Diet composition and some aspects of feeding ecology of Rastrelliger kanagurta and Selar crumenophthalmus in south coast Kenya inshore waters

Analysis of stomach contents provides insights into food composition and feeding habits of fish species, integrating ecological aspects such as behavior, habitat use, physiology, energy acquisition, and species interactions. This study examined the diets of Rastrelliger kanagurta and Selar crumenophthalmus from the ring-net fishery in inshore waters of Kenya’s south coast. The objective was to describe temporal variation in diet composition and feeding patterns of these small pelagic fishes. A total of 299 stomachs of R. kanagurta and 211 stomachs of S. crumenophthalmus were analyzed using the relative volumetric method. Feeding intensity varied seasonally, with higher activity during the NEM and reduced activity in the SEM. Peak gastrosomatic index occurred in November for R. kanagurta and January for S. crumenophthalmus, while lowest values were observed between May and July, suggesting feeding intensity increases post-spawning. The vacuity index averaged 15.7% for R. kanagurta and 16.3% for S. crumenophthalmus, indicating both species are gluttonous feeders. Fulton’s condition factor (K) ranged from 0.64–1.45 in R. kanagurta and 0.82–1.09 in S. crumenophthalmus, with lower values during spawning. Diet analysis revealed R. kanagurta is planktivorous, dominated by zooplankton (58%), while S. crumenophthalmus consumed mainly crustaceans (30.7%) and fish (30.5%). Relative importance index showed copepods (74.7%) as the key prey for R. kanagurta, whereas fish (38.1%) ranked highest for S. crumenophthalmus. Seasonal variation was evident in R. kanagurta, feeding on zooplankton during SEM and shifting to crustaceans and fish in NEM, while S. crumenophthalmus showed no significant seasonal change (p>0.05). Ontogenetic shifts were observed in both species, with juveniles feeding on zooplankton and adults shifting to larger prey. Diet diversity was lower in R. kanagurta (H’=2.00) than in S. crumenophthalmus (H’=2.93). High dietary overlap (0.92 ± 0.25) suggests coexistence is facilitated by contrasting feeding strategies: specialization in R. kanagurta and generalization in S. crumenophthalmus, reducing direct competition.