Introduction
Selective breeding is based on genetic improvement through cumulation of genetic gain over generations. So, it is required to shorten the generation interval (GI, the age of the fish when firstly used for artificial reproduction). In common carp reared outdoor under temperate climate, the GI for females is about 5 years. However, in tropical zones, common carp females can mature in 1-2 years. It is said that common carp females need to collect 10,000-12,500 °D (°D = the sum of the average daily temperatures of the water in which the fish live) of water temperatures over 15°C to reach adulthood (Horváth, 1985). It must be also remembered that potential breeding candidates of common carp should stay before own selection challenge under the pond conditions to avoid bias of selection process. In Amur mirror carp (AM) it was found that if selection for faster growth or the proportion of edible parts were done in two-year old fish, it would have similar effect as doing the selection at market size (Prchal et al., 2018).
We therefore studied whether the GI in AM can be shortened to three years using RAS.
Material and methods
Experimental fish came from a population established in 2017 by artificial spawning using a partial factorial design of 27 dams and 29 sires of AM. The progeny was kept communally in ponds under standard stocking densities and conditions (Prchal et al., 2018). In April 2019, the following groups of fish were formed:
From April 2019 to April 2020, ~ 6 females and 6 males from groups A and B were sampled monthly to monitor gonad development and condition. A sample of gonads was taken for histology. The gonadosomatic index (GSI) and the Fulton’s condition coefficient (FC) were calculated. Water temperatures in RAS and pond were recorded at hourly intervals. As monitoring showed that group A unlike group B had reached the readiness for artificial spawning, the spawning according to the standard methodology (Kocour et al., 2005) of group S was performed in May 2020 after back adaptation of fish to pond conditions from March 2020. For the comparison, fish of the same breed in age of 6 years (group C), i.e. common broodstock, were spawned together with group S. To confirm whether the fish left in the RAS until May (group A) are still ready to be spawned even if kept all the time at higher temperatures (18-21°C), 12 fish that visually looked ready to be spawned were also included in an artificial spawning a week after spawning of group S and C. Reproductive parameters were determined in females of groups S, C and A (Tab. 1). No exact parameters were recorded in males as they were almost mature already at the age of two years.
Results and Discussion
It was found that fish kept in RAS grew faster. Females of group A (2682 ± 791.6 g) were twice bigger in February 2020 than females of group B. In males the difference was also significant but a bit lower (2376 ± 772.0 g vs. 1307 ± 228.7 g). Also, FC was in both sexes higher in RAS. Gonad development started to be significantly different from October 2019 in females. The highest GSI value observed in group A was 13.3 ± 1.8 (February 2020) compare with 5.3 ± 2.5 in group B (December 2019). In group B, GSI increased significantly in April 2020 (8.4 ± 2.9) while in group A it decreased (9.7 ± 4.9). However, histologically the fish in group B were not fully mature unlike females of group A. In males, the GSI was almost identical in RAS and pond, and histologically males of both groups were mature. Most reproductive parameters of females in group C were better than in group S (Tab. 1). Still, the reproductive success (RS) of females out of group S was comparable with group C which is the most important for selection program. RS of group A was significantly lower. It shows that females after maturation shall be kept at temperatures below 15°C until preparation phase to artificial spawning. Fertilization and hatching rates in group S and C were comparable unlike group A. It was calculated that females of groups A and S collected 10,590 °D with daily average water temperatures over 15°C at the end of October 2019 and 12,900 °D in February 2020 while females of group B collected only 9,055 °D. Thus, it seems that information published by Horváth (1985) concerning °D needed for common carp females to reach the maturity is useful tool and that indoor RAS combined with pond culture are efficient in common carp for shortening of generation interval to three years.
Acknowledgements
This work was funded by project no. QK1910430 of NAAR (NAZV) of the Czech Republic, and project Biodiversity (CZ.02.1.01/0.0/0.0/16_025/0007370) under the Ministry of Education, Youth and Sports of the Czech Republic.
References
Horváth, L., 1985. Egg development (oogenesis) in the common carp (C. carpio L.) In: Muir J.F. and Roberts R.J. (Editors), Recent Advances in Aquaculture, Vol. II. Croom Helm. London and Westview Press, Boulder, CO, pp. 31-77.
Kocour, M., Gela, D., Rodina, M., Linhart, O., 2005. Testing of performance in common carp Cyprinus carpio L. under pond husbandery conditions I: top-crossing with Northern mirror carp. Aquaculture Research 36 (12): 1207-1215.
Prchal, M., Kause, A., Vandeputte, M., Gela, D., Allamelou, J.M., Girish, K., Bestin, A., Bugeon, J., Zhao, J., Kocour, M., 2018. The genetics of overwintering performance in two-year old common carp and its relation to performance until market size. PLoS ONE 13(1): e0191624.