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.

鱼的血液学研究可用于许多疾病的诊断以及对血液损害程度的调查。这些参数是检查各种鱼类的健康状况，生理和病理变化的有效且有洞察力的指标，即鱼体内的代谢紊乱，从而在环境毒理学，自然环境中缺乏和慢性应激等方面充当非特异性生物标志物。以及水产养殖系统和检查水质。对这些血液参数的评估有助于生物学家了解鱼类的体内稳态以及对营养，水质或疾病所致的严重和慢性病理生理变化的生物监测，从而确定不同物种的正常参考值并确定它们之间的系统关系。 。关于血液参数的各个方面及其与生态因素的关系的大量工作已经在世界不同地区的几种鱼类上进行了报道。每项研究都将一个或其他因素归因于不同鱼类之间血液学参数的变化。因此，在本研究中，已尝试汇编有关世界不同地区所报告的各种鱼类血液学研究的信息，并且通过该研究，已对造成该疾病的主要因素进行了概述。鱼血液学参数的变化。该评论提供了对血液学价值特征的深刻见解，表明鱼类内部环境的波动以及其他内在因素（年龄，体重，性成熟周期，