Plankton’s abundance and its implications for Sardinella lemuru catch in Prigi waters

Abstract

Sardinella lemuru is well known as a dominant fish in Bali Strait. However, since 2010 the stocks have collapsed. Surprisingly, at the end of 2019 it was caught very abundantly in Prigi waters, Trenggalek, reaching 56.42% of the production by volume and 39.44% in value. The reason for this abundance is thought to be the abundant availability of food in nature. This study aimed to analyse the abundance of plankton in relation to the abundance of S. lemuru in Prigi waters at the end of November 2019. A total of 100 S. lemuru samples were caught using a purse seiner, while marine plankton were collected using a 20μm plankton net with a vertical method at 5 sampling sites in S. lemuru fishing grounds in Prigi waters. The morphology, morphometric and meristic characters of the S. lemuru specimens were analysed, as well as the gut contents. Plankton samples collected from the sea and S. lemuru guts were identified and their abundance analysed. The S. lemuru samples had a mean total length (TL) of 16.47 ± 0.19 cm. The plankton from the seawater consists of 52.8% zooplankton and 47.2% phytoplankton, consisting of 21 genera of phytoplankton and 12 genera of zooplankton. Actinocyclus dominated the phytoplankton group with an abundance of 14,098 ind/m3, while the zooplankton group was dominated by Acartia with an abundance of 15,592 ind/m3. Correlation analysis shows the value of the correlation coefficient between phytoplankton in the waters and the S. lemuru was r = 0.920 with a determination coefficient of 84.9%. The abundance of S. S. lemuru in November 2019 was due to the abundant availability of food in the fishing area, when phytoplankton is the main food choice for S. lemuru.