Asian Journal of Agricultural Sciences 2(1): 27-34, 2010
ISSN: 2041-3890
© M axwell Scientific Organization, 2009
Submitted Date: August 27, 2009
Accepted Date: September 14, 2009
Published Date: January 10, 2010
Mortality, Exploitation rate and Recruitment pattern of Callinectes amnicola
(De Rochebrune, 1883) from Okpoka Creek, Niger Delta, Nigeria
1
J.F.N . Abowei, 2 A.D.I. George and 2 O.A. Davies
Department of biological sciences, Faculty of Science, Niger Delta University, Wilberforce
Island , Am assoma, Bay elsa State, Nigeria
2
Departm ent of fisheries and aquatic environm ent, Faculty of Agriculture, Riv ers State University
of science and Techno logy , Port Harcourt, Rivers State, Nigeria
1
Abstract: The motality, explo itation rate and recruitme nt pattern of Callinectes amnicola from Okpoka creek
was studied from January- December 2007. Total mortality (Z) value was Z=5.19 yrG 1 , M=1.30 yrG 1 , F=3.89
yrG 1 and exploitation rate (E)= 0.7495. The exploitation rate (E max ) that gives max imum relative yield-perrecruit was 0.4890. The ex ploitation rate at wh ich margina l increase occurred in the relative yield-per-rec ruit
was 10% of its value at E=0, whereas (E 0.1 ) was observed to be betw een 0.4684. The exploitation rate (E 0.5 ),
which, corresponds to 50% of the virgin (i.e., the unexploited stock) relative biomass-per-recruit was 0.3129.
The mean ratio of length-at-first capture (Lc) and asymptotic length (L4) was 0.21, while that of natural mortal
(M) and growth rate (KyrG 1 ) was 0.82. The recruitment pattern showed all year round recruitment with two
peaks (one major and the other minor). The parameters obtained were L 4 =61.8, K=1.71yrG 1 , C= 0, WP = 0,
to =0. The percentage recru itment values for the different months were: Jan uary (4.29% ); February (5.45 %),
March (7.47% ); Ap ril (15.50%); May (20.57%); June (15.68%); July (8.77%); August (7.40%); September
(0.43% ); October (1 .23% ); Novem ber (13 .21% ) and D ecem ber (0.0 0% ). C. am nicola from Okpoka creek die
more due to human exploitation than natural death, thus experiences excessive fishing pressure. This implies
that the level giving maximum sustainable yield has been overshot. There is therefore the urgent need to check
the exploitation of C amnicola in Okpoka Creek for its sustainability.
Key w ords: Callinectes amnicola, mortality, explo itation rate, recruitm ent, O kpoka C reek and N igeria
INTRODUCTION
The crustac ean, Callinectes amnicola , belongs to the
family Portunidae The species is a p opular food item in
the diet of coastal com mun ities in W est Africa. It is
caught in the creeks, lagoons and adjacent inshore marine
waters. It supports a major artesanal fishery in coastal
communities in Nigeria. Recreational fishers often seek
this interesting animal because of its availability,
econ omic value, hardiness and complex life cycle (Miller
and H oude, 19 99; Smallegange a nd Jaap Van Der M eer,
2003). It also plays a crucial role in the estuarine food
webs, providing food for many species (Laughlin, 1979;
Hines et al., 1987; Orth and Van M ontfrans, 1990;
Thomas et al., 1990; Heck and Coen, 1995). It is one of
the most abundant estuarine macro invertebrates and
supp orts valuable commercial and recreational fisheries
along the Atlantic and Gulf coasts (Guillory and Perret,
1998).
Mortality rates are of prime importance to fishery
scientists in expressing the dynamics of fish population,
(Youngs and Robson, 1978; Marshall, 1993). Total
mortality coefficient (Z), natural mortality (M), fishing
mortality (F) and the exploitation ratio (E) are vital for
determining the status of the C. amnicola fishery. These
parame ters are also used for establishing the level of
exploitation (Enin, 1995; Enin et al., 1996), hence the (Z,
M , F and E) were estimated for the samples collected
during the study.
There are several causes of mortality, which include:
age (Nikolsky, 1969), diseases and pollution (Gordon,
1978), preda tion (C hifam ba, 19 93), spawning stress
(Sparre and Venema, 1992) and Lack of food (King,
1995).
The explo itation ratio (E) enables one assess roug hly
if a stock is over fished (high E-values) or not on the
assumption that the optimal value of E (Eopt) is about
equal to 0.5. The use of 0.5 for Eopt is based on the
assumption that the sustainable yield is optimized when
F = M (Gulland, 19 71).
Recruitment is the entrance of young fish into the
exploited fishing area and became liab le to contact w ith
fishing gear (Gulland, 1983). It also refers to either the
addition of new fish to the vulnerable population by
grow th from am ong sm aller size individuals (Rick er,
1975). Recruitment is the m ajor source o f variability in
fish population (Bankole, 1990). The mean age of fish at
recruitment generally depends on the type of mesh size of
the gear used in fishing.
Corresponding Author: J.F.N. Abowei, Department of biological sciences, Faculty of Science, Niger Delta University,
Wilberforce Island, Amassoma, Bayelsa State, Nigeria
27
Asian J. Agr. Sci., 2(1): 27-34, 2010
The recruitment pattern or fish entry into the catch is
as a result of the combined effect of recruitment and gear
selectivity. The general form of recruitment curve may be
determined by a proper knowledge of the biology of the
species and estimation of the recruitment pattern obtained
by comparing the size composition of actual catches w ith
known selectivity of gear. Knife-edge recruitment model
identified in the Bev erton and H olt (1957) yield-perrecruit model (Gayanilo et al., 1995) pred icts all fish of a
certain age below the age of recruitment, which recru it
(enter) into fishery but are not exposed to fishing
mortality.
The Okpoka creek is one of the most numerous
creeks in Niger Delta. The Niger D elta estuarine waters
cover an area of about 680 km 2 . The Bonny/ New Calabar
river systems forme d abo ut 39% of the total area (S cott,
1966). The Niger Delta area is the richest part o f Nigeria
in terms of natural resources with large deposits of
petroleum products (oil and gas); (M offat and Linden,
1995; Braide et al., 2004). Similarly, the vast coastal
features which include forest swamp s, mangrove, marsh,
beach ridges, rivers, streams and creeks serve as natural
habitats for various species of flora and fauna (Alalibo,
1988; Jam abo, 2008 ).
Despite the importance of Callinectes amnicola to the
artisanal catch in Nigeria, studies on the mortality,
exploitation rates, recruitment pattern , yield-per-recru it
and biomass-per-recruit are lacking. However, estimates
of few other shellfishe s are av ailable. N otable among
them are some bivalves (Pitar tumens, Tellina nymphalis,
Senillia serilis and Tagelus adansonii) in the Andoni flats,
(Ansa, 2005), estuarine prawn (Nem atopalamon hastatus)
of the Southeast coast of Nigeria (Enin et al., 1996) and
Macrobrachium macrobrachion in the Cross-river estuary
Nigeria. The mortality, exploitation rate and recruitment
pattern of Callinectes amicola from Okpoka Creek in the
Niger Delta area of Nigeria provides information for the
effective management and sustainable exploitation the
crab in the fishery and similar water bodies.
Characteristically, the area is a typical estuarine tidal
water zone with little fresh w ater input but w ith extensive
mangrove swamps, inter-tidal mud flats, and influenced
by semi-diurnal tidal regime. In the Bonny River estuary,
the salinity fluctuates with the season and tide regime is
influenced by the Atlantic ocean (Dangana, 1985). Tidal
range in the area is about 0.8m at neap tides and 2.20m
during spring tides (N ED EC O, 1961).
It is strategically located southwestern flanks of Port
Harco urt and Okirika of R ivers State. The creek is
bounded by thick mangrove forest dominated by
Rhizophora species interspersed by White mangrove
(Avecinia sp.) and Nypa palm. Along the shores of the
creek are located the Port Harcourt Trans- Amadi
Industrial layou t, several establishments, markets, the
main Port Harcourt Zoological garden and several
commu nities. The communities are Oginigba, Woji New
layou t, Azuabie, Okujagu- Ama, Ojimba- Ama, Abuloma,
Okuru- Ama, Oba-Ama and Kalio- Ama.
Artisanal fishers mainly exploit the fisheries. The
fishers use wooden/dug-out canoes ranging in size from
3 to 8m long. T he canoes are either paddled or powered
by small outboard engines, and manned by an average of
two men. From these b oats, the fishers o perate their cast
nets, hook and lines, gillnets, crab pots, etc.
Sampling stations: Six samp ling stations were
established along a spatial grid of the Okpoka creek
covering a distance of about six kilometers. The sampling
stations were established based on ecological settings,
vegetation and human activities in the area. The sampling
station is about one kilometer apart from each other.
Station 1: Located upstream of the Port Harcou rt main
abattoir at Oginigba waterfront with living houses on the
left flank of the shoreline. Vegetation is sparse with
mainly red mangrove (Rhizophora sp.,) white mangrove,
Avicenia sp. and Nypa palm (Nypa fructicans).
Station 2: Situated at Azuabie/Port Harcou rt main
abattoir waterfront. It is located downstream of Station 1.
The bank fringing the Azuabie/abattoir is bare with no
visible plants e xcep t toilet houses, residential houses,
animal pens, boats and badg es, while at the opposite side
there are few mangrove and Nypa palm. Human activities
here include slaughtering of animals, marketing, fishing
and boat building. It is located downstream of station 1
and it is main collection point of abattoir wastes and other
human and market wastes.
MATERIALS AND METHODS
Study Area: The study was carried out in Okpoka creek,
which is one of the several adjoining creeks off the Upper
Bonny River estuary in the Niger Delta (Fig. 1). The
Bonny River Estuary lies on the So utheastern edge of the
Niger Delta, between longitudes 6°58! and 7°14!! East and
latitudes 4°19!! and 4°34! North. It has an estimated area
of 206 km 2 and extends 7 km offsho re to a depth of about
7.5 m (Irving, 1962, Sc ott, 1966 and Alalibo, 198 8). The
Bonny River is a major shipping route for crude oil and
other cargoes, and leads to the Port Harcourt quays,
Federal Ocean Terminal, Onne, and Port Harcourt
Refinery company terminal jetty, Okirika. Specifically,
the Okpoka creek lies between Longitudes 7°03! and 7°05!
East and Latitudes 4°06! and 4°24! and it is about 6
kilometers lon g.
Station 3: It is downstream from the Po rt Harcourt
abattoir at the W oji sand-Crete. It is about one kilometer
away from Station 2. The major activities here included
sand mining and loading.
Station 4: This station is located at Okujagu-A ma area.
There are no industrial activities here . Ma inly fishers
28
Asian J. Agr. Sci., 2(1): 27-34, 2010
Fig. 1: Map of Niger Delta showing Rivers State and the Study Area
occupy the area. Nypa palm dominates the marginal
vegetation while the opposite side is thickly populated
with red mangrove forest. Rhizophora racem osa and
Rhizophora mangle. The main activity is fishing, boat
ferrying and occasional sand moving.
here include jetty operations, oil and non-oil industrial
activities, boat traffic and fishing. Vegetation is few
dominated by red mangrove interspersed with wh ite
mangrove Avicenia africana.
Sample collection: The crabs for study were collected
fortnigh tly for twe lve (12 ) calendar m onths (January to
Decemb er) using the square lift net trap at each of the
sampling stations along the Okpoka creek. The lift net
trap has a square structure made of wooden stick of about
4 cm thick and an area of 4.9 m 2 . The mesh sizes of the
bag-like net were 1.2 cm to 2.0 cm multifilament nylon.
The length of the bag is 40 to 60 cm. Strong nylon cords
were wov en in a net-like fashion from the cen tre to the
Station 5: Is situated at Ojimba cum Abuloma
waterfronts. There are no com merc ial activities apart from
ferryboats operations. The shoreline fringes have m ainly
Nypa palm. The area is shallow and at low tide, the
greater part of the bottom mud flat is exposed.
Station 6: Is located in front of Kalio-am a directly
between Okpoka and Amadi creeks. The human activities
29
Asian J. Agr. Sci., 2(1): 27-34, 2010
midd le of each of the four edges. A twine of about 6m
long was attached to the centre and the other free end of
the twine was tied to a floater, which served as a marker
on the water surface to sh ow the position of the gear.
The lift net trap was baited at the centre with animal
offal and fish. The trap was operated from a hand-paddled
canoe manned by two persons; one rowing while the other
sets and hulls the trap into and from the water. The crabs
were caught trapped and most of them were observed
feasting on the bait until they were hulled into the boa t.
Sampling lasted for 4 hours on every sampling day and
samples were collected between low ebbing and low
flooding tide periods. The catches were taken to the
laboratory in a cooler and stored in a deep freezer for
further analysis.
Crabs were identified to species level carried out
using photo cards and available identification keys
(Fischer, 1978; W illiams, 1974; Schneider, 1990 ).
Therefore each crab was sorted into species, sex and the
required metric measurements were taken.
The carapace width and length w ere measured w ith
a 0.5 mm precision vernier caliper to the nearest
millimeter (mm) while weight measurement was done
using a 0.001g precision Adam (PGW series) weighing
balance to the nearest gram s (g).
The age structure of the crab samples collected and
measured during the study period was estimated based on
the modified Peterson method, known as the “Integrated
Method” (Pauly, 1983; Sparre, 1987). This was achieved
by analyzing the polymodal length-frequency distribution
plots obtained from the crab sample data grouped into
10mm class intervals. The data for each sex was pooled
together into one length-frequency plot per year. The
cumulative length-frequen cy histogram was red rawn
repea tedly for six times on the assum ption that grow th
patterns repeat them selves from year to year.
A single smoo th curve interconnecting the majority
of the peaks of sequentially arranged length-frequency
data was draw n first rising rapidly and th en decreasing
smoothly according to growth curves of fishes. The
grow th curve was fitted by eye to interconnect the plots of
the length-frequency distributions arranged in time series.
The meeting po ints of the curve with the length axis gave
the mean length of the species and the relative age of the
crab read from time ax is (Pauly, 198 3).
The application of this method according to Pauly
(1983) is based on the following assum ptions.
C
C
Similarly, that the single smoo th curv e is assu med to
represent the average g rowth of the crabs of a given stock
as the grow th pattern repeats it self from year to year.
The single grow th curv e was fitted by eye . Each discrete
mode in the distribution was presumed to be a year class.
Total mortality coefficient (z): The total mo rtality
coefficient (z) was estimated by the length-converted
catch curve proc edure of ELEFAN ll (Pauly, 1980, 1983,
1984a, b) In this analysis the percentage of samples in
length groups are pooled to sim ulate a steady -state
population. The data for the catch curve was obtained by
pooling all the monthly length-frequencies. The principle
applied in the catch curve analysis involved the use of the
natural logarithm of the numbe r of crabs, caught in each
age group (N) plotted against their corresponding relative
age (t) (Royce, 1972; Pauly, 1985; Moses, 1988). The
relative ages were calculated from L¥ and K values of the
VBGF (Pauly, 1982). The total mortality (Z) was obtained
from the slope ( b) of the descending limb of the catch
curve with the sign changed.
Instantaneous natural mortality (M) and fishing
mo rtality (F): The instantaneous natural morta lity (M)
was estimated by Pauly’s (1980, 1983) empirical formula:
Log 1 0 M = 0.0066-0.279Log 1 0 L¥+0.654Log 1 0 K
+ 0.4634Log 1 0 T
(1)
L¥ and K were taken from the VBGF w hile T was the
mean environmental temperature estimated as 29.5°C for
the study area. Fishing mortality (F) was calculated from
F = Z-M
(2)
Exploitation ratio (E): The state of exploitation of C.
amnicola was calculated from the equation:
E=
i.e,
(3)
i.e., the fraction of total mortality (Z) caused by fishing
mortality (F).
Recruitment pattern: The relative y ield-per-recruit
(Y/R) and relative biom ass-per-recruit (B/R ) were
determined by the knife-ed ge recruitmen t approach which
is identified by Beverton and H olt (1959) as yield-pe rrecruit model and incorporated into the recruitment
routine in FiSAT (Pauly, 1983; Gayanilo et al., 1995). In
FiSAT, the recruitment patterns were analyzed using the
maximum likelihood approac h of N ORSMSE P
(Separation of the Normally distributed components of
length-frequency samples) to fit the Gaussian distribution
on length-frequency data for the year pooled together
(Pauly, 1983; Moreau and Cuende, 1991; Gayanilo et al.,
1995).
The succeeding modal lengths represent the next
grow th points and reflect the average growth of the
stock.
Length growth is at first rapid and then decreases
smoothly and is best approx imated by a long
continuous curve for the whole population.
30
Asian J. Agr. Sci., 2(1): 27-34, 2010
Table 1: Estimated m ortality parameters of C. a mn icola
year
Z
M
F
E
2007
5.19
1.30
3.89
0.7495
Z = Coefficient of total mo rtality , M = Coefficient of natural mortality,
F = Coefficient of Fishing mortality. E = Exploitation ration.
RESULTS
The results of the total mortality (Z), natural m ortality
(M), fishing mortality (F) and explo itation rate (E) is
presented in Table 1. Total mortality (Z) value was
Z=5.19 yrG 1 , M=1.30 yrG 1 , F=3.89 yrG 1 and exploitation
rate (E) = 0.7495.
Table 2 shows the estim ated optimum ex ploitation
rates and related co efficient rates of C. amnicola from
Okpoka creek. The estimated optimum exploitation rates
(relative yield-per-recru it (Y/R )) and related coefficient
rates (relative biomass-per-recruit (B/R) analysis by the
knife-edge selection is presented in Fig . 2. The
exploitation rate (E max ) that gives maximum relative yieldper-rec ruit was 0.4890. The exploitation rate at which
marginal increase occurred in the relative yield-per-recruit
was 10% of its value at E=0, whereas (E 0.1 ) was observed
to be between 0.4684. The exploitation rate (E 0.5 ), which,
corresponds to 50% of the virgin (i.e., the unexploited
stock) relative biomass-per-recruit was 0.3129. The mean
ratio of length-at-first capture (Lc) and asym ptotic len gth
(L4) was 0.21, while that of natural mortal (M) and
growth rate (KyrG 1 ) was 0.82.
The recruitment pattern of C. am nicola is presented
in Fig. 3. The recruitment pattern showed all year round
recruitment with two peaks (one major and the other
minor).
The parameters obtained were L 4 =61.8,
K=1.71yrG 1 , C= 0, W P = 0 , to = 0. The percentage
recruitment values for the different mon ths were: Jan uary
(4.29%); February (5.45% ), March (7 .47% ); April
(15.5 0% ); Ma y (20.57% ); June (15.68% ); July (8.77% );
August (7.40%); September (0.43%); October (1.23%);
Nov embe r (13.21% ) and D ecember (0.00% ).
Tab le 2: Estimated optimum exploitation rates and related coefficient
rates of C. a mn icola
L c /L 4
Year
E max
E 0.1
E 0.5
M /K
2007
0.4890
0.4685
0.3129
0.21
0.82
E max = E xplo itation rate th at giv es m axim um yield -per-re cruit
E 0.1 = Exploitation rate at which relative yield-per-recruit was 10%
E 0.5 = 50% of unexploited stock.
Lc = Len gth-at-first captu re
L 4 = as ym ptotic leng th
M / K = R a ti o o f n at ur al mo rt al it y ( m) an d gr ow th ra te (K )
Fig
DISCUSSION
The estimated instantan eous rate of total mortality
(Z=5.19 yrG 1 ) instantaneous rate of fishing m ortality
(F=3 .89 yrG 1 ) and instantan eous natural mortality
(M=1.30 yrG 1 ) obtained in this study are higher than
values reported by Lee and Hsu (2003) for the swimming
crab Portunus sanguinolentus in the waters off Northern
Taiwan. They repo rted Z = 3.16/yea r, F = 1.51/ye ar, for
males a nd , Z =3 .3 7/y ear, F = 1 .5 7/y ear, and M = 1.8/year
for the females. Also the estimated exploitation rate, E =
0.7495 obtained in this study showed that fishing
mortality was higher than natu ral mortality.
The implica tion is that C. am nicola die more du e to
human exploitation than natural death. W ith an estimated
E = 0.7495 during the study, one can infer that the stock
of C. amn icola of the Okpoka creek, off Upper Bonny
River, is probably experiencing excessive fishing
pressure. This is based on the assumption that in an
optimally exploited stock, natural and fishing mortalities
shou ld be equal or E = F/Z = 0.5 (Gulland, 1971). This
agreed with the observations on M. macrobrachion in the
Cross R iver estuary by Enin (199 5).
. 2:
Relative yield-per-recruit and relative biomass-perrecruit of C. amnicola using the Knife-edge recruitment
method
The maximum exploitation rate (E max ), which gives
maximum relative yield-per-recruit, is estimated at 0.536
and greatly differs from the exploitation rate (0.7495)
estimated in this study. This further suggests that the
stock of C. am nicola is probably b eing o ver fished bo th in
31
Asian J. Agr. Sci., 2(1): 27-34, 2010
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Fig. 3: Recruitment Pattern of C. amnicola of Okpoka Creek,
Upper Bonny River
terms of yield-per-recruit and biomass-per-recruit as
suggested by Gulland, (1971). This implies that the level
giving max imum sustainable yield has been overshot, thus
driving the stock down (Enin et al., 1996).
The range of M /K ratio 0.82 is below the Beverton
and Holt (1959) range of 1.0-2.5 for fish indicating a poor
environmental state. This may be applicable to
crustaceans in the same environment. The M/K ratio is an
indirect method used by scientists to examine the
accuracy of growth parameters and is supposed to be
constant for a group of species or closely related families
or taxa (Ch akraborty, 20 01).
The recruitment pattern established in this study
revealed an all year round recruitment with two peaks
(one major and the other secondary). This observation
conforms to the assertions of Rufli and Van Lissa (1982)
and Pauly (1982) that they are characteristics of most
tropical fish stock with seasonal association.
The recruitment pattern of this crab showed peak
recruitment from A pril - June. Ansa (2005) also observed
all year round recruitment pattern for P. tumens in Andoni
flats. The all year ro und recruitm ent is to compensate for
the high mortality rates, of the C. am nicola as observed
from the results may be of this study.
The winter point (W P) of q.0 showed that growth of
the crab is poorest in the period of Decemb er – January of
the year. This may be due to the reproductive activity of
the species which come s to a peak during the dry months
of the year with favorable environmental factors as
observed by Steel and Perry (1998), Enin (1995) and
Branco and M asun ari (200 0).
The amplitude of seasonal growth oscillation (C)
observed during the study indicated that C. am nicola
experienced no seasonality in growth. Generally, the blue
crabs are considered a ‘r-selected species’ w ith certain life
history traits (i.e., high fecundity, high inter-annual
variation in abundance, rapid growth, early reproductive
maturity, high natural m ortality rates, and relatively short
life span) suggestive of a density-independent spaw nerrecruit relationship (Van Engel, 1987).
32
Asian J. Agr. Sci., 2(1): 27-34, 2010
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