Aspects of the biology and feeding ecology of the orbiculate cardinal fish Sphaeramia orbicularis (Cuvier, 1828) (Teleostei: Apogonidae) in a Kenyan mangrove forest

Abstract

The orbiculate cardinal fish Sphaeramia orbicularis is the most abundant teleost among the root system of the extensive mangrove forests bordering Gazi bay, Kenya. The species was never recorded from the bay proper and it can thus be considered to be a true mangrove resident, the sampled population clearly consisted of two cohorts : the modes were approximately 65 mm and 80 mm. Most individuals with standard lengths 40 mm had mature gonads; the number of eggs ranged from 4 700 to 10 000. S. orbicularis are carnivores, mainly feeding on small epi- and hyperbenthic crustaceans. Numerically, gammaridean amphipods and tanaids were the dominant prey categories in the stomachs of both size classes. Individuals belonging to the smaller cohort mainly supplemented their diet with harpacticoid copepods, while larger fishes also fed on postlarval brachyuran crabs and caridean shrimp. The latter two taxa were important prey items in gravimetrical terms. A preliminary analysis of the otoliths revealed 21 stress marks and 20 striations. An attempt to validate these growth rings indicated that the average age of fishes in the samples ranged from 11 (smaller cohort) to 15 (larger cohort) months.