Rasbora tawarensis is an endemic freshwater fish in Lake Laut Tawar. It is one of the major freshwater fish, the main target for fishing in the lake and of considerable commercial importance in the region. It is listed in the IUCN red list as vulnerable [1] and updated by CBSG as critically endangered due to the very restricted area of its distribution [2]. Presently, the R. tawarensis population is decreasing, indicated by decreasing catch-per-unit effort (CPUE), where the average CPUE decreased from 1.17 kg/m² of net in the 1970s to only 0.02 kg/m² of net in 2009 (Muchlisin, unpublished data). Unfortunately, many basic life history characteristics of the R. tawarensis have not been documented.

Studies on reproductive behaviour of fish are important and a basic requirement for improvement and effective fishery resources management and conservation [3–7], determination of basic life-history information and for assessing the impacts of environmental variability on the dynamics of fish populations [8]. Life history parameters such as spawning frequency and sex ratio may vary between populations of a species and temporally within population [9]. Several studies describing various aspects of reproductive biology have been conducted for many freshwater tropical cyprinids for example the snakeskin gouramy, Trichogaster pectoralis [10], Thynnichthys thynoides [3], snakehead, Channa striata [11], rainbow selebensis, Telmatherina celebensis [12], bonti-bonti, Paratherina striata [13] and serandang, Channa pleurophthalmus [14] and However, to date there has been limited study done on R. tawarensis despite its enormous fishery and ecological importance. Hence, the objective of the present study was to evaluate the spawning periodicity and sex ratio of R. tawarensis.

The GSI values of R. tawarensis for both sexes with peaks in March, September and December, presumably the likely onset of the spawning seasons largely coincided with the rainy seasons. Our observation showed that the fish migrates from the lake to river tributaries for spawning, frequently during rainstorm in the rainy season. Its spawning season occurs twice during the rainy season i.e. early and at the end of the season. In addition during the transition period where the rain falls and the rainy days were relatively higher compared to the dry season, the fish is also triggered to spawn. According to Rainboth [20] the spawning activities of Southeast Asian cyprinids are accomplished in a variety of ways; longitudinal migration from downstream to up stream and vice versa or even laterally from the stream they inhabit into temporary flooded riparian areas or tributaries. Many studies have reported high correlation of rainy season with spawning peaks of tropical fishes associated with flooding of rivers and lakes, or the monsoons. For example T. thynnoides in the Chenderoh Reservoir, Malaysia spawned during the rainy season in January, August, and November when the water level was high [3]. The spawning season of African bonytongue fish in the So River in the floodplain of West Africa occurred during the wet season (May to August) as floodwaters gradually rose [21]. In addition, the reproduction in Tor putitora was observed mainly in the autumn months of March to April and also in the monsoon months, from July to August. During these months, T. putitora migrated from the main river to the tributaries where it bred in the flooded waters [22]. In general, fishes in the tropics depend on rainfall to trigger the reproductive cycle as the stable temperature and photoperiod could not generate reproductive cues [10].

Typically, fish migrate upstream to spawn when the water level increases during the rainy season, to ensure that the current brings eggs and larvae into nursery areas on the floodplain further downstream. During this season the fish feed intensively in the flood zone, growing and building up fat layers for the following dry season, when the food is scarce [23]. The periodic floods provide increased available habitat and also releases nutrients that evoke blooms of phytoplankton and an increase in micro zooplanktonic food organisms for the hatchling fishes [24]. The increased water level, inundation of shallow areas, increase in water velocity and turbidity may be responsible for inducing the fish to spawn [25] especially for fishes in the flood plain areas [26]. This is also the main feeding and growing period for many tropical fishes, when they build up fat stores to carry them through the dry season [24]. In addition, in tropical zones, seasonal changes of environment are less extreme, and many fishes exhibit extended or continuous reproductive pattern [27]. This is in agreement with the moderate levels of GSI in January and February as observed in this study. Thus these add further support that the rainy season or rainfall plays an important role in reproductive period of tropical fishes.

It is highly likely that R. tawarensis is capable of spawning throughout the year, as mature males and females were detected throughout the year, although more abundant during certain periods with a peak season in September. A similar phenomenon was observed in C. pleurophthalmus [14] and Astyanax fasciatus [28] which had the potency to spawn throughout the year, with spawning peak influenced by water temperature and rainfall. Based on gonadal development and variation of oocyte size in the ovary, R. tawarensis can be classified as a group synchronous spawner or a fractional multiple spawners, having two or more distinct clutches of oocyte existing concurrently with each clutch at a different developmental stage. According to Redding and Patino [27], this pattern allows for multiple, distinct ovulatory events that typically follow seasonal, lunar, or diurnal cycles.

Female GSI values of R. tawarensis were consistently higher than in male, a phenomenon also observed in the freshwater catfish Oxydoras sifontesi and Pimelodus blochii from Venezuelan floodplains [18] and A. fasciatus from south-eastern Brazil [28]. The GSI is one of the main parameters used to evaluate gonadal development in fishes and this method is easier and cheaper to utilize. The high correlation of GSI with number of matured females and males could be utilised to extrapolate peak spawning season. Furthermore, the GSI and length frequency distributions provide good population-level information of reproductive performance [29].

The sex ratio of R. tawarensis fluctuated seasonally. This is in agreement with Nikolsky [30] who reported that the sex ratio may vary from year to year in the same population, but in most fish species it is close to one, for example in the rainbow selebensis, T. celebensis [12], Protopterus annectens [31] and Oreochromis niloticus [32]. However, the sex ratio of R. tawarensis showed a predominance of female, a similar trend to that reported for Tilapia mariae [33], A. fasciatus [28] and Pellonula leonensis [34]. In contrast, in Abudefduf saxatilis [35] and T. putitora [22], the number of male was higher than female.

However, the reported sex ratios may have be biased due to selectivity of fishing gear, therefore independent data from other fishing gears would be required to validate whether the samples obtained from existing gears were representative of the population [7]. Furthermore, the seasonal variation in the sex ratio observed was probably because once fertilization of eggs was completed, male possibly emigrates from spawning area towards feeding ground located in the shallow areas [32].

Gonadosomatic indices (GSI) of female R. tawarensis were highest in the months of March, September and December with the peak in September, indicating the onset of the reproductive seasons. However, matured fishes were detected throughout the year. The R. tawarensis was classified as a group synchronous spawner. In addition, the female was predominant in the population.