ. British Columbia, Valley Creek at the time of brook trout Vancouver, p. 269-280. MANN, R. H. K. 1971. The populations, growth and hatching (January and February estimates), production of fish in four small streams in sculpins 5 cm or longer were present in southern England. J. Anim. Ecol. 40: 155-190. densities of about 7000/hectare. However, RICKER. W. E. 1946. Production and utilization of no trout eggs or fry were found in the fish populations. Ecol. Monogr. 16: 373-391. ---. 1958. Handbook of computations for bio­ stomachs of sculpins. logical statistics of fish populations. Fish. Res. Board Can. Bull. 119, 300 p. LITERATURE CITED SAUNDERS, J. w., AND M. w. SMITH. 1955. Standing crops of trout in a small Prince Edward Island ALLEN, K. R. 1949. Some aspects of the production stream. Can. Fish Cult., No. 17: 32-39. and cropping of fresh waters. Trans. Roy. Soc. SHETIER, D. W. 1957. Trout stream population study New Zealand 77(5): 222-228. techniques employed in Michigan. In: ___ . 1951. The Horokiwi Stream, a study of a Carlander. K. D. (ed.), Symposium on Evalua­ trout population. New Zealand Mar. Dept., Fish. tion of Fish Populations in Warm-water BuU. 10, 238 p. Streams. Iowa State Coll., Ames, p. 64-71. ANDREASSON, S. 1971. Feeding habits of a sculpin WHITE, D. A. 1967. Trophic dynamics of a wild (Cottus gobio L. Pisces) population. Inst. brook trout stream. Ph. O. thesis, Univ. Wisc., Freshw. Res. Drottningholm, Rept. 51: 5-30. Madison.