Traits For Aquaculture Research Articles

Selection for faster growth in the freshwater crayfish Cherax destructor

The freshwater yabby, Cherax destructor, is native to south-eastern Australia and has been the basis of a small aquaculture industry since the early 1970s. However, despite the long period of culture the industry has never matured into a large-scale aquaculture producer. One of the suggested reasons explaining why the industry has not developed as expected is that inferior animals are presently being farmed. As a consequence a program was started in 1998 aimed at increasing the productivity of yabby aquaculture through genetic improvement. This program was based on founder stock from two populations previously identified as having good traits for aquaculture and involved a within-family selection protocol, coupled with a circular mating strategy, to select for faster growth rate (i.e. weight at age). A control line was also maintained to estimate the rate of genetic response. After two generations of selection, males and females from the selected families were 29.5% and 32.7% heavier than controls, respectively. This represents an average genetic gain per generation of ∼15.5%. It is concluded that a within-family selection strategy is very effective in increasing growth rate in this species.

Effect of temperature on oocyte development of Oreochromis karongae (Trewavas, 1941)

Summary Oreochromis karongae (Trewavas, 1941) is one of the indigenous Tilapia species exhibiting favourable traits for aquaculture in Malawi. However, commercial fingerling production is still a problem. An experiment was carried out to investigate the effect of raising ambient temperature to 27°C on oocyte development of the fish. Female O. karongae were reared under two temperature regimes: at room (20.3 ± 0.8°C) and at raised (26.5 ± 0.5°C) temperature for 90 days. Changes in gonadosomatic index (GSI) and oocyte developmental stages were determined every 45 days. Fish samples from the pond (22.5 ± 3.4°C) from which experimental fish were collected were used for comparison. Results showed that raising temperature to 26.5 ± 0.5°C significantly enhanced oocyte development. Higher GSIs (P ≤ 0.05) were obtained after 45 and 90 days in fish cultured at elevated temperature (0.82 ± 0.66 and 1.13 ± 0.47%, respectively) than at room temperature (0.06 ± 0.03 and 0.37 ± 0.05 %, respectively). GSI of fish samples from the pond were not different from that of fish from room temperature. After 45 days, relative frequency of mature oocytes was higher (P ≤ 0.05) in fish from raised temperature (60.42 ± 3.63%) than in fish from room (1.76 ± 0.84%) and pond temperature (2.43 ± 1.38%). After 90 days, the frequency of mature oocytes in fish from raised temperature was not different from pond fish (8.68 ± 2.40 and 10.99 ± 3.41%, respectively). Fish from room temperature had a low (P ≤ 0.05) frequency of mature oocytes (3.12 ± 2.03%). The results suggest that O. karongae has the potential to spawn throughout the year when the temperature is manipulated.

Production of transgenic silver sea bream (Sparus sarba) by different gene transfer methods.

We have been interested in developing convenient mass gene transfer methods for producing strains of silver sea bream (Sparus sarba) with superior genetic traits for aquaculture. A transgene construct carrying rainbow trout growth hormone (rtGH) complementary DNA driven by a common carp b-actin promoter was introduced into silver sea bream by electroporating the sperm with the rtGH transgene and using the treated sperm to fertilize eggs stripped from mature females. The presence of the GH transgene in presumptive transgenic individuals was detected by polymerase chain reaction (PCR) analysis. Between 56% and 70% of the animals carried the GH transgene. We refer to this method as sperm-mediated gene transfer (SMGT). Since the handling stress of stripping gametes from female sliver sea bream brood fish could cause severe mortality, an alternative gene transfer method would be highly desirable. We developed a liposome-based method to transfer the GH transgene into the fish. This method, referred as testis-mediated gene transfer (TMGT), involves injecting the liposome-transgene mixture into the gonads of male sea bream at least 48 hours before spawning. The males were mated to reproductively active females, and fertilized eggs were collected for further incubation. Between 59% and 76% of the hatched fry were found by PCR analysis to carry the rtGH transgene. The efficiency of gene transfer was improved more than 80% by injecting multiple doses of the liposome-transgene mixture into the gonads of treated males. Results of Southern blot analysis of DNA isolated from PCR-positive animals showed that the transgene was integrated into the host genome and could be transmitted to its offspring. The rtGH transgene was expressed in many of the rtGH-transgenic fish. Several P1 GH-transgenic silver sea bream exhibited significant growth enhancement compared with nontransgenic controls. Our studies showed that faster-growing silver sea bream could be produced by a variety of mass gene transfer technologies. These gene transfer technologies would be of great value to aquaculture.

Genetic improvement of marronCherax tenuimanusSmith and yabbiesCheraxspp. in Western Australia

Marron and yabbies are farmed commercially in Western Australia and are native and introduced species respectively. Genetic improvement of farmed stock for aquaculture can be achieved by choosing particular strains, by selective breeding or by producing hybrids of different strains. Studies into the taxonomy and growth of both marron and yabbies have shown variation between geographically isolated populations. This variation may offer potential for selecting strains that demonstrate desirable traits for aquaculture, such as improved growth rates. In addition to selection of wild strains that demonstrate desirable traits for aquaculture, further advances in production may be achieved by selection of characteristics in captive stock that are heritable. In marron, moderate proportions of the phenotypic variation in growth rate, tail and chelae size have been shown to be heritable (h2 = 0.3–0.6). Recent research into yabby genetics involved assessing growth and reproductive characteristics of geographically isolated populations of yabbies inhabiting a range of ecotypes throughout Australia. Growth rates and size at sexual maturity varied among populations. However, there was no significant difference in the sex ratio between these geographically isolated groups. The main focus of the yabby genetics research has been to develop a method to prevent or reduce reproduction of yabbies. Uncontrolled reproduction in this species results in high densities and smaller animals because growth is density dependent. Preventing reproduction and thereby controlling densities in yabby ponds or dams can be achieved by stocking monosex populations. The stocking of monosex populations results in an estimated 70% increase in gross income. Hybidization of yabby populations was investigated as a method of controlling reproduction. Sterile hybrids were produced by crossing yabbies from geographically isolated populations. These experiments have also revealed a hybrid cross that consistently produces only male progeny. If this all-male hybrid strain performs as well as all-male monosex pond stock, achieved by manual sexing, the production of large, and therefore higher value, yabbies will be increased in the commercial aquaculture industry.

The genetics of age and weight at sexual maturity based on individually tagged rainbow trout ( Oncorhynchus mykiss)

Age and body weight at sexual maturity are important traits for aquaculture as is the relationship between the traits. The genetics of rainbow trout age at maturity (days), an indication of the time of spawning within a season, and body weight at maturity were investigated for two consecutive spawning seasons. The relationship of the maturity traits to each other and to body weight at specific ages was evaluated. During the first spawning season, data were obtained on early maturing males (E-males), mature at an average of 301 days post-fertilization. During the second spawning season, data were collected on females, males mature for the first time (T-males), and males mature for the second time (E-males), mature at an average of 697, 685, and 673 days, respectively. Estimation of genetic parameters was accomplised using an additive genetic and maternal mixed model. Heritability estimates for age at maturity were high, 0.86 and 0.60 for the first and second seasons, respectively. The heritability estimate for body weight at maturity for the first season was also high, 0.89, but was a moderate 0.27 for the second season. For both spawning seasons, there appeared to be little genetic relationship between age at maturity and body weight measured approximately 0 and 2 months before the first mature fish was found. However, the genetic correlation between age at maturity in the second season and body weight measured on the day the last mature fish was found (740 days) was 0.56. The genetic correlations between age at maturity and body weight at maturity within the same season were 0.67 and 0.58 for the first and second seasons, respectively. For both seasons, genetic correlations between body weight at maturity and body weight measured at other specific ages were fairly high, ranging from 0.50 to 0.99. The genetic correlation between body weight at maturity for the first and second seasons was 0.51. Selection to genetically improve body weight from 0 to 2 months before the spawning season should not affect the timing of spawning. However, selection to decrease the age at maturity and alter the spawning season may affect body weight at maturity and thus characteristics of the eggs.