Assessment of Food Sources for Juvenile and Adult Penaeus monodon in Tanzania Coastal Waters

To investigate the food sources for juvenile and adult Penaeus monodon and reconstruct its diet, samples of potential food sources were collected from the nursery areas of Kondo, Mbegani and Kitame, as well as from the fishing grounds of Mwambakuni and Saadani, located in the Bagamoyo coastal waters of Tanzania in the Western Indian Ocean. The carbon and nitrogen isotopic ratios of both P. monodon muscle tissues and their respective potential food sources were determined from samples collected during the Northeast Monsoon (NEM, September–October) and Southeast Monsoon (SEM, April–May), representing the dry and wet seasons, respectively. P. monodon muscles exhibited a wide range of δ¹³C values from −20.87‰ to −12.22‰ (average of −16.23‰, n = 6) during NEM, alongside a narrower range of δ¹⁵N values from 5.32‰ to 8.86‰ (average of 6.63, n = 6). Similarly, during SEM, δ¹³C values ranged from −20.68‰ to −15.59‰ (average of −17.47‰, n = 6), while δ¹⁵N values ranged from 5.32‰ to 8.55‰ (average of 7.05‰, n = 6). Different size classes displayed distinct δ¹³C with a range of −8.27‰ but were closely related in δ¹⁵N, exhibiting an average range of only 2.66‰. The contributions of carbon and nitrogen from mangrove and terrestrial sources were found to be negligible as food sources for P. monodon across both seasons. However, both juvenile and adult P. monodon significantly relied on seagrass ecosystems for their food, with minimal connectivity (p ˂ 0.05) between the nursery and fishing grounds. Furthermore, it was observed that the primary carbon sources for P. monodon included shellfish (prawns), sergestid shrimps, plankton and algae during the SEM, with a slight shift towards algae and polychaetes during the NEM. Notably, P. monodon specialized in a specific food source during NEM, with juveniles showing significantly higher levels of cannibalism compared to adults.