J. Zool., Lond. (1976) 180, 455-465

The determination of age and growth from the scales in Burbus liberiensis (Pisces, Cyprinidae)

A . I. P A Y N E Zoology Department, Fourah Bay College, University o j Sierra Leone, Freetown,

Sierra Leone *

(Accepted 13 April 1976)

(With 1 plate in the text)

The predictability of scale check formation in Barbus liberiensis has been investigated. Scale length is shown to be linearly related to body length. The formation of the check can be interpreted in terms of the reproductive cycle and changes in somatic condition. Throughout the population check formation takes place in two phases, resorption of material from the scale edges coinciding with the early and middle phases of gonad maturation, and the formation of the check itself as a result of repair as maturation is completed. Check forma- tion is not correlated with actual spawning activity and fluctuations in food intake and temperature are insignificant.

The first scale check is formed at the end of the first year and from analysis of check frequency the mean body lengths at the end of the first, second and third years were 6.9 cm, 8.3 cm and 9.3 cm. There was no significant difference in the growth rate of males and females although the females tend to live longer and therefore attain a greater maximum size. Some individuals may not have formed a check at the end of the first year.

The difference between check formation in tropical and temperate fish and also between forest and savanna types are discussed. The predictability and the factors affecting check formation in tropical freshwater fish are considered.

Contents

Introduction . . . . . . . . . . Methods . . . . . . . . . . . . Check formation . . . . . . . . . . Age and growth . . . . . . . . . . Discussion . . . . . . . . . . . . Conclusions . . . . . . . . . . References . . . . . . . . . . . .

Page . . . . . . . . . . 455 . . . . . . . . . . 456 . . . . . . . . . . 456 . . . . . . . . . . 458 . . . . . . . . . . 461 . . . . . . . . . . 464 . . . . . . . . . . 464

Introduction The determination of the age of fish from growth checks on scales, otoliths or other

skeletal structures, a common requirement in fisheries work, presents more difficulties in tropical than in temperate areas where the procedure has become well-established. In temperate regions, the marked difference between seasons, particularly with respect to temperature and photoperiod, leads to a predictable discontinuity in seasonal growth and also to a restricted, single spawning season. Certain factors, not always precisely deter- mined, which are associated with these events, give rise to visible annual checks in the growth of structures such as the scales, usually between April and July (Balon, 1972).

*Present address: Dept of Biological Studies, Lanchester Polytechnic, Coventry, England. 455

456 A . I . P A Y N E

Such predictability cannot generally be assumed in the tropics where, for example, there may be no seasonal variations in growth and where spawning can vary in periodicity and frequency depending upon species and locality. In this situation, the timing and regularity with which check formation takes place must be rigorously examined (De Bont, 1967).

Burbus Iiberiensis Steindachner is a small cyprinid which occurs in forest streams in parts of West Africa. Definite checks were observed on the scales of a population of this species living in a stream running through the Botanical Garden of Fourah Bay College in Freetown, Sierra Leone. A critical examination of the formation and significance of these checks forms the subject of this investigation. The extent to which the factors involved in this respect are applicable to other tropical freshwater fish and the degree to which they contrast with those commonly found to operate in temperate fish are also considered.

Methods Those areas of the fish where normally shaped scales can regularly be found, and where

regenerated or replacement scales are not so common, were located. These areas proved to be: ( I ) the anterior half of the horizontal scale row directly below the lateral line, and (2) the second and third horizontal rows below the lateral line in the region of the anus. The third scale in the row immediately below the scales of the lateral line was selected as most commonly having a consistent shape and a clearly visible structure. This was designated the “key scale”. All measure- ments were made on this scale where possible; the number of spawning marks or other features were compared with at least four other scales, including the equivalent scale on the other side of the fish. If the key scale on one side was a regenerated one, then the equivalent scale on the other side was measured. If this was also regenerated, then the immediate neighbour of the key scale was used. In the majority of cases, however, it was possible to use a primary key scale.

The scales were not usually mounted as they were examined straight from the freshly killed fish and readily adhered to the slide. Using a constant power of magnification, and with an eye- piece micrometer, the length of the scale was measured from the centre, or focus, to the ventral corner of the anterior edge (see Plate I(a)). Similarly, the distance from the focus to any check was measured in the same region.

Check formation The checks on the scales of B. liberiensis are not formed by the circuli being particularly

close together, as is often found in temperate fish, but by two or three circuli being fused together irregularly (Plate I(a), (b)). This type of mark is generally associated with the resorption of scale material, known as erosion, followed by repair. Such marks have been observed among temperate fish, for example in roach, Rutilus rutilus (L.), (Jones, 1953;

PLATE I. (a) Key scale of E. liberiensis showing three checks. The scale was taken from a female of 10.3 cm total body length caught in March. The radius used for scale measurement is indicated by the dashed line. The anterior field is damaged. (b) The structure of the scale check in B. liberiensis. The regular formation of circuli is interrupted by irregular fragmentation or anastomosis of circuli in the region of the check itself. (c) Erosion along the margins of a scale from B. liberiensis caught in early May. Typically the scale should have an outline similar to the scale shown in (a). The dorsal and ventral edges (arrowed) are the most severely effected; note how the circuli have been interrupted by resorption in these areas. A comparable effect can be produced by starving fish in a laboratory for a month or more.

AGE A N D GROWTH DETERMINATION FROM S C A L E S 457

PLATE 1

458 A . I . PAYNE

Cragg-Hine & Jones, 1969-see their plate X) where they have been associated with spawning activity. In B. liberiensis erosion occurs largely along the dorsal and ventral edges of the scale (Plate I(c)) and consequently the marks are often better defined here than towards the anterior edge.

The main period of check formation must be between April and July as the greatest frequency of checks formed right at the edge of the scales occurs at this time (Table I). These new marks are only found on the scales of fish with maturing gonads, which includes a large percentage of the population at this time; by July the majority of mature individuals have completed checks at the edge of the scales.

It has been shown previously that B. liberiensis has a distinct spawning cycle (Payne, 1975). Gonad maturation, which begins during the dry season, in January, culminates in spawning from June to August. The final stages of gonad maturation coincide with the beginning of the rains in late April or early May and some upstream migration also takes place at this time. Spawning usually occurs in July and August.

There is, therefore, a distinct correlation between check formation and the final stages of gonad maturation, although it is not caused by the act of spawning itself, as the marks are usually complete by the time spawning occurs (Table I). From late April until June individuals can often be found with resorption plainly occurring along the edges of the scales giving them a somewhat emarginated appearance. Scales in a similar condition can be produced in the laboratory after starvation of a fish for at least a month. This gives some indication of the time required for resorption to become obvious and suggests that, in the stream, resorption probably begins in early April if not before.

Of the factors other than reproduction which might be implicated in regular check formation it is unlikely that shortage of food gives rise to scale erosion in the wild popula- tion as an examination of gut contents revealed no appreciable reduction in food intake over the period in question (Payne, 1975). Furthermore, there is no general reduction of condition within the population in April as might be expected if a period of starvation occurred at the time resorption is apparent. For example, mature female fish showed minimum condition in October, mature males in January and immature fish in April (Payne, 1975).

Temperature in the stream ranges from 22.6" to 25°C and the ionic content varies to give a conductivity range of 16.5 to 39 pmhos during a year. These changes are relatively minor and unlikely to tax the metabolism of the fish sufficiently to cause resorption of calcium from the scales.

Age and growth The checks on the scales of B. liberiensis are formed over a restricted period (Table I)

and are associated with the annual spawning cycle. They can, therefore, be used with some confidence to estimate growth and age in these fish.

The relationship between the key scale and body length is linear and gives the calculated regression equation of:

L=2*42 Sk+1'12

where L is total body length (cm), measured from the snout to the end of the tail and S k

is the radius of the key scale (mm). The standard error of the regression slope is &0.005

A G E A N D GROWTH D E T E R M I N A T I O N FROM SCALES

and the correlation coefficient is 0.937. The total length at check formation was calculated from the following formula (Tesch, 1968):

459

L,-C=S,(L-C) - S

where L,=length of fish when check ‘n’ was formed; L=length of fish at capture; S,= radius of check ‘n’; S=total scale radius; C=intercept on length axis (from regression equation above). An analysis of the data on body length at first, second and third check formation for a sample of one hundred individuals is shown in Table 11.

TABLE I Thefrequency with which checks occur at the edges of scales in monthly samples

of B. liberiensis

Month Total no. No. with ring

mature counted at scale edge %

October November January February March April

June July

May

31 14 20 36 12 53 20 31 19

3 0 0 0 0 8 5

19 14

10 0 0 0 0

15 25 51 14

If scale checks associated with reproduction are to be used to estimate age or growth then age and length of the fish at first maturity, when the first check is formed, must be determined. Barbus liberiensis becomes mature and spawns in the first year of life (Payne, 1975) and check formation can be observed at this first spawning. The initial check is therefore laid down at the end of the first year since a fish which is spawned in July or August itself spawns approximately 12 months later. In addition, the mode in the length frequency distribution caused by the 0 year class fish increases from 4.5 cm in November to 6.5 cm in June. The modal length in June is similar to the calculated mean length of 6.9 cm at first check formation (Table 11).

The mean total lengths at the end of each of the first three years of life for this stream population of B. liberiensis are-6.9 cm, 8.3 cm and 9.3 cm. No significant differences were found in body length at given ages between males and females. The females, however, tended to live longer. For example, of the total of 13 fishes with three checks on their scales, only two were males (Table 11). Similarly, of all the specimens over 10 cm caught, only one was a male.

Several specimens over 10 cm long were obtained with only a single mark on the scale or with the first mark formed at well above the average size. For example, one female fish measuring 10cm at capture had a single ring which was formed at 9.1 cm and another measuring 11.9 cm formed its first ring at 10.3 cm. It seems likely, therefore, that some fish do not form the first or perhaps even the second annual mark. It is possible that

460 A. I . P A Y N E

spawning does not occur in these fish for the first year, although no fish of the appropriate size has been found without developing gonads during the breeding season. It does, however, seem most improbable that the fish mentioned above did not spawn before reaching 10.3 cm, a size beyond which the majority of the population do not survive. The most plausible explanation is that certain individuals, because of a more efficient metaholism or superior competitive abilities, are able to avoid the necessity of resorbing material from the scales during gonad development. Of 47 B. liberiensis with two or more checks on the scales, 10 had apparently not formed a first annual mark.

TABLE I1 Body length at annual ring formation in B. liberiensis. The Table shows the frequency of formation at various lengths

in a sample offish. I=mid-point of 2 crn length classes; n=annual ring number

Whole population Males Females

I\n 1 2 3 4 1 2 3 4 1 2 3 4

5 . 5 5.7 5.9 6.1 6.3 6.5 6.7 6.9 7.1 7.3 1.5 7.7 7.9 8.1 8.3 8.5 8.7 8.9 9.1 9.3 9.5 9.7 9.9

10.1 10.3 10.5 10.7 10.9 11.1

Total Mean (cm) 0

4 1 5 5 4 9 9

11 12 1 7 1 9 3 1 1 6 4 1 9

9 9 6 2 3 4 5 1 3

1 1

2

1 1

2

3 4

3 5 5 6 6 1 2 2 3 2 5 1 1 6

3 6 4 1 1 1

1 1

2

1 1 4 1 4 5 2 4 1 3 1 5 2 4

6 4 5

2 2 5 1 3

4

2

1 1

90 50 12 5 45 21 2 1 36 22 14 4 6.9 8.3 9.3 10.6 7.0 8.1 - - 7.0 8.4 9.3 10.6 0.67 0.48 0.45 - 0.32 0.49 - - 0.45 0.80 - -

AGE A N D GROWTH DETERMINATJON FROM SCALES 46 1

Discussion Check formation in B. Iiberiensis takes place in two stages; first, resorption of scale

material, which occurs during April and coincides with the most rapid phase of gonad development and second, the establishment of the mark itself during July by which time gonad maturation is complete but spawning has not yet taken place. The interaction of calcium metabolism and the reproductive cycle is the most important factor in the pro- duction of this type of mark (Simkiss, 1974). Resorption during gonad maturation probably reflects an increased demand for calcium by the gonads at this time which can only be met, at least in part, by a net removal of calcium from the scales and possibly other skeletal structures. In the cichlid Sarotherodon esculentus (Graham) gonad maturation is accompanied by a measurable reduction in the calcium content of the scales (Garrod & Newell, 1958). Once the gonads are almost ripe in B. liberiensis the calcium requirement may decline which would allow the observed regeneration of the scale to take place although in the irregular fashion which gives the characteristic check structure. Females, which have rather large gonads in B. liberiensis, might be expected to have more pro- nounced scale checks because of a greater demand for calcium during gonad maturation but this cannot be confirmed by the present results.

Similar marks, associated with reproduction, have been observed among temperate fish, for example in roach, Rutilus rutilus (L.), (Jones, 1953; Cragg-Hine & Jones, 1969) and have often been referred to as “spawning marks”. This term is not suitable in the situation exemplified by B. liberiensis where the check is formed before spawning. Here, they could more accurately be called “maturation” or “breeding” marks.

The scale checks which appear regularly in B. liberiensis are fundamentally different in structure from the annual rings used to age temperate fish. The temperate annual rings result from periods of slow growth in winter, represented by scale circuli being close together, followed by accelerated growth and widely spaced circuli in the spring. This gives a mark of very different appearance from the irregularly anastomosing circuli produced by scale resorption followed by regeneration in B. liberiensis.

Check formation in two phases has been produced in laboratory feeding experiments on young of the salmonid Oncorhynchus nerka (Walb.) where starvation caused resorption of scale material and the mark itself only appeared when feeding was recommenced (Bilton, 1974). This is an analagous situation to that in B. liberiensis where the mark actually appears when gonad maturation is almost complete.

Checks appear on the scales of B. liberiensis as gonad maturation approaches comple- tion which, in turn, coincides with the beginning of the single rainy season in Sierra Leone. Check formation can be correlated most closely with reproduction than with any other factor, including starvation or temperature changes. Van Someren (1950) compared the time at which the check appeared on the scales of Salmo gairdneri (Richardson) in Kenyan mountain streams with four variables : maturity stage, feeding intensity, food supply and water temperature. A strong positive correlation was only obtained with the maturity stage, since the checks were usually found at the edge of the scales of ripe, spawning or spent fish. Such marks were consequently most in evidence among fish in the final stages of upstream spawning migration, which coincides with flood periods between April and June. Since Van Someren attributes formation of the check to maturation of the gonads, there would generally appear to be considerable similarity between S. gairdneri in streams of the Kenyan highlands and B. liberiensis in the forest streams of lowland Sierra Leone.

462 A. I . PAYNE

Holden (1955) and Garrod (1959) established that Sarotherodon esculentus in the north of Lake Victoria generally laid down two scale rings per year which could be correlated with the two spawning seasons and the two peaks in rainfall.

Holden (1955) suggested that check formation was dependant upon the condition of the fish and that there is a threshold level of condition below which check formation occurs. This situation would be most likely to arise in S. esculentus during the reproductive season with the various demands this makes upon metabolism. In B. liberiensis the somatic condition, i.e. condition of the body minus gonads, is considerably lower during gonad maturation than it is during either the non-breeding period or the period immediately before spawning (Payne, 1975). Somatic condition tends to be low or in decline during the resorptive phase of check formation and rising or high when the mark is actually formed. The decline in somatic condition, i.e. weight in relation to length, during gonad maturation reflects the shift of resources from the body to the gonads which apparently includes calcium from the scales. These observations on B. liberiensis agree with Holden’s hypo- thesis in that there does appear to be a correlation between changes in condition and the two phases of check formation seen in B. liberiensis.

Holden (1955) stated that mature S. esculentus sometimes maintained their condition during maturation and consequently did not form a check on the scales. A similar situation occurs in B. Iiberiensis where a significant proportion of the population apparently does not form the first or sometimes even the second ring.

In the Malamfatori area of Lake Chad, Northern Nigeria, Alestes baremose (Joannis) form “winter rings” in January and February associated with the cold water temperature at that time; in addition mature fish of three years and above form rings in July to Sept- ember which can be correlated with spawning (Hopson, 1972). The large seasonal tempera- ture fluctuations in Lake Chad, 14” to 34°C in shallow water and 17.4” to 32.2”C in open water, which initiate check formation in A. baremose emphasise the insignificance of changes such as those recorded in the Botanical Garden stream, Sierra Leone where the range was 22.6” to 25°C.

Hopson observed that winter and spawning rings in A. baremose were similar in both form and clarity. The checks themselves were described as consisting of breaks or irregular- ities in the circuli, and from a comparison with the plate in Hopson’s paper (1972) the structure of the checks is essentially similar to that of the checks on B. liberiensis scales. In B. liberiensis the main period of check formation, April to July, slightly precedes actual spawning in July and August. In A . baremose check formation and spawning tend to coincide, as they take place in July to September and July to August respectively. There is no indication as to whether scale resorption precedes check formation in A. baremose as it does in B. liberiensis.

Regular checks have been found on the scales of Hydrocynus vittatus Castelnau in Lake Kariba (Balon, 1971). The first check or “juvenile mark” in H . vittatus is discernable as a change in density of the circuli but in the second and subsequent checks resorption is evident. The resorption seems to result from spawning. The time at which the marks were formed in H . vittatus could not be precisely determined.

By comparison, Griffith (1975) found that for H. vittatus in Lake Bengweulu, Zambia, the wide temperature range (26.5”-18”C) leads to the production of distinct scale checks during September in addition to those resulting from reproductive activity in January, a situation similar to that found for Alestes baremose in Lake Chad (Hopson, 1972).

A G E A N D GROWTH DETERMINATION F R O M SCALES 463

Griffith suggested that the minimum temperature in Lake Kariba of 27.5" was never low enough to cause the formation of a scale check, hence the single annual check recorded by Balon (1971) for H. vittatus in this lake. In Lake Bangweulu also the female H. vittatus can survive up to 11 years whilst the males seldom live longer than four.

Three species of Syndontis in the Nile at Khartoum all formed rings on the vertebral centra (Bishai & Gideiri, 1965). These rings were more pronounced in mature individuals suggesting a possible link with reproduction. They were formed during the flood season of the Nile, in June to September, although whether this was also the reproductive period was not recorded.

Annual rings were also found on the centra of Bagrus meridionalis in Lake Malawi (Tweddle, 1975). They were most pronounced in mature fish and were probably laid down during the breeding season. Female B. meridionalis tend to live longer than males and consequently attain a larger size, a phenomenon similar to that found in Barbus liberiensis and H. vittatus (Griffith, 1975). The early demise of the males could assist in resource conservation by a population and as such would be a readily understandable adaptation in species such as B. Iiberiensis which live in headwater streams where resources are scarce. It is not so readily apparent why the same might be true for the two lake-dwelling species although it may indicate how compact niches have become in the tropics.

Generally, it seems that for several types of fish in different parts of tropical Africa, the formation of a mark on the scale or other skeletal structures can be associated with breeding and in particular with the maturation of the gonads. Where there is a wide seasonal fluctuation in temperature and the annual minimum is relatively low (probably less than 20°C) additional rings may be formed. In most of the examples given above, however, (Van Someren, 1950; Holden, 1955; Hopson, 1972; Balon, 1971; Bishai & Gideiri, 1965; Tweddle, 1975) including B. liberiensis, the authors have pointed out that immature or younger fish show a low frequency of check formation or have checks that are poorly marked.

Check formation may not invariably be associated with reproduction alone in the tropics. Daget (1952) investigated the age distribution and growth of five Alestes species in the middle Niger, an area which has a similar seasonal pattern to Sierra Leone except that the dry period is more protracted. From length frequency data it appeared that during the period of low water in the dry season, from January to June, there was virtually a cessation of growth, although during the first year the fish tended not to form a mark. It was not demonstrated exactly when check formation took place but Daget stated that it occurred during the dry season and attributed it to the poor nutritional condition of the fish at this time. Gonad maturation, which was observed towards the end of the dry season, may have been a contributory factor, however.

Lowe-McConnel (1963) found that, in the dry season pools of the Essequibo drainage area in the savanna zone of Guyana, certain fish, particularly cichlids, had rings at the edge of their scales in April, towards the end of the dry season. Since little food was available at this time and also the gonads were in the later stages of development, these two factors were considered to be responsible for the formation of the rings.

There is a distinct possibility that the factors giving rise to the seasonal checks on scales may be more complex in savanna regions, such as the middle Niger or the Essequibo basin, where rivers tend to be reduced to a series of pools in the dry season and competition for food must be severe, compared with other environments such as rivers in the forest belt,

464 A . 1. P A Y N E

large lakes or mountain streams where water supply and food availability are not so drastically curtailed. Nevertheless, marks appear on the scales at predictable times and therefore can be used in the estimation of age and growth.

Although check formation may be predictable in some species, such as B. liberiensis, this does not apply to all fish in the tropics. For example, ring formation was irregular and difficult to interpret in Sarotherodon variabilis (Blgr.) in Lake Victoria (Fryer, 1961). Similarly, S. macrochir (Blgr.) in fishponds sometimes produce a ring with spawning and sometimes without (De Bont, 1967). Hemichromis bimaculatus Gill inhabits similar environments in Sierra Leone to those of B. liberiensis but checks are never found on the scales even though H. bimaculutus has a distinct breeding season extending from the mid- dry to the early rainy season. Alestes longipinnis (Gunther) often occurs in the same streams as H . bimaculatus and yet shows clear checks on the scales similar to those of B. liberiensis. It is evident, therefore, that each individual case in the tropics must be investigated carefully and that the situation is rather less uniform than in temperate regions.

Conclusions Where regular checks have been found on the scales of tropical freshwater fish they can

most commonly be associated with gonad maturation although seasonal scarcity of food may also be a contributory factor in some savanna rivers. As reproduction often takes place during the early rains or flood period, check formation tends to coincide with these conditions. In exceptional circumstances, where wide seasonal temperature variations occur, low temperatures may have additional effects. The earliest checks are often in- distinct or do not appear in at least a proportion of young individuals of a population. The structure of the scale mark in tropical freshwater fish is different to the annual ring en- countered in temperate types. In B. liberiensis the mark is formed firstly by resorption of scale material during the period of most rapid gonad development, followed by regenera- tion of the scale when gonad maturation is almost complete.

I would like to thank Dr G. Legg of the Booth Museum, Brighton, and Mr L. S. Ahulu of the Zoology Dept., Fourah Bay College, University of Sierra Leone for their help with the photo- graphic plates.

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