Spawning behaviour between endemic delta smelt (Hypomesus transpacificus) and non-native Wakasagi (H. nipponensis) in the San Francisco estuary.

In the San Francisco Estuary, non-native Wakasagi Hypomesus nipponensis hybridize with endemic and endangered Delta Smelt H. transpacificus, and all wild-caught hybrids have a maternal Wakasagi parent. However, the mechanisms that facilitate or impede hybridization and explain sex-biased hybrid asymmetry between these two species remain unclear. We conducted a no-choice spawning experiment, in which wild-caught Wakasagi (WK) and cultured Delta Smelt (DS) spawned naturally in different species-sex combinations: five Delta Smelt females with five Wakasagi males (DS × WK), five Wakasagi females with five Delta Smelt males (WK × DS) and five Wakasagi females with five Wakasagi males (WK × WK). We compared these data to those from a previous study testing DS × DS using the same methods. We quantified the frequency of spawns and attempts, sex ratio of participants and diel timing of behaviours, as well as the fertilization success of resulting eggs. Wakasagi engaged in five times more spawns than Delta Smelt in same-species trials, and spawns were more frequent in WK × DS than in DS × WK trials. Whereas fish in WK × WK trials exhibited mostly diurnal spawns, those in all other crosses exhibited predominantly nocturnal spawns. Eggs resulting from WK × DS trials had lower fertilization success than those in all other crosses. These results did not clearly explain the hybrid asymmetry observed in nature. Nevertheless, our study demonstrated that: 1) Wakasagi exhibited a very high propensity to spawn, which may provide a reproductive advantage over Delta Smelt, 2) Wakasagi showed great plasticity in spawning behaviour, which may facilitate interspecific spawning and 3) all crosses produced fertilized eggs, demonstrating that there are no complete pre-zygotic barriers to hybridization. Ultimately, any level of Delta Smelt spawning activity that includes Wakasagi is wasted reproductive potential, and our study highlights the mechanisms by and degree to which this may occur.