Variation in the Condition of Northern Pike, Esox lucius*
Jean-Frangois Doyon,2 John A. Downing, and Etienne Magnin
Department de sciences bioiogiques, University de Montreal, C.P. 6128, Succursale *A», Montreal (Quebec) H3C 317
Doyon, J.-F., j. A. Downing, and E. Magnin. 1988. Variation in the condition of northern pike, Esox lucius. Can.
J. Fish. Aquat. Sci. 45: 479-483.
The relationship between condition and latitude, temperature, and sex of northern pike {Esox lucius) was exam
ined by regression analysis of 5210 published observations drawn from more than 38 populations. Most popu
lations showed nearly isometric growth, but the overall exponent of the power function relationship between
total body length and body weight was slightly less than 3 (p < 0.01). Multiple regression analysis showed that
females weighed about 9% more than males of equal length, that body weight for fish of equal length increased
about 5% for every 10° increase in north latitude and increased about 4% for every 10°C decrease in mean annual
air temperature, and that fish in European and Asian populations attained about 13% greater weight for a given
length than their North American counterparts.
On a 6tudie~ la relation entre la condition et la latitude, la temperature et le sexe du grand brochet (Esox lucius)
a I'aide de I'analyse de regression de 5 210 donn&s publiges, tirees de plus de 38 populations. La majority des
populations de brochets d&nontre une croissance quasi isome*trique quoique I'exposant de la relation entre la
longueur totale et le poids corporel, pour tous les brochets, soit de fac.on significative (p < 0.01) le*gerement
inferieur a 3. L'analyse de regression multiple reVele que les femelles pesent environ 9 % de plus que les males
de longueur identique, que le poids corporel des poissons d'une meme longueur augmente d'environ 5 % pour
une augmentation de 10° de latitude nord et de 4 % pour toute baisse de 10°C de temperature moyenne annuelle
de I'air, et que, pour les brochets d'une longueur donnee, les poissons europgens et asiatiques atteignent un
poids qui est d'environ 13 % supe*rieur a celui des brochets d'Ame>ique du Nord.
Received November 13, 1986
Accepted November 2, 1987
Recu le 13 novembre 1986
Accept le 2 novembre 1987
(J9012)
The northern pike (Esox lucius) is a fish of economic and
ecological importance in North America, Europe, and
Asia. In the United States, northern pike is one of the top
five sportfish species (U.S. Fish and Wildlife Service 1977).
The economic value derived from the northern pike makes it a
major focus of fisheries management organizations. The north
ern pike is a voracious predator (Kipling 1984), often found
near the margins of productive weed beds in lakes, ponds, and
rivers (Carlander 1969; Scott and Crossman 1973). Esox lucius
thrives in several types of habitats including reservoirs (Kupchinskaya 1985), large lakes (Kipling 1983a, 1983b; Kipling
and Frost 1970), small lakes (Van Engel 1940), ponds
(McCarraher 1957), rivers (Holland and Huston 1984), and
streams (Le Louarn and Bagliniere 1985). Young-of-the-year
grow rapidly eating large quantities of large invertebrates such
as amphipods, isopods, and insect larvae but graduate rapidly
to a diet composed mainly of small fishes (Holland and Huston
1984). Esox lucius is the ubiquitous top carnivore in many
aquatic ecosystems; thus, factors regulating its growth and suc
cess are of interest to the management of prey species.
Many factors influence the growth and condition of fish spe
cies. The optimality of their growth conditions is often thought
to be reflected by the relative weight of fish at a given length.
'Publication of the Groupe d'6cologie des eaux douces of l'Uni-
versitg de Montreal.
Current address: Department of Renewable Resources, MacdonaJd
College, 21111 Lakeshore Road, Sle-Anne de Bcllevue, Que.
H9X 1C0.
Can. J. Fish. Aquat. Sci., Vol. 45, 1988
Fish living under marginal environmental conditions will the
oretically weigh less at any particular body length than those
living under more luxurious circumstances. Esox lucius has
been called a northern species (Scott and Crossman 1973); thus,
one might expect greater body condition at high latitudes and
low temperatures. Females often show greater body condition
than males because of the large mass of eggs produced during
the year (e.g. Brown and Clark 1965; Johnson 1966). Body
condition might also vary between geographically isolated
stocks due to genetic inequality of stocks or geographic vari
ations in living conditions. European pike, for example, have
long been thought to grow more rapidly than North American
pike (Scott and Crossman 1973). The purpose of this research
was to test the hypotheses that body condition in Esox lucius
varies systematically on a large scale with latitude, temperature,
and sex and that there are systematic differences in the condition
of European, Asian, and North American populations.
Methods
The relative weight of fish of similar lengths has been referred
to as their condition (Ricker 1975) or ponderal index (Lagler
et al. 1977). The measurement of the condition of fish is a
standard component of most fisheries curricula (Lagler et al.
1977; Weatherly and Rogers 1978), and the morphological
measurement of the condition of fishes has not changed mate
rially in over 80 yr (cf. Fulton 1902; Buck and Hooe 1986).
Fulton (1902) observed that the mass of fishes rose approxi479
mately as the cube of the body length and therefore laid the
intuitive foundation for the coefficient of condition, k:
(1)
k = WIL3
where W is body mass (grams) and L is body length (milli
metres). Calculation of k assumes that W rises exactly as the
cube of L, or that growth is isometric.
Not all fish growth is isometric. Fulton (1902, p. 334) him
self observed that "The curves do not everywhere agree with
the rule that in similarly-shaped bodies the masses vary as the
cubes of the dimensions; the proportions appear to change
somewhat with growth." If fish growth is allometric and equa
tion 1 is used as an index of condition, then there will be an
artifactual systematic variation in4 'condition'' with body length
(reviewed by Carlander 1969, 1977). To avoid these difficul
ties, Le Cren (1951) advanced a relative or allometric (Ricker
1975) condition factor.
(2)
K » W/L*
where b is the population-fitted exponent of the power rela
tionship between L and W. k* is perfectly correlated with the
difference between the weight of a fish and the weight predicted
from a length-weight power relationship (W = aLb) fitted to
the data for all fish in the population. Because the b values are
usually found through linear regression of log W on log L
(Ricker 1975), to linearize the relationship between L and W
and to eliminate heteroscedasticity, log K values should be
approximately normally distributed.
If one were interested in finding whether a third variable, V,
has a significant effect on kn, this could be tested by analysis
of covariance (ANCOVA) or multiple regression:
(3)
logW = a + blogL + cV
as long as different populations included in the analysis showed
similar values of bin independent tests. Statistical significance
of the regression coefficient, c would indicate that V is signif
icantly correlated with the residuals of a log W - log L regres
sion and therefore that V has a significant effect on fish
condition.
In our analyses, published data were collected on total fish
length (L, millimetres) and fresh weight of individual animals
(W, grams) from several populations ofEsox lucius. Most length
and weight data were read from tables, but some data were
extracted from graphs using digital measurement of photo
graphic enlargements (Table 1). Data were also collected on the
sex of specimens and the latitude and the mean annual air tem
perature in the geographic area in which they were collected.
Latitudes were taken from publications or estimated from maps,
while temperatures were estimated as the long-term annual
average from the nearest reporting station (Wernstedt 1972).
Information on air temperature was used instead of water tem
perature because it was more readily available. Some data were
included from unspecified water bodies within large geograph
ical areas (e.g. Michigan, Minnesota, Sweden) and therefore
could not be paired accurately with exact latitudes or precise
temperatures. For these data, prevailing latitudes and temper
atures were calculated as the average of all latitudes listed for
the specific political unit by Wernstedt (1972). Fractional lati
tudes were truncated to whole degrees.
Data on more than 5200 northern pike from more than 38
populations were collected from the published literature (Table
1). Lengths of fish ranged from 68 to 1170 mm and weights
480
from 1.1 to 8200 g. Data on populations living between 41 and
65° north latitude were collected. This covers much of the circumpolar geographic distribution of Esox lucius (Scott and
Crossman 1973). Mean annual ambient air temperatures varied
between -4.3 and 10.4°C. Although some gravid individuals
are included in the sample, no systematic correlation exists
between temperature and the collection of spawning individu
als. Fewer observations (11 %) were collected for European than
North American fish.
After the data were collected, L and W data were analyzed
statistically. First, the b value (equation 2) was estimated for
each of the i populations by least squares regression:
(4)
log W = log a{ + b, log L
where at and b, are fitted constants (Nie et al. 1975). Confidence
intervals for b( were computed and comparisons were made vis
ually. Second, an overall b value was calculated using pooled
data from all populations, and this was compared with the the
oretical value of 3 using a f-test. Conclusions are compared with
those obtained using geometric regression (Ricker 1973). Third,
we fitted the multiple regression
(5)
log W = log a + b log L + cLt + dT + eS + /Eu
where Lt is latitude, 7 is air temperature, S is sex (male or
unspecified = 0; female = 1), Eu is a dummy variable (Gujarati 1978) indicating whether the population was European or
Asian (Eu = 1) or from North America (Eu = 0), and a-f are
constants fitted by least squares mulitple regression. The fit of
the model was verified using standard residual analysis (Draper
and Smith 1981). The significance of the effect of each variable
was inferred by the partial significance of its effect on log W.
No variables were retained in the regression that did not have
a highly significant (p < 0.01) effect on body mass.
Results and Discussion
Length-Weight Relationships
Individual populations showed only a narrow range of fitted
b values (Fig. 1). Length-weight exponents for individual
populations ranged between 1.62 and 3.8 but the 95%
confidence intervals of most overlapped b = 3 (Fig. 1). This
analysis agrees with Carlander (1969) in that the analysis of
individual populations shows little evidence of allometric
growth. The fact that b is generally similar in different
populations shows that data can be pooled and that a test such
as equation 3 will yield valid conclusions. The pooled
relationship between L and W (Fig. 2) is
(6)
W = 8.28 x 1O-«L"«
where n = 5210, r* = 0.99, and/?« 0.0001. The exponent
in this equation is highly suggestive of isometric growth even
though a /-test (r = 10.6; p < 0.001) shows a significant
difference from b — 3. The slope of the geometric regression
(bg - 2.97; Ricker 1973) also indicates allometric growth, but
no technique exists for testing whether the slope of the
geometric regression is different from 3 (Prepas 1984). Our
results indicate that the body form of Esox lucius becomes
slightly more fusiform with increasing size or the specific
gravity decreases slightly.
Sex, Geographic Location, and Climate
The multiple regression analysis indicates that sex, geo
graphic location, and climate (in order of significance of partial
Can. J. Fish. Aquat. Sci. Vol. 45. 1988
Table 1. Date sources on length and weight of Esox lucius. Lt is the latitude (°N), T is the mean annual
air temperature (°Q, and n is the number of fish sampled.
Site
Alimanen River
Baptizing Creek
Caniapiscau Reservoir
ChanyLake
Clear Lake
Desaulnier Reservoir
Detcheverry Lake
Dniester River
Eastmain River
Fort George River
Gilbert Lake
Helene Lake
Country
n
Lt
Finland
Canada
Canada
USSR
USA
Canada
Canada
USSR
Canada
Canada
USA
4
13
2
7
65
53
54
55
43
53
53
46
52
53
43
53
1.4
-0.2
-4.3
-0.4
7.6
-3.2
-3.2
9.9
-1.4
-3.2
8.0
-3.2
44
6.2
4.6
187
6
7
9
12
12
11
Pannriq
20
USA
USSR
USSR
USA
Canada
Canada
Switzerland
USA
Canada
30
4
Reference
Chimist 1956
Solman 1945
Boucher and Roy 1985
Berg 1962
Ridenhour 1957
Boucher and Roy 1985
Boucher and Roy 1985
Berg 1962
Boucher and Roy 1985
Boucher and Roy 1985
Priegel and Krohn 1975
Boucher and Magnin
10*7*7
Houghton Lake
Ill'menLake
Trjicha Trflkc
Lake Erie, E. Harbor
Lake Huron (G. Bay)
Lake Huron (S. Bay)
Lake Neuchatel
Lake of the Woods
LG2 Reservoir
Little Fish Lake
Loch Choin*
Michigan Lakes"
Minnesota Lakes6
Mud Creek
Nottaway River
Nouveau Lake
Opinaca Reservoir
Pikutamaw Lake
Red Rock Lake
Richelieu River
Sflkami i-s»i»
Swedish Lakes
Ventura Marsh
Waskesiu lake
Waswanipi Lake
Windermere
Pana/fa
Scotland
USA
USA
Canada
Canada
Canada
Canada
Canada
Canada
Canada
Canada
Sweden
USA
Canada
Canada
England
58
407
64
62
223
1197
3
9
7
9
40
1164
7
72
13
41
45
46
1
6
7
12
20
6
120
219
6
1285
10
47
49
53
53
56
43
46
54
50
54
52
52
53
45
53
_
43
54
49
54
1 A
1 .*r
10.4
5.8
3.4
8.1
6.3
-3.2
-0.2
8.6
7.2
5.3
0.8
-0.2
-4.3
-1.4
-1.4
-0.2
6.1
-3.2
3.8
7.6
-0.2
-0.2
9.1
ly/7
Carbine 1943
Berg 1962
Brown and Clark 1965
Wainio 1966
Wainio 1966
Chimist 1956
Carlander 1942
Boucher and Roy 1985
Solman 1945
Munro 1957
Beckman 1948
Smith 1941
Schultz 1955
Magnin 1977
Boucher and Roy 1985
Boucher and Roy 1985
Boucher and Roy 1985
Solman 1945
Foumier 1980
Boucher and Roy 1985
Svardson 1964
Ridenhour 1957
Rawson 1932
Magnin etal. 1973
Kipling and Frost 1970
•Length and weight data taken from graphs.
"Latitude and air temperature calculated as the state average of reporting stations.
effects; Table 2) all significantly affect fish condition. After the
significant effect of body length on weight, sex was the variable
having the most significant effect on W. The regression coef
ficient for S indicates that, on average, females weigh 5% more
than the average for males and animals for which sex is not
specified. In the male and unspecified category were 415 males
and 4423 for which sex was not specified. Assuming that the
sex ratio in Esox lucius is 1:1 (Kipling and Frost 1970), our
results suggest that females are about 9% heavier than males.
Analyses presented by Johnson (1966) showed that females were
between 8 and 35% heavier than males and that this percentage
increased with increased body length. Brown and Clark (1965)
found differences of 70-86% between males and females of
similar length in a spawning population. The allometric equa
tion presented by Blueweiss et al. (1978) suggests that clutch
mass in fish in general varies between 14 and 20% of body
weight for animals between 100 g and 6 kg. The discrepancy
between these values and the general regression coefficient for
the partial effect of sex on body mass at length probably stems
from two factors: (1) our analyses were necessarily performed
Can. I Fish. Aquat. Sci., Vol. 45,1988
on data collected at different times of the year and thus contain
females of varying degrees of gravidity and (2) the regression
coefficient S in Table 2 pools the effect of sex on mass for
mature and immature pike. Our analyses, however, provide evi
dence for the generality of superior weights of female pike,
although the large number of fish for which sex was not deter
mined and the variety of seasons represented in the data set
make interpretation of this difference difficult.
Geographic location was the third most important variable.
Table 2 shows that European and Asian pike are 13% heavier
at a given length than North American pike. This may reflect
genetic differences between stocks of Esox lucius or generally
better growth conditions in European and Asian habitats. Our
analyses cannot distinguish between these hypotheses.
Latitude and temperature had a highly significant effect on
fish condition. Table 2 shows that pike condition increases at
higher latitude and decreases with higher temperature. The
effects agree because temperature and latitude are generally,
although not perfectly, negatively correlated. The regression
coefficient for the effect of latitude shows that fish weight at
481
Exponent of length-weight relationship
103
Sakami lot*
PEk
lok*
t
*ll*«r
Ai(nwR*n liv«f
taOmam tlv»r
Fig. 1. Exponents of power function relationships between length and
weight of Esox lucius for the 34 populations showing significant (p <
0.05) correlations. Bars represent the approximate 95% confidence
interval for the estimated exponent.
10000
length increases by about 5% for every increase of 10°. In addi
tion, weight tends to decrease by about 4% for every 10°C
increase in mean annual air temperature. This suggests that the
mass of a male pike of 80 cm would be 3310 g near the southern
limit of its North American distribution (e.g. central Missouri)
and 3970 g near Yellowknife, N.W.T. Seasonal variation,
impossible to quantify here, may yield even greater differences
at certain times of year. Latitude and temperature can thus lead
to up to 20% differences in body weight for fish of equivalent
length when effects of season are ignored. The analysis sug
gests that northern climates represent optimal living conditions
for Esox lucius.
Esox lucius has been labelled a northern species (Scott and
Crossman 1973) and this appelation is supported by our results.
Body condition is highest at high latitudes and low tempera
tures. This variation may be linked to season length and the
physiological need for greater energy storage where summer
seasons are short. The systematic difference in condition
between North American and European/Asian populations sug
gests that North American populations may either be physio
logically less efficient or live in less affluent environments. If
the latter is true, then comparison of living conditions experi
enced by northern pike in Europe, Asia, and North America
may yield clues to successful habitat management strategies for
this important fish species.
Acknowledgments
We gratefully acknowledge the financial support of the Natural Sci
ences and Engineering Research Council of Canada and the Minister
of Education of the Province of Quebec (FCAR). We also thank two
anonymous reviewers for comments on an earlier draft of this manu
script and Duane Shodeen for practical discussions.
References
Beckman, W. C. 1948. The length-weight relationship factor for conversions
between standard and total lengths and coefficients of condition for seven
Michigan fishes. Trans. Am. Fish. Soc. 75: 237-256.
Berg, L. S. 1962. Freshwater fishes ofthe USSR and adjacent countries. Akad.
Nauk. SSR Zool. fast. 4: 1-413.
100
200
300
soo
Total body length
1000
(mm)
Fio. 2. Pooled relationship between length and weight of Esox lucius
for 5210 sets of observations drawn from more than 38 populations.
Table 2. Regression coefficients for the multiple regression model
presented in equation 5 (n = 5210; R1 = 0.993; F = 158,518;
p « 0.00001). Variables are listed in decreasing order of their partial
significance as predictors of body weight (log W, g). L is total body
length (mm), S is sex (female = 1, male or unspecified = 0), Eu is
a dummy variable indicating whether the population is European or
Asian (Eu = 1) or from North America (Eu = 0), T is mean annual
air temperature (°C), and Lt is latitude (°N).
Independent variable
\ogloL
S
Eu
T
Lt
logl0 a (intercept)
482
Regression coefficient
/-significance
2.9712
0.0205
<0.0001
<0.0001
<0.0001
<0.0001
0.0011
0.0524
-0.0016
0.0021
-5.1674
Bluewhss, L., H. Fox. V. Kudzma, D. Nakashima, R. Peters, and S. Sams.
1978. Relationships between body size and some life history parameters.
Oecologia 37: 257-272.
Boucher, R., andE. Maonin. 1977. Bioecologie du grand brocket Esox lucius
du lac Helene, territcrie de la Baie James. Socfcte* d'£nergie Baie James,
Montreal, Que. 83 p.
Boucher, R., and D. Roy. 198S. Roseau de surveillance ecologique du Complexe La Grande 1978-1984. Poissons. Soci&e' d'Energie Baie James,
Montreal, Qu6. 119 p.
Brown, E. H. Jr., and C. E Clark. 196S. Length-weight relationship of
northern pike, Esox lucius, from East Harbor, Ohio. Trans. Am. Fish.
Soc. 94:404-405.
Buck, D. H., and M. L. Hooe. 1986. Comparative growth of northern
largemouth bass and Fl hybrid largemouth bass through three growing
seasons. Trans. Am. Fish. Soc. US: 296-304.
Carbine, W. F. 1943. Egg production of the northern pike Esox lucius L., and
the percentage survival of eggs and young on the spawning grounds.
Pap. Mich. Acad. Sci. 29: 123-138.
Carlander, K. D. 1942. An investigation of Lake of the Woods Minnesota,
with particular reference to the commercial fisheries. Minn. Bur. Fish.
Res. Invest. Rep. No. 42:40 p.
1969. Handbook of freshwater fishery biology. Vol. I. Iowa State
University Press, Ames, IA. 7S2 p.
1977. Handbook of freshwater fishery biology. Vol. n. Iowa State
University Press, Ames, IA. 431 p.
Chewst, P. 1956. Le brochet. Bull. Fr. Piscic. 180: 81-96.
Draper, N. R., and H. Smtth. 1981. Applied regression analysis. Wiley, New
York, NY. 709 p.
Can. J. Fish. Aquat. Sci. Vol. 45, 1988
Fournier, H. 1980. Quelque aspect de la biologie du grand brochet (Esox
lucius L.) dans une portion de la riviere Richelieu comprise entre la frcntiere Canado-americaine et St-Paul de I'lle-aux-Noix. M.Sc. thesis, Universite' du Quebec, Montreal, Que\ 143 p.
Fulton, T. W. 1902. Rate of growth of sea fishes. Annu. Rep. Fish. Board
Scotl. 20: 326^446.
Guiarati, D. 1978. Basic econometrics. McGraw-Hill, New York, NY. 462
P-
Holland, L. E., and M. L. Huston. 1984. Relationship of young-of-the-year
northern pike to aquatic vegetation types in backwaters of the upper Mis
sissippi River. N. Am. J. Fish. Manage. 4: 514-522.
Johnson, L. 1966. Consumption of food by the resident population of pike,
Esox lucius, in Lake Windermere. J. Fish. Res. Board Can. 23: 1523—
1535.
Kipling, C. 1983a. Changes in the growth of pike (Esox lucius) in Windermere.
J. Anim. Ecol. 52:647-657.
1983b. Changes in the population of pike (Esox lucius) in Winderm
ere from 1944 to 1981. J. Anim. Ecol. 52: 989-999.
1984. A study of perch (Perca fluviatilis L.) and pike (Esox lucius
L.) in Windermere from 1941 to 1982. J. Cons. Int. Explor. Mer 41:259267.
Kipling, C, and W. E. Frost. 1970. A study of the mortality, population
numbers, year class strengths, production and food consumption of Pike,
Esox lucius L., in Windermere from 1944 to 1962. J. Anim. Ecol. 39:
115-157.
Koz'min, A. K. 1980. The biology of pike (Esox lucius L.) from Lake Lacha.
J. Ichthyol. 20: 44-48.
Kupchinskaya, E. S. 1985. Pike, Esox lucius, of Ust'Ilimskoe Reservoir. J.
Ichthyol. 25: 133-141.
Lagler, k. F., J. E. Baroach, R. R. Miller, and O. R. M. Passino. 1977.
Ichthyology. Wiley, New York, NY. 506 p.
Le Cren, E. D. 1951. The length-weight relationship and seasonal cycle in
gonad weight and condition in the perch (Percafluviatilis). J. Anim. Ecol.
20: 201-219.
Le Louarn, H., and J.-L. Bagliniere. 1985. Some biological parameters of
the pike (Esox lucius) in a salmonid dominated stream in the Scorff River,
France. Cybium 9: 75-88.
Magnin, E. 1977. Ecologie des eaux douces du territoire de la Baie James.
Socifte' d'Energie Baie James, Montreal, Que". 453 p.
Magnin, E., E. Murawska, and V. Legendre. 1973. Croissance de onze
especes de poissons de la region de Waswanipi. Bui. No. 8, Que\ Min.
Tour. Hunt. Fish. Wild. Serv., Quebec, Que1. 30 p.
McCarraher, D. B. 1957. The natural propagation of northern pike in small
drainage ponds. Prog. Fish-Cult. 19: 185-187.
Can. J. Fish. Aquat. Sci., Vol. 45, 1988
Munro, W. R. 1957. The pike of Loch Choin. Freshwater Salmon Fish. Res.
16: 1-16.
Nie, N. H., C. H. Hull, J. G. Jenkins, K. Steinbrenner, and D. H. Bent.
1975. Statistical package for the social sciences. McGraw-Hill, New York,
NY. 675 p.
Prepas, E. 1984. Some statistical methods for the design of experiments and
analysis of samples, p. 266-335. In J. A. Downing and F. H. Rigler Led.]
A manual on methods for the assessment of secondary productiv ity in fresh
waters. IBP Handb. No. 17. 2nd ed. Blackwell Scientific Publications,
Oxford.
PRIECEL, G. R., and D. C. Krohn. 1975. Characteristics of a northern pike
spawning population. Wise. DNR Tech. Bull. No. 86: 18 p.
Rawson, D. S. 1932. The pike of Waskesiu Lake, Saskatchewan. Trans. Am.
Fish. Soc. 62: 323-330.
Ricker, W. E. 1973. Linear regressions in fishery research. J.Fish. Res. Board
Can. 30:409-434.
1975. Computation and interpretation of biological statistics of fish
populations. Bull. Fish. Res. Board Can. 192: 382 p.
Ridenhour, R. L. 1957. Northern pike, Esox lucius L., populations at Clear
Lake, Iowa. Iowa State J. Sci. 32: 1-18.
Schultz, F. H. 1955. Investigation of the spawning of the northern pike in
Prince Albert National Park, Saskatchewan, 1953. Bull. Wildl. Manage.
3: 1-19.
Scott, W. B., and E. J. Crossman. 1973. Freshwater fishes of Canada. Bull.
Fish. Res. Board Can. 184:966 p.
Smith, L. L. Jr. 1941. The northern pike: its past and future. Cons. Vol. 3:
1-6.
Solman, V. E. F. 1945. The ecological relations of pike Esox lucius, and
waterfowl. Ecology 26: 157-170.
Svardson,G. 1964. The pike. Translated from "Fiske 1964" by Agneta Smith,
Ithaca, NY. 23 p.
U.S. Fish and Wildlife Service. 1977. National survey of hunting, fishing
and wildlife-associated recreation, 1975. U.S. Government Printing
Office, Washington DC. 95 p.
Van Engel, W. A. 1940. The rate of growth of the northern pike, Esox lucius
Linnaeus, in Wisconsin waters. Copeia 1940: 177-188.
Wainio, A. 1966. A study of pike (Esox lucius) in two areas of Lake Huron.
M.Sc. thesis. University of Toronto, Toronto, Ont. 72 p.
Weatherly, A. H.,andS.C. Rogers. 1978. Some aspects of age and growth.
Chap. 3. In S. D. Gerking [ed.] Ecology of freshwater fish production.
Blackwell Scientific Publications, Oxford.
Wernstedt, F. L. 1972. World climatic data. Climatic Data Press, Lemont.
PA. 523 p.
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