Monsoonal variation and its impact on the feeding habit of Bali Sardinella (S. lemuru Bleeker, 1853) in Bali Strait

Sardinella lemuru is known as a highly opportunistic and flexible forager. Their high abundance in the coastal upwelling of Bali Strait was initially attributed to their feeding habit on phytoplankton and hence attaining higher catch. It was challenged by subsequent reports which suggested zooplankton as their main diet. This difference is due to the lack of information on the one-year cycle of its seasonal feeding. Here we used a combination of the plankton in seawater and the stomach contents of S. lemuru and monsoonal oceanographic changes at Bali Strait to determine the diet composition and food selectivity in four fishing seasons of 2012–2013. The result shows that S. lemuru is an omnivorous fish, and its diet composition depends on plankton availability in the environment, size classes, and the monsoonal oceanographic change influenced by upwelling. This condition strongly supported high nutrients for phytoplankton availability in the seawater with medium diversity and moderate community stability, except in inter-monsoon-2 (Trans-2). Phytoplankton was found as the main diet item of S. lemuru during the higher abundance of phytoplankton (82.26% Rhizosolenia stolterfothii) in Trans-2. In contrast, its main diet was substituted by zooplankton (51.96% Nauplius of Paraeuchaeta norvegica) during lower phytoplankton abundance in the northwest monsoon (NW). In addition, S. lemuru has adaptive strategies in feeding habits: It not only has flexibility but also selectivity in the feeding habit, supported by the ability to perform vertical migration for plankton grazing in different depths, move to another feeding ground, or plankton might be carried by the Indonesian Throughflow (ITF) into the Bali Strait. This study provides valuable information on the feeding ecology of S. lemuru, possibly providing a scientific basis for the proper management of the S. lemuru fishery in Bali Strait.