The influence of the endogenous and exogenous factors on hematological parameters in different fish species: a review

Hematological studies of fishes are useful in the diagnosis of many diseases as well as investigation of the extent of damage to the blood. These parameters act as efficient and insightful index to examine health status, physiological and pathological changes occurring in various fishes, i.e., metabolic perturbations in fish body thereby acting as non-specific biomarkers in the field of environmental toxicology, deficiencies and chronic stress in natural as well as aquaculture systems and checking water quality. The assessments of these blood parameters help the biologists to understand the fish homeostasis and biomonitoring of severe and chronic patho-physiological changes inferable to nutrition, water quality, or disease so as to establish the normal reference values of different species and determine systematic relationships among them. Lot of work on various aspects of hematological parameters and their relation with ecological factors has been reported on several fish species from different parts of the world. Each study attributes one or other factors responsible for variation in hematological parameters among different fish species. Therefore, in the present study, an attempt has been made to compile the information about the hematological studies of various fish species reported from different parts of the world, and through this study, a general overview has been generated about the major factors responsible for the variation in hematological parameters of fish. The review provides an insight to the characteristics of hematological values showing that fluctuating internal environment of the fish, along with other intrinsic (age, body size, the cycle of sexual maturity, health condition, nutritional state, species) and extrinsic factors (temperature, stress, season, dissolved oxygen, water quality, lotic or lentic environment, stocking density, photoperiod, sampling conditions, laboratory techniques), are the reasons of huge variability of hematological parameters in fishes.