Species distribution is an indication of where fish species occur or are located in the aquatic environment. This consists of the vertical aspect (surface, mid water and bottom) and the horizontal or lateral component such as convex, central and the concave sections across the water body. Species distribution thus provides information on whether the fish species is pelagic, demersal etc., which will further inform the choice of the fishing gear to be used Abundance in this context, refers to the total catch in number or biomass of the species. Information on habitat and abundance, amongst others, are very vital for fisheries development and management.
Fish and fishery products constitute the cheapest sources of animal protein but according to Ndok (1982), they constitute only 40% of the diet of an average Nigerian. Anko and Eyo (2003), reported that Nigeria has vast potentials for fisheries development, being endowed with a maritime area of 46,300 km2, an Exclusive Economic Zone (EEZ) area of 210,900 km2 and inland waters of 12.5 million ha. But that in spite of the huge endowment, the current production level of 400,000 metric tons is at a 50% deficit to meet Nigerias fish need per annum of at least 1.5 million metric tons. Okorie (2003), observed that the bulk of fish production comes from the artisanal sector but because the local production is inadequate to meet our demand, Nigeria imports about 49.5% of fish.
The challenge is to increase domestic fish supply through the development and proper management of the fishery resources of Nigeria. However, many Nigerians are reluctant to engage in artisanal fisheries owing to the unattractive income of artisanal fishers. This could be overcome if the fishers are organized into cooperatives and are given necessary support by Government and even the private sector. This in the long run will, apart from improving the standard of living of the fishers, provide more fish to the population at cheaper prices and also bridge the supply-demand deficit gap.
Inspite of its importance, there is scanty information on the artisanal fisheries of the Nun River. The study will complement available information in the planning, formulation and execution of fisheries policies and programmes especially in the artisanal sector and to serve as future reference in the proper utilization of the fisheries potentials of the Nun River in particular and the inland waters of the Niger Delta in general.
Fishing gear such as cast nets, drift nets and seine nets are commonly used for fishing in most waters, probably due to the fact that these nets greatly influence catches (Ufodike et al., 1989). Steinberg (1964) recommended transparent monofilament net material for effective fishing in clear water. These have the characteristics of being invisible to the fish (Kennedy, 1951; Nedelec, 1975).
According to Sanisbury (1975), the factors that determine the choice of fishing gear to be used in a given fishery depend on species being fished, that is, whether pelagic, demersal etc. individual value of the species to the fishers, depth of water and fishing depth, characteristics of the seabed (if gear is to be work in contact with the bottom). Distrib ution and abundance are veritable aspects of fish stock assessment studies.
According to Sissenwine et al. (1979) fish stock assessment evaluates the effect of fishing on a fishery as a basis of fishery management decisions. Allison et al. (1997) observed that abundance, to a large extent, is a function of recruitment, which according to Clark (1979) and Bankole (1990), is the major source of variability in fish population.
Land use and other human activities influence species diversity and abundance (Victor and Dickson, 1985; Victor and Ogbeibu, 1985, 1986). Scott (1966) identified over 250 species of fish landed in the Niger Delta. Chindah and Osuamkpe (1994) studied the fish assemblage of the lower Bonny River of the Niger Delta with its adjourning creeks and observed 25 families consisting of 57 species. Alfred-Ockiya (1996) observed 28 families and 41 species in Kolo creek, Rivers State, Nigeria; Lowe-McConnel (1964) observed 44 species on the Rupenninme River. Nwadukwe (1995) observed a total of 23 species from 17 families in 2 habitats in the Lagos Lagoon in which 6 species appeared regularly.
Sikoki et al. (1998) observed a total of 24 species belonging to 15 families in the Lower Nun River using a fleet of 9 gill nets that ranged from 3/4"-7" mesh sizes. There were differences in number and biomass of fish caught due to gill net selectivity with a sharp decline in larger mesh sizes especially in 4" and 7" mesh net. It was further observed that the highest catch was recorded in the 2" category (19.6%) and the least (1.6%) in the 7" mesh size but were small and generally immature. Sikoki et al. (1998) further observed that the catch was generally low, which was attributed to fishing methods, fishing season and industrial activities, causing biological over fishing of the stock in the area.
MATERIALS AND METHODS
Study area: This study was carried out at the lower Nun River around
Anyama Ijaw in Bayelsa State (Fig. 1). The sampling area lies
between Lat. 4°51'N and 4°54'N; Long. 6°11'E and 6°13'E. The
concave bank in the study area is moderately steep sloppy with loamy bottom,
while the convex bank is relatively shallow and sandy.
||The lower Nun river, Niger Delta, showing the sample area
and sampling stations (NR1-Convex; NR2-Middle; NR3-Concave)
The tidal influence is very mild during the dry season. However, a slightly
reversed flow occurs during the rising tide at the peak of the dry season but
during the flood period there is a swift one directional current in the study
Sampling procedure: Sampling was carried out twice a month at 2 weeks interval for 12 calendar months using 3 sets of gill net with stretched mesh sizes of 8, 12 and 15 mm, respectively each measuring 35 mm in length and 3 m in depth with a surface area of 105 m2. Sampling lasted for 3 h every sampling day. The 3 drift gill nets were operated simultaneously from 3 fishing canoes, by adjusting the float line and the weighted bottom line to keep the net at the desired depth.
Specimens caught during the sampling were preserved in 10% formalin solution and taken to the laboratory for identification. Fish specimens were identified from monographs, descriptions, checklist and keys (Reed et al., 1967; Holden and Reed, 1972; Leveque et al., 1991; Olasebikan and Raji, 1998).
Distribution and abundance: Sampling was done at the surface i.e., the concave, the central and convex sections of the river, the mid water and bottom to determine the distribution and abundance of fish in the study area. All the specimens were counted to determine species abundance. Abundance score of the species were estimated following the criteria of Allison et al. (1997) as follows: 1-50 Rare; 51-100 Few; 101-200 Common; 201-400 Abundant and > 400 Dominant.
Data analysis: Analysis of Variance (ANOVA) was used to test for significant difference of fish caught with the different mesh sized gill nets and distribution. All analysis were carried out through the computer enhanced Microsoft Excel Programme.
Ichthyofauna: A total of 11, 156 specimens from 14 families consisting
of 25 species were caught in the lower Nun River during the period of sampling.
The highest diversity was observed in characidae (4) Schilbeidae (4) and Cyprinidae
(3). The relative abundance of the species are shown on Table
1. Dominant in the catch were Parailia pellucida (71.3%), Pellonula
leonensis (18.41%) and Odaxothrissa mento (7.03%), belonging to the
families of Schilibeidae and Clupeidae. The species appeared regularly
in the study. Common species include Eutropius niloticus (1.03%) and
Chrysichthys nigrodigitatus (0.82%).
||Relative abundance of fish caught in the lower Nun River
|R = Rear; C = Common; A = Abundance; D = Dominant
Petrocephalus bane ansorgii sp. (0.91%) were observed as few. All other
species rarely appeared in the catch during the investigation.
Quantitatively, Schilbeidae (Paraila pellucida, Eutropius niloticus Siluranodon auritus and Eutropius buffei) was the highest (72.4%) followed by Clupeidae (Pellonulla leonensis and Odaxothrissa mento), which formed 25.44%. Mormyridae (0.91) and Bagridae (0.82) ranked 3rd and 4th, respectively in abundance in the fishery. The least abundance of 0.01% came from Rhabdalestes septentrionalisn (Characidae), Distichodus rostratus (Distichodontidae), Silluanodon auritus (Schilbeidae), Sphyraena afra (Sphyraenidae) and Labeo coubie (Cyprindae) (0.01%). Others with a relatively low abundance in the catch consisted of Carangidae (0.03%), Gobidae (0.03%), Elopidae (0.03%) and Distichodontidae (0.04%).
||Lateral distribution of fish species across the lower Nun
||Vertical distribution of fish in the lower Nun River
The lateral distribution of the various species is shown in Table 2. All the 14 species occurred in the convex area, which also constituted the highest relative abundance of 60.08%. The concave formed the least abundance of 15.5%, while catches from the central location contributed 24.02%. Table 3 shows the vertical distribution of fish species in the study area with the highest relative abundance of 37.13% observed at the surface. The least of 29.85% occurred at the bottom while the mid water consisted of 31.91% of the total catch. Schilbeidae was more abundant at the bottom (94.02%) followed by mid water (74.41%) while the least of (51.93%) occurred at the surface. On the other hand, Clupeidae was more abundant at the surface (46.23%) followed by mid water (23.2%) with the least relative abundance of 2.16% at the bottom.
The Lower Nun River is an estuary of the River Niger thus all the characteristics of River Niger are prevalent in it. The 25 species belonging to 14 families, in the study, indicates good species diversity with the highest diversity in Characidae, Schilbeidae and Cyprinidae. The dominance of Parailia pellucida, Pellonula leonensis and Odaxothrssa mento could be attributed to gear selectivity. The result is similar to Sikoki et al. (1998), who encountered 24 species belonging to 16 families in the area. Apart from Schilbeidae and Clupeidae that had small sized sexually mature fish, the fishes caught in this study were small in size and generally immature due to the small mesh sizes of the gill nets used. This is in agreement with Nwadukwe (1995), who observed that smaller mesh sized nets caught small and immature fishes in the Lagos lagoon.
Parailia pellucida was the most abundant species in this study followed by Pellonula leonensis. Ezenwaji (2004) also observed similar high abundance in the 2 species though with Pellonula leonensis dominating in the lower Anambra river.
This study is in agreement with Sikoki et al. (1998) in terms of number
of families and species but, varies in the kind of species. Only 6 families
were common in both studies (Bagridae, Characidae, Cyprinidae, Schilbeidae,
Mormyridae and Mochokidae). Carngidae, Clupidae, Distichodontidae, Gobidae,
Elopidae, Anabantidae, Sphyraenidae and Paeneidae present in this study, were
absent in their studies. Citharindae, hepsetidae, Centropimidae, Gymnarcidae,
Claridae, Pomadasydae and Osteoglossidae reported in Sikoki et al. (1998)
were similarly not caught during the sampling. This may be attributed to gear
selectivity and fleet size of gill net since they used a fleet of 9 gill nets
that ranged from 3/4"-7" mesh sizes. Ufodike et al. (1989) reported that
gill net technology, catch, period/technique are essential in maximizing fish
catches. The relatively low species composition in this study compared with
other similar studies with high species richness such as Nwadiaro (1984), Chindah
and Osuamkpe (1994), Alfred-Ockiya (1996) and Koutrakis et al. (2000)
is due to their use of multiple gear, which include seine net, fyke net, cast
net, trap/fences, atalla, long lines and hook and lines. Allison et al.
(1997) had suggested a multiple gear approach as the best way to obtain comprehensive
icthyofaunal studies. However, Sikoki et al. (1998) attributed the low
diversity in the Nun River not only to fishing methods, but fishing season and
industrial activities resulting in biological over fishing of the stock. Allison
et al. (1997) observed that fish assemblage may differ with location
even within similar habitats due to variation in abiotic factors. Allison et
al. (1997) also related poor species diversity to human activities such
as dredging and water pollution from petroleum products occasioned by barges,
tugs and other river crafts. The impact of human activities on species richness
has also been reported by Kone et al. (2003) in the Go River (Ivory Coast)
and Gratwicke et al. (2003) in the Upper Mangame River, Zimbabwe.
More fish were caught in the convex section and the surface. This difference was as a result of the exclusive sampling at these locations between September and October due to high flood level and water velocity.
It is worthy to note that this study greatly complements previous studies on species diversity and abundance in the Lower Nun River and further highlights the significance of the use of small mesh size gill nets in fisheries research and has thus made significant contributions to knowledge on the gill net fishery of Lower Nun River.