Abstract <p>This study investigates the prevalence of neoplasia – tumour in the Indian oil sardine (<i>Sardinella longiceps</i>) along the southeastern coast of India from July 2023 to June 2024. A total of 418,485 fish samples were collected from six major landing stations, including Royapuram, Pondicherry, Cuddalore, Nagapattinam, Mandapam, and Kanyakumari. Among these, 1,451 fish (0.34%) were found to have tumours, with the highest prevalence recorded at Royapuram (0.48%) and the lowest at Kanyakumari (0.13%). Tumour occurrence peaked during the monsoon season, with significant seasonal variation observed. Tumours displayed diverse characteristics, including differences in size, colour, consistency, and location, with the majority found on the head region. Neoplasms are being considered as biomarkers of environmental pollution and the present study highlights the increased prevalence of fish tumours that can be linked to environmental carcinogens, such as heavy metals, persistent organic pollutants, and industrial effluents present in the coastal waters, which may contribute to tumour development in these fish populations. This study highlights potential health risks to marine ecosystems, food safety, and public health, and calls for integrated approaches combining fish health surveillance with environmental contaminant monitoring to better understand and mitigate the causes of tumour development in coastal fish populations.</p> Research highlights <p>This study provides a comprehensive assessment of tumour occurrence in the Indian oil sardine (<i>Sardinella longiceps</i>) along the southeast coast of India, revealing clear spatial and seasonal patterns linked to coastal environmental conditions. Tumour incidence varied significantly among landing centres, indicating localized environmental stress and heterogeneity in coastal pollution exposure. A pronounced seasonal signal was observed, with higher tumour occurrence during the monsoon period, highlighting the influence of monsoon-driven land–sea interactions and contaminant runoff. The widespread occurrence of similar external tumours across individuals and locations also raises the possibility of transmissible or environmentally mediated tumour processes, warranting further investigation. Overall, the findings support the use of Sardinella longiceps as a sentinel species for assessing pollution-related stress and ecosystem health within tropical coastal Earth system dynamics.</p>