Natural mating in Nile tilapia (Oreochromis niloticus L.) : implications for reproductive success, inbreeding and cannibalism

Abstract

Niletilapia ( Oreochromis niloticus L.) is one of the most important species among the commercially farmed tilapias. Both small-scale and commercial production of tilapia is rapidly expanding in many countries of the world because tilapias meet many of the desirable characteristics for an ideal species for aquaculture: fast growth, high acceptability to consumers and tolerance of a wide range of culture conditions. Fry production inNiletilapia, Oreochromis niloticus is typically achieved by mass-spawning of males and females stocked in large hapas, earthen ponds or tanks. Territorial behaviour and reproductive competition in such spawning setup together with the asynchronous spawning nature in tilapia could have serious implications for breeding plans that employ mass spawning for seed production. The aims of this thesis were to gain thorough understanding into the natural mating systems in O. niloticus ; the genetic consequences of the mating system (reproductive success, effective population size and inbreeding); as well as non-genetic variations (size heterogeneity and cannibalism) among fish as a result of asynchronous reproduction. In order to establish captive breeding programmes that maximize genetic gain without compromising the long-term genetic diversity of stocks, understanding of the natural mating systems is a crucial step. In Chapter 2 , we analyzed parentage and quantified male reproductive success of O. niloticus under commercial hatchery conditions in two breeding hapas each stocked with 12 males and 25 females by sampling offspring from individual mass-spawning events. Eleven highly polymorphic microsatellite markers were used to genetically characterize the mating systems in O. niloticus and quantify the reproductive success of individual males and females. In total, 760 offspring from 76 spawnings were included in the paternity analysis. Mating system inNiletilapia ranged from single pair to promiscuous mating. Multiplepaternitywas detected in 46% of the broods, with up to 4 males fertilizing a single clutch. Multiple maternity was also detected in over 8% of the clutches analyzed. There was a very high variance in male reproductive success, with one third of males siring more than 70% of the offspring. Male condition factor had a significant effect on reproductive success with better conditioned males siring a large proportion of offspring. The high reproductive variance resulted in a N e of around 16 for hapas A and B, and a low N e / N ratio, 0.43 and 0.45 for hapas A and B respectively. The rate of inbreeding, ∆F , for each hapa, was estimated to be around 3.0% per generation, which is about twice the inbreeding expected in an idealized population of the same census size. When designing of fish breeding programmes based on mass-spawning, the higher inbreeding and lower effective population size should be taken into consideration.In mating systems with skewed reproductive success, the effective population size ( N e ) of a population becomes smaller. As a result, small populations experience an increased degree of inbreeding ( ∆F ). Inbreeding depression in fish affects fitness related traits such as survival, growth, reproductive traits and other morphological traits. The effect of genetic depression on different traits in O. niloticus was investigated in Chapter 3 .Experimental fish were produced in a full-sib/half-sib mating design in which each sire was mated to two dams and each dam mated to only one sire.We mated 20 sires and 35 dams to produce35 full-sib familieswith expected inbreeding coefficients (F) of 0.0, 6.3, 9.4, 12.5 and 25%. The fry were produced and reared in hapas suspended in fertilized ponds until time of tagging. In total 1832 fish were stocked in two fertilized earthen ponds for a period of 8 months (including 3 months of over-wintering). At the end of the experiment 389 fish were harvested. Results show that level of inbreeding significantly affected early fry survival and body weight at stocking. Level of inbreeding did not have significant effect on both the pond survival and weight at harvest. Effects of pond, weight at stocking, sire and dam component significantly affected body weight at harvest and pond survival. Fluctuating asymmetry was significantly affected by the dam component and pond but not by the level of inbreeding. This suggests that FA was more influenced by environmental factors than by genetic factors. The results of this study emphasizes that inbreeding had a significant effect on survival and growth on early but not on later stage of development. The observed difference in effects of inbreeding on early and later life stages can be explained by strong natural selection on fitnessduring over-wintering.Inbred individuals that survive the selective pressure to adulthood might still suffer reduced adult survival and reproductive success. Chapter 4 investigated the effects of inbreeding on reproductive success inNiletilapia, O. niloticus under semi-natural conditions in breeding hapas. A total of 72 fish (27 males and 45 females) from nine full-sib families(8 fish/family)were divided into two and stocked in two hapas at a sex ratio of 1 and 3. Fish had inbreeding coefficient of F = 0.0, 6.3, 9.4 and 25%. Fry were collected from the mouth of incubating females on a weekly basis and parentage assignment was done on 1120 offspring from 56 spawnings using seven polymorphic microsatellite markers. Female reproductive output (fecundity), expressed as the total number of eggs per spawning, was significantly affected by the level of inbreeding and body weight. Per 10% increase of the level of inbreeding, egg number declined by 11% of the mean. Male reproductive success, calculated as the proportion of offspring sired per spawning, was affected by the level of inbreeding, condition factor, sex ratio and gonad weight of males. Per 10% increase in the level of inbreeding, male reproductive success declined by 40%. The decline in reproductive success was higher in sex ratio 1 compared to sex ratio 3 indicating that the inbreeding depression was magnified under higher male-male competition. Degree of relatedness between mating pairs (Kinship coefficient), however, did not have any effect on reproductive success of males. Overall, our findings show that inbred individuals have lower reproductive success and uncover a natural selective pressure that favors outbred males and that counterbalances accumulation of inbreeding.  Asynchronous nature of reproductive cycles of individual broodstock in O. niloticus leads to episodic fry production. This presents a problem to aquaculture in two ways: 1) massive fry losses due to size-dependent cannibalism; 2) mixed age/size populations and associated cannibalism can mask the genetic merit of individuals. Fish breeders practising mass selection might inadvertently select for cannibalism as large size, faster growth rate and robustness are those phenotypic traits which a breeder desires in the selected broodstock. Selection under such circumstances can mask genetic merit and obscure selection response in mass selection programmes. In order to develop a protocol for size grading and minimize such effects, factors influencing size-dependent cannibalism inNiletilapia,Oreochromis niloticus, were investigated in controlled conditions ( Chapter5 ). First, individual-based trials were conducted to develop a linear regression model to predict the occurrence of cannibalism based on body measurements. Oral gape (G, mm), body depth (D, mm) and weight (W, g) of 140 fish were measured and we estimated maximum prey weight (Wprey) for a given predator weight (Wpredator): Log10Wprey = 1.03Log10Wpredator−1.13. This model was verified by conducting 76 single pair trials. The revised model based on observed cannibalism is: Log10Wprey= Log10Wpredator−1.17. Several experiments involving a group of prey and predators were conducted to elucidate the relative importance of factors that influence cannibalism. Cannibalism was significantly affected by stocking density, age of fish and predator/prey weight ratio. Logit models of mortality data of both the individual-based and observations based on a group of predators and prey estimatedR50(the predator/prey weight ratio at which an encounter of a predator and prey resulted in a predation success of 50%) to be 14.6 and 14.8, respectively. The results obtained in this experiment can be used to derive simple management of heterogeneous fry groups (e.g. grading of fish). Grading will have implications for reducing fry losses, minimizing the non-genetic variations caused by size dependent cannibalism among fish and can improve the efficiency of mass selection.The results described in this thesis are discussed in Chapter 6. This study demonstrated that a thorough understanding of mating systems and reproduction parameters are crucial foroutlining strategies that ensure the maintenance of genetic diversity inbreeding schemesemploying mass spawning for fry production. With the knowledge of mating systems and the number of parents contributing to the next generation, mass selection based on mass spawning can be used as a model for low cost breeding schemes that ensure a desired level of genetic gain without compromising the genetic diversity of the stocks due to inbreeding. In addition, knowledge on mating systems and other population parameters can be used in the management of supportive breeding programmes for restoration of endangered wild fish populations