Hydrobiologia 407: 183–189, 1999. O. V. Lindqvist, H. Mölsä, K. Salonen & J. Sarvala (eds), From Limnology to Fisheries: Lake Tanganyika and Other Large Lakes. © 1999 Kluwer Academic Publishers. Printed in the Netherlands.
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Prospects for the fishery on the small pelagic Rastrineobola argentea in Lake Victoria Jan H. Wanink Institute of Evolutionary and Ecological Sciences, University of Leiden. P.O. Box 9516, 2300 RA Leiden, The Netherlands. E-mail:
[email protected]. Present address: Kluisgat 19, 9732 EM Groningen, The Netherlands Key words: Rastrineobola argentea, Lake Victoria, light fishery, beach seine, spawning areas, nurseries
Abstract The pelagic cyprinid dagaa plays a crucial role in the disrupted ecosystem of Lake Victoria. It is the main utilizer of zooplankton, a major prey for the introduced Nile perch and, after Nile perch, economically the second-most important species in the fishery. Light fishery for dagaa was started in the 1960s and boosted during the 1980s. In spite of an intensified exploitation by man, Nile perch and piscivorous birds, the dagaa population increased significantly. Spatial and temporal distribution patterns of dagaa and its potential predators restricted the harvestable fraction of the dagaa stock mainly to mature fish. An increase in recruitment to the reproducing part of the population and a reduction in generation time enhanced the prospects for a sustainable fishery. However, a recent increase in the use of mosquito seines forms a potential danger for the fishery, since dagaa seems to use the inshore waters as spawning areas and nurseries. Introduction Several lacustrine cyprinids, all being endemic to some East and Central African lakes and formerly placed in the genus Engraulicypris, have adopted a pelagic way of life. Their shape and silvery camouflage colouration resemble marine pelagic fish such as the Herring (Clupea sp.) and their pelagic existence is regarded as an adaptation to the marine-like conditions in a great lake, i.e. the occurrence of a rich and stable plankton community (Graham, 1929). In a taxonomic revision, the representative of this group of fishes in Lake Victoria, Engraulicypris argenteus Pellegrin, was renamed to Rastrineobola argentea (Howes, 1980). Although the species was locally known as ‘omena’ in Kenya, ‘mukene’ in Uganda or ‘nsalali’ in Tanzania (Graham, op. cit.) and its artisanal exploitation was reported long ago (Alluaud, 1905, cited in Graham, op. cit.), it was unknown to many fishermen until the 1960s (Chitamwebwa, 1988). A light fishery for R. argentea was started in the mid 1960s and reached commercial significance in the early 1970s (Okedi, 1981). By that time, the name ‘dagaa’, which I will use hereafter, became pop-
ular. It had been adopted from the resembling clupeids of Lake Tanganyika, which were also caught by light attraction (Okedi, op. cit.). The biology of dagaa was not extensively studied until the 1980s, when the lakes ecosystem was severely disrupted by the population boom of the introduced Nile perch (Lates niloticus L.), which eradicated most of the dominating haplochromine cichlids (Ogutu-Ohwayo, 1990a,b; Barel et al., 1991; Witte et al., 1992a,b, 1995; Goldschmidt et al., 1993). In contrast to the haplochromines, dagaa successfully survived the Nile perch predation and the increased fishing pressure (Bwathondi, 1990; Ogutu-Ohwayo, 1990b; Wanink, 1991). Various explanations for the prosperity of dagaa have been suggested. First, the disappearance of most of the zooplanktivorous haplochromines may have reduced the competition for food (Witte et al., 1992a). Secondly, being a more r-selected species in comparison with the haplochromines, dagaa was expected to cope relatively well with the disruption of the ecosystem (Bruton, 1990; Ligtvoet & Witte, 1991; Wanink, op. cit.). Thirdly, a shift from juvenile to adult mortality has probably increased the number of fish recruiting to the reprodu-
184 cing population (Wanink et al., 1999). Finally, dagaa increased its growth rate and reduced its age at first maturity (Wanink, 1996; Wanink et al., 1998). Good prospects for the dagaa fishery have been predicted from observations in the southern part of the lake, which showed that the fishery exploited merely the mature part of the population (Wanink & Goudswaard, 1994a; Wanink et al., 1999). As expected, the catches increased and dagaa became the economically second-most important fish after the Nile perch. However, a drop in the annual yield has recently been reported for the Kenyan part of the lake and attributed to an increase in the use of mosquito seines to land the fish (Riedmiller, 1994). Although the use of smaller mesh sizes increases the risk of juveniles being caught, a strong impact of beach seining would not be expected according to the idea that dagaa is a pelagic spawner (Graham, 1929). Unfortunately, the theory of Graham is based on a few floating eggs, which were possibly produced by dagaa and on only one larva, sampled at an offshore station. In this paper I compare the yield of the fishery with the catch rate of a standard sampling gear in the southern waters of the lake over a period of several years during which the fishing effort increased, but the use of mosquito seines was not common. I show that during these years the dagaa stock was not reduced by the increased fishing pressure. On the other hand, I present indications for the occurrence of inshore spawning areas and nurseries of dagaa, which illustrate the danger of the use of mosquito seines to land dagaa.
Materials and methods Catch, stock and effort Data from the official landing statistics of the three riparian countries were used to assess the total annual catch of dagaa by the commercial light fishery for the period 1968–1989 (Ogari, 1985; Acere, 1988; Bwathondi, 1988; Ogutu-Ohwayo, 1990b; A.A. Asila, in litt.). Between 1981 and 1989 the dagaa population near Mwanza (Tanzania) was regularly sampled by surface trawling at night, using a small boat with a 25-hp outboard engine and a net with a cod end of 5 mm stretched mesh. Due to size-dependent diurnal vertical migration of dagaa, this technique sampled mainly the adult fraction of the population (Wanink, 1992;
Wanink et al., 1999). However, sampling the surface area at night should give a good estimate for the harvestable stock, as the fishermen operate at the same time and place. The occurrence of virtually identical length frequency distributions of dagaa in experimental and commercial catches confirmed this assumption (Wanink & Goudswaard, 1994a). Numbers and sizes of dagaa were scored from 10 min or 15 min hauls conducted at the two main sampling stations in the Mwanza Gulf (G. & E. Witte et al., 1992b). Catches were converted to biomass, using a relationship between standard length and dry weight of dagaa (Wanink & Goudswaard, 1994b). Fishing effort for dagaa was poorly documented for many years. In this paper the available data on the number of dagaa gear used in the Tanzanian waters during 1985–1989 are used as an estimate of effort (Reynolds et al., 1995). Pelagic or inshore spawning? In March 1988, at the local peak in breeding (Wandera & Wanink, 1995), nightly surface trawls were made at 5 stations in the Mwanza Gulf. These stations were located at distances from shore ranging from 1 to 5 km (Wanink et al., 1999). Samples were stored in 4% formalin immediately after collection. At the laboratory the gonads of the fish were checked and the number of ripe/running females was scored. Many plankton samples from 1977 to 1989 (net mesh: 150 µm) were checked for eggs or larvae of dagaa. Between August 1987 and October 1988 a small mosquito seine was used to sample 3 sandy beaches in the Mwanza Gulf for dagaa larvae. The net was towed in from a distance of 25 m from shore. At each spot 3 hauls were made on a monthly base, except in July, September, November and December. Distribution of juveniles To assess the possible impact of the fishery on juvenile dagaa, the fraction of dagaa smaller than 33 mm standard length in the nightly surface trawls was determined. This is a minimum estimate of the juvenile fraction, as the value of 33 mm represents the size of the smallest mature female ever observed in the study area, while the males mature at a larger size (Wanink, unpubl.). Catches made during 1987 and 1988 at the above-mentioned 5 sampling stations and at 2 inshore stations (E and BB in Witte et al., 1992b) were used to relate the juvenile fraction in the catches to the distance from shore.
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Figure 1. Developments in the annual yield of the dagaa fisheries in the three riparian countries. Horizontal bars represent the periods of major increase in the Nile perch catches in the separate countries.
Figure 2. Developments in the stock of dagaa from the Mwanza Gulf, expressed as the catch rate of the research vessel. Mean N h−1 ± SE in the successive years are based on 2, 8, 10, 6, 18, 12, 25 and 8 trawl shots, respectively. The horizontal bar represents the period of major increase in the Nile perch catches in the Mwanza Gulf.
Results Catch, stock and effort During the first 20 years of the light fishery for dagaa, the total annual yield from the lake rose from less than 1000 to over 80 000 metric tonnes (Figure 1). Catches in the separate countries show similar trends, although in Uganda the dagaa fishery started only very recently. After a gradual increase during many years, the catches suddenly rose steeply following the explosive increase of the Nile perch population. This rise in the catches may be caused by an increase in
Figure 3. Annual yield of the dagaa fishery in the Tanzanian waters, relative to highest yield during the period 1981–1989, as a function of relative stock size and fishing effort. The inset shows the results of least squares linear regression analyses.
the abundance of dagaa or by an increase in the fishing effort for this species after the collapse of the haplochromine cichlids. Developments in the stock of dagaa from the Mwanza area can be read from the catch rates of the research vessel (Figure 2). In 1985, after the local Nile perch boom, a strong increase in the abundance of dagaa was observed, which continued until the end of the study period in 1989. In terms of biomass, the increase was less. This was due to a reduction in the average size of dagaa (Wanink, 1991). The increase in abundance of dagaa in the Mwanza area between 1985 and 1989 coincided with an increase in fishing effort for this species in the Tanzanian waters of the lake (Reynolds et al., 1995). To depict the effect of stock size and fishing effort on the yield of the fishery, the relative yield in the Tanzanian waters was plotted as functions of relative stock estimated from the catches of the research vessel and relative effort by the commercial fishery (Figure 3). Relative variables were constructed by adjusting each annual value to the highest value in the period 1981–1989, which was set to 1. When analysed separately, yield was correlated both with stock size and fishing effort. In the plateau years (1985–1989 inclusive) fishery effort alone accounted for 66% of the variation in yield (n=5, p 32 mm SL) from the trawl catches amounted to 66%, while only a few were present at the offshore stations. This suggests that dagaa move to shallow areas to spawn. The absence of eggs in hundreds of zooplankton samples made in the offshore waters between 1977 and 1989 is another argument against the theory that dagaa is a pelagic spawner. Although larvae were occasionally caught at all sampling stations, their numbers were always very low. Large numbers of larvae were only caught with the mosquito seine near the sandy shores. In August 1987 dagaa larvae with a modal length of 11 mm were abundant at all three sampling stations. After that, no larvae were caught until June 1988, when large numbers with a modal length of 13 mm were present at all stations again. Also during November–December 1979, when catches were made at one of the above-mentioned beaches, a high abundance of dagaa larvae was recorded (F. Witte & M.J.P. van Oijen, pers. com.). Distribution of juveniles After spending their larval stage in the shallow areas, juvenile dagaa are expected to migrate away from the shore, as the highest densities of adults in the Mwanza area are found at a distance of 2 km from shore (Wanink et al., 1999). Especially in the deeper
waters, juvenile dagaa should not suffer much from the fishery when it is performed at the surface, because in contrast to the adults, the juveniles move from the surface towards midwater at sunset (Wanink, 1992). However, some juveniles were always present in the nightly surface trawls, even at the deepest stations and this juvenile fraction increased strongly towards the shore (Figure 5).
Discussion Prospects for a truly pelagic fishery In a recent paper, Rabuor & Polovina (1995) have analysed the catch and effort data for different parts of the Kenyan waters of Lake Victoria. They found an increase in catch rates and fishing effort for both Nile perch and dagaa during the 1980s. Although their model suggests an increase in catch rates due to a shift in effort to more rewarding areas, rather than an increase in fish abundance, they lack direct estimates of abundance. The present paper shows that in the Tanzanian waters the landings of dagaa increased with the abundance and the fishing effort. Data to test the effect of abundance were lacking, but effort accounted for 66% of the variation in yield during the plateau years. Even if the increase in abundance of dagaa did not cause the higher yield of the fishery, its occurrence shows that the stock was not negatively influenced
187 by the intensified fishing effort. The reduced generation time of dagaa must have been an important factor in its survival of the increased exploitation by both man and animal predators (Goudswaard & Wanink, 1993; Wanink & Goudswaard, 1994b; Wanink, 1996; Wanink et al., 1999). Furthermore, the shift from juvenile to adult predation by fish and birds, has probably resulted in a larger fraction of juvenile dagaa reaching the reproductive stage (Wanink et al., 1993, op. cit.). Thus, the prospects for the fishery will depend mainly on the age structure of the catches. The fraction of juvenile dagaa in the catches depends on the distance from shore (Figure 5). During the years 1985–1989, when the catches and the fishing effort in the Tanzanian waters increased strongly (Figures 1 & 3), a trend towards offshore fishing with scoop and lift nets was observed in the Mwanza region (Chitamwebwa, 1988). This must have reduced the mortality of juvenile dagaa. Despite the use of new techniques, by the end of 1989 the beach seine was still the main gear used in the dagaa fishery (Mous et al., 1991). However, most dagaa was then probably caught in the offshore waters, since the lift nets gave a much higher catch per unit of effort than the beach seines (Mous et al., op. cit.). The high catch per unit of effort by the lift nets in the offshore waters was expected, as the highest densities and the largest sizes of adult dagaa were found at a distance of 2 km from shore (Wanink et al., 1999). An offshore light fishery will catch mainly adult dagaa, which at night replace the juveniles in the surface layer (Wanink 1992; Wanink et al., op. cit.). Thus, the prospects for a truly pelagic fishery are good, which is confirmed by the population increase during the years 1985–1989. The danger of mosquito seines Besides the spatial distribution of the fishery, the mesh sizes of the nets determine the juvenile fraction in the catches. The inshore fishery in the Mwanza Gulf during the 1980s will have been less harmful than predicted (Figure 5), since the stretched mesh size of the experimental trawl net was only 5 mm, against 8–10 mm in the beach seines and 8 mm in the scoop- and lift nets (Ligtvoet et al., 1995). Selectivity analysis showed that the 5 mm trawl net and a 10 mm beach seine retained 50% of the fishes above 44 and 53 mm standard length respectively (Wandera and Wanink, 1995). A value of 38 mm has been reported for a 5 mm beach seine (Manyala, 1993).
An inshore fishery using a 10 mm beach seine in the Mwanza region would not have caught many juvenile dagaa, since maturation started at a size of 33 mm. Even in the northern Ugandan waters, where the smallest mature specimens observed were 41 mm, such a net caught mainly adults (Wandera, 1990). When the fishery in that area switched to a 5 mm beach seine, more than 70% of the catch consisted of fishes which would not have bred (Wandera, op. cit.). A 2 mm mosquito seine, of which the use has recently increased in the Kenyan waters of the lake (Riedmiller, 1994), will be even more selective for juvenile dagaa than a 5 mm beach seine. In spite of their small meshes, mosquito seines would not be a great danger to the dagaa stock if the species was a pelagic spawner and the larvae were abundant in the offshore waters, as assumed since Graham (1929). However, this tentative assumption was reinforced only by a catch of a few eggs in a plankton net (Ndawula, cited as pers. com. in Wandera, 1993). Already in the 1950s shoals of juvenile dagaa (some of only 12 mm) were observed in the littoral zone, but most investigators seem to have missed the report of this fact, together with the comment that at least some breeding of dagaa seems to occur in the inshore regions (Fryer 1960). The distribution of ripe/running females and larvae described in the current paper supports Fryer’s conclusion and suggests that dagaa spawns in the littoral waters, which are subsequently used as nurseries by the larvae. This agrees with the recent observations on the closely related cyprinid Engraulicypris sardella Günther from Lake Malawi, which appeared to be an inshore spawner after being regarded as a pelagic spawner for many years (Thompson, 1995). In Lake Tanganyika, the clupeid Stolothrissa tanganicae Regan also appeared to be spawning in the inshore waters instead of in the supposed pelagic areas and all pelagic species were found to spend the first part of their live in the littoral waters (Roest, 1977, 1993). All these observations support the view of Greenwood (1966), who said that pelagic spawning is uncommon in freshwater fishes. I conclude that the use of mosquito seines can potentially reduce the prospects for the dagaa fishery in Lake Victoria, as it is highly selective for juveniles and fishes their main habitat. Furthermore, this technique will affect the stocks of the other commercially important fishes in the lake, the Nile perch and the Nile tilapia (Oreochromis niloticus L.), as juveniles of these species form an important by-catch (Riedmiller, 1994; Ligtvoet et al., 1995; Wanink, unpubl.). A ban on
188 mosquito seines and a stimulation of offshore fishing would increase both the yield and the sustainability of the dagaa fishery.
Acknowledgements I thank the Netherlands Foundation for the Advancement of Tropical Research (WOTRO; grant W 87-189) and the section for Research and Technology of the Netherlands Minister of Development Cooperation for funding, the people from the Tanzania Fisheries Research Institute (TAFIRI) for their hospitality and support, Mhoja Kayeba and Ruben Enoka for their hard work during the sampling sessions, Martin Brittijn for preparing the figures, and Rudi Drent, Ro LoweMcConnell and Frans Witte for their comments on the draft manuscript.
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