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The adaptability of fish species to the changing environment owes to their genetic diversity which been seriously affected not only through habitat degradation, but also through the use of inappropriate artificial reproduction and fish culture practices. The large-scale induced breeding operations by public and private hatcheries have raised concern about potential genetic degradation of cultured endemic and exotic species. Due to a lack of technical knowledge and awareness of the potential ecological consequences, widespread inbreeding depression, negative selection and genetic introgression by hazardous hybridizations has and continues to occur.
The fish Catla catla (catla) is one of the popular major carps in Pakistan, widely distributed in freshwater natural and aquaculture production systems. The mass production of fish seed through artificial breeding is being practiced mainly on the Indian major carps and some exotic species. The operation of hatcheries is driven primarily by profit, and as such, hatchery owners set their target on quantity of spawn rather than quality. They usually maintain a small number of relatively small-sized broodfish to keep the production cost at minimum which have resulted in fish seed with poor levels of survival, growth, resistance against disease etc. The poor performance of the hatchery strain could be explained by genetic erosion through inbreeding, negative selection, hybridization among major carp species (catla, rohu, and mrigal) insufficient effective breeding number in the hatcheries, or a combination of these factors.
Regular monitoring of genetic variation in hatchery stocks is necessary to determine whether breeding programs are causing genetic erosion by reducing genetic variability and improper hybridization and gene introgression. Molecular markers offer the most realistic method to assess the genetic status of the hatcheries against levels of genetic variation in wild populations. So the present project is designed to monitor the genetic health status of Catla catla strains from hatcheries and wild. To accomplish this, exhaustive sampling would be conducted collecting samples from all hatcheries and major natural populations throughout Punjab province. After isolation of genomic DNA of the tissue specimens, the recently developed catla-specific microsatellite primers would be employed to amplify the alleles of target microsatellite loci. The products amplified through PCR would be electophoretically resolved using agarose and polyacrylamide (PAGE) gel. The banding pattern on PAGE would be scored through gel documentation and the genomic data subjected to rigorous genetic analysis by various softwares would yield inferences in terms of comparative genetic diversity, level of inbreeding and differentiation among major hatchery and natural catla populations.
Using genetic markers, the reason for reduced vigor of catla hatchery stocks could be traced and the pure naturalized stock of catla could be identified for replenishing the hatchery brood stock. Incorporating the knowledge of fish population genetics is imperative to produce quality fish seed and to manage the natural fishery stock to a sustainable level. The findings of the proposed project would benefit all the fishery stakeholders and may provide a good impetus the research and development in the sector. The goal of successful future strategies for sustainable fisheries in Pakistan could only be achieved through research in the field employing modern but relevant molecular tools as exemplified by the same in various developed regions of the world.
Significance of the proposed research duly supported with review of literature and bibliography to indicate current trends in the proposed field of study
A brief account of work done in Pakistan/papers published thereof and relationship of the proposed research to the socio-economic development of the country.
The Indian major carp catla (Catla catla Hamilton 1822) is of high significance for aquaculture production in Pakistan, India, and Bangladesh. As a principal species, it contributed 469,070 tonnes to total aquaculture production of the world in 2002 (FAO, 2004). In Pakistan, it is among the most popular indigenous aquacultured species, because of its fast growth rate and good taste. The human population explosion led towards a high market demand of the fish resulting in overexploiting of natural stocks. The other main source is production through aquaculture at various scales. To replenish the overfished natural recourses and stock the fish in aquaculture production units, the seed of fish is produced mainly at private or public hatcheries, and fry are subsequently distributed. Hatcheries commonly maintain minimal broodstock populations, recruit successive generations of broodstock from within the ââ‚¬Ëœsystemââ‚¬â„¢ and spawn multiple species simultaneously in common tanks to minimize production costs.
With inbreeding, hybridisation and translocation widespread, concerns have also been raised over the potential impact of hatchery-produced Rohu seed on the genetic integrity of wild rohu populations. However, the extent of genetic differentiation among Rohu populations is largely unknown and so the significance of this issue cannot presently be assessed. The available data on Rohu population structure consists of two limited studies confined to Bangladesh and anecdotal reports from India. There is presently no data on population structure of wild and hactchary strain of Rohu in Pakistan. These objectives will be addressed through molecular genetic studies utilising nuclear DNA microsatellite markers.
The available evidences support a widely-held perception that genetic degradation of hatchery stocks is occurring, and that both inbreeding depression and inadvertent hybridization are common problems contributing to poor quality seed production. Eknath & Doyle (1990) expressed concerns that effective population size might be very low in hatchery-reared catla. Using demographic methods, they estimated the effective population (broodstock) to be remarkably lower than genetically recommended in hatcheries. More recently, studies employing allozymes (Simonsen et al., 2005) and microsatellite DNA markers (Alam & Islam, 2005) have revealed low levels of variation in catla from Bangladesh while the hatchery population exhibited a lower variation as compared to wild populations. Moreover. All together, these results indicate that low effective population sizes in hatchery leads to inbreeding and loss of genetic variation. Avoiding loss of genetic variation is an important goal in management and conservation of populations. First, it is essential to maintain heterozygosity; increased homozygosity, i.e. inbreeding, may decrease fitness of populations, so-called inbreeding depression, either through fixation of deleterious recessive alleles or due to absence of variation at loci showing overdominance (Hedrick & Kalinowski, 2000). Second, maintaining allelic variation is a prerequisite for maintaining evolutionary potential, both for enabling wild populations to adapt to changing environmental conditions and for ensuring that traits and properties of captive populations can be improved by selective breeding programmes (Frankham et al., 2002).
Microsatellites are extremely popular DNA markers marker being reliably used for genetic investigations of wide range of species. Over the past decade, microsatellite markers have been used extensively in fisheries research including population genetic structure of marine and freshwater fishes (Liu et al., 2003). Currently, these are the mainstay of modern population genetics and provide contemporary estimates of departure from panmixia, relatedness of individuals, demographic processes and phenomenon like inbreeding, gene flow, genetic drift and overall genetic health of a given population (Abbas et al., 2010).
The genetic variation detected in catla could provide a sound molecular genetic database for decision making over effective management strategies for this aquaculture species and also in evaluating the potential genetic repercussions caused by traditional hatchery operations for selective breeding. The objective of the study is to assess the level of genetic variation, and sort out the major determinants of existing genetic structure of catla and relatedness in the natural and the hatchery populations using microsatellite markers.