PART II
MANAGEMENT OF FISHERIES IN RIVER AND LAKE BASINS: BASIC AND APPLIED STUDIES FOR FISHERY MANAGEMENT INFORMATION ACQUISITION/THEORIE ET PRATIQUE DE LA COLLECTE DE DONNEES SUR L'AMENAGEMENT HALIEUTIQUE DANS LES BASSINS FLUVIAUX ET LACUSTRES

SESSION III   APPROACHES TO THE MANAGEMENT OF FISHERIES IN RIVER AND LAKE BASINS/METHODES D'AMENAGEMENT HALIEUTIQUE DANS LES BASSINS FLUVIAUX ET LACUSTRES

THE IMPORTANCE OF LONG-TERM DATA COLLECTION ON RIVER FISHERIES, WITH PARTICULAR REFERENCE TO THE CYPRINID (OPSARIDIUM MICROLEPIS, GUNTHER, 1864) FISHERIES OF THE AFFLUENT RIVERS OF LAKE MALAWI

IMPORTANCE DE LA COLLECTE DE DONNEES A LONG TERME SUR LA PECHE FLUVIALE, NOTAMMENT EN CE QUI CONCERNE LES POPULATIONS DE CYPRINIDES (OPSARIDIUM MICROLEPIS, GUNTHER, 1864) DES AFFLUENTS DU LAC MALAWI

by

D. Tweddle
Fisheries Research Officer
Fisheries Department
P.O. Box 111
Karonga, Malawi

1. INTRODUCTION

1.1 General Problems of Fishery Management

The greatest problem facing the African freshwater fisheries administrator is the lack of reliable long-term scientific data on which to base management decisions. There are a number of reasons for this. Most countries in Africa have been independent for a relatively short period of time, and prior to this scientific research consisted in many cases of short-term expeditions by scientists from colonial powers. These scientists often produced excellent results (e.g., in Malawi: Bertram, Borley and Trewavas (1942); Lowe (1952); and Jackson, et al. (1963)), but there was rarely any consistent follow-up. Since independence, countries relied heavily on expatriate help until indigenous personnel could be trained. This training process is still in operation in many countries. Under these circumstances expatriates employed on short-term contracts were expected to produce positive results in the time available. Short-term research programmes were therefore the rule. Further complications are that, where research is being funded as part of an aid project, when that project comes to an end there is often neither the money, equipment nor personnel available to continue monitoring. The problems are now being alleviated with the increasing numbers of trained indigenous personnel. Even now, however, with the development and expansion of the Fisheries Department activities, competent research personnel may find themselves promoted to positions where administrative pressures leave little time for research. The problems are not of course confined to tropical Africa. Bruton (1980) has criticized the trend in southern Africa toward multidisciplinary research teams working on water bodies (usually newly established dams) for fixed periods of time and then discontinuing the programme so that data on long-term trends are not available.

Despite these problems, a considerable body of information has been built up, especially with respect to the African Great Lakes' fisheries (e.g., Turner, 1978). Rivers have been less well monitored, although this is now changing, with CIFA playing a prominent role in coordinating various research projects.

1.2 Problems Involving Fisheries for Migratory Species

In this paper the intention is to draw attention to a category of fishing which has received little long-term scientific research, but has been the subject of some speculation in recent years: fisheries for cyprinids which live in lakes but migrate up rivers in flood to spawn. The decline of Labeo victorianus, Boulenger in Lake Victoria has been well documented (e.g., Cadwalladr, 1965). In this case fisheries records were available over a reasonable period of time and it was thus possible to correlate the decline in catch per net in rivers at opposite sides of the lake with the introduction of floating gillnets in the rivers. However, positive results of this kind facilitated by long-term records are rare.

In Malawi, Lowe (1952) recommended that some fishing effort on the Sarotherodon stocks be diverted to the abundant Labeo mesops, (Gunther) in order to relieve the pressure on the former. Now, 30 years later, the Sarotherodon stocks continue to thrive despite increased fishing pressure, while Labeo mesops has become very uncommon in much of its former range. FAO (1976) presented gillnet CPUE data for the Southeast arm of Lake Malawi. These data indicated a sharp decline in Labeo mesops and catfish CPUE since the early 1950s, while Sarotherodon remained fairly constant. The total CPUE declined from over 40 kg/100 m net/night to approximately 5 kg in the early 1970s; the decline being most marked between 1957 and 1961. The reasons for the decline are not clear. Nylon gillnets were becoming increasingly available at this time and their greater efficiency may have been the cause. However, one would have expected a similar decline in the Sarotherodon catches, but this did not occur. Increased fishing pressure in the vicinity of river mouths when the Labeo gather prior to the spawning run, has been suggested as the most likely cause of the decline in CPUE. This certainly seems a likely reason and correlates well with findings on Labeo victorianus in Lake Victoria (Cadwalladr, 1965) and Labeo altivelis, (Peters) in the Luapula River, Zambia, where excessive fishing to supply a protein diet to workers in Zairean copper mines, not only virtually exterminated the Labeo from that river, but also destroyed the annual mass spawning migration (Jackson, 1961). However the situation may not be quite as straightforward in Malawi. Catfish catches have declined to a similar extent and these, with the exception of Clarias gariepinus (Burchell), do not breed in rivers. Besides C. gariepinus the category catfish includes Bagrus meridionalis (Gunther) and a flock of at least 12 Bathyclarias spp. The species most involved in the decline are therefore not known. C. gariepinus is not an obligatory riverine breeder, breeding also in marginal flooded areas, and is unlikely to be affected by heavy fishing pressure at river mouths as suggested by Turner (1978). It seems most probable that, in the case of catfish, heavy fishing pressure on species which reach maturity at a size larger than that at which they are recruited to the fishery is the main reason for the decline. Mesh size regulations protect the Sarotherodon until after they reach maturity.

In the case of Labeo the situation is more complicated. Heavy fishing pressure prior to the spawning run is undoubtedly a factor in the decline. Another factor which should be considered, is the effect of increasing agricultural use of the river catchments. Clearance of trees and shrubs along river banks and increasing use of land for agriculture exposes the soil to more rapid erosion and faster runoff of rain water. Streams which a few years ago ran for several days or weeks at a time between well-wooded banks and over grassy flood plains are now flash flood streams carrying a heavy silt load. Smothering of eggs by siltation and lack of time for the eggs to develop before the streams dry up may well be major factors in the decline of Labeo stocks. Labeo mesops are not restricted to the large rivers and often run up the smallest streams to spawn. They are therefore not as vulnerable to the effects of river mouth fishing as the mpasa or lake salmon (Opsaridium microlepis) which is restricted to large rivers.

While CPUE figures give a good indication of the state of the fishery, they do not tell the complete story unless they are correlated with the total effort. If effort is increased fourfold and the CPUE declines to half the initial rate, the total amount of fish being caught is still twice the initial amount. A decision on allowable fishing pressure must take both the potential yield in terms of fish tonnage and economic factors into account. These facts illustrate the need for consistent long-term data collection. This cannot be too strongly emphasized: to quote from Welcomme (1979) “Long-term evaluations of catch made over a number of years have given disappointing results often because the characteristics of the fishing gear change inadvertently through the time series. It is therefore desirable to establish some form of standards for experimental fishing gears in order to ensure continuity and comparatibility of samples in time and between different systems”.

2. MPASA RESEARCH

The results of a two-year research study on the North Rukuru River mpasa fishery are presented to illustrate the difficulties in making management recommendations on the strength of data gathered over a short time span.

The Malawi Fisheries Department activities were concentrated, until very recently, on the most productive water bodies with easy access to major urban areas, i.e., Lake Chilwa, the Lower Shire marshes and the Southeast arm of Lake Malawi. The mpasa (endemic to Lake Malawi) does not appear from the records to have ever been an important commercial fish in the southern part of the lake because of the lack of suitable spawning rivers in the area. Therefore, no conclusions on present status in relation to the past can be drawn.

The Fisheries Department expanded its activities northward on Lake Malawi to Salima in 1974, Nkhota Kota in 1977, and Karonga in 1978. Statistics collection has therefore only been in existence in the northern region since 1978–79 and the whole coastline is still not fully covered.

The main mpasa rivers, listed below, are shown in Figure 1.

Figure 1 Lake Malawi, showing mpasa rivers mentioned in the text

Linthipe River

The most southerly major mpasa river is the Linthipe, south of Salima. Historical records are scarce but “in January 1939 the water in the Linthipe River was said to be boiling with mpasa” (Bertram, Borley and Trewavas, 1942), hardly an objective scientific statement but probably indicative of a thriving population. The Linthipe is still an important mpasa river and electrofishing the lower reaches during the dry season produces many fry. The size of the run is unknown because of the adverse climatic conditions and lack of qualified research personnel in the area at the time of the run. However, in 1979 there were reported to be a good number of mpasa being caught off the Linthipe mouth.

Bua River

The Bua River in Nkhota Kota district is the best known of the mpasa rivers. Fisheries Department personnel have been visiting this river regularly since 1975 to catch and tag mpasa in an attempt to determine growth rates and the extent of migration of mpasa. The distance of this river from the personnel qualified to carry out the research restricts investigations to very few trips a year. Catches of local fishermen cannot be utilized as the mpasa breeding area is within the Nkhota Kota game reserve and fishing is prohibited. Nevertheless, much useful information has been collected for comparison with the more comprehensive data collected in the past two years on the North Rukuru River. Results will be discussed later.

Luweya River

The current status of the mpasa run in this large river in Nkhata Bay district is not accurately known. The river was described by Jackson, et al. (1963), but they only saw two mpasa and could not estimate the size of the fishery. The mpasa spawning stretch is still very difficult to reach to assess the present status of the fishery, although one report from an untrained observer suggests it is still very important.

South Rukuru River

The past and present status of the run in this river in Rumphi district is unknown.

North Rukuru River

This river in Karonga district supports a thriving and very important mpasa fishery, which has been the subject of detailed study over the past two years.

Songwe River

This river is probably even more important than the North Rukuru, but it is difficult to assess as the tracks to the spawning stretches are virtually impassable till after the rains have finished. Also the river forms the border with Tanzania so that half of the catch could not be recorded. It is however proposed to make a more detailed assessment of the fishery in the next season.

2.1 Summary of Current Knowledge

It is clear from the above that no long-term data are available on the mpasa fisheries. In an attempt to assess the history of the fishery, the Nkhota Kota Fisheries Office issued a questionnaire to fishermen in the area of the important rivers. The results of this questionnaire are shown in Table 1. At first sight this table indicates a drastic decline in catches. However several points should be borne in mind:

1. Fishermen the world over are notorious for insisting that things were better in the old days.

2. If a fisherman caught over 50 fish in his net in a night he would be far more likely to remember that night many years later than the numerous nights with less catches.

3. The fishing intensity has undoubtedly increased greatly. The same amount of fish might well be caught, but spread among many more fishermen.

However despite these provisions, the circumstantial evidence does point to an overall decline in the mpasa fishery. It is against this background that the present mpasa research programme on the North Rukuru River was initiated.

3. METHODS

Regular sampling trips were made during the 1979 and 1980 dry seasons. It was quickly established that the mpasa holding stretch was restricted to approximately 5 km either side of the Karonga/Chitipa road bridge at Mwankenja. Above this the river flows through a very steep gorge. The river in this stretch consists of a mixture of deep pools and fairly severe rapids with few of the shallows and fast flowing gravel stretches which mpasa seem to prefer. We were informed by several fishermen in the area that mpasa travelled as far as a waterfall above the gorge, but thorough sampling in a 1-km stretch below the falls on 14 July 1979 failed to yield any mpasa. Several of the mpasa's smaller relatives, the Sanjika (O. microcephalus (Gunther), were taken. Sanjika are known to penetrate further up-river on spawning migrations than mpasa (Jackson, et al., 1963). Below the bridge the lower limit of the holding stretch is the point where the river flows out of the hills on to the flat, sandy coastal plain, where the river widens, slows and becomes very shallow.

Table 1

Mpasa fisheries in Malawian rivers: 1940s to present

 5 = 51+ mpasa/100 m net/night
 4 = 26–50 mpasa/100 m net/night
 3 = 11–25 mpasa/100 m net/night
 2 = 6–10 mpasa/100 m net/night
 1 = 5 or fewer mpasa/100 m net/night
 * = minor fishery within river
** = major fishery within river

Mpasa were caught on rod and line using a range of spinners which meant that different depths and habitats could be fished effectively, thereby sampling the entire population in the river. Wet fly fishing was also used effectively at the end of the season when the river was low and clear when spinners were ineffective.

Total, fork and standard lengths, sex and condition were recorded for each fish, which was weighed to the nearest 25 g for large fish and 5 g for small fish. Lengths were taken to the nearest cm below, thus 25 cm includes fish from 25.0–25.9 cm. Ovaries were taken from active and ripe females for fecundity estimates. Scales were taken from the shoulder region of all fish for examination for growth marks later. Stomach contents were examined for all fish.

In 1980 a Development Assistant was posted to Mwankenja to record catches. The mpasa holding stretch of the river was divided into four beats, each of which was sampled for four days each month. The fishing methods and catches of all fishermen seen were recorded at the time they were encountered, number and weight of each species being taken. The fishermen were then interviewed again on the following day to find out how many fish they caught after the original interview. Weights were extrapolated from averages for fish actually weighed. The catch for the three days on each beat (the fourth day being spent seeking out missing fishermen) was extrapolated to give the catch for the full month. As all fishermen on the river live along the banks because of the nature of the terrain, this recording system worked well. In addition to the statistical sampling, the Development Assistant recorded the length, weight and sex of every single mpasa he saw from the river, whether or not it came from the beat being sampled at the time. These data were kept separately and used to supplement the angling records.

Attempts to obtain juveniles from the river using electrofishing and mosquito net seines were unsuccessful. Juveniles were, however, obtained in November 1979 and January 1980. At this time a flourishing mosquito net beach seine fishery for usipa (Engraulicypris sardella (Gunther)) was in operation from the sand bar at the mouth of the North Rukuru River, and all mpasa juveniles caught incidentally in these seines were collected, data being recorded as for larger fish.

4. RESULTS

4.1 Age and Growth

Length frequency analysis

Data for the 1979 and 1980 dry season catches are shown in Figure 2. Data have been smoothed by taking a running average of three 1-cm size groups. The 1979 data include angling catches from the river. The 1980 data are a combination of angling and traditional fishing catches recorded by the Development Assistant. Data for both sexes were combined as no significant differences were evident between them.

Taking the 1979 data first, a number of year classes are clearly discernible. The smallest year class has a modal total length of 21 cm and the next 33 cm. It is more difficult to interpret the data for larger fish, but it is possible that there are further modes at 41 cm, 51 cm and 57 cm, representing older year classes.

In the 1980 data similar modes are apparent at 30 cm, 38 cm, 43 cm, 53 cm and 58 cm. The growth of individual year classes from 1979 to 1980 is indicated by arrows, corresponding year classes by dashed lines.``

It is evident that, if interpretation of the modes is correct, growth in the year was poorer than average. The distinct gap in the length frequencies, from 44 cm to 48 cm in 1979 and 46 cm to 49 cm in 1980 (less in the figure because of the effect of smoothing) helps to verify this. This gap is believed to represent a short breeding season in either 1974 or 1976, but may possibly represent nil recruitment from the 1975 spawning.

Figure 2 Length frequency data for North Rukuru River mpasa caught in 1979 and 1980. Arrows indicate growth of individual year-classes, dashed lines corresponding year-classes

The smallest year class in the river in 1979, the 21-cm mode, was virtually absent in 1980.

Data (unsmoothed) for juveniles in mosquito seine catches near the river mouth are shown in Figure 3. It is probable that in November 1979 both fry from the 1979 spawning and one year olds were present but by January the one year olds and larger fry had either moved back into the river with the early rains or had moved offshore to join the adult feeding stock. This is comparable to findings on usipa in southern Lake Malawi, where the modal length of juveniles from close inshore did not change with time, whereas offshore stocks showed steady growth (B.J. Mkoko, pers. comm.). The larger fish in the November stock approached the smallest of the 1979 river stock in size so it may be presumed that the 21-cm mode represented one-year old fish. Thus a tentative growth curve can be constructed using the 1979 data. This is shown in Figure 4. Figure 4 also shows a growth curve using the 1980 data and indicates that several more years data are needed to get a reliable picture. Two possible ages are shown for the older year classes, as it is not possible to say whether the gap in the length frequencies seen in both years is a result of nil recruitment from the 1975 spawning or a shortened breeding season in either 1974 or 1976.

Figure 3 Length frequency data for juvenile mpasa caught in beach seines at the North Rukuru River mouth

Figure 4 Growth curves constructed using the data from Figure 2. The alternative ages for the older year-classes shown are based on the possibility that the gap in the length frequencies in the mid-40 cm range is a result of a missing year-class (1975) and not the more likely explanation of a shorter breeding seasion in 1974 or 1976

Scale readings

It has not proved possible to age mpasa using marks on scales. Some marks are present and an attempt to interpret these and determine whether different river stocks have different mark patterns is still in progress.

Length and age at maturity

Data collected early in 1979, i.e., up to August indicated a very sharp dividing line between ripe and immature fish. All fish above 28 cm total length were ripe, below 28 cm immature. This length coincided with the smallest of the two-year old mode (see Figure 2) and therefore clearly indicated that mpasa reach maturity at two years old. However later in the year, in September, the one year fish were found to have developing or ripe gonads and some males were ripe-running. One-year old fish were not found in the river in any quantity in 1980.

4.2 Reproduction

Mpasa run up river during the rains. The run appears to be made at times of heavy flood. In 1980 the first mpasa were taken on rod and line in May. Prior to this the river was very high and turbid making it difficult to fish effectively and preventing the fish from seeing the spinner. The first two fish caught were females and were caught in a shallow, fairly fast cobble-bottomed stretch of river. One was ripe-running, and examination of its ovaries revealed that apart from the ripe eggs which flowed freely out of the vent under the influence of gravity the ovaries contained another set of active eggs. The second fish was in very poor condition with an inflamed vent indicating that it had recently spawned, but its ovaries were full of eggs intermediate in size between the active and ripe eggs of the other fish. There was no evidence of unshed ripe eggs from the previous spawning, suggesting that all ripe eggs are shed during the spawning session. During the dry season all fish caught were active ripe or ripe, with a high proportion of ripe-running fish from August onward. This evidence suggests that mpasa breed at least twice while in the river, once soon after running up, during or at the end of the rains and again later in the dry season when the river is low and clear.

Fecundities have yet to be determined.

4.3 Abundance

The number of mpasa ascending the river to spawn is very difficult, if not impossible, to determine. The spawning run is made during times of flood when access to the river is difficult, and a considerable amount of fishing in the lower stretches using drifting gillnets is done at night. As these fishermen, unlike those in the spawning stretch, often live well away from the river, it is virtually impossible to estimate their numbers and the sizes of their catches. Verbal reports of catches show great variation. Fishing upstream in the spawning stretch does not start until the end of the rains and weirs are not constructed until April–May. There is, as yet, no way of knowing how many mpasa run back to the lake after spawning once, before these weirs are constructed. A small number of fish have been taken in gillnets in the main lake during the dry season and the state of the gonads suggested that these fish had previously spawned although they had fully recovered condition and had therefore presumably been back in the lake for some time. The aim must be to obtain a relative rather than an absolute estimate of abundance in the river. The statistical data collected by the development assistant at Mwankenja are the first steps in this direction. Data will need to be collected consistently using a standardized recording system over several years in order to get a picture of the year to year fluctuations in catch. The first records for 1980 indicate that they year was a very poor one, with only 1.15 t of mpasa caught.

Angling success can also be used to give a rough estimate of abundance, rough because anglers vary enormously in ability and individual anglers are subject to good and bad days. Also effort is difficult to calculate as some stretches of river are more difficult to fish than others, with longer walks between fishy areas. However, as the whole river was covered evenly in both 1979 and 1980, CPUE defined as catch per trip for each of the two experienced anglers concerned, does give a guide to relative abundance. The figure for 1979 was 5.2 mpasa per trip for June and July and in 1980 1.9 mpasa per trip in May and June (fish were virtually non-existent by July), indicative of a much greater number of fish in 1979, especially as angling in that year commenced rather late and almost certainly after the peak period. Extrapolation of the relationship between angling CPUE and total tonnage of mpasa caught in 1980 to 1979 gives a very rough figure of 3.15 t of mpasa. At a price of Mlwi.K 1/kg the catch was worth Mlwi.K 3 150 in 1979 and Mlwi.K 1 150 in 1980 and provided a valuable source of high quality animal protein well inland from the lake.

The lakeshore statistical system was operation by the 1979/80 wet season so it was possible to estimate the tonnage caught in the lake within approximately 10 km either side of the river mouth as the fish gathered piror to running upriver. The figure actually recorded for January to June 1980 was 7.61 t. This is a considerable underestimate as mpasa are taken from the nets throughout the day and night and are not landed all at once, unlike the rest of the species in the traditional fisheries on which the recording system is based. Also mpasa is a highly prized (and highly priced) fish, and fishermen therefore tend to hide the fish and sell privately later to avoid local price controls. A conservative estimate of the actual tonnage caught is therefore at least 20 t.

As stated earlier it was not possible to estimate the amount of fish caught during the actual run upriver.

Comparison of North Rukuru data with data from River Bua

Length frequency data for angling catches in the River Bua in 1978 are shown in Figure 5. Enough fish were caught in that year to enable males and females to be treated separately. It appears from the data that two-year classes were present in the river, males being slightly larger (about 2 cm) on average than females. The close similarity in growth rates indicates that the decision to combine sexes for the smaller North Rukuru fish was justified. It is clear that the minimum and average sizes of mpasa in the Bua are considerably greater than in the North Rukuru. Catches in the River Bua fluctuated considerably from year to year, but so few trips were made that the data could not be statistically analysed to produce meaningful results. However, 1978 was an exceptionally good year with average catch per angler per day in early June being 15 (24 per angler on 3 June, 19 on 4 June, 13 on 5 June and 5 on 6 June). These data are underestimated as they only record the fish tagged. Others were released because they were caught too far away from the tagging station and a few kept for laboratory studies were not recorded. In 1980 the average catch per angler on the Bua (same anglers) at the end of May was five fish/day. Thus 1980 was a poor year for both the Bua and North Rukuru mpasa runs. Verbal reports indicate a similar poor year on the Lufira River.

5. DISCUSSION

One of the problems which needed to be solved in order to plan a management strategy for conserving mpasa stocks was whether the fish moved up the nearest river to spawn or whether, like salmon, they returned to their ‘home’ river. The Bua tagging programme was initiated to try to answer this question. Results, however, have been disappointing. In the few trips, which time and finance permitted, it was not possible to tag as many mpasa as we would have liked. Also, the Bua River flows for much of its course through the Nkhota Kota Game Reserve, and poachers catching tagged fish in the reserve would be unlikely to report the capture, or if they did would say the fish was caught in the lake.

The pronounced size range difference between the Bua and North Rukuru mpasa populations strongly suggests that the stocks are separate. Mpasa less than 50 cm total length are nonexistent in the Bua (fry and juveniles excepted), but fish of this size form the bulk of the North Rukuru population. As the character of the two rivers is similar, some other factor must be responsible for the disparity in size ranges. The Bua River is relatively lightly fished because it is in a game reserve. In contrast the North Rukuru is very heavily fished. Lowe visited Karonga in March 1946 and stated (Lowe, 1952): “A mpasa desirous of ascending the North Rukuru River to spawn was faced with (a) numerous crocodiles lying off the delta of the river; (b) a number of seines; (c) fishermen with scoop net in each hand chasing mpasa in the shallow water of the delta, and once in the river proper; (d) fishermen with large scoop nets fishing from platforms built every 50 yards or so along the banks for several miles of the river; (e) eight complete barriers of traps spread out over about 13 mi of river, and (f) fish netting parties, in which all the men of a village were seen to form a line across the river each with a scoop net in his hand”.

Figure 5 Length frequency data for Bua River mpasa caught in June/July, 1978

The current situation is: (a) thousands of yards of gillnets set in the vicinity of the river mouth; (b) large numbers of drifting gillnets used in the river itself; one end is tied to the operator's waist while he wades and swims downriver holding the net, about 10–15 m long, out in the current, and (c) scoop nets along the banks of the river. After the peak of the floods fish traps and weirs are constructed in side streams and backwaters and eventually, when the river is low enough, across the river itself, usually in April–May. Drifting gillnets and small seines do not appear to be used in the spawning areas until the dry season because the fishermen are uncertain of the whereabouts of the resident crocodiles. Crocodiles are now virtually absent from the lower reaches of the river and the lake around the river mouth, but there are still a few inland.

Hook fishing using mole crickets as bait and, increasingly, spinners become of major importance as the river drops, and when the river is at its lowest fish drives using scoop nets or spears are used. Thus it is extremely unlikely that any large mpasa present in the river when the weirs are constructed would survive to get back to the lake. Besides man, mpasa have to cope with predation by fish eagles, cormorants, otters and crocodiles, with various kingfishers taking their toll of fry.

It is therefore probable that the small average size of the North Rukuru mpasa population is a response to the extremely high fishing mortality, selection working in favour of the small, early-maturing fish. The evidence suggests that the mpasa has a very flexible breeding strategy which has allowed the species to survive in reasonable numbers, despite intensive fishing, whereas the nchila, Labeo mesops, which has a much more rigidly-defined breeding pattern, has suffered to a much greater degree.

Further evidence for the flexibility of the breeding strategy is shown in Figure 2, the length frequency data for the 1980 dry season. The classic pattern for any plot of length frequencies is large numbers of the smaller size groups with progressively less of the larger older fish, the difference in numbers between each year class being used to calculate mortality rates. However, despite the evidence of extremely high mortality in the river, almost as many of the larger mpasa were taken as the smaller. This strongly suggests that not all mpasa run upriver every year and that many do not run up until they are several years old. Fishing mortality in the lake as a whole, not counting the fish moving into the areas around the river mouths prior to the spawning run is negligible and the life history of the fish while in the open lake is poorly known. Two average-sized specimens (one male and one female) caught off Karonga in gillnets in October and November 1980 had stomachs full of usipa and it is believed that this is the main diet of adult mpasa. Mpasa do not appear to feed at all while on the spawning run. Juveniles and fry in the shallow weedy areas bordering the lake are opportunists carnivores, feeding primarily on winged insects but also on chironomid larvae, cichlid fry and other small fish.

The very poor spawning run in 1980 in all the rivers, for which information is available, may possibly be correlated with the apparent poor growth for all year classes in 1979–80 as suggested by the length frequency data. The usipa stock, on the available evidence, was very low during the year and catches consisted almost entirely of very large fish with no evidence of younger broods. If, as a result of this, food was scarce for mpasa, growth as a whole would be poor, with fewer fish being in good enough condition to make the spawning run.

With the apparent flexibility of breeding strategy it might be asked whether weirs have the devastating effect on the stocks suggested by Jackson, et al., (1963) who stated: “Weirs are a most primitive method of fishing and cause great harm. Their use has long not been tolerated in countries with any pretence toward being civilized. Apart from allowing very little escapement of fish, and virtually none in bad years when the water is low, they cause damming of streams which result in flooding and washing of banks with consequent silting up of the stream bed on the one hand and soil erosion on the other”. The North Rukuru River below the weirs is very sandy and very shallow over the 20-km stretch to the lake and any mpasa attempting the downstream journey would be extremely vulnerable, particularly in view of the very poor condition of the spent fish. It is probable that the majority of fish in the river when weir construction starts, would be doomed anyway and it might therefore possibly be argued that weirs are beneficial in that the fish are thereby made available for human rather than other animal consumption. However, the latter statement by Jackson, et al., (1963) is very relevant in this context. The weirs do cause severe erosion of the banks at the side of the weir, leading to loss of good agricultural land and buildings, and to siltation of potential spawning beds. They may also contribute to rendering the lower reaches of the river very shallow and sandy, thereby discouraging spent fish attempting the run back to the lake.

The change in the character of many rivers was mentioned earlier, due mainly to increased agriculture in the catchment areas. Two Southwest arm rivers, the Lisangadzi and Bwanje are excellent examples. These rivers had mpasa runs until not very long ago, but looking at them now it is very difficult to visualize them as mpasa rivers, e.g., in 1976 the Lisangadzi only flowed for three weeks. In 1977/78 it ran intermittently and yet at its peak reached a depth of over 5 m.

6. CONCLUSIONS AND RECOMMENDATIONS

6.1 Research

Short-term studies provide useful guidelines for future research. The data collected in the two years on the North Rukuru are sufficient to make very general statements on the biology of the mpasa. They have provided information on rates of growth and on length at maturity, and other general aspects of the fish's biology. They have also shown that there is still a great deal that needs to be learnt and that long-term data must be collected. These are necessary to monitor year to year fluctuations in the size of the spawning run and to determine if the mpasa population is indeed in a state of decline. Research should, if possible, be conducted on the mpasa while in the lake, but as the fish are rarely caught by any of the methods currently used in the lake, this is likely to be difficult. Year to year fluctuations in the spawning run may possibly be correlated with the state of usipa stocks over the same time scale. If so, this would be indirect proof that mpasa do feed primarily on usipa in the open lake, as suggested by the limited data available.

Further research on the breeding cycle of mpasa is necessary, to determine if the fish do breed twice or more while in the river. The spawning process has not yet been observed, but from the distribution of ripe running fish caught they appear to spawn in fast-flowing cobble-bottomed stretches of river. The eggs are negatively buoyant and are adhesive for 20 minutes after spawning (Soma, 1979 and personal observations). Thus they probably accumulate on the gravel in the slack water behind individual cobbles. Eggs have not, however, been found in the wild and the nature of the spawning stretch makes investigations rather difficult.

The prime research need is for long-term data to be collected in a consistent fashion on the major mpasa rivers, the Linthipe, Bua, Luweya, North Rukuru and Songwe. The same recording system should be used on all rivers. The personnel involved must be on very good terms with the fishermen and not be involved in enforcement of any legislation which may be enacted. As the system requires close cooperation, it is impossible to personally check every fisherman's catch at the end of the day.

Angling catches are also very important, particularly for determining the distribution of fish within the river and for investigations into the biology. Angling is also the only suitable way of catching fish for tagging, causing less damage to the fish than any netting or trapping methods.

6.2 Management of the Mpasa Fishery

It is difficult at this stage to make many recommendations. However, the following definite recommendations can be made now:

1. Any increase in agriculture in catchment areas must be accompanied by a very active land husbandry advisory service. Cultivation of the river banks should not be permitted in the upper reaches. It is no coincidence that the Bua River, whose spawning stretch and all main tributaries are protected within the Nkhota Kota Game Reserve, has the largest mpasa run at the present time, and that the North Rukuru, which also has a good run, passes for much of its course through unpopulated hilly country with deep rocky gorges.

2. A minimum size limit should be imposed on the fishery. A limit of 28 cm would protect the immature fish but would allow all mature fish to be exploited. A larger size limit, e.g., 30 cm, would protect some of the breeding stock, but would be difficult to enforce and might lead to ill feeling with the fishermen. The 28-cm limit would not deprive the fishermen of a lot of their catch, but the existence of the limit would hopefully make the fishermen aware of the need for control of the fishery for their own ultimate benefit. The stability of the lake Sarotherodon fisheries because of the size limit and minimum mesh size regulations serves as a good example to the fishermen of the value of regulations of this sort.

3. A gap should be left in the centre of fishing weirs. Laws for this have been enacted in the past and provision is made for enforcement of such a law in the present Fisheries Act if the need arises. While such laws are theoretically to allow a proportion of the fish to escape, a greater benefit might be that the main flow of the river would tend to be through the gap in the centre, thereby reducing bank erosion and river siltation.

4. It has proved possible to artificially spawn mpasa using injections of pituitary extract and to rear the fry. It is hoped to extend this work and establish a hatchery. If this is done, two avenues should be explored: (a) rearing the fish to maturity, and (b) rearing the fry to the juvenile stage and releasing them to grow naturally in the lake. If this can be done satisfactorily then the juveniles should be reared in water from a mpasa river and the hatchery should therefore be on the banks of the river, as the evidence suggests that mpasa return to the river of their birth to spawn. If the apparent decline in mpasa stocks is due to adverse effects of agriculture on spawning grounds, then augmenting stocks by rearing juveniles beyond the critical fry stage and then releasing them would help to guarantee the survival of the fish and the fishery.

In the long term it may be found necessary to impose regulations on the lake fishery near river mouths. Minimum mesh size regulations would be impractical as much of the fishing effort is for sanjika (Opsaridium microcephalus), for which the most effective mesh size is 64 mm (2 ½ in) stretched mesh, therefore setting a mesh size limit to protect mpasa until they reach adult size would eliminate this important fishery. Regulations could, however, be imposed to either: (a) limit fishing effort to X thousands of metres of gillnet total effort or Y hundreds of metres per canoe; (b) ban fishing in the vicinity of the river for a specific period each week, e.g., on Saturday and Sunday, or (c) ban fishing altogether within a certain distance, e.g., 1 km of the river mouth. Regulations of this sort would not seriously hamper the lake fishermen who could exploit other fishes, and would benefit the people inland who are dependent on the migratory cyprinids. Any regulations of this sort should only be imposed after much information has been gathered and if the accumulating evidence does point to a definite and continued decline in the mpasa fishery.

6.3 General Anadromous Fisheries Management

The data presented here indicate clearly the need for long-term research and statistics collection on the mpasa fisheries of all rivers running into Lake Malawi. Many other countries have similar fisheries for anadromous species, with similar problems. Increasing demands are being made on water resources, e.g., for agriculture, domestic water supplies, industry and hydroelectric power generation. Any or all of these could have serious adverse effects on such fisheries. It is therefore imperative that the fisheries administrator has at his fingertips accurate data on the past, present and potential value of his fishery in order to effectively argue the case for management of the water resource to the maximum benefit of all concerned. Fisheries for anadromous species may be very valuable in the cash economy and in providing top quality animal protein well inland of the major fish producing areas along the shores of lakes into which the rivers flow.

Since this paper was presented at the CIFA Seminar purse seining experiments using light attraction at night conducted by the FAO R/V ORION led to the capture of large numbers of immature mpasa in open water off the mouth of the North Rukuru River. The length frequency distribution (unsmoothed) of all fish caught on 7 February 1981 is shown in Figure 6. This plot indicates that two length groups were well represented, one with a modal length of 9–10 cm total length and the other 14–16 cm. These data strongly suggest that these groups represent two year classes the 9–10 cm fish being in their first year (approximately 9 months old) and the 14–16 cm fish in their second year. Thus the mode in the 1979 river fish length frequency at 21 cm probably consisted of two-year old fish and not one-year olds as tentatively suggested in this paper. Ages shown in Figure 4 should therefore be increased by one year.

These findings permit a slight modification of the interpretation of Figure 3. In November 1979 the majority of fish were probably one and a half years old, but by January 1980 many of these fish had moved offshore, being replaced by recruitment from the river of the juveniles from the 1979 spawning.

Figure 6 Length frequency data for juvenile mpasa caught by purse seine offshore from North Rukuru River mouth, 7 February 1981

REFERENCES

Bertram, C.K., H.J.H. Borley and E. Trewavas, 1942 Report on the fish and fisheries of Lake Nyasa. London, Crown Agents

Bruton, M.N., 1980 Fish ecology in southern Africa: the state of the art. Paper presented at L.S.S.A. Annual Congress

Cadwalladr, D.A., 1965 The decline in the Labeo victorianus Blkr. (Pisces: cyprinidae) fishery of Lake Victoria and an associated deterioration in some indigenous fishing methods in the Nzoia River, Kenya. E.Afr.Agric.For.J., 30(3): 249–56

FAO, 1976 An analysis of the various fisheries of Lake Malawi. Report prepared for the Government of Malawi by FAO of the UN acting as executing agency for the UNDP. Based on the work of J.L. Turner. Rome, FAO, FI:DP/MLW/71/516 Technical Report 1:73 p.

Jackson, P.B.N., 1961 The fishes of Northern Rhodesia. Lusaka, Northern Rhodesia, Government Printer

Jackson, P.B.N. et al., 1963 Report on the survey of northern Lake Nyasa 1954–55. Zomba, Nyasaland, Government Printer, 171 p.

Lowe, R.H., 1952 Report on the Tilapia and other fish and fisheries of Lake Nyasa 1945–47. Fish.Publ.Colon.Off.,Lond., 1(2): 126 p.

Soma, K., 1979 Preliminary report on seedling production of mpasa, Barilius microlepis. Report to Malawi Fisheries Department (Unpubl.)

Turner, J.L., 1978 Status of various multi-species fisheries of Lakes Victoria, Tanganyika and Malawi based on catch and effort data. CIFA Tech.Pap., (5): 4–15

Welcome, R. (ed.), 1979 Fishery management in large rivers. FAO Fish.Tech.Pap., (194): 60 p. (issued also in Spanish)

ACKNOWLEDGEMENTS

My colleagues Drs A.G. Seymour and D.S.C. Lewis were of great assistance in catching specimens for study and participated in many useful discussions on interpretation of the data collected. Dr J.G.M. Wilson and Messrs P. McCoy, M.M.K. Mhango and M.F.K. Nyirenda also assisted in catching fish. Mr H.K. Nyasulu, the fish recorder on the North Rukuru River, was most conscientious in carrying out his duties. The questionnaire on the history of the mpasa fisheries was organized by Dr N.G. Willoughby.

PERSPECTIVES ON THE MANAGEMENT OF SAROTHERODON TRADITIONAL FISHERIES IN THE SOUTHEAST ARM OF LAKE MALAWI, LAKE MALOMBE AND THE LINKING UPPER SHIRE RIVER

PERSPECTIVES DE MISE EN VALEUR DES PECHERIES TRADITIONNELLES DE SAROTHERODON DANS LA PARTIE SUD-EST DU LAC MALAWI, LE LAC MALOMBE ET LE BRAS INTERMEDIAIRE DE LA UPPER SHIRE

by

J.J. Chaika
Fisheries Department
P.O. Box 47
Mangochi, Malawi

1. INTRODUCTION

Fisheries management has generally aimed at finding an objective scientific basis for regulation. A number of methods applicable in the temperate countries have been tried in the tropics with very little success. Fortunately, Sarotherodon (Tilapia) spp. are known to spawn once a year. The three important species by abundance: S. saka and S. squamipinnis spawn at different periods and the spawning period of S. lidole overlaps the spawning period of the other two species. Superficial observations suggest that Sarotherodon spp. migrate between the Southeast arm and Lake Malombe via the linking Upper Shire River. What triggers the migration is unknown, but it may be for feeding or for breeding, or that Lake Malombe acts as a nursery because of its shallowness and its abundant submerged weeds.

With the exception of the adult S. lidole, the larger of the three species, the other two species are difficult to tell apart. However, the ripe males are easily identifiable because they develop breeding colours.

Thus, the management regulations of Sarotherodon fisheries are in part based on the knowledge of size at maturity and in part on their migratory and spawning periods. The fact that the mesh size limitations in the Southeast arm and the Upper Shire River on one part, and in Lake Malombe on the other, are different implies that management is dealing with two separate unit stocks. Indeed, this is the management dilemma. The area under study is the major fishing ground for the Sarotherodon spp. About 80 percent of total annual catches of Sarotherodon spp. are caught here.

The three species of Sarotherodon, S. saka, S. squamipinnis and S. lidole constitute a lucrative fishery in the Southeast arm of Lake Malawi, Lake Malombe and the linking Upper Shire River (Figure 1). In 1977, 10 688 t of Sarotherodon spp. were landed, and 78 percent of the catch was recorded from traditional fishermen.

Superficial observations suggest that the three species migrate at a certain period of the year between the Southeast arm and Lake Malombe via the Upper Shire River. The reason(s) for the migration is unknown. But it is speculated that it could be connected with breeding or search for suitable nursery ground or both. The need to protect the supposedly breeding population and the juveniles has necessitated the passing of restrictive regulations on the length of fishing gears, mesh sizes, fish size and the institution of closed seasons. There is a kambuzi (small Haplochromis spp.) fishery which is rapidly developing. Small mesh size nets are allowed for this fishery. However, the fishing gears also exploit Sarotherodon spp. The effect of the kambuzi fishery can only be assessed through observation of Sarotherodon spp. catch data over a long period. The absence of such data should not prohibit making a preliminary appraisal of some of the beliefs about the behaviour of the species, because it is on these beliefs that the regulations have been passed. Superimposed on catch per effort (Figure 2) graphs are the breeding periods of the three species.

1.1 Study Area

The Southeast arm (lat. 14°30'S, 34°50'E), an appendage of Lake Malawi, is drained by the linking of the Upper Shire River. About 16 km from the Southeast arm, the river drains into a shallow depression forming Lake Malombe (lat. 14°35'S, 35°17'E). The river between the Southeast arm and Lake Malombe has a gradient of 0.08 m/km (Figure 1).

The whole of the study area is a very rich fishing ground, due in part to its shallow depth. The deepest point of the Southeast arm is about 50 m and it gradually rises to about 5 m at the mouth of the Upper Shire River. Lake Malombe has an average depth of 5 m, and varies from 3 to 7 m. Certain areas of Lake Malombe are matted by rootless submerged weeds believed to be exclusively Najas spp.

Figure 1 Map showing Southeast arm, Upper Shire River and Lake Malombe

Figure 2 Monthly catch per effort of Sarotherodon spp. in the Upper Shire River, the Southeast arm and Lake Malombe. Superimposed are the spawning periods of the three Sarotherodon spp.

1.2 Fisheries

A fishery is arbitrarily defined to denote the type of fishing gear together with the predominant species it exploits (Table 1). Sarotherodon spp. are exclusively exploited by gillnets, chambo seines and kambuzi seines. Chambo and kambuzi seines are both beach seines. They differ only in maximum permissible length and mesh size. The operation is similar. Table 2 shows the total catches of Sarotherodon spp. from traditional fisheries in the Southeast arm, Lake Malombe and the Upper Shire River from 1975 to 1977 by type of gear.

Table 1

Southeast arm, Lake Malombe and Upper Shire River fish species by percentage by various fishing gears

Table 2

Total catch (t) of Sarotherodon spp. from traditional fisheries by type of gear in the Southeast arm, Lake Malombe and the Upper Shire River (1975–77)

Gillnets are operated only in the Southeast arm and Lake Malombe. The water current, floating weeds and logs and hippopotomi prohibit operation of gillnets in the river.

Gillnets are set in the afternoon and checked the following morning in the Southeast arm. Fish are driven into the nets in Lake Malombe. Fishing is always at night, preferably when there is no moonlight. Fishing with chambo seines is also carried out at night, also preferably when there is no moonlight.

2. SEASONAL VARIATION IN CATCHES AND MIGRATIONS

Peak catches which would denote mass migration of the three species was sought by plotting monthly catch per effort data (1976–77). Superimposed on the graphs are the breeding periods of the three Sarotherodon spp. (Figure 2). Since gillnets are not used in the Upper Shire River, catch data from chambo and kambuzi seines only are used. The unit effort is in number of pulls. If the graphs were smoothed, two peaks would be identified from the South-east arm graph. A very prominent peak appears during the month of April. There is also a peak catch in the Upper Shire during this month. If these peaks indicate spawning, then it is S. squamipinnis, and could be migrating by moving from the Southeast arm to Lake Malombe. That this may indicate a migrating population is the fact that the peak catches start a month earlier, in March, in the Southeast arm. Another peak appears in December in the Southeast arm, which may be attributed to catches of S. lidole, and/or catches of S. saka and S. squamipinnis. This is the end of the spawning period of S. saka and the beginning of the spawning period of S. squamipinnis, while it is also the peak breeding period of S. lidole.

The Southeast arm-December peak appears two months later than the October peak in the Upper Shire River. During the same time, catches in Lake Malombe also start to increase. The period coincides with the spawning period of S. saka.

The appearance of catch peaks in successively different months may suggest a mass movement of S. saka. Since the peaks coincide with the breeding periods, they may be connected with spawning.

Lowe (1952) observed that catch peaks in March–April and from September to November in the Southeast arm occurred during the spawning periods of S. squamipinnis, S. saka and S. lidole, respectively. These were also the months of lowest phytoplankton abundance.

Consequently, she concluded that Sarotherodon spp. adopt schooling behaviour at times of low phytoplankton concentration and a solitary life when food is abundant. Sarotherodon spp. are phytoplankton feeders.

3. MANAGEMENT

Despite the paucity of basic biological information regarding the Sarotherodon spp., the fishery is so important to the fishing industry in the Republic of Malawi, that it necessitated passing regulations to protect it.

The regulations set limits on the length of gears, size of meshes, depth of fishing gears, legal harvestable size and closed seasons (Table 3). The regulations are intended to protect the breeding populations and juvenile Sarotherodon spp.; juvenile Sarotherodon spp. move inshore at night. This was evidenced by large quantities caught in kambuzi seines before restricting the use of this gear to day hours only. Use of kambuzi seines in Lake Malombe is prohibited from 6.00 p.m. to 6.00 a.m. from 1 January to 31 March. This is also the period when fishing is closed in the Upper Shire River. It is assumed that young Sarotherodon spp. migrate from Lake Malombe to the Southeast arm during these months.

A question may be raised as to why smaller mesh size chambo seines and gillnets are allowed to be used in Lake Malombe when the lake appears to be a nursery ground? Or why should fishing on the Upper Shire River be closed for five months when gears of larger mesh size than those in Lake Malombe are used? Or indeed, why should the closed season in the Southeast arm be only of two months duration when similar mesh size gears as those used in the Upper Shire River are being used? This is the management dilemma. The assumption is that since the fishery on Shire is intensive and the river channel is narrow, allowing the fisheries to operate when the fish population is migrating would result in overfishing, and damage both young and adult fish due to use of kambuzi seines. The restriction on the length of beach seines to 250 m in Upper Shire is also due to narrowness of the river channel. The fishermen are not allowed to block the whole width of the river channel.

Table 3

Permissible sizes of fishing gears and fish and closure seasons imposed

4. THE MANAGEMENT DILEMMA

Any future projection of the fishery will depend on understanding of the behaviour and biology of the species. While the present management regulations are useful as a deterrent, factual evidence on such factors as the critical stage in the spawning period of Sarotherodon spp. and the relative importance of each species in the catches is important, but the young fish are so similar that it is difficult to separate one from another. The breeding males have characteristic colours. S. lidole is larger than the other two species, next largest is S. squamipinnis. This suggests that they have different growth rates.

The three Sarotherodon spp. are nest spawners. Fertilization takes place on the nest, in which case the success of a male fertilizing a female would be subject to disturbance if the nests are destroyed. Since during part of the spawning period of S. saka and S. squamipinnis the fishery in the Southeast arm and Lake Malombe is open, there could be some fishing interference with the spawning fish. The extent of this interference can only be assessed in the yearly landings. However, disturbance is caused by chambo and kambuzi seines only, and these gears are used on a few pre-selected areas; the disturbance they cause is probably minimal. Strong winds probably do just as much damage to the nests in shallow water. S. lidole is a relatively deep water spawner, probably it is free from fishery disturbance except from trawls in Lake Malawi. Due to the shallowness of Lake Malombe, S. lidole probably does not spawn in this lake.

However, a lot of S. lidole have been seen caught in the river in April (personnel observation). Some of the fish had young in the mouth of an average length of about 3.8 mm. Their appearance in the Upper Shire may be connected with taking the young to suitable nursery grounds in Lake Malombe.

5. DISCUSSION

The need for basic scientific information and yearly catch data cannot be denied. Whether such information should be exhaustive before management regulations are imposed is questionable. Considering the scarcity of resources: capital, trained manpower and money, application of deterrent regulations to a fishery that appears to be reaching its maximum sustainable yield may be necessary. The Sarotherodon fishery in the Southeast arm, the Upper Shire River and Lake Malombe is so intensive that a policy of laissez-faire would probably result in overfishing of the resource. The apparent contradictions in the application of the management tools arise from lack of detailed information. Therefore, there is a need to monitor the effects of the regulations through scientific research on the fishes and the fishery. Unless such research is conducted, there will still be a management dilemma.

REFERENCES

Fryer, G. and T.D. Iles, 1972 The cichlid fishes of the great lakes of Africa: their biology and evolution. Edinburgh, Oliver and Boyd, 641 p.

Lowe, R.H., 1952 Report on the Tilapia and other fish and fisheries of Lake Nyasa, 1905–47. Fish.Publ.Colon.Off.Lond., 1(2):126 p.