Many hormonally induced fish begin to spawn in the presence of active males after ovulation. In literature, this phenomenon is referred to either as “induced spawning” or “hormonal induced spawning”. Landinez  believe that hormonal induction followed by fish spawing occurring naturally in tanks without extrusion is a “semi-natural” spawning; however, there are problems in accepting this terminology. The term “induced spawning” is conducive to erroneous interpretations and may even be mistaken for spawning by extrusion because the extrusion process is also a form of spawning induced by a handler. However, the name “semi-natural spawning” seems confusing. Perhaps the most appropriate term is “natural spawning” because the act of spawning is only regulated by breeding. Saldaña and Ascon , working on tambaqui C. macropomum, observed high fertilization rates (above 90%) with the natural spawning technique. Similar results were found by Sirol, et al.  in P. mesopotamicus and by Varela, et al.  in R. quelen. In this study, the technique of natural spawning resulted in the spawning of more than 50% of females of different species, resulting in fertilization rates above 95%.

This value exceeds the general average fertilization rate of 54.3% obtained via extrusion. Fertilization rates differed by an even greater margin, with values of 7.5% for extrusion versus 86.1% for natural spawning. All values of fertilization rates via extrusion were lower than with natural spawn- ing. According Bromage, et al. , determining the exact time of ovulation to induce female egg production is very important to obtain high-quality eggs, which is fundamental to achieving high rates of subsequent fertilization. The appearance of female eggs before or after the moment of ovulation can result in low fertilization rates and low-quality larvae . The exact time of ovulation in females may vary according to the species, with the largest time intervals observed for cold water fish such as trout Oncorhynchus mykiss (4 – 6 days)  and Clupea harengus (14 days)  and smaller time intervals observed for subtropical and tropical fish as Carassius au- ratus (2 – 3 hours)  and Prochilodus platensis (1 hour) . Therefore, because the species evaluated in this study are neotropical,nft system the period in which the eggs are viable may be assumed to be relatively short. In natural spawning, individuals adjust and synchronize the release of gametes across a great range of maturity levels, thus ensuring a high fertilization rate. Conversely, with spawning by extrusion, the possibility of making mistakes when selecting the female to be extruded increases due to the short optimum maturation window.

This phenomenon is demonstrated in this study by the relatively low fertilization rates and the large variation in values. According to Bermudez, et al. , natural spawning creates the possibility that brood stocks may not be entirely prepared for spawning and that the female gamete may release without a corresponding release of the male semen, causing a subsequent loss of eggs. Another possibility is that females do not release their gametes because they are not ready. In this study, we observed successful spawns with high fertilization rates, which indicate that males responded to female spawning. However, it was found that some female piauçu (L. macrocephalus) and curimbatá (P. lineatus) did not spawn, indicating that their eggs were not ready. The absence of spawning in dourado (S. brasiliensis) in a natural system may be an indication that fish were not ready or that using carp pituitary extract in this system is not effective for this species. In this study, we observed higher average egg production with extrusion. However, no significant differences (P > 0.05) in egg production were found between treat- ments for the different species evaluated. Knowledge about aquaculture species of neotropical migratory fish is in its initial stages, and many studies are still needed to better understand the reproduction of these fish. The results of this study indicate that two investigated methodologies can be useful to obtain gametes from these species.Furthermore, the natural spawning method achieves a higher survival of brood stocks and high fertilization rates.