Population dynamics and ecology of spiny lobsters Panulirus argus and P. Guttatus at Bermuda

Evans, Christopher Robin (1988) Population dynamics and ecology of spiny lobsters Panulirus argus and P. Guttatus at Bermuda. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Population density in four study areas for P.guttatus was 11-34 animals in the exploited phase/hectare (ha.) from capture-recapture work. The population densities of P.guttatus for its major physiographic environments were established from capture-recapture and transect work. Greatest densities were on the Atoll-Rim and Main (10 fa.) Terrace, (34-39 animals/ha.). Lowest were on the Reef Tract's patch-reef zone (2-3 /ha.). Present size at first physical maturity of P.argus females was 81-82 mm carapace length (C.L.), from leg length index (L.L.I.) measurements and 80-83 mm C.L. from presence of eggs and/or spermatophores, and for males was 96 mm C.L. from L.L.I. analyses; P.guttatus female maturity was at 53-58 mm C.L. from C.L./tot. length of male and female. The Von Bertalanffy growth parameters for P.argus and P.guttatus, at Bermuda were: K = 0.18 and 0.15 for male and female P.argus respectively; Similarly, K = 0.22 and 0.155 for P.guttatus; L ∞ = 204 mm and 192 mm C.L. for male and female P.argus respectively; Similarly, L ∞ = 87.6 mm and 86.0 mm C.L. for P.guttatus; t_{o =} - 0.82 years and - 1.0 years for male and female P.argus respectively; Similarly, t_{o =} - 0.846 yrs, and - 1.12 yrs, for P.guttatus. Details of the biological parameters for lobsters of the Northern Fishing Grounds were established: (1) Male P.argus; Z = 1.60, E = 0.72, F = 1.15, M = 0.45; 2) Female P.argus; Z= 0.99, E = 0.72, F = 0.71, M = 0.28; 3) Male P.guttatus; Z = 0.81, E = 0.50, F = 0.40, M = 0.41; 4) Female P.guttatus; Z = 0.77, E = 0.66, F = 0.51, M = 0.26. Parameters for Argus Bank were: (5) Male P.argus: Z = 0.52, E = 0.41, F = 0.21, M = 0.31; (6) Female P.argus: Z = 0.32, E = 0.41, F = 0.13, M = 0.19, (Z, annual total mortality coefficient; E, exploitation rate; F, M, annual fishing & natural mortality coefficients). Unknown facets of the field behaviour of P.guttatus were revealed and a further insight into its ecology was gained. Knowledge of the field behavior of both juvenile and adult P.argus was advanced. More than 64 juvenile P.argus chiefly of size 20.0-37.5 mm C.L. were attracted to haulable artificial shelters casitas with built-in predator-barriers, set as a group with spacing of 20 metres, inshore in seagrass; lobster habitat was enhanced. Most (ca. 70%) of the eggs produced by P.argus at Argus Bank were laid by females of C.L. > 126 mm C.L. Trapped sex ratio of P.guttatus was 116 females: 1,866 males. Legal/sublegal ratio for P.argus was 1.15±0.176. The annual fishing mortality coefficient of P.argus for Bermuda Platform was 0.34 (from tag returns) and the reciprocal of the catchability coefficient 134,114±2421 (standard error). The number and density of P.argus post-recruits in the reef areas of Bermuda Platform was 163,000 animals and 3.7 animals/ha. or 407 kg/sq.km. (standard error of 7.5%), the median from stock assessment and tag returns. Density of P.argus on the Atoll Rim at Kitchen Shoals was 6.4/ha. and on the Easter Main Terrace 7.8/ha. from capture-recapture and transect work. Annual mean population numbers and annual equilibrium catches of P.argus (of > 92 mm C.L.) were estimates for the seasons 1975/76 to 1986/87: since 1975 there has been a trend of slowly increased annual mean population number or fluctuation around a stabilised level of about 75,000 legal-size lobsters. M.S.Y. was 31,800 lobsters/season. Optimum fishing effort was 78,600 hauls/season. Standing stock of P.argus at Argus Bank was 5,000 kg (3,000 legal lobsters). Density at Argus Bank was ca. 175 kg legal-size stock/sq.km. (1 legal animal/ha.): potential yield was 1 tonne/cal. yr. (600 animals/cal. yr.).(DX 77442)