IDENTIFYING THE SPAWNING ESTUARIES OF THE TROPICAL-SHAD, TERUBOK TENUALOSA-TOLI, USING OTOLITH MICROCHEMISTRY

The tropical shad, terubok Tenualosa toli, is known only from 2 large estuaries and the adjacent coast of northwest Borneo. Populations of t his culturally and commercially important species have fallen to dange rously low levels during the 1990s. In an attempt to conserve the spec ies, the Sarawak state government has set up fisheries reserves to pro tect fish during spawning in the estuaries of the Lupar and Lassa rive rs. Terubok are protandrous hermaphrodites that, at. 1+ years old, mig rate to the inshore areas of the adjacent coast after initially spawni ng as males. To assess the effectiveness of the conservation measures we had to confirm that terubok spawn only in the 2 known spawning area s. We did this by examining the elemental composition at the nuclei of otoliths of fish using the new technique of LA-ICPMS (laser ablation- inductively coupled plasma mass spectrometry). Otoliths of fish collec ted from 7 coastal sites and both estuarine spawning areas in 1994 had similar elemental composition. This confirmed that all fish were prob ably spawned in 1 of the 2 estuaries. However, otoliths of fish collec ted from 2 of 8 coastal sites sampled in 1995 had significantly differ ent elemental composition from the fish from the Lupar or Lassa estuar ies. These sites had not been sampled in 1994 and were at the extremes of the species' known distribution. These results indicate that the s pecies may spawn in 1 or more other estuaries outside the known distri bution. The elemental composition at the nuclei of otoliths of fish fr om the 2 known spawning estuaries differed little in each of the 2 yea rs of the study, except in 1994 when Ba-138 concentration was higher i n the Lassa. These results meant that the elemental composition of oto lith nuclei could not be used to reliably estimate the relative contri bution to the coastal fishery of fish from the 2 known spawning estuar ies. However, elemental concentrations of 6 of the 7 elements examined differed between years for the 5 sites adequately sampled in both 199 4 and 1995. These differences were not related to changes in the sex o r age composition of the samples and this suggests that the water chem istry in both the spawning estuaries may have changed in a similar way during the study. Changes in otolith chemistry such as these could, i n some instances, cause confusion in interpreting fish population stru cture from these data.