Evidence of spawning migration of protandrous longtail shad (Tenualosa macrura) in the Siak River estuarine, Indonesia

Abstract

Longtail shad (Tenualosa macrura), an endemic species of Siak River estuary, Riau is feared to be on the brink of extinction, driven by the apparent recruitment overfishing and degradation of its habitat over the past decades. Mature females were massively caught during their migration to the spawning area, specifically for their eggs which have high economic value for the export market. Despite the effort from the Indonesian government to control the level of exploitation through open-close season and area, the infringements keep happening partly due to weak supervision from law enforcers. Moreover, there was a strong indication that the current harvest control rule is not backed with robust scientific evidences, especially on where and when the spawning migration occurred. These uncertainties were bound to be answered through a hydro-acoustic study, using scientific echosounder SIMRAD EK-60, at a transducer frequency of 120 kHz, with a total area covered 173 nmi2 (590 km2) and Elementary Sampling Distance Unit (ESDU) 0.1 nmi. In addition, repetitive (following the moon phase) oceanographic surveys were conducted during 2015–2016 on four prominent locations with a total of 16 observed stations. The result revealed a strong indication of spawning migration between the new moon and full moon, from April to November with different intensities every month. Mature males (pias) and females (terubuk) were seen in the spawning area during the waxing crescent phase in April, and movement was detected from Malaka Strait to the Siak River estuary. Largest schooling spotted at a swimming layer of 5–20 m depth and highest spawning event suspected occurred close to the water's depths bottom of the river bed during May to June. In contrast, emigration reached its peak in September. The generalised additive model (GAM) showed a linear relationship between abundance and some oceanographic parameters, with temperature and phase of the moon being the most integral factors. The occurrence of a high salinity and pH at spawning areas during high tides in the new moon and full moon phases, could certainly be the driving force for spawning migration.