1.1 The collapse of a fishery
Lake Malombe is sometimes considered as a foreboding of what could happen with an African fishery if the growth of fishing effort remains unchecked (Coulter, pers. com.): around 1990 an important fishery on a group of tilapiine fish species called Chambo collapsed as a result of overfishing (FAO, 1993; Bulirani et al., 1999; Palsson, Bulirani and Banda, 1999). Annual catches of the three species that form the Chambo complex Oreochromis lidole, O. squamipinnis and O. karongae, dropped from 9 300 tonnes in 1982 to a mere 50 - 200 tonnes from 1993 onwards. Subsequently the Oreochromis fishery was almost completely replaced by a fishery on a complex of haplochromine cichlids - Kambuzi, the output of which fluctuated heavily, reached levels of around 9 500 tonnes in 1987 and 1990 but dropped to a level of around 2 800 tonnes four years later and has not recovered since (Figure 1).
Presently, the exploitation of Lake Malombe is dominated by two artisanal fisheries: a gillnet fishery and a purse-seine type of fishery, locally known as Nkacha. Gillnets are used as stationary nets and as open water seines; they are on average 750 m long with varying mesh sizes. More important - and more contested as will be discussed later - are the Nkacha nets: the open water purse seines are operated from two planked boats by seven crewmembers using it in day and night shifts. The nets are around 150 m long with a mesh size of 14 mm. Before the collapse of the Oreochromis fishery two other gear types were important as well, both beach seines. Large Chambo seines with a headline length ranging from 50 m up to 1 800 m (mean around 800 m) and a depth of 5-20 m required the labour of 10 to 30 people to operate. Kambuzi seines were nets around 200 m in length (range 50-700 m), with a depth of 2-12 m, a mesh size of 15 mm and required 6 to 20 people to operate. These expensive large seines were sometimes operated in pairs or in combination with a larger Chambo seine, where the second net was pulled around the first to catch fish escaping from it, a fishing method called Chalira. The large Chambo seines disappeared shortly after the collapse of the Oreochromis fishery, while most small meshed Kambuzi seines presently have been converted into purse seines. In 1993 the proportion of illegal gears used in the Malombe fishery - i.e. gears outside proscribed size and mesh size regulations - was between 40 percent and 75 percent for these four methods (FAO, 1993; Hara and Jul-Larsen, 2003).
FAO (1993) presented evidence that high fishing effort was the cause of the decline in Oreochromis catches since 1981 and the report warned of a possible collapse of the haplochromine fishery as well. For Oreochromis spp., an MSY of 6 000 tonnes per year was calculated, which was reached from 1984 until 1989, at the estimated effort levels.
FIGURE 1 A. Total catch from Lake Malombe as a proportion of the total catch of Malawi. B. Total catch of Lake Malombe broken down by species and species groups.
Nevertheless, a sharp decline in catch occurred, with effort decreasing further in 1990 and 1991 while neither catches nor CPUE improved. The possibility of recruitment overfishing caused by excessive fishing of the parent stock, was ruled out, as effort levels exploiting the parent stock were low compared to the years between 1976 and 1983, when catch levels were maintained. FAO concluded that excessive fishing of the 0+ and 1 year juveniles by small meshed beach seines and purse seines was responsible for the collapse: 70 percent of all Oreochromis spp. caught were taken by these gears. Small meshed beach seines also were thought to harm Oreochromis spp. during breeding and brooding periods, as these seines were dragged through habitat utilized by Oreochromis, destroying extensive under-water weed beds (Tweddle, Alimoso, and Sodzabanja, 1994). Oreochromis spp. that escaped was subsequently fished further away from the shore by gillnets: fishing mortality peaked at both sizes of around 10 cm (0+ and 1+ year of age) and 25 cm and larger (3+ year of age). This is a typical case of technical interaction between different fisheries: small meshed nets, in particular Kambuzi seines and Nkacha nets exploit the juvenile stages in the inshore area, while larger meshed gillnets nets exploit the three year and older fish. In short: the Oreochromis stocks were thought to have collapsed by a combination both of excessive fishing of the juvenile stock followed by a high but in itself not unsustainable fishing pressure on the parent stock. The collapse possibly was accelerated by destruction of Oreochromis nursery habitat, though the evidence is scant.
In addition all models suggested that haplochromine - Kambuzi - stocks were overexploited by the purse-seine fishery. Nevertheless the Kambuzi fishery remained profitable. Therefore a complete collapse of the Malombe fishery was not outside the realm of the possible: the future of the Malombe fishery was bleak. Recommendations to overcome the situation included: limit access through licensing; limit numbers of gears to present levels; forbid fine meshed beach seines and stimulate conversion of them into purse seines; gear and mesh size regulations for all gears (e.g. the purse seines should have headline length of 150 m and a minimum mesh of 25 mm); no closed season. Continued monitoring was considered of the utmost importance. Since then much work has gone into an attempt to regulate fishing effort through co-management arrangements (Donda, 2000).
1.2 Lake Malombe: production, productivity and physical environment
Lake Malombe had a very high fishery production of between 60-320 kg/ha (average 193 kg/ha) that lead to a long-term annual average fish yield of around 7 500 metric tonnes. The fishery contributed around 15 percent to the total fish catch of Malawi during the 1980s with a peak of 22 percent in 1982, but levels dropped to between 5-10 percent in late 1990s (Figure 1). The 390 km2 lake is shallow, twice as long as wide, and lies in the outflow of Lake Malawi through the Upper Shire River. The average depth is 5-7 m with a maximum of around 17 m. Except for areas of submerged vegetation and Typha swamps found around the in and outflow of the Shire River, it is a fairly featureless open water body. Small-scale fishing only started in the 1960s after the destruction of a large crocodile population (Tarbit, 1972; Tweddle, Alimoso and Sodzabanja, 1994). Dense weed beds, reported on already in the 1940s, and lakeshore reeds were cleared in the 1970s and 1980s to facilitate seining. Currently few weed beds occur in the lake (Weyl pers. obs.). The lake is fully mixed, is fairly turbid with an average visibility of 2.4 m.
Rainfall and runoff are believed to contribute significantly to the productivity of the lake, which is much higher than that of the South East arm of Lake Malawi, where the Shire River originates (FAO, 1993). Average annual water levels measured in the Upper Shire at Mangochi, 6 km from its entrance into Lake Malombe have decreased by around 3.5 m over the period from 1978 to 1999 (Figure 2). During most of the decade of 1990s the area received little rainfall that resulted in a complete recession of Lake Chilwa further south in Malawi (Zwieten and Njaya, 2003). If rainfall and runoff are important for the productivity levels of Lake Malombe, then increased fishing effort and declining productivity must have at least acted in conjunction. However, possibly as a result of the overwhelming evidence for overfishing, the possibility of changing productivity of the stocks and the lake as a result of decreasing water levels, increased runoff and sedimentation due to deforestation around the lake (mentioned in discussions with older fishermen as a cause of decreased productivity of Oreochromis) or habitat destruction received limited attention in the discussion on the causes of fluctuations and decline in fish stocks. Habitat destruction and the effect of changes in rainfall and runoff were hinted at as causal factors, but never investigated. This is important when evaluating the success of management measures directed to limiting fishing effort: if these factors contributed to lowered productivity or even to structural changes in the pathways of biological production of the lake, expected effects of management measures may take place at a slower rate, may not be as large as expected or will not take place at all.
In this paper we will not address these issues on the level of biological processes. Here we will discuss whether fisheries authorities have the capacity to timely detect important changes and trends based on available monitoring information on catch rates, effort developments and changes in environmental drivers - those indicated by water levels: can changes and trends in fish stocks can be perceived timely and causes of change and variability in catch rates be interpreted meaningfully with the information systems available? Three questions arise:
is there evidence for possible changes in productivity of Lake Malombe as a result of environmentally driven factors?
is it possible to detect the combined effects of productivity changes and increased effort through the existing catch and effort and water level monitoring systems in Malawi?
what consequences do the answers to these questions have for the resource management of Lake Malombe?
FIGURE 2. A. Annual average (thick line) and mean monthly (thin line) water levels in the Upper Shire River at Mangochi B. Long term average monthly water levels and highest and lowest recorded monthly water levels.
 Observations in 2003
indicate that a large part, possibly up to one third, of the catch of the Nkacha
purse-seine fishery on Kambuzi now consists of snails (M. Banda, pers. rem.;
P.A.M. van Zwieten, pers. obs).|
 Though mesh size may be as small as 6 mm (Weyl, pers. obs).
 See Banda et al. (2002) for developments on proposed management strategies from a biological point of view.
 Van den Bossche and Bernascek (1990) give an average depth of 4 m. But Figure 2 it can be inferred that average annual water levels over a 20-year period ranged over 3.5 m. Adam in the lower Shire River regulates the minimum water level.
 Compare with the South East-arm of Lake Malawi: visibility 7.8 m; Lake Malawi, 12.5 m (Secchi disc readings).