Ecological impacts of common carp cyprinus carpio L. 1758 (pisces: cyprinidae) on natural fish species in Lake Naivasha, Kenya
This study was carried out from July 2008 to July 2010 in Lake Naivasha (0045'S; 36ﾰ21'E), mainly to investigate possible impacts of ecological disruptions caused by Common carp Cyprinus carpio on the naturalised fish species (Oreochromis leucostictus, Tilapia zillii and Micropterus salmoides), and its consequent implications on the fisheries ofthe lake. C. carpio is one of the world's worst invaders currently exhibiting a biogeographic distribution. It is the latest alien entrant into Lake Naivasha systems, where within a period of only six years, it had already established itself and contributed over 95 % of the gillnet fishery. In so doing, the species has effectively displaced the naturalised fish populations, which currently only contribute 4.22 %, 0.77 % and 0.01 % for M salmoides, O. leucostictus and T. zillii. To estimate possible ecological interactions, diel samples of all the major fish species were obtained by use of panels of gillnets of varying mesh sizes launched at major habitat types of the lake. Simultaneous abiotic parameters were also collected. Habitat preference and suitability indices were computed from fish abundances. Age and growth parameters were estimated from scale annuli using marginal increment ratio analysis, and von Bertalanffy growth model. Trophic dynamics, prey prominence, niche breadth, trophic niche partitioning and dietary overlap were computed to establish possible interspecific interactions. Ex situ functional response experiments established foraging behaviour of both C. carpio and the tilapiines. Life-history strategies were based on gonadal examination of maturity recrudescence. Invasion propensity of C. carpio, and the corresponding response of naturalised fish populations were determined from Winemiller and Rose triangular Model of species survival strategies. Key results from this study indicated that C. carpio was ubiquitous in all habitat types in the lake, while the naturalised fish species, exhibited restricted distribution, with the tilapiines occurring only in the inshore and littoral habitats. M salmoides preferred rocky habitats. The ubiquity of the carp presents it with a better survival range than the naturalised fish species. The common carp also exhibited strongest growth performance of <D' = 12.65, compared to 11.93 and 10.96 for M salmoides and 0. leucostictus, respectively, still pointing at a better survival scheme by the carp. Seasonal opening of Lake Naivasha facilitated faster growth and bigger sizes in C. carpio, but significantly hindered growth in 0. leucostictus (F = 96.7, P < 0.001); T. zillii (F = 35.8, P < 0.001) and M salmoides (F = 5.9, P < 0.05). This implies that with the larger population of C. carpio in the lake, the fishery might be ecologically and economically more beneficial with continuous cropping of its stocks. In addition, a mean surface water temperature of 22ﾰC was found to hasten growth in all the fish species. However, temperatures of above 24 ﾰC impaired the growth in C. carpio, while temperatures of below 19ﾰC hindered growth in the tilapiines and M salmoides. The study presents for the first time, a validation scheme for scale annulus deposition for C. carpio, M salmoides and 0. leucostictus in the afrotropical systems. The study also confirmed a significant trophic-niche competition between all the species in the lake, except at the resource-rich mouth of River Malewa, indicating that trophic competition functionally occurs only where food resources are limited. It was observed that the presence of C. carpio in Lake Naivasha did not only disrupt hierarchical feeding by the tilapiines, but it also contributed to a decreased water transparency from c. 70 cm in 2002 to below 30 ern in 2009, mainly resulting from the fish's benthic foraging scheme. Such decline in water transparency is expected to pose an ecological disruption especially on light-dependent feeding mechanism of M salmoides, and photosynthetic processeses of submerged macrophytes in the lake. Except for M salmoides, C. carpio and the tilapiines matured in less than one year, with T. zillii presently maturing at 116 mm Lr compared to 130 mm Lr in the 1970s. T. zillii also exhibited a depressed fecundity of below 2000 oocytes compared to c. 6000 oocytes in 1970s, indicating high level of plasticity. Whereas C. carpio, 0. leucostictus and T. zillii displayed asynchronous breeding in the lake, M salmoides exhibited synchronised breeding, which mainly occurred at the last half of the year. Fecundity, length at maturity and asymptotic length explained rapid population establishment and invasive potential of C. carpio. On the other hand, parental care (mainly large oocytes and mouth-brooding schemes) by 0. leucostictus apparently cushioned its population from the impacts of the common carp. T. zillii, which is a substrate spawner of smaller oocytes, and a non-mouth brooder, was faced with more threats resulting from the environmental disruptions by the common carp's benthic foraging scheme. The susceptibility of T. zillii to the negative impacts of C. carpio was also corroborated through a significant inverse relationship between the carp's biomass, and that of T. zillii (If = 0.86, F = 35.97, P < 0.001). This was not true in the relationship between the biomass of C. carpio and that of the more resilient O. leucostictus (R2 = 0.36, F = 3.38, P > 0.05). The entry, and consequent rapid establishment of C. carpio population in Lake Naivasha was, however, noted to result into a dramatic economic elevation of fisheries revenue from Kshs. 351,700 (USﾷ $ 4,400) before the establishment of C. carpio, to Kshs. 1,153,300 (US $ 14,400), after its establishment, with the carp contributing 85 % ofthe total revenue. Comparatively, M salmoides, o. leucostictus and T. zillii currently contributes dismal values of revenue of 13.31 %, 1.65 % and 0.04 %, respectively. From the afore mentioned key results, it can be concluded that higher fecundity, faster growth, larger size at maturity, more spawning bouts, benthivory and ability of C. carpio to transcend all habitats in Lake Naivasha, facilitated its rapid establishment, and a consequent vigour to displace naturalised fish species in the lake. Habitat modification through amplified water turbidity by C. carpio, is a main threat to the survival of most of the fish biotopes in the lake. This study was, however, not able to separate the contribution of C. carpio to the current increasing high water turbidity in the lake from those of anthropogenic origin. Therefore, future comprehensive investigations are recommended to explore this to enable pragmatic and focused interventions to curb any further environmental deterioration in the lake. It cal). also be concluded that continuous cropping of C. carpio through commercial fishing, may be more beneficial not only to reduce the number of the common carp below threshold of 320 kg.ha' to facilitate environmental recovery, but also to hasten recovery of stocks of tilapiine populations. The study finally demonstrates necessity of critically examining characteristics, patterns and processes of species invasion potential, before their introduction into new environments. Correspondingly, in order to preclude further ecological disruptions from C. carpio in other aquatic ecosystems in the country, a strict ban on its translocation to new ecosystems is strongly recommended.