Causes of genetic degradation in fisheries

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Fishery is the most important economic activity in villages and towns along the coast. It is the sole source of employment and income generation. According to the latest estimates, the total area covered by fish ponds is about 60 500 ha (Sindh, 49 170 ha; Punjab, 10 500 ha; NWFP, 560 ha; and the other provinces (Balochistan, Azad Jammu Kashmir and Northern Area), 240 ha). About 13,000 fish farms have so far been established across Pakistan, varying considerably in size. The fisheries sector contributes around 1 per cent to the GDP of Pakistan. The recent government data reported that Pakistans total fish production for 2006-07 was about 590,000 metric tons where 400,000 metric tons came from marine resources whereas 190,000 metric tons were produced from inland resources including aquaculture. There are 531 species of fish in Pakistan. About 233 of them are of fresh water. Mahseer is the national fish of Pakistan.

During the last several decades, anthropogenic activities such as environment deterioration, overexploitation, hydrological alterations, and inappropriate stocking practices have negatively influenced the fisheries resources at various levels. Genetic monitoring is imperative to understand captive breeding systems to detect the geographical structure of genetic diversity and to underpin the factors contributing to fitness of species. Humans-induced pollution, siltation due to deforestation, loss of nursing and breeding points, disturbance in the migration ways and careless capturing of brooders in breeding season, contribute to decline in wild fish populations. Overfishing is the main factor behind declining fish stocks and degraded fish habitats and threatened productivity of aquatic biodiversity hotspots, making them more vulnerable to climate change. Furthermore, global ecological changes, such as warming and shifts in precipitation, nitrogen deposition, and runoff patterns are superimposed upon these threats. Habitat alteration contributed to 71 per cent of extinctions, overfishing contributed to 29 per cent, and pollution contributed to 26 per cent. More than 70 per cent of the worlds fisheries are under the stress of significantly depleted, over exploited and the rest are fully exploited.

The predictable consequences of inadequately managed breeding programme are loss of diversity and outbreeding depression. Hatchery stocks of fish and captivity bred animals have association with depressed performance, survival and reproduction. Individual decreases the fitness due to these losses, during the expression of inbreeding depression. These losses additionally increase in the number of alleles within individuals and also decreasing the effective population size (Ne). Use of small number of brood fish generationafter generation may lead to reduce genetic variability and indiscriminant hybridization causes loss of genetic purity of thespecies.

To sustain the diversity of fish populations, it is compulsory to take into account their natural breeding systems. The factors that diminish the effective size of population size (Ne) are population history, high reproductive variation among individuals, sex-ratio biases, harem formation and inbreeding. Smaller populations are more vulnerable to increase selfing, random genetic drift and mating among related organisms. Moreover, the mating system may not only be affected by genetic exchange among populations but also between generations. Allelic differences among geographically isolated groups can identify prolific source populations and isolated sink populations and allow investigation of subpopulation relatedness and invasion pathways. Additionally, inferences based on traditional ecological studies and genetic approaches can be combined to inform predictive demographic models.

Dispersal of individuals may be conditioned by life-history and behavioural traits, as well as by geographical barriers imposed by the geomorphology of a given area. This is particularly relevant in freshwater fishes. However, geomorphology may not always remain stable throughout evolutionary time, and physical changes in thelandscape may markedly influence dispersal and thus gene flowrates, affecting the degree of among-population divergence. Genetic drift and reduced dispersal would cause a continuous loss of the heterozygosity within respective colonies and a simultaneous increase in their differences. The consequence of drift and dispersal along with the meta population events are responsible for the reduction of total diversity (HT). Frequent local extinction and recolonization actions would result in loss of difference within and between them; as a result these the total diversity approaches to zero.

High levels of management, husbandry and rigorous assays for testing culture performance are the major factors for the improvement of genetic work which requires very good facilities for the segregation of fish stock from each other and from wild fish stock. High fecundity and fastest generation times are the significant traits for the improvement in the fish genetics. Recognization and the administration of the genetic diversity is imperative within the species because the breeding and selection of desired traits can lead to enhance in the frequency of that attribute for the economic, social or cultural value within the particular species.

Adequate knowledge of basic genetics is helpful for the person who is dealing with improvement in fish genetics and has the ability to solve the complicated problems related to the conservation and effective management of the fishes. Genotype environment interaction is less complicated in warm blooded terrestrial livestock because homeostasis is highly developed as compared to fish genetic improvement, where it finds more complication. Without understanding the basis of inheritance of the performance traits for which improvement has been sought, some of the culturists have pursued sporadic and unplanned programs of individual selection and hybridization.

The writers are associated with the University of Agriculture, Faisalabad, Pakistan.


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