Nile Perch fishing in Lake Victoria is done using gillnets and long lines. These gears have been deployed in the lake indiscriminately to the extent that in early 2000s the species showed signs of over-fishing. To address this situation, a slot size of 50–85 cm total length (TL) was instituted by the partner states to protect immature fish, and large adults to replenish the stocks while at the same time harvesting mature individuals. Catch Assessment Surveys, have been conducted regularly by the partner states between July 2005 and December 2008 to generate data for setting policies, development planning, and management intervention. A study involving two-stage stratified sampling on the Tanzanian side of the lake showed that Nile Perch caught in the minimum legal 5” gillnets were immature by more than 80%. In contrast Nile Perch of ≥85 cm total length, targeted by long-lines, were seldom encountered to validate the upper limit of the slot size. We advise to abolish the upper slot size limit and introduce a lower legal limit of 7” for gillnets.
The fishery of Lake Victoria, largest tropical lake in the world (68,800 km2), was predominantly subsistence based, until the advent of gillnets in the Kenyan waters in 1905. The gear targeted two indigenous tilapia species, Oreochromis esculentus and Oreochromis variabilis, but other larger fishes such as catfishes, mormyrids, Protopterus aethiopicus, and Labeo vitorianus were also caught (Fryer, 1960).
Increasing fishing pressure on the indigenous tilapiines led to a decline in catches from about 25 to 5 fish per net; this led to the recommendation to introduce a 5 inch (127 mm) mesh limit, based on the 1927-8 fishery survey by Graham (1929). To boost catches, three tilapine species: Oreochromis niloticus, Tilapia zillii and Tilapia melanopleura were introduced into the lake. In addition, the voracious Nile Perch, Lates niloticus (Harrison, 1991) was introduced to convert the “trash” haplochromines into table fish (Ogutu-Ohwayo, 1990; Harrison, 1991). The Nile Perch established itself and, after a number of years, became the dominant species in the lake. Nile Perch catches and exports boomed in the late 1980s (Ogutu-Ohwayo, 1990). By the 1990s the lake still supported a lucrative fishery, but this has now declined.
The fisheries of Lake Victoria are considered to be in a state of flux and have possibly not yet stabilized (Marshall, 2008). Environmental factors such as eutrophication (Balirwa et al., 2003), that cause self-shading by algae, may collapse primary productivity and severe deoxygenation of deeper waters may reduce sustainability of the lake fishery. Nile Perch are sensitive to low oxygen concentrations, but there is no evidence to suggest that the environment is an immediate threat to the fishery. There has been an increased fishing effort for Nile Perch, especially with smaller mesh-sized nets, and small size (≥ no. 10) long-line hooks which catch a high proportion below the size at first maturity (Lm50 _50–70 cm). This suggests an unsustainable Nile Perch fishery because fish are caught before they contribute to the stocks. Food availability is not a limiting factor since the Cyprinid Rastrineobola argentea, which is a prey for Nile Perch, has shown to increase while Nile Perch stocks are declining (Kayanda et. al., 2009). Furthermore, the fishery of Lake Victoria is open-access, where anyone can enter into the fishery, provided that one has a fishing license (which is however, not monitored). Some fishers deploy efficient illegal fishing gears such as beach seines, cast nets and monofilaments are still being used (Njiru et al., 2008; Mlaponi et al., TAFIRI, Tanzania, unpublished) and methods such as the use of drifting gillnets, to get more fish irrespective of their sizes.
Signs of over-fishing (Cowx et al., 2002) were confirmed by research: hydro-acoustic surveys (Kayanda et al., 2009) and Catch Assessment Surveys (CAS). The decline in catches of Nile Perch was noticed earlier (Mkumbo, 2002) after fishing intensity had increased to meet the demand of fish processing factories which had been increasing in the riparian countries to supply fillets (chilled and frozen) to Europe, Middle East and North America (Njiru et al., 2009).This increase created fears that the fishery might not be sustainable without appropriate management measures (Matsuishi et al., 2006; Njiru et al., 2007). Already, by 2000, Uganda had set a legal minimum size for Nile Perch of 460 mm, while in Kenya they had set a minimum size of 250 mm (Mkumbo, 2002). Studies carried out between 1998 and 2000 to evaluate the status of Nile Perch stocks in the lake led to the suggestion to harmonize the regulations on minimum size for Nile Perch. They recommended slot size limits of 50 to 85 cm TL for the processing factories. This size range was regarded to be good enough to keep the factories running, while protecting immature fish, harvesting mature individuals and also allowing some large ones to escape to replenish the stocks (Mkumbo, 2002). Consequently, 5” was agreed as the minimum gillnet mesh size. Despite these management measures, Nile Perch catches from the 5” gillnets comprised up to 88% immature fish (Mkumbo, 2002, TAFIRI, 2005). Gear size regulation was harmonized for gillnets, but this was not the case for hook-sizes. Hook sizes of 8 to 11 were recommended for Tanzania, reflecting concern about the impact of the long line fishery on the spawning stock of Nile Perch. The upper slot size limit (85 cm TL) was intended to protect the mature/spawning females. These spawning individuals are few in the population, but because of their relatively high fecundity, they contribute hugely to the spawning success of the stocks. Mkumbo (2002) recommended an upper limit of 87 cm TL, but for unknown reasons this was left at 85 cm TL.
This paper adopts a combination of catch statistical data and length-frequency analysis to examine the combined impact of the two main gears (gillnets and long line) on the Nile Perch stocks in Lake Victoria. It also assesses the usefulness of the 50–85 cm TL slot size regulation for managing the Nile Perch fishery, and suggests alternative management action for the eventual sustainability of the lake's fishery.
Materials and Methods
This study is based on catch assessment survey (CAS) data which were collected from 59 landing sites, representing 10% of the landing sites in the Tanzanian side of Lake Victoria (Figure 1). These landing sites are the primary sampling units (PSUs) where CAS are conducted according to the Lake Victoria Standard Operating Procedures (SOP) (LVFO, 2007). A two-stage stratified sampling design was adopted, using PSUs at one stage, and stratified samples of secondary sampling units (SSUs) i.e. the different types of crafts carrying specific gear types and sizes, randomly selected by field enumerators. The regionally harmonized data collection forms were used according to the SOP.
Biological data analysis
Crafts were categorized into effort groups (numbers of crafts/canoes using a particular gear type with specific mesh size or hook size).The mean weight of Nile Perch caught by a specific mesh or hook size (gillnet or longline) was obtained by dividing the weight of the catch by the total number of Nile Perch caught. All data from the eleven surveys were combined from 2005 to 2008 to get the number of fish caught by gear type, size and weight.
The numbers of specimens and the total weight in the catch of a particular size and type of gear were calculated to obtain an average weight, which was converted into an average length, using the length - weight relationship:
W = aLb where W is the weight of the fish in grams, and L is the total length of fish in cm. The constants a and b were 0.0042 and 3.26, respectively (LVFO, 2008a).
A t-test was used to compare mean lengths of Nile Perch converted from weights using the length weight relationships, and the mean lengths of Nile Perch from the corresponding mesh size retained in the experimental gillnets conducted by Asila (Asila, Moi University, Kenya, unpublished).
The overall length-frequency distribution of the catch from different gear sizes of gillnets and long lines are presented in Figures 2 and 3, respectively. Nile Perch caught in gillnets had a modal length of 44. 6 cm TL, and 62.5 cm in the long lines. This indicated that relatively more juvenile Nile Perch were caught by the gillnets.
Gillnets of <2.5 to 11” captured Nile Perch, but due to the small numbers of samples, our analysis was limited to the catches from the 2.5–7.0” meshed nets. To illustrate the effects of gear size, the proportional numbers of fish caught in gillnets of mesh sizes between 2.5” and 7” and above are presented in Figure 4 and Table 1. Individuals caught in 7” gillnet mesh size and above (up to 11”) were combined. Most of the fishes caught in gillnets were below the minimum slot size of 50 cm, while long lines harvested bigger sized fishes above the slot size.
In order to compare the results of experimental fishing (Asila, Moi University, Kenya, unpublished) and our study, the mean lengths obtained for the various mesh sizes in the two data sets were compared statistically. As shown in Table 2, there was no significant difference in mean catch lengths (Student's t-test, P = 0.1829). Incidentally, the mean length of fish caught in ≤2.5” was 19.7 cm TL from the experimental gillnets, while that from the catch data was 29.6 cm TL. Possibly some large-sized fish were entangled when caught. Such fish increase the mean length. Tangling is less dependent on mesh size, since gillnet selectivity is mainly based on fish which are gilled or wedged (Asila, Moi University, Kenya, unpublished).
The mean lengths of fish caught in each mesh size are presented in Figure 5. These increase with the mesh size. The mean length of fish caught in mesh sizes over 7” showed high variance, possibly due to small sample sizes. Somewhat higher variance was also observed in the 5.25” and 6.5” gillnets. This also occurred in the catches of large hooks (sizes 3 and 4) (Figure 6). The smallest hook size (number 14) also showed high variance, again due to small sample size. (It was encountered in three crafts only).
The fish caught by the lower limit of 5” were legal (50–85 cm) by only 34.6%. In contrast, of those caught in the 7” meshed nets 88.2% were within the legal range. This indicates that the 5” mesh size recommended earlier has some deficiency. But, in fact, the minimum mesh size of 5” for gillnets suggested by Graham (1929) for tilapia in Lake Victoria was later adopted as the minimum legal size for catching Nile Perch!
Fisheries management advice is usually based on the sizes at sexual maturity to determine the minimum harvestable size. Unfortunately, this has not been the case for Nile Perch. In order to control juvenile fishing and protect the adult spawning stock, the East African Council of Ministers, under the auspices of LVFO, adopted the slot size for Nile Perch, allowing only fish between 50 and 85 cm TL to be caught and landed in all three East African countries. However, the sizes at first maturity of both sexes of Nile Perch were considerably higher (i.e. Lm50 of 54 cm TL for males and 76 cm TL for females in 1999) (Lake Victoria Fisheries Research Project (LVFRP), 2001; Mkumbo, 2002) than the lower limit set. This has led to improper management of the Nile Perch fishery culminating in catching immature fish. Fishing of juveniles reduces the size/amount to be recruited and the probability of surviving to spawning size. In the long run it leads to instability in recruitment and a possible collapse of a fishery (Pitcher and Hart, 1982; King, 1995). Using a modal length–mesh size relationship of y = 10x – 1.7143 (r = 0.998, Asila, 1999), the current recommended minimum mesh size of 127 mm (5”) for gillnets has the modal catch length of 48.3 cm TL, but more than 88% of the catch from these nets is immature. By instituting a minimum 7 inch mesh gillnet, using the same model, the mean fish length would be 68.3 cm TL, and they would all be mature.
Our data showed that the variance of gillnet mesh size 7” overlapped with that of gillnets down 5” (Figure 5), suggesting that there was no significant difference in the catching efficiency of the two mesh sizes. One possible explanation is that some fish were caught by tangling, independently from the size of the mesh. Another reason may be the great difference in sample sizes between the 5” (n = 7091) and 7” (n = 414) nets.
According to the Frame Surveys conducted between 2000 and 2008, the 5” gillnets made up about 43.7% of all gillnets operated in the Tanzanian side of Lake Victoria, against only 4.4% for the 7 inch gillnet. The number of gillnets operated in the lake between 2000 and 2008 ranged from 226,169 in 2000 to 583,691 in 2004, but dropped to 395,847 in 2008 (LVFO, 2008b, 2008c). This drop in the recent years is associated with fishers pulling out of the fishery due to declining catches (Frame Survey Report, 2008). The use of the legal 5” gillnets clearly puts great pressure on the Nile Perch stocks.
Biological information on the size at first maturity led to recommendations for adopting the 7″ gillnets as the legal minimum mesh size for Nile Perch. The data we have did not support the need for an upper slot limit of 85 cm TL for Nile Perch. Thus we considered that the regulation should be repealed. More effort should be directed to curb growth over-fishing by protecting the younger individuals of less than 50 cm TL to reach a factory processing size. We therefore suggest that:
because of the high demand of Nile Perch from the fish processing factories, there is a need to establish and monitor the growing number of investment- driven fish factories (e.g. setting a quota in number of factories) in order to set a balance between resources availability and demand.
the number of (legal) fishing gears entering into the fishery be limited through enhanced licensing protocols.
co-management be promoted through Beach Management Units (BMUs)
the selectivity of hooks (and bait–types) and size be monitored so that a clear advice is given to safeguard the long line fishery.
The need to manage Lake Victoria fisheries resources began when the native gill net fishery based on two native tilapiine cichlids, Oreochromis variabilis and Oreochromis esculentus, was overexploited due to intensive fishing pressure in the late 1920s by nylon gill nets. A minimum gillnet mesh size regulation to protect the indigenous Tilapiines was set at 5”, which was later adopted to protect the large exotic predatory species, Nile Perch (Lates niloticus). The circumstances that led to adopting the 5” mesh size as the lower limit in subsequent years for the management of Nile Perch are not clear. The 5” mesh size has been found to catch ≥80% immature fish. For sustainable management of the Nile Perch fishery, our data support the instituted 7” minimum mesh size limit by the Council of Ministers. However, we appeal the upper slot size limit regulation of 85 cm TL which is virtually redundant, because the upper limit regulation cannot be used. More effort should be directed to curb growth over-fishing by protecting the younger individuals of less than 50 cm TL to reach a factory processing size. More so, there is a need to introduce quotas to the fish processing factories in order to set a balance between resources availability and demand. A community based approach using Beach Management Units through collective responsibility to eliminate destructive illegal gears and methods was adopted. We also proposed setting quotas to fish processing factories to enhance management of the Nile fishery at a sustainable level.
The implementation of a Fisheries Management Project for Lake Victoria (IFMP) was made possible through the EU who funded the project. Fishers, enumerators and Fisheries Officers in the riparian districts surrounding the lake are thanked for their cooperation.