Hematological features of the European pilchard and round sardinella (Clupeiformes) from the Central-Eastern Atlantic

A comparative study was conducted of the hematological parameters in the peripheral blood of two abundant clupeid fish species from the Central-Eastern Atlantic: European pilchard (Sardina pilchardus) and the round sardinella (Sardinella aurita). These species are of significant commercial importance, yet they differ in their ecological characteristics. S. pilchardus is a temperate-water species associated with shelf waters and seasonal migrations, while S. aurita is more thermophilic, inhabiting warmer, more stable oceanic waters. A total of 18 specimens were sampled during a research survey conducted off the coasts of Morocco and Mauritania in 2017. The analysis of blood samples was conducted in accordance with standard methodologies, employing the staining of blood smears and subsequent cell differentiation. The results of the study demonstrated significant interspecific differences. The European pilchard exhibited a higher level of active erythropoiesis, as indicated by a significantly higher proportion of normoblasts (6.0±0.5% compared to 3.1±0.7% in the round sardinella). This adaptation is likely to be a response to seasonal shelf migrations, which require an efficient oxygen supply. In contrast, the leukogram of the round sardinella exhibited a considerably elevated count of band (3.9±0.3%) and segmented neutrophils (5.1±0.4%), indicating an augmented state of innate cellular immunity, potentially associated with its habitation in warmer waters characterised by a higher microbial load. The absence of eosinophils in both species indicates a low parasitic load in their habitat. The identified hematological differences are indicative of the ecological particularities and adaptive strategies of these species. The reference data on the cellular composition of blood that has been obtained provides a valuable basis for the bioindication of the physiological status of these crucial commercial species in a changing environment.