Demographic characteristics of African river prawn, (Macrobrachium vollenhovenii, Herklots 1857) in...

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Archiva Zootechnica 18:1, 15-25, 2015 15

Demographic characteristics of African river prawn,

(Macrobrachium vollenhovenii , Herklots 1857) in the

Lower Taylor Creek, Niger Delta, Nigeria

T. Kingdom†

Department of Fisheries and Aquatic Studies, Niger Delta University, Wilberforce Island,

P.M.B. 071, Yenagoa, Bayelsa State, Nigeria

SUMMARY

The growth, mortality, recruitment and current rate of exploitation ofMacrobrachium vollenhovenii in the Lower Taylor Creek, Niger Delta, Nigeria

were studied for 24 months (June 2008 – May 2010) using length frequency

data and FAO – ICLARM Stock Assessment Tool (FISAT II). Growth parameters

obtained from seasonalized von Bertanlanfy growth function were asymptotic

length, L∞ = 12.08 cm total length, growth co-efficient (K/year = 2.70),

oscillation constant C = 0.95 and winter point, WP = 0.15 (February) for M.

vollenhovenii. Using length – converted catch curve, the total mortality (Z) was

estimated at 5.31/year , the natural mortality (M) was 4.46/year , while fishing

mortality (F) was estimated at 0.85/year for M. vollenhovenii. The exploitationrate (E) obtained was 0.16 suggesting M. vollenhovenii is being under – fished.

Keywords: Basket traps, ELEFAN I, FISAT II, shrimps, Taylor Creek

INTRODUCTION

The African River prawn, Macrobrachium vollenhovenii is native to eastern

Atlantic, with a fishery that is viable in most of the countries in the sub – region

of West African (Nwosu and Wolfi, 2006). Willfuhr-Nast et al (1993) had

recommended this species for aquaculture cultivation, based on its size, as an

equivalent of the now widely cultured Macrobrachium rosenbergii, de Man,

1879 (FAO, 2000). They are principally freshwater species although their adults

migrate annually to estuaries to spawn (New and Singholka 1985). As a

consequence, this species is abundant in rivers that are directly or remotely

connected to the ocean. In the Lower Taylor Creek, the species is an important

component of the Ingo trap fishery, constituting about 52% by weight of the

total catch (Kingdom and Hart, 2013). Apart from the report on the abundance

† Corresponding author e-mail: [email protected]



T. Kingdom16

(Kingdom et al , 2013) and reproductive biology (Kingdom and Erondu, 2013) of

M. vollenhovenii , little is known about the biology or ecology of the prawn in

this Creek.

This study was therefore, carried out to provide population parameters,

with a view of evolving management strategies for its sustainable exploitation

in the Creek.

MATERIAL AND METHODS

Study area

The monthly length –frequency samples analysed in this study were

collected from four randomly selected stations on the lower Taylor Creek (a

freshwater creek), situated between 501'N; 617

'E and 502

'N; 618

'E (Figure

1), for twenty four months (June 2008 –May 2010) using basket traps (three

traps per set) at all the stations. The traps, which were made from canes, were

31 –62 cm in length, with a mouth opening of 3 –4 cm in diameter and

rectangular mesh sizes of about 3 by 0.5 cm.

L is the asymptotic length, or the mean length the fish of a given stock

would reach if they were to grow indefinitely;

K is the growth co-efficient (constant), indicating the rate at which L is

approached; to is the age the fish would have been at zero length.

From the final estimates of L and K, the growth performance index (ꞌ) ofthe species, using Munro and Pauly (1983) and Pauly and Munro (1984):

ꞌ= Log 10 k +2 Log 10 L

and the longevity (tmax) of the shrimps (Pauly, 1983)

tmax = 3/k

To obtain an estimate of the instantaneous rate of total mortality (Z), a

length-converted catch curve analysis (Pauly, 1984) was performed. In order to

compute the instantaneous rate of natural mortality (M) for Macrobrachium

vollenhovenii , Pauly (1980) predictive formula was used, thus:

Log 10 M = 0.0066-0.279Log10L +0.6543Log10k+0.4634 Log 10TT being the mean environmental (ambient) temperature

Length frequency data was obtained by measuring individual prawns by

sizes (total length-from tip of rostrum to tip of telson) to the nearest 0.1 cm.

These were later grouped into 1 cm class intervals for analysis. The FAO-

ICLARM STOCK ASSESSMENT TOOL (FISAT II) software (FAO, 2005) was utilized

to estimate the population parameters. The estimation of growth parameters

was based on the Von Bertalanffy growth formula expressed in the form:

(Lt = L [1-exp (-k(t-to)] (Pauly and Gaschutz, 1979)

Where:Lt is the predicted length at age t;




Fig. 1: Map of Niger Delta showing Bayelsa State and Taylor Creek the Study Area

The mean ambient temperature used here was 28.74

0

C (Kingdom, et al ,2013).

The instantaneous rate of fishing mortality (F) was derived from the

relationship:

F =Z-M

and the exploitation rate (E) from,

E=F/Z

i.e. the fraction of total mortality (Z) caused by fishing mortality (F).

Probabilities of capture was calculated from the ascending left arm of the

catch curves which consists of incompletely selected and for incompletelyrecruited prawns. The method consists of backward extrapolation of the right



T. Kingdom18

descending side of the catch curve to include prawns that ought to have been

caught had it not been for the effect of incomplete selection and/or

recruitment.

ELAFAN II routine of FISAT II was used to obtain recruitment patterns by

backward projection unto time axis of the available length frequency data by

means of growth parameters (Moreau and Cuende, 1991).

Table 1: Length frequency data (total length, cm) for M. vollenhovenii of the Lower

Taylor Creek grouped in 1cm class intervals (June 2008 – May 2010)Mid

length

(cm)

J

08 J A S O N D

J

09 F M A M J J A S O D

J

10 F M A M

4.5 26 10 11 52 5 2 9 6 4 14 14 12 18 13 6 8 2 5 7 4 2 3 4

5.5 27 18 15 51 5 2 9 8 7 15 14 14 16 16 6 14 4 7 10 10 5 9 8

6.5 28 19 17 48 7 2 10 8 8 15 15 14 19 19 6 12 5 8 11 13 7 13 9

7.5 17 18 13 29 7 1 6 5 8 7 15 16 17 19 6 13 4 6 11 16 10 15 10

8.5 11 12 7 17 6 1 1 2 6 6 11 11 11 15 3 10 4 6 8 14 9 14 8

9.5 8 11 7 12 2 0 1 2 7 5 5 6 6 11 1 8 3 4 6 14 9 14 6

10.5 5 3 3 11 4 0 1 0 3 4 5 3 5 8 1 2 2 3 3 8 6 8 3

11.5 3 2 0 7 3 0 0 1 2 2 4 3 2 2 1 2 1 2 2 5 4 4 2

Total 125 93 73 227 39 8 37 32 45 68 83 79 94 103 30 69 25 41 58 84 52 80 50 1595

Fig. 2: Estimation of L∞ and Z/K of M. vollenhovenii from Lower Taylor Creek using Powell –

Wetherall plot (L∞ = 12.75cm total length, Z/K = 2.18, r = -1.00). Black plots used for analysis

while yellow plots were not used. (L= total length, Lꞌ= cut-off length, L∞=asymptotic length,Z=total mortality, K=growth coefficient)




RESULTS

Population parameters

The monthly length – frequency data used in the estimation of population

parameters are presented in Table 1. The Powell – Wetherall plot gave

preliminary estimates of L∞ = 12.75 cm total length, Z/K = 2.18 (r = -1.00) for

M. vollenhovenii (Fig. 2). The restructured form of the length frequency data is

presented as output of ELEFAN I in Fig. 3. Growth parameters for M.

vollenhovenii : L∞ = 12.08 cm total length, K/year = 2.70, C = 0.95 and WP =

0.15 (February). From these results, performance index, phi prime (ɸꞌ) was

1.07 for M. vollenhovenii , while longevity tmax was 13.33 months. The length –

converted catch curve (Fig. 4) estimated the instantaneous rate of total

mortality (Z) as 5.31/year for M. vollenhovenii . The instantaneous rate of

natural mortality (M) as estimated from Pauly’s (1980) empirical formula was

4.46/year for M. vollenhovenii ; so the instantaneous rate of fishing mortality

(F) was 0.85/year for M. vollenhovenii. These resulted in the exploitation rate

of 0.16 for M. vollenhovenii .

Fig. 3: Growth curve superimposed over the restructured length frequency histograms for M.

vollenhovenii (L∞ = 12.08cm total length, K = 2.70/year, C = 0.95, WP = 0.15, Rn = 0.238)

From the probabilities of capture estimation, the length-at-first capture(Lc) for M. vollenhovenii (Fig. 5) was 4.83 cm. The recruitment pattern of the

species is shown in Fig. 6. One major recruitment peak per year was

established for M. vollenhovenii.

DISCUSSION

The L∞ of 12.75 cm of M. vollenhovenii is far lower than the estimates

provided previously in other regions. Etim and Sankare (1998) recorded 18 cm

in Fahe Reservoir, Cote d’ Ivoire, Nwosu and Wolfi (2006) 21.36 cm for malesand 19.8 cm for females in the Cross River Estuary, Falaye and Abohweyere



T. Kingdom20

(2008) 18.81 cm in the Lagos – Lekki Lagoon system and Okogwu et al (2010)

23.65 cm in Asu River, all in Nigeria.

.Fig. 4: Length – converted catch curve of M.

vollenhovenii of the Lower Taylor Creek.

Estimated Z = 5.31 per year i.e. slope of curve.

Black plots used for analysis, while yellow

plots were not used

Fig. 5: Selection curve of M. vollenhovenii of

the Lower Taylor Creek

(Estimated Lc = 4.83 cm total length, where Lc

was estimated length-at-first-capture)

The growth curves estimated for the M. vollenhovenii indicated seasonalgrowth, and that in a certain period of the year, growth drastically slows down

(0.95). An estimated winter point of 0.15 for M. vollenhovenii implies that their

poorest growth occurred in February. Nwosu et al (2007) and Abohweyere and

Falaye (2008) had already reported September for M. vollenhovenii . These

reports were all from brackish water environments, similar to the 0.60

(August) estimated for M. vollenhovenii in Asu River, a freshwater environment

(Okogwu et al , 2010). The poorest growth of M. vollenhovenii in February may

be associated with the migration of prawns from the brackish water

environment into the freshwater rivers during the dry months of February and

March, as the salinity in the brackish water environment increases beyond

their range of tolerance (Marioghae, 1982). A poor condition of aquatic

organisms is usually observed during the period of migration, which may be

reflected in a drastically reduced growth rate (Nwosu et al, 2007). Moreover,

the months of February and March are about the warmest in the year, with

surface water temperatures generally above 30ºC, a situation which reduces or

stops feeding of organisms (Helffman et al, 1997).




Fig. 6: Recruitment pattern of M. vollenhovenii of the Lower Taylor Creek, indicating

two peaks of unequal magnitude within an arbitrary year

The estimated values of the growth coefficient (K) of 2.70 year-1

reported

for M. vollenhovenii is unusually very high and different from estimates

reported for other Macrobrachium species in Nigeria (Enin, 1995; Nwosu and

Wolfi, 2006; Nwosu et al, 2007; Abohweyere and Falaye, 2008; Nwosu 2008;

Okogwu et al, 2010). However, it is still lower than 3.19 year-1

reported by

Gabche and Hockey (1995) in the Lobe River, Cameroun.

Isaac (1990) had demonstrated that growth parameters estimated from

ELEFAN I could be biased as a result of individual growth variability, seasonaloscillations in growth, the restructuring procedure, size –dependent selection,

variable recruitment period and large size –class interval.

Moreau et al (1986) had already recommended that growth performance

index, phi prime (ɸ') be used for comparing related species because it shows

little variation among related taxa from different regions. A large difference in

ɸ' among a number of stocks of the same species or related species is an

indication of the unreliability of the estimated growth parameters. The ɸ'

(1.07) estimated for M. vollenhovenii might indicate unreliability of the

estimated growth parameters; this could have been responsible for theunusually very high K value (2.70 year

-1) in this study.

Prawns are short lived animals and high mortality is characteristic of this

group. Longevity estimates showed that their life span ranges from 0.9 to 3

years (Gabche and Hockey 1995; Enin 1995; Etim and Sankare 1998; Nwosu

and Wolfi, 2006; Alhassan and Armah, 2011). The longevity of 13.33 months in

this study is within this range. The instantaneous rate of total mortality (Z) was

5.31 year-1

. This is comparable with the values reported by Etim and Sankare

(1998), Nwosu and Wolfi (2006), Nwosu et al (2007), Okogwu et al (2010) and

Alhassan and Armah, (2011) for Macrobrachium species. Emigration may alsobe responsible for the high natural mortality of M. vollenhovenii , even though



T. Kingdom22

it may be compensated by immigration (Laevastu and Favourite, 1988). The

low fishing mortality of M. vollenhovenii shows it is not the main targeted

species of the shrimps in the Lower Taylor Creek. The 0.16 exploitation rate in

this study suggest that there might be under – fishing of M. vollenhovenii in the

Lower Taylor Creek , following the rule of Gulland (1971) that states that over –

fishing occurs when the exploitation rate is greater than 0.5. A similar study

also reported under-fishing for Brackish River Prawn, M. macrobrachion,

(Kingdom and Erondu, 2012) and over-fishing for Niger River Prawn, M.

felicinum, (Kingdom and Hart, 2012) in this same area.

The length –at-first capture (4.83 cm) for M. vollenhovenii was close to the

4.43 cm reported for males, but lower than 7.34 cm for females of M.

vollenhovenii in the Cross River (Nwosu and Wolfi, 2006) and 13.44 cm for M.

vollenhovenii in Asu River (Okogwu et al , 2010). These differences may be due

to the differences in the mesh sizes of the fishing gear used. Year –round

recruitment has been described to be a normal phenomenon for tropical fish

and shrimp species (Quasim, 1973; Weber, 1976). In this study, M.

vollenhovenii had two annual recruitment pulses of unequal magnitude. This

situation had already being observed in the same species in other water bodies

in Nigeria (Enin et al, 1996; Nwosu and Wolfi, 2006; Nwosu et al, 2007;

Abohweyere et al, 2008; Okogwu et al, 2010) and Ghana (Alhassan and Armah,

2011). This also confirms the earlier report of Kingdom et al. (2013), that M.

vollenhovenii had seasonal fluctuations in abundance in the lower Taylor

Creek.

CONCLUSIONS

The study revealed the month of poorest growth to be in February and the

existence of two annual recruitment pulses of unequal sizes. The study also

concludes that M. vollenhovenii in the Lower Taylor Creek is under-fished.

ACKNOWLEDGEMENT

Appreciation goes to Food and Agriculture Organization of the United

Nations that provided the FAO – ICLARM Stock Assessment Tool (FISAT II)

software that was used for the prawn population analyses.

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