Chub mackerel (Scomber japonicus Houttuyn, 1782) were obtained from the commercial fishermen in Çanakkale, Turkey, in July, 2017. We examined a total of 40 fish (20 non-infested and 20 infested) and assessed the biometric indices, haematological parameters and serum biochemical variables. The hepatosomatic index and gonadosomatic index of infested chub mackerel fish were lower than those of non-infested fish. Blood haematocrit ratio and haemoglobin concentration in naturally parasite-infested chub mackerel fish were significantly lower than those in non-infested fish. However, white blood cell counts of the parasite-infested chub mackerel fish were higher than those of healthy ones. Serum total protein, globulin, glucose, cholesterol, triglyceride, urea, chlorine and iron levels in naturally parasite-infested chub mackerel fish were significantly lower than those in non-infested fish. Moreover, serum lactate dehydrogenase, aspartate aminotransferase, alkaline phosphatase and alanine aminotransferase activities of the parasite-infested chub mackerel fish were higher than those in healthy ones. Therefore, observed variations in haematological parameters, serum biochemical variables and biometric indices influenced by the parasite, A. pegreffii, may potentially increase sensitivity of the chub mackerel, Scomber japonicus, to diseases and environmental conditions.