VPP 322: Aquatic Animal Diseases, Health Care and Management (1+1)

POPULATION DYNAMICS AND FISHERY CATCHES

• To fishery biologist what is more important to know is whether a population is changing, increasing or regressing, rather than to know its actual size at any particular time. Thus indices of relative abundance are more useful than actual abundance. These indices are time relatives for example the number of fishes caught per day, or the percentage of sample plots occupied by a species of fish. Catch per unit effort reflects best the fluctuations.
• Three types of fluctuations in catches are recognized: (i) irregular, (ii) short range and (iii) long range.
  • The irregular fluctuations is one which occurs from year to year. It is caused by the effect of occurs over a period of a few year. It is caused by the effect of weather upon the fishing activity.
  • The short range fluctuation then declining for a similar period. This type arises due to a shift in the age composition or a variation in the size of the individuals indiscriminate catches (with regard to year class). Distribution and feeding conditions available for the younger stages (eggs, fry etc.). Presence of fish enemies, competitors for food,outbreak of epidemic diseases and parasitic infection among fish or its food organisms.
  • Long range fluctuations occur over long periods of several years. Showing changes from good to poor catches. Climatic changes are though to be the chief reasons.. Prior to this, the fishery gave excellent catches in succession for a similar period.
  • Cyclic fluctuations seen in some fisheries like the fishery of the Indian sardines and that of north sea hearing are found to be directly influenced by the cyclic fluctuation in the availability of particular food organism such as the diatoms and copepods respectively.