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The length-weight relationship, condition factor and reproductive biology of Pseudotolithus (P) senegalensis (Valenciennes, 1833) (croakers), in Tombo western rural district of Sierra Leone.

INTRODUCTION

The croakers, drums and meagres are the most important sciaenid species in Sierra Leone. Pseudotolithus senegalensis (croakers) are found along the West African Coast from Senegal to Gabon [1]. They occur in the warm water above the base of the thermocline [1] and are primarily marine but also occur seasonally in brackish water areas. They inhabit mud, sandy and rocky bottoms from the shoreline to 70-m depth. The smaller and younger ones prefer shallow waters and move to mid-waters when bottom temperature falls below 18[degrees]C. Pseudotolithus senegalensis (Valenciennes, 1833) occurs in similar habitats as Pseudotolithus (fonticulus) elongatus. The two species are jointly harvested by artisanal and industrial fisheries, using set gillnets, beach seines, long lines and bottom trawls. P. senegalensis and P. typus form approximately 30% of catch of the trawl fishery on the continental shelf [2].

These fish species are of considerable economic importance and contribute significantly to national food security, provide employment and revenue to the larger proportion of the Sierra Leone population. Sierra Leone provides certain incentive in the form of exemption of custom duties and internal taxes to promote certain activities including exploitation of its fishery resources. This incentive is, however, not without negative consequences on its fishery sector. Available data from scientific analysis and resource surveys conducted on Sierra Leone waters revealed that the demersal's fish resources are fully if not overexploited [3]. As such, a rational management of these resources requires an in-depth knowledge of its biology and ecology. Knowledge on their biology is important for management and sustainable exploitation of the stock. In the context of a multi-species and multi- fleet, the knowledge of the state of exploitation of a given resource is important and necessary for a proper management of that population [4].

Some scientific investigation has been carried out on the status of the population and reproductive biology of P. senegalensis. Such investigation includes the effects of long-term exploitation of demersal's fish populations off the coast of Sierra Leone, West Africa by Coutin and Payne [5]; the growth, maturity and mortality of the sciaenidae of the tropical West Africa and those on biological data of West African croakers carried out by Longhurst [6,7]. The reproductive biology and production characteristics of three croaker species of the Guinea coast were studied by Zuyev and Giragosov [8]. The dynamics of P typus in the gulf of Guinea was studied and it was discovered that the fishing effort targeting the sciaenids is well over the sustainable level [9].

Information on the reproductive biology of P. senegalensis in the coastal waters of Sierra Leone is very scanty. Therefore, this present study seeks to document information on the length-weight relationship of P. senegalensis. Knowledge of length-weight relationship helps in estimating the standing stock or biomass thereby establishing the yield by converting one variable into another as is often done during field studies, calculating condition indices, comparing the ontogeny of fish population from different regions and in trophic studies [10, 11].

It also aims to determine the spawning behavior of the species using gonad development stages, Gonado-Somatic Index (GSI) and Hepato-Somatic Index (HIS) of specimens collected directly from artisanal fishermen at Tombo--Western fishing district of Sierra Leone.

MATERIALS AND METHODS

Study Area

The study was carried out at the Tombo fish landing sites within the Yawribay Proposed Marine Protected Area (YPMPA) of Sierra Leone (Figure 1). On average, the two fish landing site is 52 km away from Freetown. Tombo is among the largest fishing communities in the Western Area Fishing district. The study area is located south of Freetown and it lies between latitudes 7[degrees]52' and 8[degrees]20' N and longitude 12[degrees]45' and 13[degrees]10'W. The total area is about 29,505 hectares characterized by intertidal mud sediments [12].

The climate of the study area is tropical with two well-defined seasons; the dry season (November to April) and the rainy (May to October). The heaviest rainfall occurs in July, August, and September with average annual rainfall of approximately 122.7inches (311.6cm). Mean monthly temperatures are 27.0[degrees]C and 28.6[degrees]C for the rainy season and dry season, respectively. Within the continental shelf area, mean monthly temperatures ranges from 26[degrees]C to 27[degrees]C during the rainy season and 28[degrees]C to 29[degrees]C during the dry season.

The heaviest rains occur in July and August and the mean monthly amount of rainfall reaches its maximum in July and August [13]. The dry season (November to April) is dominated by the north east trades winds from the north while the monsoon or rain bearing winds dominate during the rainy (May to October).

Sources of data

Length and weight relationship determination

Monthly samples were collected at Tombo fish landing sites. Fish specimens for the study were collected from artisanal fishers and middle men at the landing sites. Sampling of landed catches was done once in a month for a period of six months. The fishers used a wide range of fishing gears such as hooks and line, long line, castnets, gillnets , seine nets and traps.

During the sampling period, the total lengths of fish samples were measured on a stainless steel measuring board to the nearest 0.5cm. To reduce bias, samples were collected at random from different boats of artisanal fishers. Corresponding weight of each fish were also measured using an electronic balance scale to the nearest 0.5g. Daily measurements of length and weight frequencies were recorded into a data sheet and latter transcribed into data file. Furthermore, sub-samples of 10 fish were drawn at random every month from each site and preserved in a cooling box using ice cubes. The preserved samples were taken to the laboratory for detailed studies on the reproductive biology.

Laboratory analysis

Sub-samples taken to the laboratory were treated individually to determine the sex, weight of gonad, and liver. In the laboratory, standard length of each sub-sample was measured to the nearest 0.5cm using a measuring board and the gutted weight of corresponding sample measured to the nearest 0.5g. Specimens were dissected and the gonads were carefully removed with the aid of a forcep after dissection. The gonad and the liver were weighed separately to the nearest 0.5g. The sexes of dissected specimens were identified by examination of the gonads. The proportion of the two sexes relative to one another was used to calculate the sex ratio.

Information on standard length, gutted weight and weight of gonad were recorded into a data sheet for data analysis.

Data Analysis

The total length (TL) of the fish was measured from the tip of the anterior or part of the mouth to the caudal fin using an improvised measuring board made from wood. Fish weight was measured with an electronic scale, to the nearest gram. The mean lengths and weights of the classes were used for data analysis using the format accepted by FISAT [14].

The relationship between the length (L) and weight (W) of fish was expressed by the equation given by [15].

W = [aL.sup.b]

ln W = lna + bln L

Where

W = Weight of fish in (g)

L = Total length (TL) of fish in (cm)

a = Constant (intercept)

b = slope (change in weight per unit change in length)

The "a" and "b" values were obtained from a linear regression of the length and weight of the fish measured.

Length in exponent 3 expressed as a percentage was used to calculate the condition factor estimated from the relation below:

K= 100 W/[L.sup.3]

Where,

K = Condition factor

W = Weight of fish (g)

L = Length of fish (cm)

The weight of each fish and of its gonad was used to determine the gonado-somatic index (G.S.I) following [16].

GSI = weight of gonad/Weight of gutted fish x 100

Hepatosomatic index (HIS) was also calculated after [16] using the relationship below:

HIS = Weight of liver/Weight of gutted fish x 100

RESULTS

The length-weight relationship and condition factor obtained for the species is presented in Table 1. Sex ratio of male to female is (7:8); fish length range from 24.3cm (minimum) to 54cm (maximum) while the weight is from 112g (minimum) to 1000g (maximum). The condition factor which is a measure of the suitability of the environmental factor for growth of the species is 0.95. The a-value obtained was 6.27 while the b - value was 3.25. Both 'a' and 'b' values recorded showed positive allometric growth for the fish species.

Legend

Y - axis = Weight of sampled fish in grams; X - axis = Length in centimeters The results for the regression of length and weight are represented in Figure 2 above. There is a linear relationship between the length and the weight as indicated by the high 'r'. The [r.sup.2] determined for the species is 0.883 at W = 6.5[L.sub.3.4].

Legend

Y - axis = Gonado - Somatic Index of fish; X - axis = Period of sampling in Months

Legend

Y - axis = Hepatosomatic Index of fish; X - axis = Period of sampling in Months

Figures 3 and 4 show the Gonadosomatic index (GSI) and Hapatosomatic index (HIS) of the fish. Mean monthly Gonadosomatic index and Hepatosomatic index was at peak in March and low in January of the sampling period. It was apparent from the results that gonadal development peak in March, which is the beginning of the onset of the rainy season and decreases progressively. The rainy season is obviously the best period for most fishes to spawn because of the high abundance of food materials for the fry and fingerlings.

DISCUSSION

In this study, growth of fish showed positive allometry as t-test for departure from 3 which is the value of isometric growth showed significant difference. There is however no theory that suggested that estimated 'b' value is expected to be below 3 (negative allometry) [17]. Allometric growth is negative (b < 3) if the fish gets relatively thinner as it grows bigger (growth in age with reduction in size) and positive (b > 3) if it gets plumper as it increases in age [18]. The 'r' value of 0.95 revealed a strong linear relationship between length and weight. Allometric growth has been observed in other species such as 2.84 for female Clarias gariepinus; 2.92 and 2.97 for Penaeus notialis and P. monodon, respectively; and 2.92 for Nematopalaemon hastatus [19, 20, 21]. The occurrence of absolute isometric growth (b = 3) in nature is occasional [22, 23]. Length -weight relationship gives information on the condition and growth patterns of fish. The change of b values depends primarily on the shape and fatness of the species although other factors may be responsible for the differences in the parameters of the length--weight relationship among seasons and years such as temperature, salinity, food, sex, time of year and stages of maturity [24].

The mean condition factor which is a measure of the suitability of the environmental factor for growth of the species obtained for this study was 0.98 while the mean monthly condition factor ranged from 0.66 to 0.99. These results vary slightly from the condition factor (K) of between 0.77-0.81 reported for Clarotes filamentosus in in lake Oguta and K--value ranging from 0.49 to 1.48 obtained for the same species in Adoni River [25, 26]. The values obtained from this study showed that the species was in good condition. Certain factors are known to affect the wellbeing of fish [14]. These factors include data pulling, sorting into classes, sex, stages of maturity and state of the stomach [14]. Values of the condition factor vary according to season and are influenced by environmental conditions [25]. The growth conditions for the site where the research was carried out appeared to be more favourable.

Gonadosomatic index was high in March-May, the highest gonadosomatic index was in March (3.70) and this suggested that this could be the spawning period of P. senegalensis. March-May falls within the wet season and at this period, rainfall is high, water levels in the coastal waters increase and food availability is also high, thus providing favourable conditions for spawning to occur.

The hepatosomatic index recorded in this study showed similar pattern of occurrence as was observed for gonadosomatic index, highest hepatosomatic index was also recorded in March (2.80). Another period of high spawning activity occurred in December. The least spawning activity seemed to have occurred in January which had the lowest GSI value. The spawning period for Pseudotolithus senegalensis in the Cameroon is similar to that of P. elongates in the cross River estuary and that of P. typus of the Cameroon [27]. The only difference is the peak spawning period of P senegalensis and P. typus. They both have their first peak in March, April and May while the second peak is in November and December whereas P. elongates has its first peak in December, January and February and the second peak in July and September [28]. The value of coefficient of correlation 'r' estimated, indicated that the relationship between the length and weight of the species was significant. The sex ratio of the sampled fish as indicated in Table 1 showed that the female fish were more dominant especially in the months of March, April and May which is the peak spawning time. The female fish might have come close to the shore and other spawning grounds to spawn where they were caught by the artisanal fishermen.

The sex ratio observed for P. senegalensis in Tombo, Western rural district of Sierra Leone pointed to the fact that there were probably more females than males (1.15:1). This result was similar to what was obtained for B. auritus in Cape coast Ghana where the number of females exceeded that of the males [29]. The sex ratio of 1: 1.09 (male to female) obtained for trout sweetlips grunt Plectorhynchus pictus by Al-Ogaily and Hussain [30] is similar to the values obtained in this present study.

The results of this preliminary study clearly showed that P. Senegalensis does not have any obligatory month for spawning, but seems to exhibit multiple spawning behaviour. The spawning peaks appear to be from March-May and November-December, with March to May being the peak. It is a known fact that the length-weight relationships and condition factor of fishes are important management tools and the results of these parameters in the western area fishing district of Sierra Leone has shown that the environment is suitable for the survival and reproduction of P. senegalensis. Since P. senegalensis is part of the commercially important species of Sierra Leone, it is important to have a record of their length and weight from different habitats for proper management decision. Sustainable management of the species also require that the environment where the fish grows be protected against anthropogenic pollution. Imposition of closed fishing season especially during the two peak spawning period might be a sound recommendation for the species to recruit effectively.

ACKNOWLEDGEMENT

The authors wish to aknowledge,with thanks, the Sierra Leone Ministry of Fisheries and Marine Resources for granting the permission to access the water bodies and also to have unrestricted contact with the fishing communities. We are indebted to the fishery observers and the fishers for their assistance and for sparing their catch and time without which it would not have been possible to accomplish this research.

REFERENCES

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[3.] Ministry of Fisheries and Marine Resources. MFMR Statistics Unit--Unpublished, 2008.

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[5.] Coutin PC and AI Payne The effects of long-term exploitation of demersal fish populations off the coast of Sierra Leone, West Africa, 2006. DOI: 10.1111/j.1095-8649.1989.tb03058.x.

[6.] Longhurst RA Bionomics of the scianidea of tropical West africa. J. Cons.. Int. Explor. Mer. 1964; 29 (3): 302-334.

[7.] Longhurst AR Synopsis of biological data on West African croakers (P. typus, P. senegalensis and P. elongatus). FAO Fish. Synopsis 1966: 35.

[8.] Zuyev GV and VE Giragosov The reproduction biology and production characteristics of three species of croakers (sciaenidae) along the Guinea coast (Abstract), 1990.

[9.] Isebor CE and PO Abohweyere The population dynamics of Pseudotolithus typus in the Gulf of Guinea. In: Abstract of First Pan African Fisheries Congress on Sustainable Development of Fisheries in Africa held in Nairobi, Kenya 31st July-4th August 1995, P135.

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[19.] Anyanwu PE, Okoro BC, Anyanwu AO, Matanmi MA, Ebonwu BI, Ayaobu-Cookey IK, Hamzat MB, Ihimekpen F and SE Afolabi Lengthweight relationship, condition factor and sex ratio of African mud catfish (Clarias gariepinus) reared in indoor water recirculation system tanks. Research Journal of Biological Sciences 2007; 2 (7): 780-783, 2007. ISSN: 1815-8846. URL: http://www.medwellonline.net accessed January, 2014.

[20.] Yakub AS and EJ Ansa Length-Weigth relationship of the pink shrimp Penaus notialis and giant tiger shrimps P. monodon of Buguma Creek in the Niger Delta, Nigeria.The zoologist 2007, vol.5:47 53.

[21.] Enim UI Length-weight parameters and condition factor of two West African prawns. Rev. Hydrobiol. Trop. 1994; 27 (2): 121-127.

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[25.] Ajayi T The age and growth of the tongue sole, Cynolossus canariensis (stend 1982). In: Proceedings of the 2nd Annual conference of the Fisheries Society of Nigeria (FISON) New Bush source 1982, pp: 219.

[26.] Nwadiaro CS and PU Okotie Biometric characteristics: length weight relationships and condition factors in Chrychthysfilamentosus, Pisces, Bagridae from Oguta lake Nigeria. Biol. Afr. 1985; 2: 48-56.

[27.] Braga FM de S Estudo entre fator de Condicao e relacao peso/comprimento para alguns peixes marinhos. Rev. Brazil. Bio. 1986; 46 (2): 339 346.

[28.] Youmbi-Tientcheu J and T Djama Food habits of two sciaenid fish species P. typus and P. senegalensis off Cameroon. NAGA (ICLARM) 1993; 17 (1): 40-41.

[29.] Asabere-Ameyaw A Observations on the reproductive biology and recruitment of the big eye grunt Br achy deuterus auritus (Pisces: Haemulidae), in Ghana. Journal of the Ghana Science Association, 2001; 3: 14-21.

[30.] Al-Ogaily SM and A Hussain Biology of grunt Plectorhynchus pictus (Thunberg, 1972 Haemuliudae, Teleostei, Percoidei) from the red sea (Jizan area), Fisheries Research. 1990; 9: 119-130.

Olapade JO (1) * and S Tarawallie (1)

* Corresponding author email: Julius.olapade@yahoo.co.uk / fem66@hotmail.com

(1) * Julius O. Olapade (Ph.D) Acting Head--Department of Aquaculture and Fisheries Management, School of Forestry and Horticulture, Njala University, Njala PMB Freetown--Sierra Leone

(1) Tarawallie Sheku--Department of Aquaculture and Fisheries Management, School of Forestry and Horticulture, Njala University, Njala PMB Freetown--Sierra Leone.

Table 1: Length-weight relationship and condition factor (k) for
Pseudotolithus senegalensis

Sex ratio       N     a      b      r      Mean

Male   Female                              CF

1      1.15     412   6.27   3.25   0.95   0.98

Sex ratio       Length cm          Weight (g)

Male   Female   Min   Max   Mean   Min   Max    mean

1      1.15     24.3  54    36.5   112   1000   360.5
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Author:Olapade, J.O.; Tarawallie, S.
Publication:African Journal of Food, Agriculture, Nutrition and Development
Article Type:Report
Geographic Code:6SIER
Date:Oct 1, 2014
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