## 1 Introduction  
Pesticides, otherwise referred to as plant protection products (PPPs), by deliberate use and applications in pest control and crop protection have recorded elevated occurrences in the environment [1, 2, 3]. The rapid expansion of various types of integrated farming systems for better income and livelihood of farmers is also gaining importance across several developing climes [4]. Though integrated systems require fewer pesticides than the conventional ones, they are used lavishly in co-culture farming techniques [5], the eventual contamination of aquatic ecosystems with PPPs either due to spray-drift, leaching, runoff, and/or accidental spills and from aquacultural applications [6; 7; 8] could culminate into risks of debilitating effects and mass mortalities of non-target species [9; 10; 11; 12; 13; 14; 15]. In addition, risks of sustained toxicity to local biota due to their persistence and high retention within environmental matrices is also a concern [16; 17; 18; 19].  
Diazinon (dimplyate) is an organo-thiophosphate derivative used widely as an agricultural and household insecticide. Although it is inactivated by photochemical oxidation and expected not to be persistent, it has an estimated moderate to high toxicity to freshwater fish, estuarine, and marine fish and potential high toxicity to birds [20]. Its documented high toxicity to bees and other beneficial insects [20] portends risks of ecological effects, including invertebrate taxa loss and trophic cascades [21], which could also impact the survival of local fish populations. Significant risks of cancer, lung lesions, and cytogenetic effects in humans under chronic or prolonged exposures to diazinon have also been demonstrated [22; 23]. Model refinement and validation for diazinon PBPK/PD (physiologically based pharmacokinetic/pharmacodynamic) models using a series of in vivo pharmacokinetic and pharmacodynamic studies in the rat showed fairly rapid oral absorption followed by metabolism and distribution of the active oxon metabolites [24]. The toxic potential of this pesticide has been demonstrated using a number of biomarkers, including hematological changes (blood glucose, lipid, and serum enzyme profiles) and cross taxa histopathology [17; 25; 26; 27; 28; 29; 30]. In vitro studies on cell lines suggest that it causes oxidative stress through free-radical generation and promotes DNA fragmentation [31; 32]. Thyroid disruption has been implicated in organophosphate exposures [33], while growth inhibition due to diazinon exposures has been reported [34].  
Following reports of pesticides impacting and shaping the occurrence of global freshwater biodiversity, various criteria based on ecologically relevant risk estimates have been explored to protect aquatic life from pesticide stress [9; 14; 29; 35; 36; 37; 38; 39; 40]. Furthermore, the use of integrated biomarker endpoints in evaluating and visualizing chemical exposure effects on aquatic organisms is still not widely applied [41]. The present study sought to assess the chronic toxic impact of diazinon on hematological indices (hemoglobin, whole erythrocyte count, whole leucocyte count, mean corpuscular hemoglobin), morphometric indices (condition index, liver-body index, specific growth rate), endocrine indices (somatotrophic hormone, liothyronine, thyroxine) and biochemical indices (whole serum protein, whole serum glucose) of air-breathing catfish, _Clarias batrachus_. In addition, the gross effect of diazinon on fish using IBR (integrated biomarker response) as a summarizing index was also evaluated.