CHAPTER 1. INTRODUCTION. Along with growth and spawning season, year at sexual maturity counts as another

Transcription

1 CHAPTER 1. INTRODUCTION 1.1 Year at Sexual Maturity in Rainbow Trout Along with growth and spawning season, year at sexual maturity counts as another important trait in farming rainbow trout. Year at sexual maturity has become of great economic importance in the production of rainbow trout and salmon. Early maturation before reaching market size results in a decline of growth rate and feed efficiency as well as reduced flesh quality, and a consequent reduction in market value. The onset of sexual maturation and spawning activities depresses growth for a few weeks to a few months, however mature fish generally resume their normal growth rate. Still, the incidence of early maturity will be costly for farmers. The ability to control the timing of maturation would make it easier to produce a product of uniform quality and a more regular supply to the market. Studies have shown that year at sexual maturity and growth rate are negatively correlated and because of this relationship, it seems difficult to have selection for rapid growth rate and late sexual maturation simultaneously. Since these two traits are heritable, a strain with high growth rate and late maturation should be possible. It would seem that the selection strategy for year at sexual maturity should be to reduce the frequency of fish that become sexually mature before market size rather than to select for an increased year at sexual maturity.

2 1.2 Thesis Objectives Previous studies have compared the three pure strains of rainbow trout used in this study for various traits, including growth, time of spawning, survival and maturation. This study expands on these findings. Using data obtained from pure and crossed populations of rainbow trout held at the Alma Aquaculture Research Station, this study was initiated to investigate: 1. The effect of fertilization week within strains of dam 2. Strain comparisons A. pure strains B. pure & hybrid strains C. reciprocal hybrids 3.The effect of weight on sexual maturity 4. The effects of sex on year at sexual maturity 5. Genetic parameters. 6. The relationship between YSM and weight at different ages. 7. A breeding scheme to produce a strain with fast growth and late sexual maturation.

3 CHAPTER 2. LITERATURE REVIEW 2.1 Introduction Age at maturity has been defined in two ways and each of these definitions should be considered as for one trait. The first is sexual maturity status within a single spawning season (Gall & Huang 1988) and the second is year at sexual maturity without considering age (day) of the fish within years.the second definition, is considered in this study. Body weight and year at maturity are two related and important traits for economical fish production. Selection for increased body weight at a specific age is the most commonly used method for improving growth rate. However, several studies have shown that selection and environmental factors which increase fish size also act to decrease year at sexual maturity, which may not be suitable for fish producers (Crandell and Gall 1993, Herbinger and Friars 1991, Iwamoto, Alexander and Hershberger 1984). Fish that mature early are generally males (Gardner 1967, Naevdal 1983), comprise a small proportion of the population, and may die or have an increased mortality rate at maturity. The onset of sexual maturation disrupts growth of the maturing individual resulting in immature fish being larger in body size than mature fish by the time the spawning season is completed (Tveranger 1985). A combination of genetic and environmental factors may be involved in the incidence of early maturing males (precocious male), (Garrison 1971, Childs & Low 1972, Hager & Noble 1976 and Bilton 1978). Alm (1959) suggested that growth rate is a major factor affecting year at sexual maturity and both traits are under genetic control.

4 Gjedrem (1985) determined that there is a significant negative genetic correlation between age at sexual maturity and body weight just prior to maturity for rainbow trout. Tharp (1983) has shown that age at sexual maturity and growth rate are related and this relation is heritable, but selection for rapid growth rate and late sexual maturity may be incompatible objectives. 2.2 Year at Sexual Maturity Non-Genetic (Environmental) Factors Affecting Year At Sexual Maturity PHOTOPERIOD Several studies have confirmed that early maturing males are among the fastest growing individuals and one of the main environmental factors controlling the timing of production in the temperate zone is photoperiod. The seasonally changing photoperiod is the principal environmental cue for the timing of sexual maturation in salmonid (Bromage 1993). Scott & Sumpter (1983) observed that long day lengths are responsible for initiating the annual cycle of gonadal development. Satterlin (1976) has shown that spawning season of male brook trout, which is normally during fall can be changed to the spring, under long photoperiod condition. Based on a study of Skarphedinsson, Bye and Scot (1984), long photoperiods enhance early sexual development in male rainbow trout. This early maturation is apparently not a result of rapid growth.

5 TEMPERATURE Temperature may have effects similar to those of photoperiod, but it appears to be more important in determining spawning activity, gamete viability, and embryo survival. Thus, temperature is best thought of as a short-term environmental signal coordinating gamete release, fertilization and embryo-genesis. Temperature and photoperiod effects may interact, but at a given photoperiod, high and low temperature advance or delay spawning, respectively. Highly domesticated stocks reared under constant temperature generally spawn at 2 years of age with some stocks having a high proportion of males and a few females mature at 1 year. However some stocks reared in cold water usually spawn at later ages (3-4 yr). Iwamoto, Alexander & Horshberger, (1984) stated that among progeny from normal males reared at 15oC the ratio of precocious males to all progeny was 0.018:1 while the ratio was 0.014:1 for progeny reared at ambient temperature. STOCKING DENSITY AND OXYGEN LEVEL In addition to other non-genetic factors, rearing density and oxygen levels influence the incidence of maturation in male Atlantic salmon. In an experiment by John Berg and Sigholt (1996) the influences of different oxygen levels and rearing density have been investigated. Oxygen levels were low , intermediate and high mg/l. They found that highest incidence of maturation occurred among males at lowest rearing density at the intermediate and high oxygen levels. The lowest growth rate was seen at the lowest oxygen level and there was no correlation between the growth rate and sexual

6 maturation. The same experiment (John Berg and Sighot 1996) showed that sexual maturation among males can be depressed by high stocking density (Table 2.3.1) Genetic Factors Affecting Year at Sexual Maturity ADDITIVE AND NON-ADDITIVE GENETIC EFFECTS Both additive and non-additive genetic factors seem to play a role for mean year at first maturation; however for the proportion of fish maturing in their second year, only non-additive factors seem to be important. Donaldson (1959) altered the year of maturation of female rainbow trout by selective breeding, implying that additive genetic factors control this trait. A study of Atlantic salmon (Ritter & Newbould 1977) showed genetic factors to be important in determining the age at which the salmon are destined to mature. PATERNAL EFFECT Early maturing male parents have a big impact on incidence of early maturing male offspring. Five and a half times as many precocious males were present in the progeny of early maturing male parents than there were from normal male parents. Also, male: female ratios were slightly, but not significantly, different in the progeny from the two types of male parents (Ritter, Former, Misra, Goff, Baily and Baum 1986). A study by Glebe & Saunders (1986) has shown significant sire effects on precocious maturation of Coho salmon. However, there has been little work done examining the importance of the maternal genotype. Glebe (1979) hypothesized a strong

7 maternal effect on early growth and predicted a significant maternal influence on precocious maturity. PLOIDY In an experiment by Thorgard and Gall (1979), female triploid rainbow trout had string-like ovaries, while male fish had partially developed testes, (like normal males). Also, Lincoln and Scott (1983) have shown that ovarian, but not testicular development, will end at 5 months of age in triploid fish. To avoid early maturing males, it has been recommended that female triploids should be produced, but it should be noted that female triploid rainbow trout grow more slowly than male triploids. HERITABILITY ESTIMATES Heritability of age at sexual maturity of salmonids has been estimated (Crandell and Gall, 1993 and Gjerde and Gjedrem, 1984). Variation due to the different estimation methods has been shown. Heritability of age at sexual maturity in salmon has been estimated by Crandell and Gall (1993) to be Also, heritabilities of year at sexual maturity of salmon and rainbow trout have been estimated to be 0.39 and 0.21, respectively (Gjerde and Gjedrem 1984). It has also been shown that additive genetic factors play a key role (Gjerde and Gjedrem, 1984 and Gjerde, 1986) but the influence of non-additive genetic effects must be considered (Gjerde, Simianer and Refsite,1993).

8 CORRELATION Different studies have shown that genetic and phenotypic correlations between year at sexual maturity and body weight are generally negative. Gjerde, Siminaer and Refstie (1993) found that the correlations between body weights at different ages were general high, particularly the genetic correlation, but decreased as the time elapsed between the weight measurements increased. They also estimated genetic and phenotypic correlations for Atlantic salmon and found that genetic correlations of percent early maturity with weights until 16 months in sea water were all positive. Crandall and Gall (1993) estimated genetic correlations between age at sexual maturity (days) and body weight at maturity and body weight at specific ages for two consecutive spawning seasons. The genetic correlation between age at sexual maturity for the first and second season was 0.06 (Tables and 2.3.3). GENOTYPE AND ENVIRONMENTAL EFFECT In a study conducted by Alexander and Hershberger (1984) the relative effects of genotype and initial freshwater rearing on the incidence of sexual precocity in Coho salmon were examined. They showed that within a given temperature treatment (ambient), weight and length of normal-sired and precocious-sired progeny were not significantly different. However exposure to the 15oC rearing water, even for a short period, resulted in a significant difference in weight between the two treatment groups. Among the sources of variation, male parents had the largest effect on the incidence of male sexual maturity. The percentage of male progeny from precocious males was

9 53.7% vs 47.5% from normal male parents. Among the male progeny from normal males parents, reared at 15oC, the ratio of precocious to all male was :1, while the ratio was :1 for the one reared at ambient temperatures. Although the percentage of precocious male progeny was higher from precocious male parents, the same temperature relationship was observed.

10 2.3 TABLES Table Proportion of sexual maturation among males at different stocking density and oxygen levels Stocking density AOL AOL AOL % - 4.1% % 7.6% 20.6% AOL =average oxygen level Source: Berg and Sighot (1996) Table Estimated genetic and phenotypic correlations of body weight with early and normal sexual maturity for Atlantic salmon Months in seawater and trait Genetic correlation of early maturation r g Phenotypic correlation of early maturation r p Genetic correlation of normal maturation r g Phenotypic correlation of normal maturation r p Body weight ± ± ± ± ± ± ± ± Growth rate -4 to ± ± to ± ± to ± ± to ± Source: Gjerde, Simianer and Refstie (1993)

11 Table Estimated genetic and phenotypic correlations among age (days) and body weight at sexual maturity for the first (age one and WT. one) and second spawning seasons (age two and WT. two) and body weight at 180, 278, 544, 628, and 740 days for rainbow trout. TRAITS AGE ONE r r g p WT-ONE r r g p TRAITS AGE TWO r r g p WT-TWO r r AGE ONE AGE TWO WT WT WT WT WT TWO WT Source: Crandell and Gall (1993) g p