ABSTRACT
Studies on the population dynamics of cichlid fish species in Opi lake were carried out between August, 2014 and January, 2015. Also, physico-chemical parameters of the lake were analyzed using appropriate methods. Morphometric relationships and seasonal variations of the fish species were determined. A total of 235 cichlid fish, comprised of 5 species, namely: Tilapia zillii (28.09%), Sarotherodon melanotheron (51.49%), Oreochromis niloticus (14.04%), Pelmatochromis guentheri (5.96%) and Hemichromis fasciatus (0.43%) respectively were sampled and used for the study. The above listed cichlid fishes showed no correlation (P>0.01) and (P>0.05) with values of the physico-chemical parameters determined. S. melanotheron however, showed positive correlation (P<0.05) with phosphate. The fish species sampled showed that there were more females than males. Ripe gonads were recorded much in all the species except in O. niloticus where mature gonads were greater in number. In T. zillii, the smallest sexually matured females were 8.1 cm total length. The length–length relationship had a b–value of 0.76 when compared to the length–weight (L-W) relationship that showed positive allometric growth of b>3. Growth parameters of T. zillii showed asymptotic length of = 1.27 cm Tl, growth curvature (K) = 0.8 yr-1, growth performance index Q1 = 0.11, age–at–length zero (to) = 0.48 yr and the higher potential longevity (tmax) = 3.75 yrs. Mortality parameters showed that the total mortality (Z), natural mortality (M), fishing mortality (F) and exploitation rate (E) were 3.36 yr-1, 1.755 yr-1, 1.605 yr-1 and 0.48 yr-1 each. The smallest sexually matured female in S. melanotheron was 9.8 cm Tl. The length-length relationship had a b–value of 0.79 and L–W relationship showed positive allometric growth of b>3. The growth parameters showed that was 37.38 cm Tl, K = 0.8 yr-1, Q1 = 3.1, to = 0.188 yr and tmax = 3.75 yrs. Also, mortality parameters showed that Z = 4.54 yr-1, M = 0.70 yr-1, F = 3.34 yr-1 and E = 0.85 yr-1 respectively. O. niloticus had sexually matured female of 11.5 cm Tl. Whereas length-length relationship gave a b–value of 0.82 while L–W relationship showed positive allometric growth of b>3. Bertalanffy growth parameters were = 5.27 cm Tl, K = 0.8 yr-1, Q1 = 1.35, to = 0.32 yr and tmax = 3.75 yrs. Mortality estimate were Z = 3.39 yr-1, M = 1.175 yr-1, F = 2.21 yr-1 and E = 0.65 yr-1 respectively. Also P. guentheri had sexually matured female of 11.6 cm Tl. Length-length relationship had a b–value of 0.84 and L–W relationship showed negative allometric growth of b<3. The Bertalanffy growth parameters were = 22.0 cm Tl, K = 0.8 yr-1, Q1 = 2.59, to = 0.22 yr and tmax = 3.75 yrs. Mortality estimates were Z = 2.6 yr-1, M = 0.80 yr-1, F = 1.80 yr-1 and E = 0.21 yr-1. H. fasciatus was only sampled once towards the end of the study (January) which showed that the species appear endangered. Fishing mortality was substantially high in all the species sampled which is undesirable since it can lead to the collapse of the cichlid fish species in Opi lake.
CHAPTER ONE
1.1 Introduction and Literature Review
Cichlid fishes belong to the family Cichlidae, in the order Perciformes. The family is both large and diverse, and at least 1,650 species have been scientifically described by Froese and Daniel (2012), making it one of the largest vertebrate families. The actual number of species is therefore unknown with estimates varying between 2,000 and 3,000 (Stiassny et al., 2007). Cichlids belong to the sub order Labroidei, along with the wrasses (Labridae), Damselfishes (Pomacentridae), and Surf perches (Embiotocidae) families (Stiassny et al., 2007). Cichlids are among the most popular freshwater fish kept in the aquarium and are classified by Nelson (2006) as follows:
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Suborder: Labroidei
Family: Cichlidae
Nelson (2006) recognized eight subfamilies of Cichlids: Astronotinae, Cichlasomatinae, Cichlinae, Etroplinae, Geophaginae, Heterochromidinae, Pseudocrenilabrinae and Retroculinae. Sparks and Smith (2004) reported that cichlid taxonomy is still being debated, and classification of the genera cannot yet be definitively given. A comprehensive system of assigning species to monophyletic genera is still lacking, and there is no complete agreement on what genera should be recognized in this family (Nelson, 2006). Some of the most well known cichlid species are Angelfish, Oscar, Jack Dampsey and Discus (Stiassny and Lamboj, 2010).
The different cichlid species have developed in diversity in other to fit their own specific niches, and different cichlid species can therefore vary significantly when it comes to size, temperament (natural behavior), feeding habit, preferred water quality (Froese and Daniel, 2012).
Cichlids live in fresh and brackish waters and can be found in Asia, Africa and South of 300 N in American (Helfman et al., 1997). Several species of Tilapia, Sarotherodon and Oreochromis can disperse along brackish coastlines between rivers (Loiselle, 1994). Frank (2005) reported that most extreme habitat for cichlid fishes are the warm hypersaline lakes, where members of the genera Alcolapia and Danakilia are found. Lake Abaeded in Eritrea encompasses the entire distribution of D. dinicolai, and its temperature ranges from 29 – 45oC (Stiassny and Lamboj, 2010; Norlander, 2009; Graves et. al., 2002).
Population dynamics is a change in the number of fishes in a population or the vital rates of a population over some time. Enyenihi et al. (1986) reported that a good understanding of fish population dynamics (i.e., how mortality, growth and recruitment interact to affect abundance) is required to inform fisheries management on sustainable yield on the case of the endangered Kootenai River white sturgeon (Acipenser transmontanus).
Growth and Mortality rates have been studied in selected species but because of the very high degree of speciosity, dynamic pool model cannot be satisfactorily applied to demersal stocks (Oliver, 1999). Also, Oliver (1999) stated that demersal fisheries exploit a number of species which are so evenly mixed on the fishing ground that it is difficult to single out individual species as dominant.
Scientific understanding of population dynamics and stocks status are entirely fishery-dependent i.e. independent research surveys are not used to monitor their populations (Graves and Macdowell, 2006). Therefore, estimates of historical biomasses are based on the relative measures of catch per unit of effort (CPUE) obtained which are assumed to be proportional to exploitable abundance, because the populations are not sampled at random, unbiased fashion. A complete reliance on fishery-dependent data may introduce many potential sources of error. However, CPUE time series and assessment models fitted to CPUE data are currently the best available estimates of historical relative abundance (Graves and Macdowell, 2006). Tobler (2005) stated that despite a potential promising trend in recent years for white marlin (Parachromis friedrichsthalii), the relative biomasses estimated for each species have declined substantially from 70–50% level. Stiassny et al. (2007) noticed that when Lake Turkana was raising the shoals of Tilapia ‘dispersed’ and the yield was little, he called the period of inundations “off season”, and usually did not fish from August to December.
Seasonal dispersion of Tilapia associated with inundation has been observed in a Sudan floodplain (Hickley and Baily, 1987; Baber et al., 2002) and in Lake Mweru (Reid, 1990), and according to Sparre and Hart (2002), there are regular seasonal variations in catches of cichlid fish species in the East African Great Lakes, generally with decreased catches in drier months (January-Febuary). The pattern in Lake Turkana, however, appears out of phase with this generalization and the biological significance for these dispersions is unknown (Sparre and Hart, 2002).
Baber et al. (2002) stated that geographical distribution of the shoaling pelagic clupeid, Llisha africana encompasses the Atlantic coastal waters of the gulf of Guinea between Senegal and Zaria. This species is most abundant in the Nigerian coastal waters (Reid, 1990), where it constitutes an important catch component of the inshore artisanal fishery and forms a considerable proportion of the daily dietary protein intake of the riparian human populations. Although no overall stock depletion of I. africana is apparent in the Qua iboe estuary (Reid, 1990), personal communication with long standing artisanal fishermen indicated a general decline in stock and sizes of fish caught, being attributed to over-exploitation. Regrettably these claims have hitherto not been supported by scientific studies (Enyenihi, et al., 1986).
According to Davies et al. (2009), effective population size is the number of breeding individuals in an idealized allele frequency under random genetic drift and the amount of inbreeding as the population under consideration.
Small population size results in increased genetic drift, while population bottlenecks result when population size reduces for a short period of time (Berryman, 2002).
Overpopulation may indicate any case in which the population of any species of animals might exceed the carrying capacity of its ecological niche (Davies et al., 2009).
Lowe and Trewavas (1982) stated that the breeding sites are a limiting factor for Tilapia since the substrate is often too soft and flocculent for nest-building. Also, Lewis (1997) believed that the density of spawning O. niloticus in Kolding Gulf was more dependent on the availability of the spawning sites than on the number of fish potentially capable of spawning. It might be that the number of nest building sites increases with increasing Lake level. However, Dudley (2000) stated that after impoundment of the Kafue Gorge dam, Zambia found that high ‘dry season’ water level caused by the dam had decreased the reproductive success of Oreochromis species.
