ABSTRACT

The artisanal Callinectes amnicola fishery of the Qua Iboe River Estuary and Imo River Estuary of south eastern Nigeria were studied for twelve months (from June 2008 - May 2009), with respect to their species composition, reproductive biology and population structure and dynamics. Callinectes amnicola was the only species encountered in both estuaries. Sex ratio favoured males in the Imo River Estuary (1:0.89, 4.62, 1 df, p < 0.05) and females in Qua Iboe River Estuary (1:1.35, x2 26.26, 1 df, p <0.05). Population characteristics such as sex, moult stage, colour morphotypes, and development stage significantly influenced the carapace length (CL), carapace width (CW) and total weight (TW) of crabs in both estuaries (p ><0.05); while spatial differences in salinity (low in Imo River estuary - 0.53% and moderate in Qua Iboe River estuary -17.4 %o) significantly influenced population structure (p ><0.05). Crabs of Qua Iboe River estuary at higher salinity were heavier and longer than those of Imo River estuary at lower salinity (p >< 0.05). Crabs sampled were 26.6 - 87.0 mm CL (62.48 ± 0.23), 53.7- 187.0 mm CW (124.5 ± 0.51) and 9 -323 g (126.39 ± 1.43) in Qua Iboe River Estuary and 15.5 - 81.0 mm CL (58.83 ± 0.28), 43.1 -168.9mm CW (117.00 ± 0.56) and 4-307 g (109.85 ± 1.36) in Imo River Estuary, and exhibited a high composition of adults (80 - 90%). Male crabs were heavier and longer than female crabs (p < 0.05). The carapace length -weight relationship exhibited positive allometry (b > 3) in Qua Iboe River estuary and isometry (b = 3) in Imo River estuary. Females exhibited negative allometry (b < 3) in both CW and CL - weight relationships. The size at sexual maturity in males was 90.05 mm CW in Qua Iboe River estuary, and 92.7 mm CW in Imo River estuary. Females attain sexual maturity earlier in Imo River estuary at 100.5 mm CW and at 115.0 mm CW in Qua Iboe River Estuary. Ovigerous crabs constituted 13% and 10% of all females sampled in Qua Iboe River and Irno River estuaries, respectively. The size of ovigerous females exhibited bi-modal peaks at 107.55 mm CW and 132.55 mm CW in Qua Iboe River estuary and 162.55 and 187.55 mm CW in Imo River estuary. Ovigerous females did not occur throughout the year but peaked in the months of May and December in both estuaries. Absolute fecundity ranged between 12,000 and 8,785,000 ripe eggs per spawn per female and averaged 1,001,926 ripe eggs per spawn per female at Qua Iboe River estuary and 851,669 ripe eggs per spawn per female at Imo River estuary. Fecundity was significantly influenced by egg volume, gonad weight, and total weight in that order of magnitude (p <0.05). Male and female C. amnicola spawn between September and December, rest between January and April and recover for spawning between May and August. The current level of exploitation of the fishery was investigated based on size frequency analysis of trap and gillnet catches. Estimates of growth parameters were derived from ELEFAN I procedure of FiSAT software. The asymptotic length (Lc), curvature parameter (K) and longetivity (tmax) were 176.66 mm CW, 83.8 mm CL, 0.46 year and six years and six months, respectively, for males and 193.93 mm CW, 91.93 mm CL and 0.36 year and eight years four months, respectively, for females in Qua Iboe River estuary. The corresponding values were 175.93 mm CW, 85.18 mm CL, 0.43 year and seven years, respectively, for males and 160.18 mm CW, 76.18 mm CL, 0.99 year and three years lifespan, for females in Imo River estuary. The amplitude of seasonal oscillation (C) as well as winter point (WP) were zero (C 0.00; WP = 0.00), indicating absence of seasonality and continuous growth without cessation periods. Total and natural mortality derived from catch curve analysis indicate that C. amnicola from the study area die more due to human exploitation than natural death, thus experiencing excessive fishing pressure. Maximum exploitation rate, corresponding to the maximum relative yield per recruit and calculated with the relative yield per recruit model of Beverton and Holt, was Emax 0.799.The computed exploitation rate, calculated from fishing mortality, F and total mortality, Z, indicates overexploitation (E>0.5) but below predicted values (E = 0.6 — 0.8). Yield per recruit analysis suggests that yield could be maximized, if E was increased to 0.8 to increase food supply, within precautionary limits and under the precondition that recruitment was independent of stock size. An estimated annual catch of 4.2t and 6.7t were obtained while mean annual catch per unit of effort (CPUE) was 6.88 (± 0.32, standard error, SE) and 7.15 (± 0.44, SE) kg/hour; monthly average earning per fisher was 4,853.69 (± 262.22, SE) and 12,729.04 (± 1,102.35, SE), and monthly average earning per boat was 9707.39 (± 452.44, SE) and 21,476.63 (± 1,676.23, SE) in Qua Iboe and Imo River estuaries, respectively (p