Increase dominance of common carp, <i>Cyprinus carpio</i> L: the boon or the bane of Lake Naivasha fisheries?

Abstract

Lake Naivasha fisheries depend on introduced species with variable catch composition, determined mainly by fishing intensity, water levels and changes in aquatic macrophyte densities (Mucidri et al., 1994; Hickley, et al., 2004). The mean annual commercial species composition of the fin-fish landed for the period 1987–2000 was dominated by Oreochromis leucostictus (Trewavas), Black Bass, Micropterus salmoides (Lacépède), and Tilapia zillii (Gervais) (Hickley et al., 2004). Other fish species of no commercial importance found in the lake are Barbus amphigramma (Boulenger) and Poecilia reticulata (Peters). Lake Naivasha fishery was closed in 2001 in bid to increase catches especially of M salmoides and T. zillii which were dwindling (Ojuok & Mugo, 2002). After reopening of the fishery in 2002, common carp, Cyprinus carpio (L.) was discovered in catches contributing <1% (Hickley et al., 2004). Since then, C. carpio catches have continued to increase and the species now dominate the fishery of this lake. Introduction of the C. carpio in Lake Naivasha was accidental, with fish possibly having escaped from fish farms adjacent to River Malewa, the main inflow into the lake, and into which C. carpio fingerings had been stocked in 1997 (Hickley et al., 2004). Increased catch of C. carpio has resuscitated the lake fishery from a possible near collapse. However, globally, wherever it has been introduced, C. carpio has had negative impacts on natural aquatic environments (Hickley et al., 2004). There are fears that if the carp continues to dominate the Lake Naivasha fishery, it will further aggravate the already stressed lake environment and its fishery (Hickley et al., 2004). The aim of this study was, therefore, to avail more information on the characteristics of common carp, which can be used to assess the possible impacts on the Lake Naivasha fisheries and possible management measures required. Currently, C. carpio is the most important commercial fish in Lake Naivasha. Its catch increased from 0.9 t (< 1%) in 2002 to 133.4 t (95 %) in 2006 (Fig. 1), and the earnings rose from US$ 600 (0.75%) in 2002 to US$ 58 000 (82%) in 2005. The domination of C. carpio could be attributed to ban of gillnets <4 inches (102 mm) and favourable conditions in the lake. Although the smaller sized tilapiines and M. salmoides could be present in the lake in sizeable quantities, the dismal contribution to the commercial catches may be attributed to ban on the use of gillnets <4 inches. The recommended gillnets of ≥4 inches are unable to catch large quantities of the smaller sized tilapiines and M. salmoides. Trends in catches of the major commercial fish species in Lake Naivasha. Ol, Oreochromis leucostictus; Tz, Tilapia zillii; Ms, Micropterus salmoides; Cc, Cyprinus carpio Gut analysis revealed that C. carpio has a diversified diet feeding on a variety of food items, which include plant material (40%), plants seed (17%) and detritus (12%) (Fig. 2). The flexibility in the diet ingested probably indicates that food may not be a limiting factor in Lake Naivasha. Length frequency indicates a population structure dominated by juvenile fish (< 36 cm TL, 53%), although with substantial number of mature individuals indicating good recruitment (Fig. 3). Spawning is unlikely to be limited because the normal temperature of Lake Naivasha is 21–24°C, above the required 18°C (Cowx, 2001), and C. carpio is not selective in its choice of substratum for attachment of eggs (Petr, 2000). Food ingested by the Cyprinus carpio, in Lake Naivasha. Fish rems, fish remains; Inverts rems, invertebrates remains Length frequency distribution of Cyprinus carpio commercial catch from Lake Naivasha in 2002–2005. Vertical arrows indicate length at 50% maturity (Lm50); n, sample size The asymptotic length (L∞), fishing mortality (F), exploitation rate (E), maximum life span (tmax), growth parameter (φ′) show an increase, while growth curvature (K) showed a decrease over the years (Table 1). Population parameters obtained in the study indicate a species growing to big sizes at a slow growth rate. The population parameters obtained in the study were within the range of the species in its native regions of Europe For example, C. carpio attains L∞ of 60.7, 63, 74.8, 82 cm TL, K of 0.16, 0.20, 0.16 0.12 year−1 in France, Spain, Kazakhstan and Croatia respectively (http://www.fishbase.org, 2006). The potential success of C. carpio in Lake Naivasha is unlikely to be restricted. The species is hardy, tolerant of degraded aquatic environment and thrives well in turbid waters (Scott & Crossman, 1973). The increase in C. carpio could have several detrimental effects on other Lake Naivasha fishery (Hickley et al., 2004). The preclusion of fishers who targeted the smaller tilapines in the lake with gillnets <4 inches would have probably led to increase poaching subsequently threatening the whole fishery. Carp is a benthivore and its feeding behaviour, which involves sucking in sediments with prey items and retaining food organisms whilst, sediments particles are expelled has detrimental effects on the ecosystem (Parkos, Santucci & Wahl, 2003). It is this habit of feeding on bottom sediments, which uproots aquatic plants, suspends the sediments and increases water turbidity, that makes carp unwanted species in some water bodies (Petr, 2000; Parkos et al., 2003). Any effects of C. carpio on submerged macrophytes will be additive to the already detrimental impact of Procambarus clarkii (Girard) in Lake Naivasha (Hickley & Harper, 2002). Additionally, when macrophytes are lost, wind increases sediment resuspension, that in turn reduces light availability for phytoplankton and submergent macrophytes. Reduction in algae and macrophytes may threaten the food base for phytoplanktivorous O. leucosticus and herbivorous T. zillii while increased turbidity might hamper the visual feeding of M. salmoides. Disturbance of the lake bottoms by C. carpio would seriously affect T. zillii which lays adhesive eggs on the lake bottoms with pebbles or sand and with abundant vegetation (Hickley & Harper, 2002; Hickley et al., 2004). Breeding of M. salmoides and O. leucostictus, which build their nests on muddy bottoms of shallow water could also be adversely affected by feeding behaviour of C. carpio. Common carp and T. zillii both laying sticky eggs will be competing for substratum to attach the eggs (Petr, 2000). Unlike T. zillii, the carp is unselective on the substratum used for egg attachment (Petr, 2000), and this could offer it a better competitive advantage especially in disturbed environment where it thrives better. Although direct predation on adult fish species is unlikely, there could be some noticeable effects of presence of C. carpio if the omnivorous species starts to prey on juveniles and eggs of the other species in the lake. This study revealed that C. carpio included fish (11%) and fish eggs (3%) in its diet, which could further aggravate the situation for the other species in the lake. In terms of size, C. carpio which attains L∞, of 61–97 cm TL is the giant of the lake compared to M. salmoides, O. leucostictus, T. zillii which attain maximum sizes of 54, 42 and 29 cm TL respectively (M. Njiru, personal observation). The large sizes could probably be more advantageous for C. carpio in defending territories for breeding and feeding. If this would be the case, the other ‘weaker’ fishes will further be alienated from their preferred sites. Several countries have reported adverse ecological impact after introduction of C. carpio (Petr, 2000; Miller & Crowl, 2006). Will this be the case in Lake Naivasha? Already C. carpio is the dominant species in the lake (Fig. 2). However, the real impact by the species may depend on anthropogenic effect and management measures instituted to sustain the fisheries. Lake Naivasha fishery was closed in 2001 to rejuvenate the fishery due to decline in catches with the greatest threat being high capacity. Studies conducted thereafter recommended reduction in fishing boats from 100 to 45 boats each with 10 nets of 4 inches and above (Ojuok & Mugo, 2002). However, because of high rate of unemployment in the region, poaching and use of illegal gears have continued, and this could be contributing to high exploitation rate (Table 1). Continued unregulated abstraction of water by surrounding horticultural farms, not only reduces the amount of the water in the lake, but also siphons larvae and juvenile fish threatening fish recruitment. There is, therefore, a need to regulate the amount of water taken out of the lake and use of sieves would reduce uptake of young fish. For sustainability of the fisheries, there is need to involve all the stakeholders in the management of the lake, because top down management coordinated by the central government has not worked in Kenya (Njiru et al., in press). Community involvement in management of Lake Victoria through Beach Management Units drastically reduced illegal fishing methods and gears (Njiru et al., in press). In Lake Naivasha, the stakeholders are being involved through the Fisheries Department though the gains have been minimal. Poaching, pollution from horticultural farms and unregulated water abstraction still continues. However, despite all these odds, C. carpio has continued to thrive and increase in the lake. Will the increased dominance of C. carpio, be the boon to the fishers of Lake Naivasha or the bane to the other fIshery consisting of O. leucostictus, T. zillii and M. salmoides? Unfortunately, only time will tell.