J . Fish Biol. (1983) 22, 631-646
The biology of bream, Abramis brama (L), and its natural
hybrid with roach, Rutilus rutilus (L), in the River Exe
I. G. Cowx
Department of Biological Sciences, University of Exeter, England
(Received 9 June 1982, Accepted 24 June 1982)
Hybrids between bream, Ahrumis brama, and roach, Rutilus rutilus. occur in the lower
reaches of the River Exe and were positively identified by their meristic features and shape
of the pharyngeal bone. The growth of bream and hybrids was determined by backcalculation from scales. Annual checks were laid down in early June. The mean length
for age of female bream was significantly larger than that of males for fish 6 years of age
and older. This divergence in growth rate was associated with the sexual maturation of the
fish. Data for the sexes were combined and compared with the growth rate of hybrids and
roach from the same region. Hybrid growth was similar to that ofbream for the first 6 years
of life but was intermediate between that of the two parent species in the older age groups.
Some hybrids with developing gonads were found. A change in the diet of bream from
planktonic feeding to a benthophagic habit with age was noted. Detritus, substrate and
chironomid larvae formed the bulk of the diet of hybrids.
I. INTRODUCTION
The ecology of bream, Abrumis brumu (L.), in British and Irish waters has
received relatively little scientific attention (Hartley, 1947; Leeming, 1967;
Kennedy & Fitzmaurice, 1968; Gee, 1978; Smith, 1978; Coles, 1979; Goldspink,
198 1 ) compared with the voluminous works on other cyprinid species. Goldspink
(1 98 1) attributed this paucity to the species’ restricted geographical distribution,
longevity, schooling habit and sporting value, altogether making it a difficult
species to sample and study effectively.
Similarly, despite their common occurrence in the wild (Regan, 191 1 ; Wheeler,
1969; Maitland, 1972; Burrough, 1981), little attention had been paid to the
ecology of natural hybrids between species of Cyprinidae (Smith, 1978; Cowx,
1980). The majority of work on cyprinid hybrids to date has concentrated almost
entirely on their identification (Wheeler, 1976; Child & Solomon, 1977; Wheeler
& Easton, 1978; Swinney & Coles, 1982) probably because it is an essential
preliminary in such studies and hence a justifiable subject for investigation. However, considering the enormous potential associated with hybridization of animal
stocks (Pirchner, 1969) the ecology of cyprinid hybrids has been neglected.
This paper describes observations made on the biology of bream and its natural
hybrid with roach, Rutilus rutilus (L.), in the lower reaches of the River Exe,
Devon. The River Exe in its lower reaches differs from other British waters in
which bream has been studied in that it supports good coarse and game fisheries
Address for correspondence: Severn-Trent Water Authority, Abelson House, 2297 Coventry Road,
Birmingham, England.
63 I
0 1983 The Fisheries Society of the British Isles
0022-1 I12/83/060631+ 16 $03.00/0
632
I . G . COWX
and represents the most southerly British water studied. The study forms part
of a more detailed investigation into the ecology and management of the coarse
fish populations of the River Exe catchment (Smith, 1978; Cowx, 1980).
11. MATERIALS AND METHODS
Bream were originally stocked into the Exeter Canal in 1913 by Exeter City Council,
with the assistance of a local angling club. The species gradually spread along the canal
and was first recorded in the River Exe in 1929, since when it has become common in
the lower reaches of the river and has hybridized with the endemic roach population.
The River Exe in its lower reaches, in and below Exeter, is slow flowing, wide (60 m)
and deep (3-4m) and has been subjected to extensive flood alleviation works, i.e. it is
typical of the bream zone of lowland rivers. Flood alleviation works and construction of
many weirs has reduced the current speed sufficiently to allow the finer silts to settle and
these, with dead leaves, form the substrate. This muddy bottom supports good plant
growth, predominantly Elodea canadensis (Michx), Myriophyllum alternijlorum (D.C.)
and Potamogeton natans (L.).
A total of 164 bream and 45 roach-bream hybrids were taken from the River Exe during
a monthly sampling programme extending from September 1976 to November 1977. Gillnetting (mesh sizes 17.5-75 mm) was the principal sampling method employed although
a few fish, mainly fry, were taken by electrofishing. Fork length (F.L.-mm) and weight
(g) were measured and scales detached for age and growth studies. The anterior third of
the gut was removed for analysis by the point’s method (Hynes, 1950) and the sex and
maturity state of the fish were noted.
To distinguish between fish types, i.e. bream, hybrids and roach, meristic data (fin ray
and scale counts) were recorded from all bream and suspected hybrids. Pharyngeal bones
were dissected from the fish, cleaned by maceration in hot water, bleached and preserved
dry. In addition, similar data were recorded for 50 roach taken from the River Culm, a
tributary of the Exe. These roach were considered true bred as the presence of bream has
never been recorded in the Culm.
111. RESULTS
IDENTIFICATION OF BREAM, ROACH AND THEIR NATURAL HYBRID
In fish in which the meristic features of the parents are distinct, identification
of the hybrid offspring should present no problems (Wheeler, 1969). Generally,
these progeny exhibit meristic characteristics intermediate between those of the
parent species. Unfortunately recognition of hybrids is not always so straightforward. Wheeler (1969, 1976) suggested that cyprinids are capable of complex
interbreeding between parent and offspring, resulting in a diverse array of
characters continuous between those of the parent species. In such cases positive
identification of the hybrids may be difficult. Wheeler (1 969) suggested the
pharyngeal teeth provide a trenchant character by which the fish can be dis,tinguished. In the then unforeseeable event of complex interbreeding occurring
in the River Exe community, examination of the pharyngeal teeth, in addition
to the meristic characters, was carried out.
Meristic data
Data on four of the meristic characters collected for the three fish types are
shown in Table I; t-test analysis revealed significant differences (P<O.OO 1)
BIOLOGY OF B R E A M A N D H Y B R I D S
633
TABLE
I. Comparison of the meristic characteristics of roach, R . rutilus, bream, A . brama,
and their natural hybrid
Character number
Statistic
Lateral line scales
Range
Scales between dorsal fin
origin and lateral line
Scales between anal fin
origin and lateral line
Anal fin rays
Roach
Bream
(N=50)
(N=164)
4048
5 3-64
Hybrids
(N=45)
h4eanfS.E. 44.41 +O-25 57.13f0-51
48-55
50-50f0.54
7-9
10-15
8.20 k0-22 I 2.70 f0-1 8
4-5
7-10
Mean+s.E. 4.61 f0-14 8.31 +O-47
Range
8-14
23-30
Mean fS.E. I 1-45f0-19 26.42 k0.26
8-1 1
9.30 f0-21
5-7
6-33k0.19
16-2 1
17.95f0.17
Range
Mean fS.E.
Range
between fish types for each of these characters. No other differences between fish
types were detected.
Pharyngeal teeth
The pharyngeal bone (Fig. 1) provides another positive character by which the
three fish types can be distinguished. The characteristic features of the pharyngeal
bones of each fish type are as follows.
(i) Bream: Teeth are hook-shaped with a smooth masticatory platform and
arranged in a single series of five on each bone (i.e. tooth configuration of
5 : 5). The teeth are set high up on the bone which itself has a long ventral
extension, pars ventralis.
(ii) Roach: Teeth are found in a single row of five or occasionally six ( 5 : 5 or
5 : 6) on a bone which is very much stockier in appearance than the bream
pharyngeal bone and by comparison has a small pars ventralis. The roach
teeth are conical in shape but with a smooth, concave grinding surface.
(iii) Hybrids: The bone shape is intermediate between those of the parent types.
The teeth, which are basically roach-like, are laid out in a single series of five
or six each side although on occasions a double row was observed with a single
tooth in the second row (5 : 5 or 6 : 5 or 6 : 5 + 1).
To confirm the different shapes of the pharyngeal bones the relationship
between the length of the bone and the length of the edentated segment was determined (Fig. I). Measurements were made to the nearest 0.01 mm using a
micrometer screw gauge. A preliminary plot of the data showed the relationship
between bone length and edentated length to be curvilinear. Logarithmic transformation of the data (Fig. 2) gave a straight line best described by the following
equations:
Bream: Log y=0.899 log x-0.336,
Hybrid: Log y = 1.063 log x-0.390,
Roach: Logy= 1.072 log x-0.242,
where y is the length of the edentated segment of the bone, and x is the total length
of the pharyngeal bone.
634
1. G . COWX
FIG.I . Pharyngeal bones from: (a) bream A . brama; (b) roach-bream hybrids; (c) roach R. rurilus. Magnification x 3. X = length of pharyngeal bone; Y = length of edentated segment of pharyngeal
bone.
In all cases the correlation coefficient approximated to unity. Analysis of
covariance revealed a significant difference ( P <0.0 1) between the relationships
for each fish type.
Comparison of the meristic features and pharyngeal teeth of bream and roach
in the present study (Table I & Fig. 1) with the characteristic features ofthe species
established by other workers (Horoszewicz, 1960; Wheeler, 1969; Maitland, 1972)
confirms that roach and bream from the lower reaches of the River Exe have been
identified correctly. In addition, the existence of a third discrete fish type was
established. By virtue of its intermediate form between those of roach and bream
it was presumed to be the hybrid progeny of natural interbreeding between the
roach and bream populations of the lower reaches of the River Exe. However,
in the absence of independent evidence derived from controlled breeding (artificially crossing roach with bream in the laboratory and examining the characteristic
features of the progeny) these fish cannot be positively identified as roach-bream
hybrids.
BIOLOGY OF BREAM A N D HYBRIDS
635
I
I
I
I I I I
5 6 7 8 910
1
15
Length of pharyngeal
L
I
1
20 25 30
bone (mm)
FIG.2. Relationship between pharyngeal bone length and the length of the bone occupied by teeth
in roach, R. rutilus, A; bream, A . brama, 0 ;and roach-bream hybrids, x .
AGE AND GROWTH
Age determination
The detailed description of bream scales given by Segerstrfile (1932, 1933), who
distinguished between widely spaced edges of sclerites and the formation of an
annual check, has led to their general acceptance for age and growth studies,
(Hartley, 1947; Steinmetz, 1974; Goldspink 1 9 7 8 ~ .198 1). Similarly, roach
scales have been thoroughly described and generally accepted for use in age and
growth studies (Segerstrile, 1932; Hartley, 1947; Jones, 1953; Wallin, 1957;
Cragg-Hine & Jones, 1969). Unfortunately, no such description of roach-bream
hybrid scales, which have a similar appearance to roach scales, has been made
and their use for age and growth studies has, therefore, not been validated. This,
in addition to the statement of Hellawell (1974), who stressed it was imperative
to establish the validity of using scales for age and growth studies in each new
population investigated, has resulted in the need to verify the use of scales from
River Exe bream and hybrids for age and growth studies.
Verification of the annual nature of check deposition on the scales of River Exe
hybrids and bream, and hence validation of their use for age and growth studies,
was undertaken by observing the seasonal growth pattern on the scales. The mean
number of rings outside the last annulus in all 11-V-year-old fish was determined
for each month of the year (Fig. 3). From the limited data available check for-
636
1. G .
cowx
J JASONDJ FMAMJJASON
Month
FIG. 3. Formation of the annual check in bream and roach-bream hybrids: mean number of rings
outside last check for all 11, 111, IV, and V year old fish. x -Mean and 95% confidence limits;
0 values for individual fish. Pecked line fitted by eye to suggest trend.
mation in both bream and hybrids was found to be an annual event restricted
to a short, well-defined period in June. In view of the annual formation of the
scale check the use of scales for the determination of age of bream and hybrids
in the present study was considered valid. 1 June was considered the birthday
of all fish.
Growth
The growth of bream and hybrids was determined exclusively by backcalculation from scale measurements made under a microscope fitted with an eyepiece graticule. Measurements were made along the caudal axis of both bream
and hybrid scales since measurements along this radius proved to be more closely
related to fish length than other scale radii. The relationships, which take the form
of simple linear regressions, are:
Bream: F.L. =3.99 Sr+ 15-66,r z 0 . 9 4 ,
Hybrid: F.L. =2.99 Sri-33.34, r ~ 0 . 9 6 ,
where F.L. = fork length (mm), Sr = scale radius (arbitrary eyepiece units,
12 epu = 1 mm), r = correlation coefficient.
Back-calculations were made by substituting measurements of scale radii to
respective annuli in the appropriate regression equation. Back-calculated growth
was determined for male and female bream separately but all hybrid fish data were
BIOLOGY O F BREAM A N D H Y B R I D S
637
combined. Insufficient fish of any particular year class were caught to calculate
growth of individual year classes.
The back-calculated mean lengths for age of male and female bream and
hybrids are given in Table 11. Comparison of the growth rates of male and female
bream reveals a difference in the pattern of growth between sexes. Females show
a relatively constant rate of growth throughout life with a slight decrease after their
sixth birthday. By contrast, males exhibit similar growth to females in the younger
age groups but have a pronounced decline in growth rate after their sixth birthday;
t-test analysis reveals no significant difference in the growth rate between sexes
for the first 6 years of life (P>0.05) but thereafter females grow at a significantly
greater rate (P<O*Ol). No previous observation on the variation in growth rate
between sexes has been documented.
The difference in growth rate between sexes is illustrated in Fig. 4. This figure
also compares the growth of River Exe bream with that in other British waters.
Growth of River Exe fish in the first 6 years of life is comparable with that documented for other British populations, but the marked decline in growth rate thereafter was not apparent in the other populations examined. In the subjective
assessment of growth rate by Backiel & Zawisza (1 968), bream which take 12 years
to reach 400 mm in length, as was observed in the River Exe, were considered
to have a medium growth rate.
In Fig. 5 the back-calculated growth curve for River Exe roach-bream hybrids
is illustrated and compared with the composite (i.e. male and female data combined) growth curves for bream and roach (Cowx, 1980) from the lower reaches
of the river. The hybrid growth curve is of interest because up until 7 years of
age hybrid growth matched that of bream (P>0.05) but thereafter bream grew
significantly faster ( P < 0.00 1) than hybrids. Roach, by comparison, exhibited a
slower growth rate throughout life than either bream or hybrids (P<O*OOl). It
therefore appears that in the older age groups the growth of hybrids is intermediate
between the growth curves calculated for the parent species. A similar situation
was found by Smith (1978) amongst the mixed roach bream populations of the
adjacent Exeter Canal.
The annual growth curves for male and female bream and roach-bream hybrids
(sexes combined) were tested for agreement with the von Bertalanffy model using
Ford-Walford plots (Fig. 6 ) as described by Mann (1973). Estimates of ultimate
length (L,) and growth coefficients for anabolism ( E ) and catabolism ( K ) were
calculated and are presented in Table 111. The data for bream from the River Exe
did not satisfy the von Bertalanffy growth model. The points (L, and &+,) for
the younger age groups (both sexes) appear to run ' parallel ' to the line of equality.
Accordingly, separate values of L , based on fish older than V years were calculated and are presented in Table 111. New values for the growth coefficients
obtained from length data pertaining to mature fish gave conflicting results for
the two sexes; the L , value being reduced for males (from 457.2 to 407.6 mm)
but increased for females (from 627.5 to 720.4 mm).
REPRODUCTIVE BIOLOGY
Examination of the gonads of bream from the River Exe showed that mature
fish comprise only a relatively small proportion (approximately I 1% ofthe bream
Bream
(males)
Bream
(females)
Hybrids (sexes
combined)
Species
Length age (mm)
VII
VIII
IX
x
XI
XI1
XI11
XIV
97.2 149.3 194.3 219.3 241.0 167.0 296.3 301.2 317.0 332.8 347.8
VI
57.8
v
1 1 1.3 176-2 228.6 264.3 296-5 313.8 334.2 361.3 394.5 423.5 438.5 464.1 472-8
IV
58.3
111
1076 154.7 193.8 248.8 277.6 283.0 295.0 304.9 314.9 342.8 354.8 382.7
I1
53.8
I
TABLE
11. Growth of male and female bream, A . brama, and hybrids in the lower reaches of the River Exe
639
B I O L O G Y OF B R E A M A N D H Y B R I D S
1
I
1
1
1
1
1
1
1
1
1
1
1
1
IIm~vmmmlxxxIxIIm
Age (years)
FIG.4. Comparison of the growth ofbream in the River Exe with other British waters. (a) Wraysbury
17 (Gee, 1978), (b) Coosan Lough (Kennedy & Fitzmaurice, 1968), (c) River Exe-females,
(d) River Exe-males, (e) Norfolk Broads (Hartley, 1947), (0 River Witham (Coles, 1979), (g)
Cheshire Meres (Goldspink, I98 I).
caught in the River Exe) of the whole population and only superficial observations
were possible. Female bream appear to attain maturity in their sixth year of life
( V + ) whereas males mature a year later. These age groups were the youngest
breeding fish encountered. The late maturation of River Exe bream compares
favourably with other bream populations examined; Neubar (1926), Hartley
( 1947), Kennedy & Fitzmaurice (1 968) and Backiel & Zawisza (1 968) all found
bream did not mature until 5 years of age.
Generally progeny of a cross between two species are infertile, however, all
older roach-bream hybrids of both sexes (> IV years of age) from the Exe were
found to have well developed gonads (Tables IV, V). This was particularly evident
in the females which had large numbers of apparently ripe eggs in their ovaries.
DIET
The general components of the diet of bream and hybrids are given in Table
VI. Also included is a summary of the diet of roach from the lower reaches of
the Exe (Cowx, 1980) for comparison. These data were compiled from fish caught
1. G . COWX
640
I
IImmvvrmmlxxxImm
Age (years)
FIG. 5. Comparison of the growth of bream, roach and roach-bream hybrids in the lower reaches
of the River Exe.
at regular intervals throughout the year to minimize any variation in diet due to
season and thus give a true impression of the diet.
Detritus, debris and substrate constitute the major part of the diet of all bream,
comprising approximately 50% of the diet. A marked change in the faunistic
component of the diet was noted with increase in size. Planktonic crustacea (32%)
was the principal component in the diet ofjuvenile fish (< 120 mm) although chironomid larvae (10%) were also important. Chironomid larvae (30%) and benthic
crustacea (5%) predominated in the diet of older fish. Hartley (1947), Backiel &
Zawisza (1968) and Kennedy & Fitzmaurice (1968) similarly noted a change in
the diet of bream from planktonic feeding to a benthophagic habit with age.
The composition of the diet of River Exe hybrids was found to be restricted
to a few items of which molluscs and diptera larvae were the main faunistic
components. The remainder of the diet was principally detritus and substrate
(72%) and vegetation (10%) suggesting that hybrids have an omnivorous benthophagic habit. When compared with the diet of roach it appears that the diet of
the two are similar except that hybrids have a greater intake of detritus whilst
roach graze more intensively on algae and have a higher intake of Ephemeroptera
nymphs.
One significant omission from the diet of River Exe bream and hybrids was
aquatic Oligochaetae. These are quickly digested and little evidence of their consumption remains in the stomachs. Detection of their chaetae amongst the mass
of detritus was found to be impractical. However, infestation of the fish by the
BIOLOGY OF B R E A M A N D H Y B R I D S
64 1
60C
40C
20c
C
1
1
1
1
1
1
1
1
c
--
60C
t
\r
Q)
-
40C
c
0
-!
LL
f
200
0
-
0,
r
5
LL
0
6 00
400
200
Hybrids all fish
L , = 407 rnrn
I
0
200
I
I
400
I
I
I
600
Fork length (F L ) a t age f (mm)
FIG.6. Ford-Walford plots of male and female bream and roach-bream hybrids from the River Exe.
L , derived by calculation.
cestode Caryophyllaeus laticeps which has Tubificidae as its intermediate host,
provides indirect evidence as to whether fish have been feeding on tubificids
(Kennedy, 1969); 7.8% of the bream and 15.5% of the hybrids examined were
infected with C. luliceps inferring that they must feed on tubificids.
IV. DISCUSSION
The existence of hybridization in the River Exe was not unexpected. Since the
spread of bream into the lower reaches of the River Exe in the late 1920’s their
coexistence with roach within the same habitat and a temporal overlap of spawn-
642
1. G.
cowx
TABLE
111. Ultimate length (mm), L , , and von Bertalanffy growth coefficients for anabolism, E, and catabolism, K, for bream, A . hrama, and
roach-bream hybrids from the River Exe
Species
Bream (based on all age groups)
Males
Females
Bream (based on fish older than V)
Males
Females
Roach-bream hybrids (sexes combined)
L,
E
K
457-2
625.5
63.6
66.1
0.14
407.6
7204
407.0
80.6
494
0.20
0.06
0.15
61.4
0.11
TABLE
IV. Fecundity of female roach-bream hybrids from the River Exe
Length
(mm)
Weight
(8)
Gonad
weight (8)
Age
(years)
242
243
250
254
305-3
288.5
307.7
342.5
12-03
13.60
17.39
12.79
VI
VI
VI
vl+
Fecundity
(eggdfema le)
+
+
+
49 320
53 640
73 040
55 550
TABLEV. Growth in absolute testis weight (g)
with age in roach-bream hybrids from the River
Exe
Age group
11
I11
IV
V
v1
VII
Number of fish
examined (N)
Mean testis
weight (9)
6
4
5
3
1
1
0.79
1.91
2.63
2.68
2.34
15.65
ing season of the two species provide conditions conducive to hybridization. The
high degree of hybridization observed in the River Exe, with some 21% of all
bream and hybrids examined being of hybrid origin, reflects the extent to which
the two parent populations interbreed. Human interference, mainly through
channelization of the river and removal of submerged vegetation, probably
increased the likelihood of interbreeding between the two species by reducing the
number of potential spawning sites.
Wheeler (1 969) suggested that in some cases roach-bream and roach-rudd
(Scardinius rrythrophthalmus) hybrids can be fertile and if back-crossed to a
643
BIOLOGY OF BREAM AND HYBRIDS
TABLE
VI. Comparison of the percentage composition of the diet of bream, A . brama,
roach, R . rutilus, and their natural hybrid in the River Exe
Food items
Oligochaetes
Mollusca
H ydrobidae/Sphaeridae
Crustacea
Planktonic
Asellus aquaticus
Gammarus sp.
Ephemeroptera nymphs
Caenis sp.
Ephemera danica
Trichoptera larvae
Coleoptera larvae
Noterus sp.
Dytiscus sp.
Hemiptera
Diptera larvae
Chironomidae
Others
Aerialherrestrial insecta
Unidentifiable aquatic insecta
Algae
Macroph yta
Detritus, debris, substrate
Number of fish examined (N)
Bream
12mm
Bream
12mm
Roach
Hybrids
0.9
1 .o
1.2
5.1
6.8
31-9
0.4
2.2
2.6
5.9
0.8
0.6
2 .o
2-0
0.1
6.8
0-9
0.2
0-4
0.2
0.4
I .2
0.3
0.4
0.4
10.4
1 *o
0.5
2.4
0.1
0.8
49.2
46
29.8
2.1
0.4
3.2
56.7
42
2.4
3.9
4.8
1-1
15.0
9.3
41.8
534
5.4
0.8
0-6
1.6
10.0
71.6
45
parent type could give rise to populations of complex ancestry. The F, backcrossed derivative should show morphological features intermediate between
those of the hybrid and the parent; in a freely interbreeding community of two
species a continuous range of morphological features between the parent types
would be expected. For example, Burrough (198 1) found a range of characters
in the pharyngeal teeth of the mixed roach-rudd population of Slapton Ley,
Devon, and concluded complex ancestry. No continuous range of characters was
evident in the River Exe roach-bream community strongly suggesting the absence
of introgressive breeding. Whether this absence of introgression in the River Exe
populations was because the hybrids were infertile or because n o F, generation
fish were caught is unknown. Both male and female roach-brcam hybrids with
highly developed gonads were taken from the Exe (Tables IV, V) but there was
no evidence to indicate if they were fertile inter se or with either parent species.
As it is unlikely, with such a high degree of hybridization (2I0/o), that the hybrids
did not mix with fish of the parent species during the spawning period it is probable that hybrid infertility is the reason for the absence of complex interbreeding
in the Exe populations.
The growth pattern of bream in the River Exe is of interest. No previous record
of between sex variation in growth has been recorded for this species. Faster
644
I . G . COWX
growth of female fish has, however, been described frequently in growth studies
on other cyprinid species, particularly roach (Cragg-Hine, 1969; Mann, 1973;
Goldspink, 1978b).
In the River Exe bream population, the variation in growth pattern between
sexes was the result of a rapid decline in growth rate of male fish at approximately
6 years of age which was not paralleled in the female growth curve. The most
likely factor responsible for the decline in growth rate was sexual maturation of
the fish; the inflexion in growth rate occurring about the age at which maturity
was attained. The actual mechanism behind these distinct growth patterns is uncertain but, because of the close association with sexual maturity, it is probably
a strategy for maximizing the reproductive capacity of the species. Backiel &
Zawisza (1 968) stated that bream achieve maturity at a specific length rather than
age. Early fast growth of all fish will ensure the population mature at an early
age and enhance the proportion of sexually mature fish in the population. Once
mature it is advantageous for females to continue growing becaue fecundity
(number of eggs per female) is positively correlated with size of fish (Bagenal,
1967; Backiel & Zawisza, 1968). In males, however, continued fast growth is unnecessary since the gonad products of a single mature fish are sufficient to fertilize
the eggs of several females, irrespective of size. Backiel & Zawisza (1968) suggested the slowing down of the growth rate of Ural river delta and Sea of Azov
bream (both sexes) after the first 3-4 years was, similarly, the result of sexual
maturation.
It is clear that the poor fit of the bream growth data to the von Bertalanffjl
growth model, as expressed in the Ford Walford plots, is a function of the diphasic
growth pattern associated with maturation. Under these conditions the values of
L , calculated appear to be related to the change in growth rate after maturation
rather than to high population density and/or low food availability as suggested
by Beverton & Holt (1957). Examination of the data presented by Zawisza (1 961)
and Backiel & Zawisza (1968) on growth of beam in Polish waters revealed a similar pattern where the lower points relating to early growth run parallel to the line
of equality. This pattern of fit to Ford Walford plots is not restricted to bream
growth data. Growth of River Stour chub (Mann, 1976) also exhibit a similar
pattern but no values of the von Bertalanfi growth coefficients for the older fish
were determined. Hickley & Bailey (1982) also identified similar growth in River
Eden chub and derived separate values for older fish. In view of these findings
it is suggested that Ford Walford plots are an inadequate method of expressing
fish growth, particularly in long-lived species such as bream and chub, unless consideration for the growth pattern of the fish is made.
In many animal populations the progeny of interbreeding exhibit superior
characteristics when compared to the parental lines, a phenomenon termed
heterosis or hybrid vigour. The cause of heterosis is thought to be a complementation of favourable genes from each parent stock. When these favourable genes
are dominant the performance of the hybrid will be superior to that of both parent
lines. The growth of roach-bream hybrids from the River Exe was intermediate
between those of both parent species suggesting absence of any heterotic effect
relating to growth in the hybrid form. Heterosis has, however, been known to
occur in fish hybrids; Svardson (1949) attributed the rapid growth oftrout, Salrno
trutta L., and char, Salvelinus alpinus L., hybrids in Norway to the phenomenon.
BIOLOGY OF BREAM A N D H Y B R I D S
645
I wish to thank Dr C. R. Kennedy for his supervision of this work and for critically
reviewing the manuscript. I am indebted to South-West Water Authority, particularly Dr
E. R. Merry, for their unending co-operation, and to Mr P. Shears and Mr A. Shephard
for their continued assistance with the field work. The work was undertaken during the
tenure of a NERC Studentship in conjunction with South-West Water Authority. The
views expressed are those of the author and not necessarily of the Authority.
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