International Research Journal of Environment Sciences________________________________ ISSN 2319–1414Vol. 2(7), 5-14, July (2013) Int. Res. J. Environment Sci. International Science Congress Association 5 Species Richness and Diversity of Ichthyofaunal communities of the Lower Cross River floodplain, NigeriaEkpo I.E. and Udoh J.P.* Department of Fisheries and Aquatic Environmental Management, University of Uyo, P.M.B. 1017, Uyo - 520001, NIGERIA Available online at: www.isca.in Received 2nd February 2013, revised 15th April 2013, accepted 10th June 2013 AbstractThe ichthyofaunal composition of the floodplains of the Lower Cross River was investigated and compared in order to promote its management. A total of 5211 fish were sampled for 12 consecutive months from commercial landings of artisanal fishers from three sampling stations. We estimate 77 species distributed into 52 genera, 29 families and 9 orders, both of freshwater (88.66%) and euryhaline (marine intrusive) fishes (11.34%), with averagely 1-3 species per genus. The fishes composed mainly Perciformes, the least being Polypteriformes and Clupeiformes.. The five most abundant fish families are the Bagridae � Cichlidae � Mormyridae � Mugilidae and Cyprinidae. Study suggests growth and recruitment overfishing with populations generally bigger upstream than their counterparts downstream and elucidates contributions of floodplains as nursing and spawning grounds in river fisheries and productivity. Keywords: Bagridae, classification, conservation, fish sizes, rare species Introduction One characteristic feature of most tropical rivers, like the Cross River, is the high endemic multi-species diversity of the fish fauna. The Niger Delta with Cross River Barrier lagoon system (Nigeria) was classified in the “A List” as areas unanimously recognized to be of Outstanding Universal biodiversity Value (OUV) by a UNESCO biodiversity study team. Similarly the Cross River Estuary (Nigeria, Cameroon) was classified in the “B List” as areas identified to have significant components of OUV. The “C List” comprises areas that may be of OUV but lack adequate information for assessment. These wetland systems comprise part of the West African Flyway, a major migratory bird route that provides year round habitat for many bird species. The Cross river is a major hydrographic feature in the Gulf of Guinea and possesses attributes of OUV in relation to Nigeria’s fish biodiversity (updated from 239 fish species in 46 families and 511 species in 121 families to 648 fish species in the fourth National Biodiversity Report). This paper provides multi-gear and multi-species ichthyofaunal composition of the floodplains of the Lower Cross Riverin comparison with others in order to share, update information, and provides data for further analysis and to promote discussions for the management of the fishery to avoid risk of fish stock collapse and loss of invaluable ecosystem goods. Material and Methods The main channel of the Cross river has a total surface area of 70,000 km of which 50,000 km is at the lower reaches. At bankful, the Lower Cross River (LCR) is approximately 7m deep and inundates an area (floodplain) of approximately 8000 km. The floodplain contains numerous swamps, pools and lagoons that are often isolated from the main river, sometimes in the dry season. The river channels, floodplain pools, lakes and marginal swamps provide a range of habitats for different fish species. Three sampling stations (S1-3) were established along the extensive floodplains in the freshwater zone of the inshore waters of the LCR (379437.913mE and 558778.199mN), Southeastern Nigeria at Esuk Nnyanyaha, S (along a meander of the CR tributary), Ikot Offiong, S (along the main channel) and Nwaniba, S (around the river mouth), (figure-1). The river is subject to seasonal flooding between July and October. The area experiences, the wet season in April – October and the dry in November-March. Most of the flood areas dry up as the water recedes5,6. The commercial landings of artisanal and subsistence fishers along the floodplains from the three sampling stations were randomly sampled bimonthly, over a period of 12 calendar months. Fish samples were identified and measured to the nearest 0.1cm total length (TL) and 0.1g total weight (TW); some samples were preserved in 10% diluted formaldehyde. Keys were used in fish identification6-8. Fish condition factor was calculated as:  \n . The pooled data of catches by all gear types in each sampling station was used in assessing abundance by calculating the index of preponderance (IP9-10. \r\n. Fishes with IP values less than () 0.50 were regarded as being of relatively insignificant contribution while those with IP values greater than (&#x-3.3;女) 0.50 were regarded as being significant contribution. Ecological indices11-13 were used to classify the environment, describe the structure of the community and compare the International Research Journal of Environment Vol. 2(7), 5-14, July (2013) International Science Congress Association sampling stations. The relative floodplain diversity (RFD) each of the 3 habitats was calculated as  ,where , Gand S are respectively, numbers of families, genera and species, and = 348 (sum of numbers of families, genera and species in floodplain of all the three habitats investigated. Comparative analysis of fish diversity, with similar African rivers, in relation to basin area was established using overall biological diversity i species richness, N16:  !"#&'()*+,-../',01.2).0 ; where, BA = Biological diversity, A = Surface area of river basin, km &'()*+3,&'()*+/)24.003 5 -../'3,-../)2/)24.00 3 01.2).03,01.2).0/)24.003 5 N = Number of species and S = Surface area of river basin, km. Figure-1 Map of the lower reaches of the Cross River showing sampling stations and flood plains Results and Discussion Species composition, diversity and ecological significance Table- 1 provides a broad overview of the ecological indices and ichthyofaunal composition of the lower Cross River system which reveal a polydiverse ecosystem accommodating about 77 fish species, 52 gene ra, 29 families and 9 orders of both fresh and euryhaline (marine intrusive) species. Earlier investigations estimated 37, 45, 45 and 166 species6,17,15,18 , respectively, and 23 species in 18 families19 . The observed differences may be attributed to the in vestigation periods (1857 and 1992) number of researchers, museum (preserved) specimens used and the length of the Cross River system/floodplain sampled. Other reasons include influenced of rainfall5,6,20- 23 discharge and surface area of river basin 15,24 heterogeneity15 , gradual and abrupt changes in physical parameters25, river zonation26 and river continuum information adds to the baseline information needed in measuring future changes in species biomass an Environment Sciences_______________ _________________________ International Science Congress Association The relative floodplain diversity (RFD) 14 of  6 are respectively, numbers = 348 (sum of numbers of families, genera and species in floodplain of all the three Comparative analysis of fish diversity, with similar African rivers, in relation to basin area was established using overall biological diversity i ndices, B15 and ; where, = Biological diversity, A = Surface area of river basin, km ; 5  3 5 5  ; N = Number of species and S = Surface area of river basin, Map of the lower reaches of the Cross River showing sampling stations and flood plains composition, diversity and ecological significance : 1 provides a broad overview of the ecological indices and ichthyofaunal composition of the lower Cross River system which reveal a polydiverse ecosystem accommodating about 77 ra, 29 families and 9 orders of both fresh and euryhaline (marine intrusive) species. Earlier investigations , respectively, and . The observed differences may be vestigation periods (1857 and 1992) 18, the number of researchers, museum (preserved) specimens used and the length of the Cross River system/floodplain sampled. Other 23 , volume of river 15,24 , hydrographic , gradual and abrupt changes in physical and river continuum 27. Current information adds to the baseline information needed in measuring future changes in species biomass an d number. Table- 1 also indicates that of the three sampling stations, S recorded the maximum RFD (biodiversity and condition factor (K = 9.24). RFD, the greater the resemblance of the habitat to overall taxa composition of LCR. The preference of S higher plankton richness; ii. fairly stable and favourable hydrographic conditions for fish survival and growth; location around a meander of the LCR: Meanders are known to produce a succession of habitats of varying depths and bottom types encouraging the development of distinct groups adapted to such conditions; and iv. more fishes from the marine environment navigate upstream up to S Some characteristic species found only in S include Micralestes humilus, Citharinus lates, Petrocephalus bovei, Heterotis niloticus, Chromidotilapia batesii, Synodontis nigrita, Bothygo bius soporator, Mugil cephalus senegalensis . The lower species mix at S slow tidal influence coupled with salt ingression, which poses a challenge to the survival of purely freshwater species. The species composition in table- 2 indicates a total of 5211 fish specimens were sampled - 4620 (88.66%) being freshwater species and 591 (11.34%) - euryhaline fishes. In terms of number the Siluriformes ( Bagridae, Bothidae, Clariidae, Malapteruridae, Mochokidae and Schilbeidae) number, 59.52%; followed by the Perciformes ( Channidae, Cichlidae and Nandidae Osteoglossiformes ( Mormyridae, Notopteridae, Osteoglossidae and Pantodontidae ), 5.58%. The least occurring species was of the Polypteriformes ( Polypteridae euryhaline fish orders, the Perciformes, 95.76%� Pleuronectiformes ( Cynoglossidae Clupeiformes (Clupeidae ), 1.70%, were the least occurring order. Cichlidae was the fish taxa of high bio significance and richness contributing 13 species (18.0%) with 11, 9 and 7 species spread across sampling stations 1 to 3, respectively; followed by Mormyridae Bagridae - 6 species (7.8%). The Cross River/floodplains also show richer fish diversity when compared to other rivers as in table- 3. The main river channel and floodplains accommodate about twice the number of fish species expected using Daget and Ilits’22 formula. The high species richness and heterogeneity observed in this study area confirms the contributions of floodplains to river fisheries and productivity Atlantic Ocean, affords it the presence of some marine intrusive (euryhaline) species. Other African rivers also contributed species to the CR viz-a-viz Marcusenius mento basin), Heterotis niloticus, Brycinus nurse, Citharinus latus Oreochromis niloticus (Nilo- Sudanian river basin), and Pantodon bucholzi and Clarias buthupogon These introductions probably result from ancient hydrographic linkages and inter- connections. The absence of niloticus in the main channel of the LCR had earlier been reported11 but its occurrence in this study may be due to improved sampling technique, suitability of floodplain habitat or one of ecological significance - as a monospecific fish. _________________________ ______ ISSN 2319–1414 Int. Res. J. Environment Sci. 6 1 also indicates that of the three sampling stations, S 1 38.51 %) in fish species condition factor (K = 9.24). The higher the RFD, the greater the resemblance of the habitat to overall taxa The preference of S may be due to i. fairly stable and favourable hydrographic conditions for fish survival and growth; iii. its location around a meander of the LCR: Meanders are known to produce a succession of habitats of varying depths and bottom types encouraging the development of distinct groups adapted to more fishes from the marine navigate upstream up to S 1 than the other stations. Some characteristic species found only in S and not elsewhere Micralestes humilus, Citharinus lates, Petrocephalus bovei, Heterotis niloticus, Chromidotilapia batesii, Synodontis bius soporator, Mugil cephalus and Cynoglosus . The lower species mix at S , may be due to the slow tidal influence coupled with salt ingression, which poses a challenge to the survival of purely freshwater species. 2 indicates a total of 5211 fish 4620 (88.66%) being freshwater euryhaline fishes. In terms of Bagridae, Bothidae, Clariidae, Malapteruridae, Mochokidae and Schilbeidae) occurred most in number, 59.52%; followed by the Perciformes ( Anabantidae, Channidae, Cichlidae and Nandidae ), 19.62% and then the Mormyridae, Notopteridae, Osteoglossidae ), 5.58%. The least occurring species was of Polypteridae ), 1.46%. Of the three euryhaline fish orders, the Perciformes, 95.76%� Cynoglossidae ), 2.55%; while the ), 1.70%, were the least occurring order. Cichlidae was the fish taxa of high bio diversity significance and richness contributing 13 species (18.0%) with 11, 9 and 7 species spread across sampling stations 1 to 3, respectively; followed by Mormyridae - 9 species (11.7%) and 6 species (7.8%). The Cross River/floodplains also show richer fish diversity when compared to other rivers as in 3. The main river channel and floodplains accommodate about twice the number of fish species expected using Daget formula. The high species richness and heterogeneity in this study area confirms the contributions of floodplains to river fisheries and productivity 28. It’s link to the Atlantic Ocean, affords it the presence of some marine intrusive (euryhaline) species. Other African rivers also contributed Marcusenius mento (Guinean river Heterotis niloticus, Brycinus nurse, Citharinus latus and Sudanian river basin), and Clarias buthupogon (Zairean faunae). result from ancient hydrographic connections. The absence of Heterotis in the main channel of the LCR had earlier been but its occurrence in this study may be due to improved sampling technique, suitability of floodplain habitat or as a monospecific fish. 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International Science Congress Association 7 Table-1 Summary of species taxa and diversity in the Lower Cross River floodplain, NigeriaNumber of Taxa/ Diversity Indices Sampling stations Total Esuk Nnyanyaha, S Ikot Offiong, S Nwaniba, S No. of Orders 8 7 8 9 No. of Families 26 21 22 29 No. of Genera 44 36 35 52 No. of Species 64 51 49 77 Species diversity, H 0.3199 4.6393 2.2960 1.0095 Species evenness, J 0.0769 1.1799 0.5840 0.4595 Species similarity, D 0.8796 0.9250 0.7940 0.6756 Margalef index, d 18.5616 15.807 15.4515 20.59 Relative Diversity, RFD 38.51 31.03 30.46 100 No. of fish sampled 2478 1456 1278 5212 Table-2 Occurrence, mean total length, weight and condition factor of fish communities in sampling stations in the Lower Cross River, Nigeria Order/Family/SpeciesSampling Stations Esuk Nnyanyaha (S = 64) Ikot Offiong (S = 51) Nwaniba (S = 49) N MTL,cm MTW, g K N MTL, cm MTW, g K N MTL, cm MTW, g K Fresh Water Species Characiformes - - - - - - - - - - - - Characidae - - - - - - - - - - - - 1 Brycinus nurse Ruppell 23 9.40 8.40 1.01 28 9.45 13.35 1.58 24 10.00 8.45 0.85 2 Brycinus Longipinnis Gunther 31 9.10 8.40 1.01 43 8.10 9.90 1.86 18 9.25 6.90 0.87 3 Brycinus macrolepidotus Valenciennes 8 6.20 3.50 1.47 - - - - 5 11.55 13.50 0.88 4 Micralestes Humilis Boulenger 23 9.80 16.85 1.79 - - - - - - - - Citharinidae - - - - - - - - - - - - 5 Citharinus latus Muller & Troschel 1 24.60 42.50 0.29 - - - - - - - - Dischodontidae - - - - - - - - - - - - 6 Ichthyborus monodi Pellegrin 2 17.60 23.50 0.43 8 18.80 27.70 0.42 1 18.10 15.00 0.25 Hepsetidae - - - - - - - - - - - - 7 Hepsetus odoe Bloch 6 22.25 86.45 0.78 3 18.70 53.90 0.82 4 18.45 47.00 0.75 Cypriniformes - - - - - - - - - - - - Cyprinidae - - - - - - - - - - - - 8 Barbus callipterus Boulenger 98 6.30 3.70 2.68 70 7.70 6.85 1.5 20 5.00 2.40 1.92 9 Labeo batesi Boulenger 2 11.85 22.25 1.33 10 11.15 21.95 1.58 - - - - 10 Labeo coubie Ruppell 5 7.25 6.25 1.64 3 8.85 13.65 1.97 - - - - Cyprinodontiformes - - - - - - - - - - - - Aplocheilidae - - - - - - - - - - - - 11 Aphyosemion bivittatum Lonnberg- - - - - - - - 9 4.75 2.45 2.29 International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 2(7), 5-14, July (2013) Int. 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International Science Congress Association 8 12 Epiplatys sexfasciatus Gill 21 4.45 0.85 0.96 38 5.05 2.45 1.90 25 5.10 2.20 1.66 13 Epiplatys bifasciatus Steindachner 11 2.80 0.55 2.51 11 2.25 1.15 10.10 44 3.45 1.30 3.17 14 Epiplatys grahami Boulenger- - - - - - - - 2 6.15 2.65 1.10 Osteoglossiformes - - - - - - - - - - - - Mormyridae - - - - - - - - - - - - 15 Brienomyrus brachyistius Gill 90 8.75 6.20 0.93 41 9.65 9.75 1.08 26 10.58 10.18 0.80 16 Gnathonemus petersii Gunther 4 22.00 65.15 0.61 10 20.70 54.55 0.62 6 21.25 61.65 0.64 17 Isichthys henryi Gill 11 7.85 1.90 0.39 15 8.00 2.75 0.54 6 12.10 6.90 0.39 18 Mormyrops deliciosus Leach 11 16.60 39.90 0.87 12 9.05 7.75 1.05 - - - - 19 Mormyrus rume Valenciennes 12 2795 168.65 0.77 8 21.70 50.50 0.49 - - - - 20 Petrocephalus bovei Valenciennes 64 9.65 6.50 0.72 - - - - - - - - 21 Petrocephalus ansorgii Boulenger 11 17.30 11.10 0.21 18 14.45 10.00 0.33 1 10.4 9.80 0.87 22 Pollimyrus adspersus Gunther - - - - - - - - 2 5.20 4.90 3.48 Notopteridae - - - - - - - - - - - - 23 Papyrocranus afer Gunther1 34.70 28.80 0.07 3 21.00 37.65 0.41 14 23.35 41.25 0.32 24 Xenomystus nigri Gunther1 16.20 25.60 0.60 1 8.10 17.00 3.20 5 12.10 20.40 1.5 Osteoglossidae - - - - - - - - - - - - 25 Heterotis niloticus Cuvier 59 50.65 3510.0 2.70 - - - - - - - - Pantodontidae - - - - - - - - - - - - 26 Pantodon buchholzi Peters - - - - - - - - 4 9.85 11.75 1.23 Perciformes - - - - - - - - - - - - Anabantidae - - - - - - - - - - - - 27 Ctenopoma kingsleyae Gunther 13 10.20 7.50 0.71 15 10.10 22.55 2.19 4 9.45 12.75 1.46 28 Ctenopoma nebulosumNorris & Teugels7 10.45 28.10 0.25 5 12.50 32.10 1.64 5 10.85 19.55 1.33 Channidae 29 Parachanna africana teindachner 7 14.04 27.30 0.98 5 17.30 29.15 0.56 44 14.40 27.30 0.91 30 Parachanna obscura Gunther - - - - 3 10.80 11.35 0.90 11 15.65 20.00 0.52 Cichlidae 31 Chromidotilapia batesii Boulenger 51 14.75 36.30 1.13 - - - - - - - - 32 Chromidotilapia guntheri Sauvage 43 64.75 62.15 0.02 46 15.80 37.90 0.96 40 13.25 34.90 1.50 33 Hemichromis bimaculatus Gill 6 5.80 2.00 1.03 - - - - - - - - 34 Hemichromis fasciatus Peters 40 10.00 62.15 6.22 30 7.70 15.15 3.32 22 8.90 19.50 2.77 35 Hemichromis guttatus Gunther 11 6.70 3.00 0.00 7 6.70 805 2.68 - - - - International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 2(7), 5-14, July (2013) Int. 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International Science Congress Association 9 36 Oreochromis niloticus L. - - - - - - - - 3 9.5 7.0 0.82 37 Pelvicachromis pulcher Boulenger 27 9.60 19.25 1.00 40 8.80 17.90 2.63 19 9.45 14.90 1.77 38 Sarotherodon galilaeus L. 58 17.10 203.70 4.03 30 12.00 55.00 3.18 - - - - 39 S. melanotheron Ruppell 15 12.40 61.55 3.22 26 13.35 44.60 1.87 - - - - 40 Thysochromis ansorgii Boulenger 18 7.85 7.00 1.45 10 7.80 10.35 2.18 28 9.00 12.45 1.71 41 Tilapia. mariae Boulenger 37 52.50 80.90 0.06 33 12.90 16.40 0.76 20 12.10 59.00 0.05 42 Tilapia zilli Gervais 49 9.70 30.70 3.36 62 9.80 12.70 1.35 4 8.65 9.45 1.46 Nandidae - - - - - - - - - - - - 43 Polycentropsis abbreviata Boulenger - - - - - - - - 4 5.95 5.25 2.49 Siluriformes - - - - - - - - - - - - Bagridae - - - - - - - - - - - - 44 Chrysichthys aluuensis Risch 35 23.00 251.50 2.07 18 27.55 89.20 0.43 - - - - 45 Chrysichthys auratus Geoffroy-Saint-Hilaire 782 15.75 65.30 1.67 231 14.00 33.80 1.23 564 14.00 51.95 1.89 46 C. nigrodigitatus Lacepede 241 53.15 4124.0 2.75 190 51.90 2770.0 1.98 65 50.00 6450.0 5.16 47 Parauchenoglanis akiri Risch 19 8.30 9.30 1.63 - - - - 26 10.35 10.65 0.95 48 Parauchenoglanis fasciatus Gras - - - - 20 10.10 11.00 1.07 18 9.90 9.25 0.95 49 Parauchenoglanis guttatus Lonnberg - - - - - - - - 11 10.95 10.85 0.83 Clariidae - - - - - - - - - - - - 50 Clarias buthupogon Sauvage 3 15.05 23.80 0.09 7 14.55 19.0 0.62 - - - - 51 Clarias gariepinus Burchell 36 34.35 589.70 1.45 18 18.50 47.0 0.74 6 19.65 44.90 0.59 52 Clarias macromystax Gunther 5 20.45 78.50 0.92 - - - - 4 21.40 67.85 0.69 53 Heterobranchus longifilis Val. - - - - - - - - 1 26.00 152.0 0.86 Bothidae - - - - - - - - - - - - 54 Citharichthys stampfilis Steindachner15 12.85 0.70 7 14.5 19.0 0.62 - - - - - Malapteruridae - - - - - - - - - - - - 55 Malapterurus electricus Gmelin 32 13.70 53.10 2.07 40 13.90 32.50 1.21 60 14.40 77.75 2.60 Mochokidae - - - - - - - - - - - - 56 Synodontis nigrita Cuvier &Val.20 14.95 60.75 1.82 - - - - - - - - 57 Synodontis schallBloch & Schneider 11 20.25 157.60 1.92 15 21.65 105.15 1.04 - - - - Schilbeidae - - - - - - - - - - - - 58 Schilbe intermedius Ruppell - - - 10 15.35 36.45 1.01 3 14.7 26.50 0.83 59 Schilbe mystus L. 32 12.90 10.80 0.50 - - - - 1 8.80 10.50 1.54 Polypteriformes - - - - - - - - - - - - Polypteridae- - - - - - - - - - - - 60 Erpetoichthys calabaricus Smith - - - - 5 28.15 44.90 0.20 58 28.35 37.50 0.16 International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 2(7), 5-14, July (2013) Int. 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International Science Congress Association 10 Marine Intrusive (Estuarine) Species Clupeiformes - - - - - - - - - - - - Clupeidae - - - - - - - - - - - - 61 Odaxothrissa mento Regan 3 11.80 14.00 0.85 - - - - - - - 62 Pellonula leonensis Boulenger 3 5.25 1.90 1.31 - - - - 4 5.75 1.85 0.97 Perciformes - - - - - - - - - - - - Carangidae - - - - - - - - - - - - 63 Caranx hippos L. 30 9.85 24.50 2.50 22 7.75 13.00 2.79 10 7.30 12.30 3.94 64 Trachinotus goreensis Cuvier 26 9.90 16.60 1.72 21 9.40 12.50 1.51 - - - - 65 Trachinotus teraia Cuvier 38 8.05 6.70 1.28 40 8.05 6.90 1.32 - - - - Eleotridae - - - - - - - - - - - - 66 Bostrychus africanus Steindachner- - - - - - - - 2 11.35 13.80 2.94 67 Eleotris senegalensis Steindachner17 12.75 34.65 0.19 9 11.50 28.40 1.87 68 Eleotris vittata Dumeril1 8.50 4.00 0.00 2 8.90 10.75 1.52 7 9.60 17.70 2.00 Gobiidae 69 Bothygobius soporator Cuvier &Val.11 26.10 32.30 5.26 - - - - - - - - Lutjanidae 70 Lutjanus endecacanthus Bleeker4 11.80 35.40 2.16 - - - - - - - - Mugilidae 71 Liza falcipinnis Valenciennes 72 25.95 51.00 0.29 53 18.4 40.95 0.66 6 11.75 15.65 0.01 72 Liza grandisquamisValenciennes 87 25.95 52.82 0.30 60 12.4 32.15 1.69 5 11.45 8.50 0.57 73 Mugil cephalus L.22 17.40 66.35 1.26 - - - - - - - - Pomadasidae (=Haemulidae) - - - - - - - - - - - - 74 Pomadasys jubelini Cuvier 13 11.90 33.25 1.97 8 12.9 32.2 1.50 - - - - Pleuronectiformes - - - - - - - - - - - - Cynoglossidae - - - - - - - - - - - - 75 Cynoglossus senegalensis Kaup 15 27.65 105.10 0.50 - - - - - - - Mean ± Std. Dev.16.80 ± 12.33 167.25 ±668.22 3.53 13.56 ± 7.68 80.34 ± 84.72 3.22 12.79 ± 7.74 193.29 ±932.32 9.24 N = Number of fish specimen caught, S = Number of fish species caught, MTL = Mean total length, MTW = Mean weight of fish caught K = Mean condition factor Table-3 Diversity and taxa – richness in relation to river basin area in some West African rivers River Surface Area, KmTaxa Number Taxa Diversity Overall Diversity Index Families Genera Species D families D genera D species B A N Mono 22,000 18 35 59 0.289 0.356 0.408 1.053 61 Oueme 50,000 30 64 106 0.314 0.384 0.431 1.129 75 Ogun 22,370 28 60 91 0.333 0.409 0.450 1.192 61 Cross 70,000 41 97 166 0.333 0.410 0.458 1.201 81 LCRF 8,000 29 52 77 0.375 0.440 0.483 1.298 47 = Biological diversity of river basin15 , N = Expected number of species16, LCRF = Lower Cross River Floodplains/this study International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 2(7), 5-14, July (2013) Int. Res. J. Environment Sci. International Science Congress Association 11 Table-4 Species composition of catches from the lower Cross River floodplain, Nigeria: Pooled data from three sampling stations S/N Fish Species N Wt (g) % N % Wt Ave. Wt (g) No. of fish Kg-1IP Fresh water species 1. Aphyosemion bivittatum 9 22.05 0.17 0.00 2.45 408.2 0.00 2. Barbus callipterus 188 882.70 3.61 0.04 4.70 213.0 0.02 3. Brienomyrus brachyistus 157 1222.43 3.01 0.06 7.79 128.4 0.02 4. Brycinus macrolepidotus 13 95.50 0.25 0.00 7.35 136.1 0.00 5. Brycinus nurse 75 769.80 1.44 0.04 10.26 97.4 0.01 6. Brycinus l ongipinnis 92 810.40 1.82 0.04 8.81 113.5 0.01 7. Chromidotilapia batesii 51 185.13 0.98 0.01 3.63 275.5 0.00 8. Chromidotilapia guntheri 129 5811.85 2.48 0.27 45.05 22.2 0.07 9. Chrysichthys nigrodigitatus 496 1939434.00 9.52 90.39 3910.15 0.3 86.68 10. Chrysichthys aluuensis 53 10408.10 1.02 0.49 196.48 5.1 0.05 11. Chrysichthys auratus 1577 88172.20 30.26 4.11 55.91 17.9 12.53 12. Citharichthys stampfilis 46 720.00 0.88 0.03 15.65 63.9 0.00 13. Citharinus latus 1 42.50 0.02 0.00 42.50 23.5 0.00 14. Clarias gariepinus 60 22344.60 1.15 1.04 372.41 2.7 0.12 15. Clarias buthupogon 10 204.40 0.19 0.01 20.44 48.9 0.00 16. Clarias macromystax 9 663.90 0.17 0.03 73.77 13.6 0.00 17. Ctenopoma kingsleyae 32 486.75 0.61 0.02 15.21 65.7 0.00 18. Ctenopoma nebolusum 17 454.95 0.33 0.02 26.76 37.4 0.00 19. Epiplatys bifasciatus 66 75.90 1.27 0.00 1.15 869.6 0.00 20. Epiplatys grahami 2 5.30 0.04 0.00 2.65 377.4 0.00 21. Epiplatys sexfasciatus 84 165.95 1.16 0.01 1.98 506.2 0.00 22. Erpetoichthys calabaricus 63 2399.50 1.21 0.11 38.09 26.3 0.01 23. Gnathonemus petersii 20 611.46 0.38 0.03 30.57 32.7 0.00 24. Hemichromis bimaculatus 6 12.00 0.12 0.00 2.00 500.0 0.00 25. Hemichromis fasciatus 92 1529.50 1.77 0.07 16.63 60.2 0.01 26. Hemichromis guttatus 18 87.35 0.35 0.00 4.96 201.5 0.00 27. Hepsetus odoe 13 868.40 0.25 0.04 66.80 15.0 0.00 28. Heterobranchus longifilis 1 152.00 0.02 0.01 152.0 6.6 0.00 29. Heterotis niloticus 59 3510.00 1.13 0.16 59.49 16.8 0.02 30. Ichythyborus monodi 11 283.60 0.21 0.01 25.78 38.8 0.00 31. Isichthys henryi 42 103.55 0.81 0.00 2.47 405.6 0.00 32. Labeo coubie 8 72.20 0.15 0.00 9.03 110.8 0.00 33. Labeo batesi 12 263.60 0.23 0.01 21.97 45.5 0.00 34. Malapterurus electricus 132 3465.70 2.53 0.16 26.26 38.1 0.04 35. Marcusenius mento 14 455.35 0.27 0.02 32.53 30.7 0.00 36. Micralestes humilus 23 387.55 0.44 0.02 16.85 59.4 0.00 37. Mormyrops deliciosus 23 531.90 0.44 0.03 23.13 43.2 0.00 38. Mormyrus rume 20 2427.80 0.38 0.11 121.39 8.2 0.00 39. Oreochromis niloticus 3 21.00 0.06 0.00 7.0 142.9 0.00 40. Pantodon bulcholzi 4 11.75 0.08 0.00 2.94 340.4 0.00 41. Papyrocranus afer 18 719.25 0.35 0.03 39.96 25.0 0.00 42. Parachanna obscura 14 254.05 0.27 0.01 18.15 55.1 0.00 43. Parachanna africana 56 1538.05 1.07 0.07 27.45 36.4 0.01 44. Parachenoglanis akiri 45 453.60 0.86 0.02 10.08 99.2 0.00 45. Parachenoglanis fasciatus 38 386.50 0.73 0.02 10.17 98.3 0.00 46. Parachenoglanis guttatus 11 119.35 0.21 0.01 10.85 92.2 0.00 47. Pelvicachromis pulcher 86 1518.85 1.65 0.07 17.66 56.6 0.01 48. Petrocephalus ansorgii 30 311.90 0.58 0.01 10.40 96.2 0.00 49. Petrocephalus bovei 64 416.00 1.23 0.02 6.50 153.9 0.00 50. Pollimyrus adspersus 2 9.80 0.04 0.00 4.90 204.1 0.00 51. Polycentropsis abbreviata 4 21.00 0.08 0.00 5.25 190.5 0.00 52. Sarotherodon galilaeus 88 13464.60 1.69 0.63 153.01 6.5 0.11 53. Sarotherodon melanotheron 41 2081.85 0.77 0.10 50.78 19.7 0.01 International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 2(7), 5-14, July (2013) Int. Res. J. Environment Sci. International Science Congress Association 12 54. Schilbe intermedius 13 444.00 0.25 0.02 34.15 29.3 0.00 55. Schilbe mystus 33 356.10 0.63 0.02 10.79 92.7 0.00 56. Synodontis nigrita 20 1215.00 0.38 0.06 60.75 16.5 0.00 57. Synodontis schall 26 3310.85 0.50 0.15 127.34 7.9 0.01 58. Thysochromis ansorgii 50 578.10 1.07 0.03 10.32 96.9 0.00 59. Tilapia mariae 90 4614.50 1.73 0.22 51.27 19.5 0.04 60. Tilapia guineensis 32 949.50 0.61 0.04 29.67 33.7 0.00 61. Tilapia zilli 115 2329.50 2.21 0.11 20.26 49.4 0.03 62. Xenomystus nigri 7 149.60 0.13 0.01 21.37 46.8 0.00 Marine Intrusive (Estuarine) Species 63. Bathygobius soporator 11 355.30 0.21 0.02 32.30 31.0 0.00 64. Bostrychus africanus 2 27.60 0.90 0.00 13.8 72.5 0.00 65. Caranx hippos 62 144.00 0.06 0.05 18.45 54.2 0.01 66. Cynoglossus senegalensis 15 1576.50 0.29 0.07 105.10 9.5 0.00 67. Eleotris senegalensis 26 844.65 0.50 0.04 32.49 30.8 0.00 68. Eleotris vittata 10 149.40 0.19 0.01 14.94 66.9 0.00 69. Liza gradisquamis 152 6566.84 2.92 0.31 43.20 23.2 0.09 70. Liza falcipinnis 131 5936.25 2.51 0.28 45.31 22.1 0.07 71. Lutjanus endecacanthus 4 141.60 0.08 0.01 35.40 28.2 0.00 72. Mugil cephalus 22 1459.70 0.42 0.07 66.35 15.1 0.00 73. Odaxothrissa mento 3 42.00 0.06 0.00 14.00 71.4 0.00 74. Pellonula leonensis 7 13.10 0.13 0.00 1.87 534.8 0.00 75. Pomadasys jubelini 21 689. 85 0.40 0.03 32.85 30.4 0.00 76. Trichinotus teraia 78 530.60 1.19 0.03 6.80 147.0 0.01 77. Trichinotus goreensis 47 694.10 0.13 0.03 14.77 67.7 0.00 5211 2145588.06 100 100 411.74 2.43 100 N = number of fish caught Wt = weight of fish caught IP = index of preponderance Figure-2 Length frequency of fish catches along the Lower Cross River Floodplain, Nigeria Earlier reports5,18,22 indicate that at least 33 marine species (from 28 genera, 16 families and 8 orders) penetrate the fresh waters of the Lower Cross River system. In this study, the marine intrusive species constitute about 20% of species, eight families-28% (Clupeidae, Carangidae, Eleotridae, Gobiidae, Lutjanidae, Mugilidae, Haemulidae and Cynoglossidae) and three orders-33% (Clupeiformes, Perciformes and Pleuronectiformes). Brackishwater species like Mugil cephalus, Pomadasys jubelini and Caranx hippos co-occurring in the freshwater zone (like their marine counterparts), probably entered with the tidal waves, which are still observable as far as . Some of these fishes were probably searching for food, spawning sites to lay eggs and/or nursery grounds for growth, and have become adapted. The longitudinal distribution of C. nigrodigitatus stretching from the mouth of the river inwards to the upper reaches suggests its ability to tolerate wider range of salinity. Therefore, it is a good candidate for brackish water culture along with Mugil cephalus, Pomadasys jubelini andCaranx hippos. 1015202530 35 051015202530354045505560Fish Total Length, mm% Catch International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 2(7), 5-14, July (2013) Int. Res. J. Environment Sci. International Science Congress Association 13 The indices of preponderance in table-4 indicate the most important fish family is Bagridae (IP=99.26; Siluriformes) occurring in all the sampling stations: S (IP=54.88), S(IP=98.52) and S (IP=98.15). The top five families in terms of numbers are the Bagridae, 42.6% (IP=99.26), especially Chysichthys auratus contributing 30.26% and C. nigrodigitatus- 9.52%; Cichlidae, 15.49% (IP=0.28) Chromidotilapia guntheri- 2.48% and Tilapia zilli - 2.21%; Mormyridae, 7.14% especially Brienomyrus brachyistus - 3.01%); Mugilidae, 5.85% especially Liza falcipinnis - 2.51% and Cyprinidae, 3.99%, especially Barbus callipterus - 3.61%. In terms of gross weight of fish caught, the order is Bagridae, 95% � Cichlidae, 1.55% � Clariidae, 1.09%. In terms of size (i.e., number of individuals per kg), the bagrid catfish, Chysichthys nigrodigitatus (0.3) surpassed all other species (� Clarias gariepinus-2.7 � C. aluuensis-5.1). At the species level in table-4, C. nigrodigitatus and C. auratusare the most significant. Other important species include Heterotis niloticus and Clarias gariepinus. Chrysichthysremains the main fishery in the lower Cross River as observed in this study and others5,23,28 though we obtained a higher average number of 0.3 fish (C. nigrodigitatus) per kg compared to an earlier value of 0.4. Populations were generally bigger upstream than their counterparts downstream. The mean sizes of individual species increase with increase in number of species upstream, from the transitional zone at the river mouth, S to S(S3 = 49, S = 54 and S1 = 64 species, with mean TL = 12.79, 13.56 and 16.08cm, respectively). Specimens of the dominant species such as Chrysichthys auratus showed mean total lengths of 15.75, 14.00 and 14.00cm; C.nigrodigitatus - 53.15, 51.90 and 50.00cm and Chromidotilapia guntheri 64.75, 15.80 and 13.25cm in S to S, respectively. The survey indicates that the smaller-sized C. auratus with average weight of 55.91kg were caught in higher number (30.26%) than the larger-sized C. nigrodigitatus (3910.15g; 9.52%). The size range in figure-2 suggests that fishing gears with narrow selectivity were employed hence most fish sampled fall within 5-20cm, the juvenile class range. The species of fish inhabiting floodplain rivers usually cover a wide range of sizes which normally span three orders (i.e., 1.5cm to 1500cm). It also suggests that the floodplain is highly utilized by juvenile of commercially important fishes from the sea possibly as nursery and feeding grounds. However, the fishery is still vulnerable to (growth and recruitment) over-fishing (particularly during the spawning season) due to the persisting indiscriminate interception, massive capture in very large numbers and complete removal of young and gravid C. nigrodigitatus catfish during their upstream migration with the rise in river level and the inundation of the floodplains, particularly in May – June22. Such over-fishing decreases the resilience of the fish fauna making it more susceptible to environmental fluctuations, climate change and anthropogenic perturbations. Gears that exclude juveniles and fingerlings should therefore be encouraged. ConclusionThe Cross River floodplain has been shown to be a nursery and feeding ground for diverse species, including monospecific and rare species of fish hence gears that exclude juveniles and fingerlings should therefore be encouraged and future development along the floodplain should be subjected to environmental scrutiny to maintain the environmental health and integrity of the ecosystem. Species diversity studies should be a continuous work to determine the health of the fishery and enhance conservation measures. References 1.Hillary A., Kokkonen M. and Max L., eds., Proceedings of the World Heritage Marine Biodiversity Workshop, Hanoi, Viet Nam, February 25 – March 1, 2002, UNESCO World Heritage Centre, Paris, 92 (2003)2.Ita E.O., Inland fishery resources of Nigeria CIFA Occasional Paper No. 20, FAO, Rome, 120 (1993)3.Obasohan E.E. and Oronsanye J.A.O., Biodiversity and sustainability of freshwater fishes in Nigeria, In Ansa, E. 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