Greener Journal of Agricultural Sciences

Vol. 9(1), pp. 23-31, 2019

ISSN: 2276-7770

Copyright ©2019, the copyright of this article is retained by the author(s)

DOI Link: http://doi.org/10.15580/GJAS.2019.1.011719017

http://gjournals.org/GJAS

 

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Estimates of Population parameters for Sardinella maderensis (Lowe, 1838) in the coastal waters of Ghana

 

 

Samuel K.K. Amponsah 1*; Patrick K. Ofori-Danson 2; Francis K.E. Nunoo 3; Godfred A. Ameyaw 4

 

1* Department of Fisheries and Water Resources, University of Energy and Natural Resources, Box 214, Sunyani, Ghana. Email: samuel.amponsah@uenr. edu. gh

2 Department of Marine and Fisheries Sciences, University of Ghana, Box LG 25, Legon, Ghana.

oforidanson@ gmail. com

3 Department of Marine and Fisheries Sciences, University of Ghana, Box LG 25, Legon, Ghana. Email: fkenunoo@ug. edu. gh

4 Australian National Centre for Ocean Resources and Security, University of Wollongong, NSW 2522, Australia.

Email: gameyawasiedu@ gmail. com

 

ARTICLE INFO

ABSTRACT

 

Article No.: 011719017

Type: Research

DOI: 10.15580/GJAS.2019.1.011719017

 

This study examined some population parameters of Sardinella maderensis landed along the eastern coast of Ghana, based on length-frequency data from June, 2014 to January 2015.  Overall, 1401 samples of   S. maderensis were measured for standard length and resultant data analysed with FiSAT II. The asymptotic length (L) and growth rate (K) were 23.63 cm SL and 0.61 yr-1 respectively.  The theoretical age at birth (t0) and growth performance index (ϕ) were -0.284 yr-1 and 2.532 respectively. The recruitment pattern was continuous with two recruitment pulses. Total mortality rate (Z), natural mortality rate (M) and fishing mortality rate (F) were 2.96 yr-1, 1.30 yr-1 and 1.63 yr-1 correspondingly. Fishing mortality rate surpassed the optimum fishing rate which showed that the assessed fish species is under high fishing pressure. The estimated exploitation rate (Ecurr) was 0.55, implying that the stock is over-exploitation. VPA outcome revealed higher harvesting rate on individuals with length between 10 – 11 cm. The spawning biomass which was below the 30% of unexploited biomass indicated future recruitment failure of the stock. As a result, urgent management interventions such as the application of biological reference points and mesh size regulations are urgently recommended for sustainable exploitation of Sardinella maderensis.

 

Submitted: 17/01/2019

Accepted:  21/01/2019

Published: 31/01/2019

 

*Corresponding Author

Samuel K.K. Amponsah

E-mail: samuel.amponsah@ uenr.edu.gh

 

Keywords: Madeiran sardinella, population parameters, Ghana, MSY, VPA, overfishing

 

 

 

 

                               


1.0 INTRODUCTION

 

Fishing is central to the livelihoods of both artisanal coastal and inland fishing households in Ghana (Sarpong et al., 2005). Marine artisanal fisheries are practiced within the EEZ of Ghana and account for 85% of the total fish catches in Ghana (Nunoo et al., 2014). Among these species, 65% are mostly small pelagics (Nunoo et al., 2014). Ansa-Emmim (1973) reported that Sardinella maderensis and other species within the Clupeids family serve as important small pelagics exploited from the Ghanaian coast to fishing households in Ghana. Sardinellas including Sardinella maderensis accounts for more than 40% of the total domestic marine fish contribution. The abundance of these small pelagics is heavily linked to both minor and major upwelling activities in the Ghana-Ivorian marine ecosystem as well as rainfall (Koranteng, 1989; Binet, 1982).

Ecologically, juveniles of Sardinella maderensis are concentrated in the coastal waters from where they gradually move offshore as they grow older. A great majority of the adults remain confined over the shallow half of the continental shelf. Adults of Sardinella maderensis are much more sedentary with limited traveling distances along the coast (Gheno, 1970). They are most abundant in two regions to the north and south of the Gulf of Guinea respectively (FAO, 1971). Throughout the year, Sardinellas (particularly Sardinella maderensis and Sardinella aurita) supply the main part of the catch for the coastal artisanal fleets and they also form part of the catch of the long-distance fishery seiners when they operate sufficiently close to inshore (Brainerd, 1991). Despite having lower catches in the Ghanaian coastal fishery than Sardinella aurita, Sardinella maderensis which is said to be accessible for most months of the year are a very cheap source of animal protein to vulnerable fishing households (Nunoo et al., 2014; Muta 1964). Thus, its continuous occurrence in the coastal waters enhances household nutrition in vulnerable fishing communities. Its fishery also constitutes extreme economic activities of many fishers including fishermen, fish processors and traders in Ghana.

However, its population, together with Sardinella aurita currently suffers from declining catches due to the consistent application of light in fishing - an intervention introduced to ensure all-year-round supply of sardines and other unsustainable fishing practices. These aforementioned threats, as a result, make sustainable management of Sardinella maderensis stocks within Ghana’s coastal waters problematic. As such, this study seeks to address some aspects of population parameters of Sardinella maderensis off the coast of Ghana with the aim of providing a basis for proper management.

 

 

2.0 MATERIALS AND METHODS

 

2.1 Study area

 

This study was carried out in four fish landing sampling stations, namely Vodzah, Denu, Jamestown and Tema based on two staged sampling criteria (Figure 1).


 

 

Figure 1: Map showing the sampling sites

 

 

 


2.2 Data collection

 

Fish samples were obtained on monthly basis from fishermen within the study areas from June 2014 to January 2015. Obtained samples were kept on ice blocks and sent to the laboratory at the Department of Marine and Fisheries Sciences, University of Ghana for analysis. Weight and standard lengths were obtained using electronic scale and the 100-cm measuring board respectively. Identification of samples was carried out using Fischer et al. (1981) and Kwei & Ofori-Adu (2005). In all, 1401 samples of Sardinella maderensis were assessed.

 

2.3 Methods

 

Monthly length frequency data was obtained and standard length (SL) measured throughout the study period to estimate the various population parameters.

 

2.3.1 Growth parameters

 

Growth indicators for the assessed fish species was carried out using the Von Bertalanffy Growth Function (VBGF). To ascertain growth or mortality dominated population, the Powell-Wetherell Plot was applied (Pauly, 1984). The equation: Lt =L(1-e-K(t-t0)) (Pauly, 1979) was used in estimating the growth function. Age at birth (to), longevity (Tmax) and growth performance index (ϕ) were estimated following procedures outlined by Pauly (1983).

 

2.3.2 Mortality parameters

 

Mortality parameters including Total mortality rate (Z), Natural mortality rate (M) and Fishing mortality rate (F) were estimated using Pauly (1980). The mean surface temperature (T) of 25.7°C was applied in estimating natural mortality rate (M). The optimum fishing mortality rate (Fopt) and exploitation level (E) were estimated as 0.4M (Pauly, 1984) and F/Z (Gulland, 1969) respectively. Exploitation rates (Emax, E0.1 and E0.5) were computed following Ahmad et al. (2018). The impact of exploitation on yield was done based on the interrelationship between exploitation rate (Emax) and critical length ratio (Lc50/L∞).

 

 

2.3.3 Length at first capture (Lc50) and maturity (Lm50)

 

The length at first capture of the assessed fish species was calculated from the length-converted catch curve. The length at first maturity (Lm50) was estimated as 2*(L∞)/3 (Hoggarth et al., 2006),

 

2.3.4 Recruitment pattern

 

Backward projection of the obtained length frequency data was used in establishing the recruitment pattern (Mudoidiong et al., 2017; Nurul-Amin et al., 2008). Midpoint of the smallest length interval was used as length at first recruitment (Lr50) (Gheshlaghi et al., 2012).

 

2.3.5 Virtual Population Analysis (VPA)

 

Length structured VPA was performed using values of L∞, K, M, F, a (constant) and b (exponent) for the species as inputs (Gayanilo et al., 2005). The t0 value was approximated to be zero. The direct exponential relationship between the weight (W) and length (L) was used to obtain the values of the constants, a and b (exponent) (Pauly, 1984).

 

2.3.6 Maximum Sustainable Yield (MSY)

 

MSY was estimated as 0.5x (Y+MB), where B is the average biomass calculated from cohort analysis in the same year, M is the natural mortality and Y the annual yield. Annual yield (Y) was calculated as , where W is weight and C is the catch (Sparre and Venema, 1998).

 

2.4 Data Analysis

 

Obtained length frequency data were grouped and fed into FiSAT II (FAO-ICLARM Stock Assessment Tools) software for estimating the population parameters. Yield software package by Branch et al. (2000) was used in plotting length at age.

 

 

3.0 RESULTS

 

3.1 Growth parameters

 

Estimated growth parameters were 23.63 cm SL and 0.61 per year for asymptotic length (L) and growth rate (K) correspondingly. The restructured Length frequency data superimposed with the estimated growth curve exposed three cohorts within the harvest (Figure 2).


 

 

 

Figure 2: Reconstructed length frequency superimposed with growth curves

 

 


The growth function for the assessed species was Lt = 23.63 (1-e-0.61 (t-(-0.284))). The growth performance index (ϕ) and Z/K ratio were 2.532 per year and 3.98 respectively (Figure 3a). Age at birth (to) and longevity (Tmax) were -0.284 and 5 years respectively (Figure 3b).

 

3.2 Probability of capture and Length at first maturity (Lm50)

 

Length-at-first capture (Lc50) and maturity were 5.3 cm and 15.7 cm respectively (Figure 3d). Critical length at first capture (Lc) was 0.22.

 

3.3 Recruitment pattern

 

Recruitment pattern was all year-round with minor peak in August-September and major peak took place in May-June (Figure 4a). The length at first recruitment (Lr50) was 5.5 cm SL. 

 

3.4 Mortality parameters  

 

From the Jones and van Zalinge plot, Total mortality rate (Z) was estimated as 2.96 (Figure 3c). Natural and Fishing mortality rates were estimated as 1.33 (M) and 1.63 (F) per year respectively. Optimum fishing mortality rate and exploitation rate (E) were 0.53 per year and 0.55 respectively. Emax, E 0.1 and E0.5 were 0.46, 0.36 and 0.28 respectively (Figure 4b).


 

 

Figure 3: A) Powell Wetherell plot B); Length at age plot; C) Length-converted catch curve D; Length at first capture

 


 

 

3.5 Virtual population analysis (VPA)

 

Recruits estimated into the population was 6.54 x 107 with the highest harvesting intensity occurring within lengths, 10 cm and 11 cm with fishing mortality rate (F) of 0.27 yr-1 (Table 1). Peak of fishing mortality rate (1.33) ensued within the length range of 22 cm to 23 cm. Values of constants ‘a’ and ‘b’ were calculated as ‘0.0087’ and ‘3.3’.

 


 

Figure 4: A) Recruitment pattern; B) Relative yield and relative biomass per recruit (B'/R)

 

 

Table 1: Survivors and catches of Sardinella maderensis from VPA output in FISAT II

Mid-Length

Catch (in numbers)

Survivors

(N)

Fishing mortality (F)

Steady-state Biomass (tonnes)

5.5

25298

65450676

0.0045

13.6

6.5

79057

58008828

0.0152

21.99

7.5

366824

50999980

0.0759

32.64

8.5

354175

44207320

0.0797

45.31

9.5

720999

37943252

0.1785

59.42

10.5

980306

31849070

0.2719

73.74

11.5

499640

26073218

0.1564

88.14

12.5

338364

21325562

0.1196

102.8

13.5

199224

17223176

0.0799

116.75

14.5

170763

13707016

0.0785

128.96

15.5

177088

10641459

0.0948

137.83

16.5

148627

7981172

0.0949

142.16

17.5

126491

5748505

0.0988

141

18.5

126491

3919562

0.126

132.84

19.5

126491

2457690

0.1715

116.09

20.5

110680

1350241

0.2277

90.24

21.5

91706

592976

0.3554

56.04

22.5

79057

158114

1.33

15

 

 tons (Table 2 threatsctices tend to occur at fish nusery grounds where many juveiles seek shelter and feed ril

3.6 Maximum Sustainable Yield (MSY)

 

Total biomass and yield were estimated as 3404.38 tons and 225.55 tons respectively. Using a mean body weight of 0.0745g, MSY was calculated as 2377 tons (Table 2).

 

 

 

Table 2: Estimates of the total biomass (tons), the yield (tons) and the MSY (tons) for Sardinella maderensis

Mid-Length

Catch

XL1, L2

N

E

 F

Z

Biomass/kg

Body weight/kg

Yield

5.5

25298

1.06

65450676

0.003

0.0045

1.32

418826

0.0032

81.38

6.5

79057

1.07

58008828

0.011

0.0152

1.35

387483

0.0053

422.94

7.5

366824

1.07

50999980

0.054

0.0759

1.41

360058

0.0083

3049.12

8.5

354175

1.08

44207320

0.057

0.0797

1.41

331067

0.0123

4342.48

9.5

720999

1.08

37943252

0.118

0.1785

1.51

300921

0.0174

12515.67

10.5

980306

1.09

31849070

0.170

0.2719

1.60

268602

0.0238

23306.49

11.5

499640

1.10

26073218

0.105

0.1564

1.49

238000

0.0317

15829.76

12.5

338364

1.11

21325562

0.082

0.1196

1.45

210770

0.0413

13960.99

13.5

199223

1.12

17223176

0.057

0.0799

1.41

185759

0.0527

10497.41

14.5

170763

1.13

13707016

0.056

0.0785

1.41

162062

0.0662

11298.33

15.5

177088

1.15

10641459

0.067

0.0948

1.42

139167

0.0819

14497.84

16.5

148627

1.18

7981172

0.067

0.0949

1.43

116678

0.1000

14862.73

17.5

126491