Identifying the spawning estuaries of the tropical shad, terubok Tenualosa toli, using otolith microchemistry

Abstract

The tropical shad, terubok Tenualosa toli, is known only from 2 large estuaries and the adjacent coast of northwest Borneo. Populations of this culturally and commercially important species have fallen to dangerously low levels during the 1990s. In an attempt to conserve the species, the Sarawak state government has set up fisheries reserves to protect fish during spawning in the estuaries of the Lupar and Lasso rivers. Terubok are protandrous hermaphrodites that, at 1+ years old, migrate to the inshore areas of the adjacent coast after initially spawning as males. To assess the effectiveness of the conservation measures we had to confirm that terubok spawn only in the 2 known spawning areas. We did this by examining the elemental composition at the nuclei of otoliths of fish using the new technique of LA-ICPMS (laser oblation-inductively coupled plasma mass spectroraetry). Otoliths of fish collected from 7 coastal sites and both estuarine spawning areas in 1994 had similar elemental composition. This confirmed that all fish were probably spawned in 1 of the 2 estuaries. However, etoliths of fish collected from 2 of 8 coastal sites sampled in 1995 had significanfiy different elemental composition from the fish from the Lupar or Lasso estuaries. These sites had not been sampled in 1994 and were at the extremes of the species' known distribution. These results indicate that the species may spawn in 1 or more other estuaries outside the known distribution. The elemental composition at the nuclei of otoliths of fish from the 2 known spawning estuaries differed little in each of the 2 years of the study, except in 1994 when 138Ba concentration was higher in the Lasso. These results meant that the elemental composition of otolith nuclei could not be used to reliably estimate the relative contribution to the coastal fishery of fish from the 2 known spawning estuaries. However, elemental concentrations of 6 of the 7 elements examined differed between years for the 5 sites adequately sampled in both 1994 and 1995. These differences were not related to changes in the sex or age composition of the samples and this suggests that the water chemistry in both the spawning estuaries may have changed in a similar way during the study. Changes in otolith chemistry such as these could, in some instances, cause confusion in interpreting fish population structure from these data.