Length-based analysis of population dynamics and stock identification in the sardine fisheries around East Java, Indonesia

Abstract

The biology and population dynamics of Sardinella longiceps and S. fimbriata were studied around East Java, Indonesia. The total catch of Sardinella spp. in East Java was 56 000 tons in 1993, and more than 50% of this catch was landed by purse seiners. Samples of purse seine catches were taken from three landing sites around East Java, at the Madura Strait, the Bali Strait and at the Indian Ocean. Biological data were collected on a monthly basis, during a period of 2 years, from March 1990 to March 1992. Patterns in reproductive activity were determined on the basis of changes in geometric mean gonado-somatic index (GSI) of mature fish. Length-based methods were used to estimate growth and mortality. Peak catches of sardines from the Madura Strait and the Bali Strait were recorded at the beginning of the rainy season around November and December. The Indian Ocean produced peak catches during the dry season, from July through October. Considerable differences exist between monthly catches from peak seasons and monthly catches from off-seasons. Patterns in reproductive activity could be recorded for the fish from the Madura Strait and both species of Sardinella showed peak reproductive activity there around November and December. Both species were estimated to grow to lengths of around 12, 17 and 19 cm in their first, second and third year respectively. Based on the age structures in the catches from different sampling sites, in combination with the migration patterns of the fish, it was concluded that both S. longiceps and S. fimbriata stocks can be divided into northern and southern populations. The northern populations were sampled in the Madura Strait and were assumed to be from the Java Sea, Makassar Strait and/or Flores Sea. These stocks consist of three year classes and do not seem to be under excessive fishing pressure. The southern stocks, well known from the Bali Strait sardine fisheries, are concluded to be under very heavy fishing pressure. These stocks consist of only two year classes and hardly any fish reach the length of maturity. Regulations controlling fishing effort are needed urgently for these fisheries.