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The biology of the catfish Cnidoglanis macrocephalus (Plotosidae) in an Australian estuary
Estuarine,
Coastal
and Shelf
Science
(1985) 21,895-909
The Biology of the catfish Cnidoglunis macrocephalus (Plotosidae) in an Australian Estuary
S. A. Nel,
I. C. Potter
School of Environmental Western Australia 6150, Received
22 June
1984
and N. R. Loneragan
and Life Australia
Sciences,
and in revised
form
Murdoch 21 January
Keywords: teleosts; catfish; estuaries; growth fisheries; Australia West Coast
University,
Murdoch,
1985
rates; gonads; diet; estuarine
This paper describes the age structure, growth, diet and aspects of gonadal development in the cobbler, Cnidoglanis macrocephalus (Valenciennes), in the large Swan estuary in south-western Australia between August 1982 and June 1984. Analysis of otolith annuli showed that while the 0+ to 3+ age classes were regularly represented in monthly samples, the 4+ and more particularly the 5 + and 6 + were much less abundant. The weighted means for the back calculated lengths at the end of the first to fourth years of life were 181 mm (=26g), 314mm (=156g), 418mm (E410g) and 518mm (=833g) respectively. The mean length at the end of the second year of life was similar to the minimum legal size for capture by commercial fishermen (320mm). The von Bertlanffy growth curve calculated from the back calculated lengths was L,=917 [l ~e-o-2011+o~11) 1. The relative weight of mulluscs, crustaceans and polychaetes in the intestine varied markedly between small and large fish, apparently reflecting differences in the size of these prey. The large mean diameter of mature eggs (X = 7.4 mm) was correlated with a low mean absolute fecundity (2078). Trends shown by egg size, gonadosomatic index and time of appearance of spent females indicate that spawning takes place between October and December. The attainment of sexual maturity is both age- and size-dependent. Although sexually maturing and occasionally spent fish were found in the lower estuary, meristic values, commercial catch statistics and other data indicate that the cobbler found in the Swan estuary are part of a population which typically spawns at sea.
Introduction
The eel-tailed catfishes (Plotosidae) are restricted to marine, brackish and fresh waters of the Indo-west Pacific region from Japan to Australia (Sterba, 1966; Lindberg, 1974; Nelson, 1976). The cobbler, Crzidoglunis macrocephalus, a plotosid endemic to Australia, is found southwards from the Abrolhos Islands (28”S, 114”E) and eastwards as far as Kingston (37”S, 140”E) (Kowarsky, 1976, Figure 1). This speciesalso occurs on the east coast of Australia from Kirra (28”S, 153”E) southwards to Jervis Bay (35”S, 151”E). Cnidoglanis macrocephalus is found in both marine and estuarine environments in south-western Australia where it is sometimes present in sufficiently large numbers to 895 0272-7714/85/120895+
15 $03.00/O
0 1985 Academic Press Inc. (London) Limited
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c
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IIS’SO 1
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115~55 /
Figure 1. Map of the Swan estuary showing the location of funnel trap sites (TN l-8). Lower and upper estuary and the distribution of Cnidoglanis macrocephalus in Australia’ (taken from Kowarsky, 1976) are stippled. Black rectangle on map of Australia shows region illustrated in the inset.
make a major contribution to local commercial fisheries (Kowarsky, 1975; Lenanton, 1982, 1984; Lenanton et al., 1982, 1984; Potter et al., 1983). While information on the biology of the cobbler is presented in the Ph.D. thesis of Kowarsky (1975), the details of growth in his study were not based on fish which had been aged, and to a large extent the same generalization applies to the data given in Lenanton et al. (1984). On the basis of comparisons between such features as fin ray counts, parasite faunas and patterns of distribution within systems and regions, Kowarsky (1975) concluded that the estuarine and marine populations of C. macrocephalus in south-western Australia might not intermix. The current study of C. macrocephalus has analysed otolith annuli to provide data on the annual and seasonalpatterns of growth, the number of year classespresent and the age at which this species enters the commercial fishery in the large Swan estuary in south-western Australia. A study has also been made of the food of the cobbler to determine whether the diet changed with body size and how it related to those of other commercial fish in the system. The seasonalpatterns of distribution, gonadal development and commercial catches were analysed to determine the breeding period and attempt to ascertain whether or not spawning typically occurred within or outside the estuary. Materials
and methods
Cobbler were collected from the Swan estuary (Figure 1) using funnel traps laid overnight both by ourselves and by a commercial fisherman in the period between August 1982 and June 1984. While our trapping programme employed eight sites (TN l-8) in the lower, middle and upper estuary, that of the commercial fisherman was restricted to a variety of sites in Melville Water in the middle estuary (Figure 1). Our sampling programme involved the use of up to eight traps throughout the estuary on at least nine nights in each month between August 1982 and January 1984. Less frequent trapping
The biology of Cnidoglanis macrocephalus
897
was employed between February and June 1984. The traps, which were made of 13 x 25 mm galvanised wire mesh, measured 0.9 m in length and 0.4 m in width. Wire with a 2 mm mesh was placed over the trap between December 1982 and March 1983 in an attempt to catch small cobbler. The total length and weight of fish were recorded to the nearest 1 mm and 100 mg. Gonads were weighed to the nearest 10 mg, except in the case of those small fish where it was not possible to ascertain macroscopically whether the gonad was an ovary or testis. Gonadosomatic indices (GSIs) were calculated from the equation WI/W, x 100, where W, = weight of the gonad and W, =weight of the fish (seeDe Vlaming et al., 1982, for a discussion of the use of GSIs). The stage of gonadal development was recorded using the seven stages described by Laevatsu (1965). At least five ovaries representing each of maturity stages 2-7, except stage 6 (ripe or spawning) which was found on only two occasions, were placed in Gilson’s fluid for 24 h and then stored in 70’?,, alcohol (Simpson, 1951). The mean diameter of eggswas then recorded to the nearest 0.05 mm. Estimates of the absolute fecundity (Bagenal, 1978) of fish with ovaries at maturity stages5 and 6 were calculated from the number of large eggs in three separate ovarian subsamplesof known weight and the total weight of the ovary. Otoliths were removed from 368 females, 295 males and 183 small individuals whose sex could not be determined macroscopically. Since careful examination of the rings on the otoliths demonstrated that these were more clearly defined in the small, flattened asteriscaethan in the large lapillae or elongated, spiral sagittae, the asteriscaewere used exclusively for ageing in this study. The otoliths were mounted in DPX and examined under a dissecting microscope using reflected light and a dark background. Distinct narrow and dark but transparent rings (hyaline zones) could be clearly observed between wider rings (opaque zones). All otolith measurementswere made to the nearest 0.05 mm. Care was taken to ensure that measurementsfrom the centre of each asteriscus to its periphery and to the outer edge of each hyaline zone was always taken along the sameaxis. The distance between the outer edge of the outermost hyaline zone and the periphery of the asteriscuswas then calculated. This marginal increment was then expressed as a percentage of the distance between the two outermost hyaline zones, or in the caseof fish with only one ring, as the distance between the centre of the asteriscusand that ring. The otolith measurementsfor each fish were used to back calculate its length at age 1, 2, 3 etc. (see Ricker, 1975; Bagenal & Tesch, 1978). The back calculated lengths were then fitted to the von Bertalanffy equation by a non-linear technique (see,e.g. Gallucci & Quinn, 1979) using the NONLINEAR routine of SPSS (Robinson, 1984). The equation is L,:=L,(l-ek’r-Lg)),where L, is length at age t (years), L, is the mean length of the asymptote predicted by the equation, K is the growth coefficient and to is the hypothetical age at which fish would have zero length if their growth had followed that described in the equation. Since cobbler do not have a well-defined stomach, the analysis of their diet was based on the contents of the first third of the intestine. The food was removed, blotted dry with absorbent paper and weighed. The individual prey items were then identified and weighed. Since cobbler tend to crush molluscs into small pieces, the numbers of the different prey items could not be determined. However, the number of intestines which contained each prey item was noted. Data on both the mean percentage weight and the percentage frequency of items in the intestine have been recorded separately for 0 + and older fish.
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1983
Figure 2. The mean percentage marginal growth increment (+ on otoliths. This value represents the distance outside the last percentage of the distance between the two outermost hyaline hyaline ring is present the distance between that check and the
1984
95% confidence limits) annulus expressed as a rings or when only one centre of the asteriscus.
Counts were alsomade of the number of rays in the fused dorsal, caudal and anal fin in 50 and 35 fish with a wide range in body size taken, respectively, from the Swan estuary and a local inshore coastal area &km to the north of the mouth of the estuary. Data on the commercial catchesof cobbler in the Swan estuary were obtained from the catch summaries provided by the Australian Bureau of Statistics and the Western Australian Department of Fisheries files. The legal minimum length for the capture of cobbler is 320 mm. Results Distribution
of catches
Analysis of variance showed that the mean catch per trap net differed among regions of the estuary (P< 0.001). The mean catches were consistently greater in the middle than the lower estuary, with catches per trap in the four seasonsranging from 3.3 to 6.0 in the middle region and from 0.8 to 2.5 in the lower region. While a few fish were caught in the upper estuary, none of these were taken in the winter and spring and only one exceeded 385 mm. These data are consistent with those obtained from catches taken during an extensive survey of the fish fauna of the Swan estuary between 1977and 1982, using gill nets at night and beach seinesand otter trawls during the day. Age determination
The round asteriscaeof C. macrocephalus, which ranged in diameter from 0.55 mm in a 70-mm fish to 2.7Omm in a 683~mm fish, showed well marked hyaline zones. In most cases,these circuli became detectable in November, a feature illustrated by the fact that the mean marginal growth increment fell sharply to a minimum value in this month in both 1982and 1983 (Figure 2). The presenceof only one marked trough in the trends for the mean values for the marginal growth increment in both years confirm that only one
The biology
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TABLE 1. Back-calculated mean total lengths aged from otoliths. F represents the weighted
Estimated age at capture If 2+ 3+ 4+ 5+
Number of fish
Mean length at capture (k9593 CL)
352 153 79 19 9
298.6 (6.8) 398.3(11.6) 477.1 (16.7) 588.3 (15.6) 617.9(17.5) n x 950”
Mean
CL
at each armulus for 6 12 C. macrocephalus mean length at ages l-5
back-calculated
lengths
at successive
annuli
2
3
4
5
180.7 181.9 181.2 1845 172.9
313.8 312.9 327.8 313.1
414.1 433.4 421.3
519.4 513.7
580.6
612 181.1 2.63
260 314.5 4.37
107 418.1 9.22
28 517.5 17.95
9 580.6 41.38
181.1
132.7
104.3
99.4
63. I
I
Annual increment
899
major circulus is laid down annually. Since November is also believed to be the middle of the spawning period (see later), fish caught in this month with asteriscae having for example one, two and three annual circuli (annuli) can be considered as having just become one, two and three years old. The relative contribution of age classesl-5 to the samplescollected during this study are shown in Table 1. Two 6 + fish were also caught. Length-frequency
histograms
andgrowth
curves
The length-frequency histograms based on fish of known age caught between August 1982 and June 1984 show that, except in the caseof the 0+ age classin some months, the sizes of the individuals representing the different age classesoverlap considerably (Figure 3). The smallest fish caught during the study, measuring 70 mm and weighing 1.8 g, was collected in February 1983. This fish and all others less than 120 mm were taken in the lower estuary between February and May of either 1983or 1984. The largest fish, measuring 683 mm (= 1880 g), was collected from Melville Water in the middle estuary in November 1982. The trends shown by the mean lengths of the group hatched in late 1982 showed that their growth in 1983 was rapid in the summer and autumn, slowed during the winter and early spring and started increasing again in the late spring (Figure 4). These trends essentially parallel those exhibited by the 1981 age class in 1982-1983, except that growth started increasing rather earlier in the spring of 1982. The mean lengths of the 1981 and 1982 year classesat the commencement of their second year of life were 207 mm ( E 40 g) and 234 mm ( = 60 g) respectively. The sequential mean lengths for fish at the commencement of their third and fourth years of life in November 1982 were 337 and 420 mm, corresponding to weights of 201 and 416 g. Growth varied considerably between years. For example, the length of 1 + fish was approximately 100 mm greater in June 1984 than in June 1983. Apart from three fish from the 1976 year class, all other individuals caught during this study came from the 1977-1983 year classes(Figure 4). Von Berta1an.y
growth
equation
The following equations demonstrate that the length of the fish (L) is correlated with the radius of the asteriscus (Z?) in both the males and females of Cnidoglanis macro-
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YEAR
CLASSES
1980
1981
1979
n
1982
1983
1982
1983 N= 19
JAN N= 50 25
1984 FE8 N= 82
FE8 N=70
0 25
MAR/APR
0 I 0
I 100
8 200
I 300
8 400
I 500
I 800
LENGTH (mm) Figure 3. Length-frequency histograms been determined from otohth annuli.
1 700
0 0
a. 100
8 200
0 300
LENGTH for Cnidoglanis
macrocephalus
0 400
a 500
1 500
1 700
(mm) whose
age had
and for all individuals including those young fish that could not be sexed macroscopically: cephalus
Males Females All fish
L=549.18 R-72.87 (n=295, r=O.gl,P
Since analysis of covariance showed that there was no significant difference (P > 0.05) between the above body length-otolith radius regression equations for males and females, the third equation basedon all fish has been used subsequently. The line relating body length and otolith radius for all fish was linear but did not pass through the origin. It is therefore justifiable (seeBagenal & Tesch, 1978) to usethe above
The biology
--~
of Cnidoglanis
901
macrocephalus
700
600
500
z
4oc
f P 3
300
200
I00
0 1983
1982
Figure 4. The 1983 spawning
mean lengths seasons.
I984
(+ 1 standard
error)
for fish resulting
from
the 1976 to
Age (years)
Figure 5. The growth cure for Cnidoglanis macrocephalus derived from the von Bertalanfiy equation shown on the figure, together with the points representing the back-calculated weighted mean lengths (0) and 95% confidence limits at each annulus. The number of values contributing to each weighted mean is recorded.
regression equation and the mean distance from the centre of the otolith to each annulus to back calculate the body lengths at age 1, 2, 3, 4 and 5. The von Bertalanffy growth curve
derived
from
individual
95”b confidence limits for
L
lengths
was
L,=
917
(1-e-“~20ctto~11))
(Figure
5).
co,k and r0 were + 146, +0.047 and + 0.10 respectively.
The
The
902
S. A. Nel et al.
I.
I
0
*
2
I
b
I
4,
Egg diameter
a
I
6
I
8
I
I
IO
(mm)
Figure 6. Frequency histograms for the diameter of ‘eggs’ in maturity Cnidoglunis macrocephalus. Data are provided on the number of ovaries the time when the various maturity stages were found in samples.
mean back calculated lengths at each a~ulus growth curve (Figure 5).
stages 2-7 of examined and
(Table 1) lay close to the von Bertalanffy
Gonadal development, fecundity and egg size The histograms for the egg diameter of the developing virgin and spent fish (stage 2) showed two distinct size groups producing modal classes at O-O.4 and 1.5-1.9mm (Figure 6). A group corresponding to the first of these categories, which histological sections showed were primary oocytes, was present in all subsequent stages. While the modal class of the group of larger oocytes increased to 2.0-2.4 mm in stage 3, the distribution subsequently became trimodal with the appearance of groups which in stages 4-7 produced modal classes of between 1.0-l .4 and 2.0-2.4 mm. The modal size of the group of largest oocytes rose to 35-3.9 in stage 4 and 7.0-7.4 mm in stage 5. The range of oocyte diameters in the only two running-ripe individuals (stage 6) caught during the study were virtually identical to those in the previous stage. A few residual large eggs were found in spent animals. The largest egg diameter recorded in this study was 9.5 mm. The relationship between the stage of gonadal development and the age and length of individuals caught between August and November was examined to determine the probable age and size at first maturity. From the data in Figure 6, and from comparable information on the development of the testis, it was assumed that all fish which were going to reach maturity in the immediately ensuing months would have attained at least stage 3 by August. While maturity indices of 3 or greater were found in all 2 + and older fish examined in this period, this was only otherwise found to be the case with late 1 + males and females which were greater than 385 and 405 mm respectively. The proportion of late 1 + males and females that fell in this category were, however, only 22.5 and 14.0% respectively.
The biology
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903
Spent femoles Small females 4 . , . . . . . . . ,. . . . .i.. . . . . . . . ., . . . ., . . . . . . . . . . . . . . . . . . . . . . . IO 36 30 22 31 I 24 2 9 12 33 20
JASONOJFMAMJ
._
Figure 7. Mean gonadosomatic index ( f 1 standard error) for small (0 + and small 1 + ) and large (large 1 + and 3 2 + ) Cnidoglanis macrocephalus. Large spent females are recorded separately.
In view of the finding that sexual maturation is related both to age and size, the mean GSIs have been calculated separately, first for 0 + and 1 + males and femaleswhich were
less than 385 and 405 mm respectively, and secondly for larger 1+and all 2-t- and older fish (Figure 7). Since there was no evidence that the GSIs followed different trends in 1982 and 1983, and asonly a small number of larger fish were caught in some months, the GSI data for these two years have been pooled. The mean GSI for larger females rose from 5.8 in July to 10.1 in September and then increased sharply to 24.7 in November. Although the means of ‘ unspent ’ fish declined in the following two months, they were still greater than 11.5. The recently spent fish had a mean GSI of less than 2.5 when they first appeared in November. These values declined in subsequent months. While the trends shown by the ovaries of the younger and smaller females were similar to that just described, the maximum mean GSI, which was also recorded in October, was only 0.45. In contrast to the situation with the larger and older females, the maximum mean GSI for males was very low (0.37) and was recorded in December rather than in October (Figure 7). The mean GSI for the group of 0 + and smaller I + male fish never exceeded 0.1. The presence of the highest mean GSIs in October in females and in December in males (Figure 7) indicates that spawning occurs in the period between the middle of spring and early summer. Such a view is consistent with the high GSIs found in some females in November and December and with the observation that spent females first appeared in November (Figure 7). It is also consistent with the observation that mature
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(stage 5) and ripe (stage 6) ovaries were only found between October and December (Figure 6). The mean absolute fecundity ( f 1 s.e.) of 44 females whose ovaries corresponded to either stage 5 or 6 was 2078 + 109.5. The relationship between fecundity (F) and the body weight (W) and body length (L) of these fish, whose weights and lengths ranged from 324 to 1880 g and from 380 to 683 mm, can be described by the following equations: log F= 0.3372 + 0.9627 log W (n = 44, r = 0.87, P < O.OOl), log F = - 3.6992 + 2.5404 log L (n = 44, r = 0.80, P< 0.001). Diet
The diet of cobbler consisted almost exclusively of molluscs, crustaceans and polychaetes (Figure 8). Marked differences were found, however, between the percentage weight of various items in the intestinal contents of small and large fish. Thus, the contribution made by the mollusc Sanguinoluriu biradiutu to the food of fish greater than one year old (28.3%) was much higher than in fish in the first year of life (2.50/b). The reverse situation was found with the crustaceans Corophium minor, Melitu mutildu and Purutunuis sp., with the percentage contribution to the weight of the food being between 3.0 and 10.5 times greater in the case of the young fish. While the type of differences found with percentage weight in the caseof S. birudiutu were paralleled by the data on the frequency with which they occurred in the intestine, this was not the casewith the crustaceans. Thus, in contrast to the situation with percentage weight, C. minor was present in a slightly greater number of the intestines of the older than younger fish and M. mutildu occurred in the same percentage number of intestines of the two age categories. The polychaete Cerutonereis erythrueensis was found more frequently than any other speciesin the intestine of older fish and it alsomade an important contribution to the weight of the food of both young and older fish. Fin ray counts
The mean number of rays in the fused dorsal, caudal and anal fin (+ 95% confidence limits) in 50 cobbler from the Swan estuary (222.4 + 3.90) did not differ significantly (P> 0.05) from that obtained for 35 cobbler from a nearby inshore marine environment (223.8 f 5.01). Seasonal
trends in commercial
catches
Previous work has shown that the weight of catch per boat for fish in the Swan-Avon and Peel-Harvey system is a good reflection of the catch per unit effort or CPUE (Lenanton et al., 1984). The CPUE for cobbler in the Swan estuary, which are taken by gill nets and traps, show a marked seasonalvariation, with the greatest values occurring in the mid-autumn to early spring months of April to September (Figure 9). The CPUE, which parallels closely the trends shown by total catch, falls markedly during the late spring and summer (November to February). In contrast to the marked seasonalvariation in catches, the mean monthly catch for each of the years between 1972 and 1983 did not differ markedly. Discussion This paper provides the first data on the age and growth of either marine or estuarine representatives of the family Plotosidae based on a large number of fish of known age.
The biology
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n
Mollusco
Crustoceo
Figure 8. Histograms showing the mean percentage weight of various intestine of Ctkfoglunis macrocephalus and the percentage number which those items were found.
Polyhoeto
Algo
food items in the of intestines in
Comparisons can be made, however, with data on the freshwater plotosid, Tandanus in eastern Australia where ageing used annuli in the dorsal spinesrather than in otoliths (Davis, 1977a). Our data show that up to seven age classesof the cobbler, Cnidoglanis macrocephalus, were present in samples taken from the Swan estuary between mid-1982 and mid-1984. While this closely parallels the situation in Tandanus randanus, the lengths of these two catfish at the sameage differ quite markedly. Thus, the back caIculated mean lengths of 181 mm and 518 mm after one and four years of life in C. macrocephalus in November 1983 are considerably greater than the comparable values of randanus,
906
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846
85Oc 800 6OOI
I I
588 1
ASASASASASASASASASASASA
1972 1973 1974 1975 1976 1977 1973 1979 1980 1981 1982 1983 Figure 9. The mean catch per boat of Cnidoglunis inacrocephalus for each month between January 1972 and July 1983. Autumn (A) and spring (S) months are denoted by black rectangles. The dotted line joins the points representing the mean monthly catch for each year. 0 0 0, annual mean monthly catch per boat; O-0, monthly catch per boat.
only 92 mm and approximately 420 mm for T. tandanus. The maximum length of 683 mm and the L, in the von Bertanlanffy equation of 917 mm for C. mucrocephalus are also greater than the L,,, of 574 mm and L, of 613 recorded for T. tandanus. Although C. macrocephalus attains a greater size than T. tandunus, its fecundity (533-3551) is less than that recorded by Davis (19776) for the latter species (2000-20600). This feature can be related to the greater mean diameter of the mature eggs in the cobbler (4.3-9.2) than in the freshwater catfish (2.3-3.1 mm) (Davis, 1977c). The GSIs for males show that the small number of large eggs in the cobbler is paralleled by the presence of a relatively very small testis. The presence of high mean GSIs for females of C. macrocephalus in October, November and December indicate that the spawning period is relatively protracted. The evidence for peak spawning in late spring or early summer is consistent with the conclusions drawn from GSIs for cobbler caught in the early 1970s by Kowarsky (1975). Since the back calculated mean length of cobbler at age 2 was 314 mm and the legal minimum length for capture is 320 mm, this species on average started entering the fishery at the end of the second year of life. From our data, which showed that the attainment of sexual maturity was related both to age and size, it is evident that only the largest of these late 1 + fish would have been approaching sexual maturity. The analysis of intestinal contents demonstrates that, like the freshwater catfish T. tundunus (Davis, 1977d), the cobbler is a carnivore whose diet changes with increasing size. The relatively greater amounts of Corophium minor, Melita matilda and Purutunuis sp. in the intestine of small rather than large cobbler presumably reflects the relatively small size of these crustaceans. Conversely, the presence of relatively greater quantities of Sanguinoluria birudiata in larger fish can probably be attributed to the larger size of this mollusc. Small cobbler (90-126 mm) in a local marine environment, were also found to feed predominantly on amphipods, with one species (Allorchestes compressa), which
The biology of Cnidoglanismacrocephalus
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was not found in Swan fish, occurring in all fish examined and contributing 75.5% to the mean percentage volume (Lenanton et al., 1982). Despite our capture of both mature and spent females in the lower estuary in November and December, there must still be somedoubt as to whether the majority of the large cobbler caught earlier in the year would have spawned in the estuary. For example, the numbers of both mature and spent fish in these and the two following months were low, a feature consistent with the concomitant marked decline that occurs in the commercial catchesof large cobbler which always takes place at this time. Another feature which suggests that spawning typically occurs outside rather than inside the estuary was our capture of only two cobbler lessthan 98 mm in length during the current study. A relative paucity of small cobbler also characterized Kowarsky’s (1975) samples from the estuary and those taken by ourselves using beach seines,prawn nets and otter trawls on many occasionsbetween 1977 and 1984. By contrast, cobbler ranging from 40 to 70 mm were abundant in samplestaken from a local inshore marine environment in January 1977 and February 1984 (Lenanton et al., 1982, personal communication). It is also worth noting that the mean number of rays in the fused dorsal, caudal and anal fin of fish taken in 1984 from the Swan estuary and a local marine environment did not differ significantly. This contrasts with the situation recorded by Kowarsky (1975), a feature taken by that author as indicating the presence of distinct marine and estuarine populations. The above data suggest that while the C. macrocephalus found in the Swan may sometimesbreed in the estuary, they typically breed at sea.However, there can be little doubt that the cobbler has become particularly well adapted to colonizing the large estuarine system of the Swan for protracted periods. It is thus probably relevant that conditions in the Swan estuary tend to be much more stable than is frequently the casein estuaries in many other temperate regions of the world. This can be attributed in part to a very small maximum tidal height (< 80 cm in the lower estuary) and to the restricted periods of high freshwater discharge (Spencer, 1956; Hodgkin, 1974; Prince et al., 1982). These result in small tidal water movements and the maintenance of relatively high salinities throughout much of the year. Moreover, even during periods of high freshwater discharge in the winter, the wide bays of the middle estuary (Figure 1) help dissipate any flushing effects and the marked haloclines which develop at these times provide regions in the deeper waters where fish with a preference for high salinities can find refuge. The relatively stable conditions in the Swan estuary, and indeed other large southwestern Australian estuarine systems such as the Peel-Harvey, are also utilized extensively by the yellow-eye mullet, Aldrichettu forsteri, and sea mullet, Mugil cephulus (Chubb et al., 1981; Potter et al., 1983; Lenanton et al., 1984). Both these speciesreach the legal minimum length for capture after spending several months in the Swan and Peel-Harvey systemsand, together with the cobbler and Perth herring, form the basisof the commercial fishery in these systems (Lenanton, 1984). Although these four species occur in larger numbers and have a relatively large body length in the Swan estuary, the potential competition for food is to some extent avoided by resource partitioning. For example, while the cobbler is a carnivore, the yellow-eye mullet is an omnivore and the Perth herring and seamullet are detritivores (Thomson, 1954,1957). The yellow-eye mullet has occasionally been caught in a running-ripe condition in the lower Swan estuary (Chubb et al., 1981), aswas the casewith cobbler on two occasionsin this study. The protracted period during which these two speciesand the seamullet and Perth herring remain in the Swan and Peel-Harvey estuarine systemscontrasts with the
908
S. A. NeZ et al.
situation found with most marine fish which utilize large temperate estuaries in the northern hemisphere such as that of the River Severn. Although representatives of older year classes of many marine species are present in this latter estuary, the populations of the more abundant teleosts tend to be dominated to a greater extent by the 0+ year class and are markedly seasonal in occurrence (Hardisty & Huggins, 1975; Claridge & Gardner, 1978; Claridge & Potter, 1983, 1984). It may thus be significant that in the Severn, the tidal effect is very large and there are no wide basins with relatively protected small embayments (Department of Energy, United Kingdom, 1981). In summary, it is proposed that the cobbler usually breeds in coastal waters and that while the resultant young may sometimes remain for the rest of their life cycle in marine environments, they frequently utilize for extended periods the excellent conditions provided by the Swan estuary.
Acknowledgements Our gratitude is expressed to those colleagues and friends who helped with the sampling and to R. C. J. Lenanton and G. Fooks who kindly provided us with cobbler. R. C. J. Lenanton and J. Kowarsky provided constructive criticism, N. G. Hall gave statistical advice and R. J. G. Manning drew the figures. Financial support was provided by the Western Australian Department of Fisheries. References Bagenal, T. B. 1978 Aspects of fish fecundity. In Ecology ofFreshwater Fish Production (Gerking, S. B., ed.). Blackwell Scientific Publications, Oxford. pp. 75-101. Bagenal, T. B. & Tesch, F. W. 1978 Age and growth. In Methods for Assessment of Fish Production in Fresh Waters (Bagenal, T. B., ed.), 3rd ed. Blackwell Scientific Publications, Oxford. pp. 3-19. Chubb, C. F., Potter, I. C., Grant, C. J., Lenanton, R. C. J. & Wallace, J. 1981 The age structure, growth rates and movements of sea mullet, MugiZ cephalus L., and yellow-eye mullet, Aldrichetta jorsteri (Valenciennes), in the Swan-Avon river system. Australian~ournal of Marine and Freshwater Research, 32,605-628. Claridge, P. N. & Gardner, D. C. 1978 Growth and movements of the twaite shad, Alosa jallax (Lacepede), in the Sevem Estuary. Journal of Fish Biology, 12,203-211. Claridge, P. N. & Potter, I. C. 1983 Movements, abundance, age composition and growth of bass, Dicentrachus labrax (L), in the Sevem Estuary and inner Bristol Channel. Journal of the Marine Biological Association of the United Kingdom, 63,228233. Claridge, P. N. & Potter, I. C. 1984 Abundance, movements and size of gadoids (Teleostei) in the Sevem Estuary. 3ournal of the Marine Biological Association of the United Kingdom 64,771-790. Davis, T. L. 0. 1977a Age determination and growth of the freshwater catfish, Tandanus tar&anus, in the Gwydir River, Australia. Australian Journal of Marine and Freshwater Research, 28,119-137. Davis, T. L. 0. 19776 Reproductive biology of the freshwater catfish, Tandanus tandanus Mitchell, in the Gwydir River, Australia. I. Structure of the gonads. Australian Journal of Marine and Freshwater Research, 28,139-158. Davis, T. L. 0. 1977c Reproductive biology of the freshwater catfish, Tandanus tandanus Mitchell, in the Gwydir River, Australia. II. Gonadal cycles and fecundity. Australian Journal of Marine and Freshwater Research, 28,159-169. Davis, T. L. 0. 1977d Food habits of the freshwater catfish, Tandanus tandanus Mitchell, in the Gwydir River, Australia, and effects associated with impoundment of this river by the Copeton Dam. Australian Journal of Marine and Freshwater Research, 28,455-465. Department of Energy, United Kingdom 1981 Tidal Power from the Seoern Estuary. Energy paper No. 46. Her Majesty’s Stationery Office, London. De Vlaming, V., Grosman, G., Chapman, F. 1982 On the use of the gonadosomatic index. Comparative Biochemistry and Physiology, 73A, 31-39. Gallucci, V. F. & Quinn, T. J. II 1979 Reparameterizing, fitting and testing a simple growth model. Transactions of the American Fisheries Society, 108, 14-25.
The biology
of Cnidoglanis
macrocephalus
909
Hardisty, M. W. & Huggins, R. J. 1975 A survey of the fish populations of the middle Sevem Estuary based on power station sampling. InternationalJournal ojEnvironmenta1 Studies, 7,227-242. Hodgkin, E. P. 1974 Biological aspects of coastal zone development in Western Australia. I. General aspects. In The Impact of Human Activities on Coastal Zones. Australian Government Publishing Service, Canberra. pp. 93-l 11. Kowarsky, J. 1975 Strategy and zoogeographic implications of the persistence of the estuarine cat fish Cnidoglanis macrocephalus (Val.) (Plotosidae) in Australia. Ph.D. Thesis, University of Western Australia. Kowarsky, J. 1976 Classification of the name and distribution of the plotosid catfish Cnidoglanis marcocephalus. Copeia, 1976,593-594. Laevatsu, T. 1965 Manual of methods in fisheries biology. Fascicule 9. Section 4. Research on fish stocks. F. A.O. Manuals in Fisheries Science No. 1, F.A.O., Rome. 5 1 pp. Lenanton, R. C. J. 1982 Alternative non-estuarine nursery habitats for some commercially and recreationally important tish species of south-western Australia. Australian Journal of Marine and Freshwater Research, 33,881-900. Lenanton, R. C. J. 1984 The commercial fisheries of temperate Western Australian estuaries: early settlement to 1975. Report of the Fisheries Department of Western Australia, 62, l-82. Lenanton, R. C. J., Robertson, A. I. & Hansen, J. A. 1982 Nearshore accumulations of detached macrophytes as nursery areas for fish. Marine Ecology Progress Series, 9,51-57. Lenanton, R. C. J., Potter, I. C., Loneragan, N. R. & Chrystal, P. J. 1984 The age structure and changes in abundance of commercially important teleosts in a eutrophic estuary. Journal of Zoology, London, 203. Lindberg, G. U. 1974 Fishes of the World: A Key to Families and a Checklist. John Wiley, New York. 545 pp. Nelson, J. S. 1976 Fishes of the World. Wiley-Interscience, New York. Potter, I. C., Loneragan, N. R., Lenanton, R. C. J., Chrystal, P. J. & Grant, C. J. 1983 Abundance, distribution and age structure of fish populations in a Western Australian estuary. Journal of Zoology, London, 200,21-50. Prince, J. D., Potter, I. C., Lenanton, R. C. J. & Loneragan, N. R. 1982 Segregation and feeding of atherinid species (Teleostei) in south-western Australian estuaries. Australian Journal of Marine and Freshwater Research. 34.865-880. Ricker, W. E. 1975 Computation and interpretation of biological statistics of fish populations. Bulletin of the Fisheries Research Board, Canada, 191, l-382. Robinson, B. 1984 SPSS subprogram NONLINEARNonlinear regression SPSS No. 433. Vogelback Computing Centre, Northwestern University. 28 pp. Simpson, A. C. 1951 The fecundity of the plaice. Fisheries Inoestigation, London (Ser. 2) 17, l-27. Spencer, R. S. 1956 Studies in Australian hydrology. II. The Swan River. AustralianJournal ofMarine and Freshwater Research, 7, 193-253. Sterba, B. 1966 Freshwater Fishes of the World. Vista Books, London. 878 pp. Thomson, J. M. 1954 The organs of feeding and the food of some Australian mullet. AustralianJournal of Marine and Freshwater Research, $469-485. Thomson, J. M. 1957 The food of Western Australian estuarine fish. Fisheries Bulletin of Western Australia, 7.1-13.
Coastal
and Shelf
Science
(1985) 21,895-909
The Biology of the catfish Cnidoglunis macrocephalus (Plotosidae) in an Australian Estuary
S. A. Nel,
I. C. Potter
School of Environmental Western Australia 6150, Received
22 June
1984
and N. R. Loneragan
and Life Australia
Sciences,
and in revised
form
Murdoch 21 January
Keywords: teleosts; catfish; estuaries; growth fisheries; Australia West Coast
University,
Murdoch,
1985
rates; gonads; diet; estuarine
This paper describes the age structure, growth, diet and aspects of gonadal development in the cobbler, Cnidoglanis macrocephalus (Valenciennes), in the large Swan estuary in south-western Australia between August 1982 and June 1984. Analysis of otolith annuli showed that while the 0+ to 3+ age classes were regularly represented in monthly samples, the 4+ and more particularly the 5 + and 6 + were much less abundant. The weighted means for the back calculated lengths at the end of the first to fourth years of life were 181 mm (=26g), 314mm (=156g), 418mm (E410g) and 518mm (=833g) respectively. The mean length at the end of the second year of life was similar to the minimum legal size for capture by commercial fishermen (320mm). The von Bertlanffy growth curve calculated from the back calculated lengths was L,=917 [l ~e-o-2011+o~11) 1. The relative weight of mulluscs, crustaceans and polychaetes in the intestine varied markedly between small and large fish, apparently reflecting differences in the size of these prey. The large mean diameter of mature eggs (X = 7.4 mm) was correlated with a low mean absolute fecundity (2078). Trends shown by egg size, gonadosomatic index and time of appearance of spent females indicate that spawning takes place between October and December. The attainment of sexual maturity is both age- and size-dependent. Although sexually maturing and occasionally spent fish were found in the lower estuary, meristic values, commercial catch statistics and other data indicate that the cobbler found in the Swan estuary are part of a population which typically spawns at sea.
Introduction
The eel-tailed catfishes (Plotosidae) are restricted to marine, brackish and fresh waters of the Indo-west Pacific region from Japan to Australia (Sterba, 1966; Lindberg, 1974; Nelson, 1976). The cobbler, Crzidoglunis macrocephalus, a plotosid endemic to Australia, is found southwards from the Abrolhos Islands (28”S, 114”E) and eastwards as far as Kingston (37”S, 140”E) (Kowarsky, 1976, Figure 1). This speciesalso occurs on the east coast of Australia from Kirra (28”S, 153”E) southwards to Jervis Bay (35”S, 151”E). Cnidoglanis macrocephalus is found in both marine and estuarine environments in south-western Australia where it is sometimes present in sufficiently large numbers to 895 0272-7714/85/120895+
15 $03.00/O
0 1985 Academic Press Inc. (London) Limited
896
S. A. Nel et al.
r
c
I
IIS’SO 1
I
115~55 /
Figure 1. Map of the Swan estuary showing the location of funnel trap sites (TN l-8). Lower and upper estuary and the distribution of Cnidoglanis macrocephalus in Australia’ (taken from Kowarsky, 1976) are stippled. Black rectangle on map of Australia shows region illustrated in the inset.
make a major contribution to local commercial fisheries (Kowarsky, 1975; Lenanton, 1982, 1984; Lenanton et al., 1982, 1984; Potter et al., 1983). While information on the biology of the cobbler is presented in the Ph.D. thesis of Kowarsky (1975), the details of growth in his study were not based on fish which had been aged, and to a large extent the same generalization applies to the data given in Lenanton et al. (1984). On the basis of comparisons between such features as fin ray counts, parasite faunas and patterns of distribution within systems and regions, Kowarsky (1975) concluded that the estuarine and marine populations of C. macrocephalus in south-western Australia might not intermix. The current study of C. macrocephalus has analysed otolith annuli to provide data on the annual and seasonalpatterns of growth, the number of year classespresent and the age at which this species enters the commercial fishery in the large Swan estuary in south-western Australia. A study has also been made of the food of the cobbler to determine whether the diet changed with body size and how it related to those of other commercial fish in the system. The seasonalpatterns of distribution, gonadal development and commercial catches were analysed to determine the breeding period and attempt to ascertain whether or not spawning typically occurred within or outside the estuary. Materials
and methods
Cobbler were collected from the Swan estuary (Figure 1) using funnel traps laid overnight both by ourselves and by a commercial fisherman in the period between August 1982 and June 1984. While our trapping programme employed eight sites (TN l-8) in the lower, middle and upper estuary, that of the commercial fisherman was restricted to a variety of sites in Melville Water in the middle estuary (Figure 1). Our sampling programme involved the use of up to eight traps throughout the estuary on at least nine nights in each month between August 1982 and January 1984. Less frequent trapping
The biology of Cnidoglanis macrocephalus
897
was employed between February and June 1984. The traps, which were made of 13 x 25 mm galvanised wire mesh, measured 0.9 m in length and 0.4 m in width. Wire with a 2 mm mesh was placed over the trap between December 1982 and March 1983 in an attempt to catch small cobbler. The total length and weight of fish were recorded to the nearest 1 mm and 100 mg. Gonads were weighed to the nearest 10 mg, except in the case of those small fish where it was not possible to ascertain macroscopically whether the gonad was an ovary or testis. Gonadosomatic indices (GSIs) were calculated from the equation WI/W, x 100, where W, = weight of the gonad and W, =weight of the fish (seeDe Vlaming et al., 1982, for a discussion of the use of GSIs). The stage of gonadal development was recorded using the seven stages described by Laevatsu (1965). At least five ovaries representing each of maturity stages 2-7, except stage 6 (ripe or spawning) which was found on only two occasions, were placed in Gilson’s fluid for 24 h and then stored in 70’?,, alcohol (Simpson, 1951). The mean diameter of eggswas then recorded to the nearest 0.05 mm. Estimates of the absolute fecundity (Bagenal, 1978) of fish with ovaries at maturity stages5 and 6 were calculated from the number of large eggs in three separate ovarian subsamplesof known weight and the total weight of the ovary. Otoliths were removed from 368 females, 295 males and 183 small individuals whose sex could not be determined macroscopically. Since careful examination of the rings on the otoliths demonstrated that these were more clearly defined in the small, flattened asteriscaethan in the large lapillae or elongated, spiral sagittae, the asteriscaewere used exclusively for ageing in this study. The otoliths were mounted in DPX and examined under a dissecting microscope using reflected light and a dark background. Distinct narrow and dark but transparent rings (hyaline zones) could be clearly observed between wider rings (opaque zones). All otolith measurementswere made to the nearest 0.05 mm. Care was taken to ensure that measurementsfrom the centre of each asteriscus to its periphery and to the outer edge of each hyaline zone was always taken along the sameaxis. The distance between the outer edge of the outermost hyaline zone and the periphery of the asteriscuswas then calculated. This marginal increment was then expressed as a percentage of the distance between the two outermost hyaline zones, or in the caseof fish with only one ring, as the distance between the centre of the asteriscusand that ring. The otolith measurementsfor each fish were used to back calculate its length at age 1, 2, 3 etc. (see Ricker, 1975; Bagenal & Tesch, 1978). The back calculated lengths were then fitted to the von Bertalanffy equation by a non-linear technique (see,e.g. Gallucci & Quinn, 1979) using the NONLINEAR routine of SPSS (Robinson, 1984). The equation is L,:=L,(l-ek’r-Lg)),where L, is length at age t (years), L, is the mean length of the asymptote predicted by the equation, K is the growth coefficient and to is the hypothetical age at which fish would have zero length if their growth had followed that described in the equation. Since cobbler do not have a well-defined stomach, the analysis of their diet was based on the contents of the first third of the intestine. The food was removed, blotted dry with absorbent paper and weighed. The individual prey items were then identified and weighed. Since cobbler tend to crush molluscs into small pieces, the numbers of the different prey items could not be determined. However, the number of intestines which contained each prey item was noted. Data on both the mean percentage weight and the percentage frequency of items in the intestine have been recorded separately for 0 + and older fish.
898
S. A.
Nel et al.
J 1982
1983
Figure 2. The mean percentage marginal growth increment (+ on otoliths. This value represents the distance outside the last percentage of the distance between the two outermost hyaline hyaline ring is present the distance between that check and the
1984
95% confidence limits) annulus expressed as a rings or when only one centre of the asteriscus.
Counts were alsomade of the number of rays in the fused dorsal, caudal and anal fin in 50 and 35 fish with a wide range in body size taken, respectively, from the Swan estuary and a local inshore coastal area &km to the north of the mouth of the estuary. Data on the commercial catchesof cobbler in the Swan estuary were obtained from the catch summaries provided by the Australian Bureau of Statistics and the Western Australian Department of Fisheries files. The legal minimum length for the capture of cobbler is 320 mm. Results Distribution
of catches
Analysis of variance showed that the mean catch per trap net differed among regions of the estuary (P< 0.001). The mean catches were consistently greater in the middle than the lower estuary, with catches per trap in the four seasonsranging from 3.3 to 6.0 in the middle region and from 0.8 to 2.5 in the lower region. While a few fish were caught in the upper estuary, none of these were taken in the winter and spring and only one exceeded 385 mm. These data are consistent with those obtained from catches taken during an extensive survey of the fish fauna of the Swan estuary between 1977and 1982, using gill nets at night and beach seinesand otter trawls during the day. Age determination
The round asteriscaeof C. macrocephalus, which ranged in diameter from 0.55 mm in a 70-mm fish to 2.7Omm in a 683~mm fish, showed well marked hyaline zones. In most cases,these circuli became detectable in November, a feature illustrated by the fact that the mean marginal growth increment fell sharply to a minimum value in this month in both 1982and 1983 (Figure 2). The presenceof only one marked trough in the trends for the mean values for the marginal growth increment in both years confirm that only one
The biology
of Cnidoglanismacrocephalus
TABLE 1. Back-calculated mean total lengths aged from otoliths. F represents the weighted
Estimated age at capture If 2+ 3+ 4+ 5+
Number of fish
Mean length at capture (k9593 CL)
352 153 79 19 9
298.6 (6.8) 398.3(11.6) 477.1 (16.7) 588.3 (15.6) 617.9(17.5) n x 950”
Mean
CL
at each armulus for 6 12 C. macrocephalus mean length at ages l-5
back-calculated
lengths
at successive
annuli
2
3
4
5
180.7 181.9 181.2 1845 172.9
313.8 312.9 327.8 313.1
414.1 433.4 421.3
519.4 513.7
580.6
612 181.1 2.63
260 314.5 4.37
107 418.1 9.22
28 517.5 17.95
9 580.6 41.38
181.1
132.7
104.3
99.4
63. I
I
Annual increment
899
major circulus is laid down annually. Since November is also believed to be the middle of the spawning period (see later), fish caught in this month with asteriscae having for example one, two and three annual circuli (annuli) can be considered as having just become one, two and three years old. The relative contribution of age classesl-5 to the samplescollected during this study are shown in Table 1. Two 6 + fish were also caught. Length-frequency
histograms
andgrowth
curves
The length-frequency histograms based on fish of known age caught between August 1982 and June 1984 show that, except in the caseof the 0+ age classin some months, the sizes of the individuals representing the different age classesoverlap considerably (Figure 3). The smallest fish caught during the study, measuring 70 mm and weighing 1.8 g, was collected in February 1983. This fish and all others less than 120 mm were taken in the lower estuary between February and May of either 1983or 1984. The largest fish, measuring 683 mm (= 1880 g), was collected from Melville Water in the middle estuary in November 1982. The trends shown by the mean lengths of the group hatched in late 1982 showed that their growth in 1983 was rapid in the summer and autumn, slowed during the winter and early spring and started increasing again in the late spring (Figure 4). These trends essentially parallel those exhibited by the 1981 age class in 1982-1983, except that growth started increasing rather earlier in the spring of 1982. The mean lengths of the 1981 and 1982 year classesat the commencement of their second year of life were 207 mm ( E 40 g) and 234 mm ( = 60 g) respectively. The sequential mean lengths for fish at the commencement of their third and fourth years of life in November 1982 were 337 and 420 mm, corresponding to weights of 201 and 416 g. Growth varied considerably between years. For example, the length of 1 + fish was approximately 100 mm greater in June 1984 than in June 1983. Apart from three fish from the 1976 year class, all other individuals caught during this study came from the 1977-1983 year classes(Figure 4). Von Berta1an.y
growth
equation
The following equations demonstrate that the length of the fish (L) is correlated with the radius of the asteriscus (Z?) in both the males and females of Cnidoglanis macro-
900
S. A.
Nel
et al.
1978
YEAR
CLASSES
1980
1981
1979
n
1982
1983
1982
1983 N= 19
JAN N= 50 25
1984 FE8 N= 82
FE8 N=70
0 25
MAR/APR
0 I 0
I 100
8 200
I 300
8 400
I 500
I 800
LENGTH (mm) Figure 3. Length-frequency histograms been determined from otohth annuli.
1 700
0 0
a. 100
8 200
0 300
LENGTH for Cnidoglanis
macrocephalus
0 400
a 500
1 500
1 700
(mm) whose
age had
and for all individuals including those young fish that could not be sexed macroscopically: cephalus
Males Females All fish
L=549.18 R-72.87 (n=295, r=O.gl,P
Since analysis of covariance showed that there was no significant difference (P > 0.05) between the above body length-otolith radius regression equations for males and females, the third equation basedon all fish has been used subsequently. The line relating body length and otolith radius for all fish was linear but did not pass through the origin. It is therefore justifiable (seeBagenal & Tesch, 1978) to usethe above
The biology
--~
of Cnidoglanis
901
macrocephalus
700
600
500
z
4oc
f P 3
300
200
I00
0 1983
1982
Figure 4. The 1983 spawning
mean lengths seasons.
I984
(+ 1 standard
error)
for fish resulting
from
the 1976 to
Age (years)
Figure 5. The growth cure for Cnidoglanis macrocephalus derived from the von Bertalanfiy equation shown on the figure, together with the points representing the back-calculated weighted mean lengths (0) and 95% confidence limits at each annulus. The number of values contributing to each weighted mean is recorded.
regression equation and the mean distance from the centre of the otolith to each annulus to back calculate the body lengths at age 1, 2, 3, 4 and 5. The von Bertalanffy growth curve
derived
from
individual
95”b confidence limits for
L
lengths
was
L,=
917
(1-e-“~20ctto~11))
(Figure
5).
co,k and r0 were + 146, +0.047 and + 0.10 respectively.
The
The
902
S. A. Nel et al.
I.
I
0
*
2
I
b
I
4,
Egg diameter
a
I
6
I
8
I
I
IO
(mm)
Figure 6. Frequency histograms for the diameter of ‘eggs’ in maturity Cnidoglunis macrocephalus. Data are provided on the number of ovaries the time when the various maturity stages were found in samples.
mean back calculated lengths at each a~ulus growth curve (Figure 5).
stages 2-7 of examined and
(Table 1) lay close to the von Bertalanffy
Gonadal development, fecundity and egg size The histograms for the egg diameter of the developing virgin and spent fish (stage 2) showed two distinct size groups producing modal classes at O-O.4 and 1.5-1.9mm (Figure 6). A group corresponding to the first of these categories, which histological sections showed were primary oocytes, was present in all subsequent stages. While the modal class of the group of larger oocytes increased to 2.0-2.4 mm in stage 3, the distribution subsequently became trimodal with the appearance of groups which in stages 4-7 produced modal classes of between 1.0-l .4 and 2.0-2.4 mm. The modal size of the group of largest oocytes rose to 35-3.9 in stage 4 and 7.0-7.4 mm in stage 5. The range of oocyte diameters in the only two running-ripe individuals (stage 6) caught during the study were virtually identical to those in the previous stage. A few residual large eggs were found in spent animals. The largest egg diameter recorded in this study was 9.5 mm. The relationship between the stage of gonadal development and the age and length of individuals caught between August and November was examined to determine the probable age and size at first maturity. From the data in Figure 6, and from comparable information on the development of the testis, it was assumed that all fish which were going to reach maturity in the immediately ensuing months would have attained at least stage 3 by August. While maturity indices of 3 or greater were found in all 2 + and older fish examined in this period, this was only otherwise found to be the case with late 1 + males and females which were greater than 385 and 405 mm respectively. The proportion of late 1 + males and females that fell in this category were, however, only 22.5 and 14.0% respectively.
The biology
2 73 c u Tn
o-
of Cnidoglanis
macrocephalus
903
Spent femoles Small females 4 . , . . . . . . . ,. . . . .i.. . . . . . . . ., . . . ., . . . . . . . . . . . . . . . . . . . . . . . IO 36 30 22 31 I 24 2 9 12 33 20
JASONOJFMAMJ
._
Figure 7. Mean gonadosomatic index ( f 1 standard error) for small (0 + and small 1 + ) and large (large 1 + and 3 2 + ) Cnidoglanis macrocephalus. Large spent females are recorded separately.
In view of the finding that sexual maturation is related both to age and size, the mean GSIs have been calculated separately, first for 0 + and 1 + males and femaleswhich were
less than 385 and 405 mm respectively, and secondly for larger 1+and all 2-t- and older fish (Figure 7). Since there was no evidence that the GSIs followed different trends in 1982 and 1983, and asonly a small number of larger fish were caught in some months, the GSI data for these two years have been pooled. The mean GSI for larger females rose from 5.8 in July to 10.1 in September and then increased sharply to 24.7 in November. Although the means of ‘ unspent ’ fish declined in the following two months, they were still greater than 11.5. The recently spent fish had a mean GSI of less than 2.5 when they first appeared in November. These values declined in subsequent months. While the trends shown by the ovaries of the younger and smaller females were similar to that just described, the maximum mean GSI, which was also recorded in October, was only 0.45. In contrast to the situation with the larger and older females, the maximum mean GSI for males was very low (0.37) and was recorded in December rather than in October (Figure 7). The mean GSI for the group of 0 + and smaller I + male fish never exceeded 0.1. The presence of the highest mean GSIs in October in females and in December in males (Figure 7) indicates that spawning occurs in the period between the middle of spring and early summer. Such a view is consistent with the high GSIs found in some females in November and December and with the observation that spent females first appeared in November (Figure 7). It is also consistent with the observation that mature
904
S. A. Nel
et al.
(stage 5) and ripe (stage 6) ovaries were only found between October and December (Figure 6). The mean absolute fecundity ( f 1 s.e.) of 44 females whose ovaries corresponded to either stage 5 or 6 was 2078 + 109.5. The relationship between fecundity (F) and the body weight (W) and body length (L) of these fish, whose weights and lengths ranged from 324 to 1880 g and from 380 to 683 mm, can be described by the following equations: log F= 0.3372 + 0.9627 log W (n = 44, r = 0.87, P < O.OOl), log F = - 3.6992 + 2.5404 log L (n = 44, r = 0.80, P< 0.001). Diet
The diet of cobbler consisted almost exclusively of molluscs, crustaceans and polychaetes (Figure 8). Marked differences were found, however, between the percentage weight of various items in the intestinal contents of small and large fish. Thus, the contribution made by the mollusc Sanguinoluriu biradiutu to the food of fish greater than one year old (28.3%) was much higher than in fish in the first year of life (2.50/b). The reverse situation was found with the crustaceans Corophium minor, Melitu mutildu and Purutunuis sp., with the percentage contribution to the weight of the food being between 3.0 and 10.5 times greater in the case of the young fish. While the type of differences found with percentage weight in the caseof S. birudiutu were paralleled by the data on the frequency with which they occurred in the intestine, this was not the casewith the crustaceans. Thus, in contrast to the situation with percentage weight, C. minor was present in a slightly greater number of the intestines of the older than younger fish and M. mutildu occurred in the same percentage number of intestines of the two age categories. The polychaete Cerutonereis erythrueensis was found more frequently than any other speciesin the intestine of older fish and it alsomade an important contribution to the weight of the food of both young and older fish. Fin ray counts
The mean number of rays in the fused dorsal, caudal and anal fin (+ 95% confidence limits) in 50 cobbler from the Swan estuary (222.4 + 3.90) did not differ significantly (P> 0.05) from that obtained for 35 cobbler from a nearby inshore marine environment (223.8 f 5.01). Seasonal
trends in commercial
catches
Previous work has shown that the weight of catch per boat for fish in the Swan-Avon and Peel-Harvey system is a good reflection of the catch per unit effort or CPUE (Lenanton et al., 1984). The CPUE for cobbler in the Swan estuary, which are taken by gill nets and traps, show a marked seasonalvariation, with the greatest values occurring in the mid-autumn to early spring months of April to September (Figure 9). The CPUE, which parallels closely the trends shown by total catch, falls markedly during the late spring and summer (November to February). In contrast to the marked seasonalvariation in catches, the mean monthly catch for each of the years between 1972 and 1983 did not differ markedly. Discussion This paper provides the first data on the age and growth of either marine or estuarine representatives of the family Plotosidae based on a large number of fish of known age.
The biology
of Cnidoglanis macrocephalus
905
n
Mollusco
Crustoceo
Figure 8. Histograms showing the mean percentage weight of various intestine of Ctkfoglunis macrocephalus and the percentage number which those items were found.
Polyhoeto
Algo
food items in the of intestines in
Comparisons can be made, however, with data on the freshwater plotosid, Tandanus in eastern Australia where ageing used annuli in the dorsal spinesrather than in otoliths (Davis, 1977a). Our data show that up to seven age classesof the cobbler, Cnidoglanis macrocephalus, were present in samples taken from the Swan estuary between mid-1982 and mid-1984. While this closely parallels the situation in Tandanus randanus, the lengths of these two catfish at the sameage differ quite markedly. Thus, the back caIculated mean lengths of 181 mm and 518 mm after one and four years of life in C. macrocephalus in November 1983 are considerably greater than the comparable values of randanus,
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846
85Oc 800 6OOI
I I
588 1
ASASASASASASASASASASASA
1972 1973 1974 1975 1976 1977 1973 1979 1980 1981 1982 1983 Figure 9. The mean catch per boat of Cnidoglunis inacrocephalus for each month between January 1972 and July 1983. Autumn (A) and spring (S) months are denoted by black rectangles. The dotted line joins the points representing the mean monthly catch for each year. 0 0 0, annual mean monthly catch per boat; O-0, monthly catch per boat.
only 92 mm and approximately 420 mm for T. tandanus. The maximum length of 683 mm and the L, in the von Bertanlanffy equation of 917 mm for C. mucrocephalus are also greater than the L,,, of 574 mm and L, of 613 recorded for T. tandanus. Although C. macrocephalus attains a greater size than T. tandunus, its fecundity (533-3551) is less than that recorded by Davis (19776) for the latter species (2000-20600). This feature can be related to the greater mean diameter of the mature eggs in the cobbler (4.3-9.2) than in the freshwater catfish (2.3-3.1 mm) (Davis, 1977c). The GSIs for males show that the small number of large eggs in the cobbler is paralleled by the presence of a relatively very small testis. The presence of high mean GSIs for females of C. macrocephalus in October, November and December indicate that the spawning period is relatively protracted. The evidence for peak spawning in late spring or early summer is consistent with the conclusions drawn from GSIs for cobbler caught in the early 1970s by Kowarsky (1975). Since the back calculated mean length of cobbler at age 2 was 314 mm and the legal minimum length for capture is 320 mm, this species on average started entering the fishery at the end of the second year of life. From our data, which showed that the attainment of sexual maturity was related both to age and size, it is evident that only the largest of these late 1 + fish would have been approaching sexual maturity. The analysis of intestinal contents demonstrates that, like the freshwater catfish T. tundunus (Davis, 1977d), the cobbler is a carnivore whose diet changes with increasing size. The relatively greater amounts of Corophium minor, Melita matilda and Purutunuis sp. in the intestine of small rather than large cobbler presumably reflects the relatively small size of these crustaceans. Conversely, the presence of relatively greater quantities of Sanguinoluria birudiata in larger fish can probably be attributed to the larger size of this mollusc. Small cobbler (90-126 mm) in a local marine environment, were also found to feed predominantly on amphipods, with one species (Allorchestes compressa), which
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was not found in Swan fish, occurring in all fish examined and contributing 75.5% to the mean percentage volume (Lenanton et al., 1982). Despite our capture of both mature and spent females in the lower estuary in November and December, there must still be somedoubt as to whether the majority of the large cobbler caught earlier in the year would have spawned in the estuary. For example, the numbers of both mature and spent fish in these and the two following months were low, a feature consistent with the concomitant marked decline that occurs in the commercial catchesof large cobbler which always takes place at this time. Another feature which suggests that spawning typically occurs outside rather than inside the estuary was our capture of only two cobbler lessthan 98 mm in length during the current study. A relative paucity of small cobbler also characterized Kowarsky’s (1975) samples from the estuary and those taken by ourselves using beach seines,prawn nets and otter trawls on many occasionsbetween 1977 and 1984. By contrast, cobbler ranging from 40 to 70 mm were abundant in samplestaken from a local inshore marine environment in January 1977 and February 1984 (Lenanton et al., 1982, personal communication). It is also worth noting that the mean number of rays in the fused dorsal, caudal and anal fin of fish taken in 1984 from the Swan estuary and a local marine environment did not differ significantly. This contrasts with the situation recorded by Kowarsky (1975), a feature taken by that author as indicating the presence of distinct marine and estuarine populations. The above data suggest that while the C. macrocephalus found in the Swan may sometimesbreed in the estuary, they typically breed at sea.However, there can be little doubt that the cobbler has become particularly well adapted to colonizing the large estuarine system of the Swan for protracted periods. It is thus probably relevant that conditions in the Swan estuary tend to be much more stable than is frequently the casein estuaries in many other temperate regions of the world. This can be attributed in part to a very small maximum tidal height (< 80 cm in the lower estuary) and to the restricted periods of high freshwater discharge (Spencer, 1956; Hodgkin, 1974; Prince et al., 1982). These result in small tidal water movements and the maintenance of relatively high salinities throughout much of the year. Moreover, even during periods of high freshwater discharge in the winter, the wide bays of the middle estuary (Figure 1) help dissipate any flushing effects and the marked haloclines which develop at these times provide regions in the deeper waters where fish with a preference for high salinities can find refuge. The relatively stable conditions in the Swan estuary, and indeed other large southwestern Australian estuarine systems such as the Peel-Harvey, are also utilized extensively by the yellow-eye mullet, Aldrichettu forsteri, and sea mullet, Mugil cephulus (Chubb et al., 1981; Potter et al., 1983; Lenanton et al., 1984). Both these speciesreach the legal minimum length for capture after spending several months in the Swan and Peel-Harvey systemsand, together with the cobbler and Perth herring, form the basisof the commercial fishery in these systems (Lenanton, 1984). Although these four species occur in larger numbers and have a relatively large body length in the Swan estuary, the potential competition for food is to some extent avoided by resource partitioning. For example, while the cobbler is a carnivore, the yellow-eye mullet is an omnivore and the Perth herring and seamullet are detritivores (Thomson, 1954,1957). The yellow-eye mullet has occasionally been caught in a running-ripe condition in the lower Swan estuary (Chubb et al., 1981), aswas the casewith cobbler on two occasionsin this study. The protracted period during which these two speciesand the seamullet and Perth herring remain in the Swan and Peel-Harvey estuarine systemscontrasts with the
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situation found with most marine fish which utilize large temperate estuaries in the northern hemisphere such as that of the River Severn. Although representatives of older year classes of many marine species are present in this latter estuary, the populations of the more abundant teleosts tend to be dominated to a greater extent by the 0+ year class and are markedly seasonal in occurrence (Hardisty & Huggins, 1975; Claridge & Gardner, 1978; Claridge & Potter, 1983, 1984). It may thus be significant that in the Severn, the tidal effect is very large and there are no wide basins with relatively protected small embayments (Department of Energy, United Kingdom, 1981). In summary, it is proposed that the cobbler usually breeds in coastal waters and that while the resultant young may sometimes remain for the rest of their life cycle in marine environments, they frequently utilize for extended periods the excellent conditions provided by the Swan estuary.
Acknowledgements Our gratitude is expressed to those colleagues and friends who helped with the sampling and to R. C. J. Lenanton and G. Fooks who kindly provided us with cobbler. R. C. J. Lenanton and J. Kowarsky provided constructive criticism, N. G. Hall gave statistical advice and R. J. G. Manning drew the figures. Financial support was provided by the Western Australian Department of Fisheries. References Bagenal, T. B. 1978 Aspects of fish fecundity. In Ecology ofFreshwater Fish Production (Gerking, S. B., ed.). Blackwell Scientific Publications, Oxford. pp. 75-101. Bagenal, T. B. & Tesch, F. W. 1978 Age and growth. In Methods for Assessment of Fish Production in Fresh Waters (Bagenal, T. B., ed.), 3rd ed. Blackwell Scientific Publications, Oxford. pp. 3-19. Chubb, C. F., Potter, I. C., Grant, C. J., Lenanton, R. C. J. & Wallace, J. 1981 The age structure, growth rates and movements of sea mullet, MugiZ cephalus L., and yellow-eye mullet, Aldrichetta jorsteri (Valenciennes), in the Swan-Avon river system. Australian~ournal of Marine and Freshwater Research, 32,605-628. Claridge, P. N. & Gardner, D. C. 1978 Growth and movements of the twaite shad, Alosa jallax (Lacepede), in the Sevem Estuary. Journal of Fish Biology, 12,203-211. Claridge, P. N. & Potter, I. C. 1983 Movements, abundance, age composition and growth of bass, Dicentrachus labrax (L), in the Sevem Estuary and inner Bristol Channel. Journal of the Marine Biological Association of the United Kingdom, 63,228233. Claridge, P. N. & Potter, I. C. 1984 Abundance, movements and size of gadoids (Teleostei) in the Sevem Estuary. 3ournal of the Marine Biological Association of the United Kingdom 64,771-790. Davis, T. L. 0. 1977a Age determination and growth of the freshwater catfish, Tandanus tar&anus, in the Gwydir River, Australia. Australian Journal of Marine and Freshwater Research, 28,119-137. Davis, T. L. 0. 19776 Reproductive biology of the freshwater catfish, Tandanus tandanus Mitchell, in the Gwydir River, Australia. I. Structure of the gonads. Australian Journal of Marine and Freshwater Research, 28,139-158. Davis, T. L. 0. 1977c Reproductive biology of the freshwater catfish, Tandanus tandanus Mitchell, in the Gwydir River, Australia. II. Gonadal cycles and fecundity. Australian Journal of Marine and Freshwater Research, 28,159-169. Davis, T. L. 0. 1977d Food habits of the freshwater catfish, Tandanus tandanus Mitchell, in the Gwydir River, Australia, and effects associated with impoundment of this river by the Copeton Dam. Australian Journal of Marine and Freshwater Research, 28,455-465. Department of Energy, United Kingdom 1981 Tidal Power from the Seoern Estuary. Energy paper No. 46. Her Majesty’s Stationery Office, London. De Vlaming, V., Grosman, G., Chapman, F. 1982 On the use of the gonadosomatic index. Comparative Biochemistry and Physiology, 73A, 31-39. Gallucci, V. F. & Quinn, T. J. II 1979 Reparameterizing, fitting and testing a simple growth model. Transactions of the American Fisheries Society, 108, 14-25.
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Hardisty, M. W. & Huggins, R. J. 1975 A survey of the fish populations of the middle Sevem Estuary based on power station sampling. InternationalJournal ojEnvironmenta1 Studies, 7,227-242. Hodgkin, E. P. 1974 Biological aspects of coastal zone development in Western Australia. I. General aspects. In The Impact of Human Activities on Coastal Zones. Australian Government Publishing Service, Canberra. pp. 93-l 11. Kowarsky, J. 1975 Strategy and zoogeographic implications of the persistence of the estuarine cat fish Cnidoglanis macrocephalus (Val.) (Plotosidae) in Australia. Ph.D. Thesis, University of Western Australia. Kowarsky, J. 1976 Classification of the name and distribution of the plotosid catfish Cnidoglanis marcocephalus. Copeia, 1976,593-594. Laevatsu, T. 1965 Manual of methods in fisheries biology. Fascicule 9. Section 4. Research on fish stocks. F. A.O. Manuals in Fisheries Science No. 1, F.A.O., Rome. 5 1 pp. Lenanton, R. C. J. 1982 Alternative non-estuarine nursery habitats for some commercially and recreationally important tish species of south-western Australia. Australian Journal of Marine and Freshwater Research, 33,881-900. Lenanton, R. C. J. 1984 The commercial fisheries of temperate Western Australian estuaries: early settlement to 1975. Report of the Fisheries Department of Western Australia, 62, l-82. Lenanton, R. C. J., Robertson, A. I. & Hansen, J. A. 1982 Nearshore accumulations of detached macrophytes as nursery areas for fish. Marine Ecology Progress Series, 9,51-57. Lenanton, R. C. J., Potter, I. C., Loneragan, N. R. & Chrystal, P. J. 1984 The age structure and changes in abundance of commercially important teleosts in a eutrophic estuary. Journal of Zoology, London, 203. Lindberg, G. U. 1974 Fishes of the World: A Key to Families and a Checklist. John Wiley, New York. 545 pp. Nelson, J. S. 1976 Fishes of the World. Wiley-Interscience, New York. Potter, I. C., Loneragan, N. R., Lenanton, R. C. J., Chrystal, P. J. & Grant, C. J. 1983 Abundance, distribution and age structure of fish populations in a Western Australian estuary. Journal of Zoology, London, 200,21-50. Prince, J. D., Potter, I. C., Lenanton, R. C. J. & Loneragan, N. R. 1982 Segregation and feeding of atherinid species (Teleostei) in south-western Australian estuaries. Australian Journal of Marine and Freshwater Research. 34.865-880. Ricker, W. E. 1975 Computation and interpretation of biological statistics of fish populations. Bulletin of the Fisheries Research Board, Canada, 191, l-382. Robinson, B. 1984 SPSS subprogram NONLINEARNonlinear regression SPSS No. 433. Vogelback Computing Centre, Northwestern University. 28 pp. Simpson, A. C. 1951 The fecundity of the plaice. Fisheries Inoestigation, London (Ser. 2) 17, l-27. Spencer, R. S. 1956 Studies in Australian hydrology. II. The Swan River. AustralianJournal ofMarine and Freshwater Research, 7, 193-253. Sterba, B. 1966 Freshwater Fishes of the World. Vista Books, London. 878 pp. Thomson, J. M. 1954 The organs of feeding and the food of some Australian mullet. AustralianJournal of Marine and Freshwater Research, $469-485. Thomson, J. M. 1957 The food of Western Australian estuarine fish. Fisheries Bulletin of Western Australia, 7.1-13.