The Biology and Life-history Strategies ofIlisha(Teleostei: Pristigasteridae) in the Coastal Waters and Estuaries of Sarawak

Many genera and species of clupeoids are poorly known, especially in the subtropics and tropics where their diversity is high and they are usually among the largest component of estuarine and coastal artisanal and subsistence fisheries. One such genus,Ilisha, is caught by fisheries along the coast of Sarawak (South China Sea). Specimens over 500 mm standard length (SL) are a delicacy and command very high prices but largeIlishaare now rare and there is concern for the state of the stocks. The objectives of the present work, the first detailed study of any Indo-West PacificIlisha, were (1) to determine which species ofIlishaoccur in Sarawak, and which of those reach a large size and (2) to describe the biology and ecology of each species, and relate these to distribution patterns and life-history strategies. A total of six species ofIlishawere recorded from Sarawak, but only one,Ilisha filigera, reaches a large size and hence is commercially valuable. The adults of all species live in coastal waters, but onlyI. filigeraandIlisha pristigastroidespenetrate to the middle reaches of estuaries (salinity down to about 9, turbidities as high as 1000 NTU). The juveniles of these two species andIlisha elongataandIlisha megalopterawere recorded in low salinities (1–5) in the upper reaches of estuaries. In all species there is ontogenetic diet switching, usually from zooplankton to fish or larger nekton such as penaeids. Ilisha melastomais primarily a mollusc feeder; the other five are primarily piscivorous with Crustacea of secondary importance. In most species, the males mature at a smaller size than the females, but there are otherwise no clear differences in size.Ilisha filigeraspawns only in the wet season, while the rest spawn mainly in the dry. From length-at-age data (using otoliths),Ilishacan be divided into three broad life-history groups: (1) small species growing to about 200 mm SL in length and living for less than 2 years:I. kampeniandI. melastoma; (2) medium-size species growing to about 400 mm SL and living for about 2 years:I. elongata,I. megalopteraandI. pristigastroides; and (3)I. filigera, which reaches more than 700 mm SL (making it one of the largest clupeids in the world) and lives for more than 4 years. The life-history strategies of the genus are discussed in relation to diet, reproduction and growth. While small species such asI. melastomacan probably withstand heavy fishing pressure, large species that mature later, such asI. elongataandI. pristigastroides, are more vulnerable to overfishing. The present scarcity of the large, long-lived species,I. filigera, suggests that it cannot withstand current levels of exploitation.