GENETIC PARAMETERS OF REPRODUCTIVE TRAITS FOR TILAPIA Oreochromis niloticus IN BRACKISH WATER AND FRESHWATER

Introduction

A selective breeding program for growth rate of tilapia in brackish water is conducted in Indonesia. While focusing on growth trait, the reproductive performance of tilapia in brackish water is also considered important. To avoid the potential threat to the saline aquatic ecosystem, reduced or even repressed reproductive performance of tilapia in brackish water is desired. However, studies comparing reproductive performance between environments and estimates of genetic parameters for reproduction traits are very limited in any aquaculture species. Objective of this study were to investigate: 1) the reproductive performance of tilapia in brackish water and freshwater and 2) the genetic parameters of tilapia reproduction traits.

Materials and methods

Offspring of 91 full sib families were produced and per family 20 fingerlings were randomly chosen for communally grow-out in a brackish water (20-25ppt) and another 20 in freshwater. After the grow-out period of 120-147 days, fish were harvested and 6 fish per family were recorded for gonad weight and maturation score in each environment. Gonads were weighed and  maturation stage was macroscopically determined according to Legendre & Ecoutin (1989). Gonadosomatic index was determined as GSI = gonad weight*100/BW. Gonad weight (GW) and GSI were analyzed separately for each sex. Genetic parameters were estimated from bivariate animal models in ASReml version 4.1 (Gilmour et al., 2015) accounting for fixed effects of pond and harvest weight. For genetic analysis maturation was reclassified as mature (1) or immature (0) (Legendre & Ecoutin, 1989) and analyzed for males and females together with sex as additional fixed effect.

Results

At harvest time, the mean weight was higher in brackish water (300.84±67.48) and significantly different (P<0.05) compared with the freshwater (238.13±56.65). Gonad weight for male and female and GSI for female in brackish water were higher and significantly different compared to freshwater (P<0.05). GSI of males showed no significant difference (P>0.05) between brackish water and freshwater (Table 1).

Most males were mature (~80% were stage 3) in both brackish water and freshwater (Table 2), while for the female, about half the fish were mature fish (stage 4-5) in both brackish water and freshwater. For males, heritabilities of GW and GSI were higher in freshwater, than brackish water and the genetic correlations were high. For females the opposite was found with higher heritabilities in brackish water and a moderate genetic correlation for GW (Table 3). The genetic correlation for GSI in females was high, but estimated with a very high standard error. Maturation was analyzed for both sexes combined and found to have a low heritability in brackish water and freshwater, 0.12 ±0.07 and 0.04 ± 0.07, respectively and a moderate genetic correlation, 0.47 ±0.74.

Discussion

The final weight in brackish water was higher (P<0.05) compared with the freshwater and can explain the higher GW and GSI in brackish water. Males showed higher maturation stages compared to females, both in brackish water and freshwater. This was expected from comparing them at the same age. Heritability of maturation traits in brackish water was higher for males and lower for females in comparison to freshwater. The heritability of maturation in brackish water (0.12) was in good agreement with an earlier report from Egypt, 0.13 (Charo-Karisa et al., 2007). Our estimate from freshwater was however much lower at 0.04±0.07. Heritabilities for maturation were lower than the result from Thoa et al. (2015) who reported heritabilities of 0.40±0.06 in saline water and 0.32±0.06 in freshwater. Genetic correlations estimates had high s.e. because of the limited size dataset.

Conclusions

All reproductive traits showed higher values in brackish water compared to freshwater.Tilapia is a species with a high reproductive rate and their maturation was not compromised by the brackish water environment. To reduce the potential threat from escapees to the brackish water ecosystem, selection for reduced maturation in brackish water environment is most promising for the female reproductive traits that have the higher heritabilities and lower genetic correlations with reproductive traits in freshwater.

References

Charo-Karisa, H., Bovenhuis, H., Rezk, M.A., Ponzoni, R.W., van Arendonk, J.A.M., Komen, H., 2007. Phenotypic and genetic parameters for body measurements, reproductive traits and gut length of Nile tilapia (Oreochromis niloticus) selected for growth in low-input earthen ponds. Aquaculture 273, 15–23.

Gilmour, A.R., Gogel, G.J., Cullis, B.R., Welham, S.J., Thompson, R., 2015. ASReml User Guide Release 4.1 Functional Specification.

Legendre, M., Ecoutin, J.M., 1989. Suitability of brackish water tilapia species from the Ivory Coast for lagoon aquaculture. 1. Reproduction. Aquatic Living Resources 2, 71–79.

Thoa, N., Ninh, N., Knibb, W., Nguyen, N., 2016. Does selection in a challenging environment produce Nile tilapia genotypes that can thrive in a range of production systems?. Scientific Report 6, 21486. https://doi.org/10.1038/srep21486