Reproductive seasonality and fecundity of the spiny lobster Panulirus homarus homarus (Linnaeus, 1758) (Decapoda: Achelata: Palinuridae) in Kenya: implications for sustainable management

Abstract

Abstract The scalloped spiny lobster Panulirus homarus homarus (Linnaeus, 1758) is one of the most commercially important lobsters exploited in Kenya. Despite its local importance and wide distribution, detailed knowledge on its reproduction in East Africa is lacking. The study reported herein sought to determine its spawning season based on seasonal variations in female gonadosomatic index (GSI), temporal distribution of ovarian maturity stages, and the proportion of ovigerous females in the monthly samples captured off Mambrui, Kenya. The study also assessed size composition, sex ratio, fecundity, and reproductive potential. A total of 2,454 lobsters were collected during the study period (January 2000-December 2001), covering a size range of 26–115 mm carapace length (CL). Neither the overall sex ratio nor the monthly sex distributions deviated significantly from the expected ratio of 1:1, except for a few months when males and females alternately dominated the samples with no discernible seasonal trends. Analysis of the cyclical changes in reproductive activity shows strong spawning seasonality in synchrony with the Northeast monsoon season in both years. Simultaneous breeding of a wide size range of females (54–90 mm CL) commenced in November and intensified over the following months until April. Outside the Northeast monsoon season, only 10 females, out of the total 517 sexually mature females sampled during the 2000 and 2001 Southeast monsoon seasons, bore eggs, and all were large individuals (>66 mm CL) encountered in the 2001 samples. Fecundity was positively and linearly related to female size (CL) and best described by the equation E = 10,400 CL – 472,040 (R2 = 0.929, N = 59). Estimated fecundity varied between 81,000 and 474,000 eggs per spawn in female lobsters (54 mm–91 mm CL). Female lobsters between 66 mm and 80 mm CL contributed 62% of the estimated total egg production of the population. The widespread retention of egg-bearing females in a fishery where the main fishing season falls during the spawning season, can have detrimental effects on the spawning stock abundance and egg production with increased risk of recruitment failure and stock collapse.