Theoretical Contributions to Understanding the Mechanisms of Thermal Tolerance in Tropical Aquaculture Fish Species Relevant to Climate Change Adaptation in Nigeria

Rising water temperatures in Nigerian fish ponds and natural water bodies driven by climate change represent an escalating threat to aquaculture productivity and wild fisheries, yet the physiological and molecular mechanisms of thermal tolerance in Nigeria's primary aquaculture species have not been theoretically examined or experimentally characterised. This study makes original theoretical and empirical contributions to understanding thermal tolerance mechanisms in Clarias gariepinus and Oreochromis niloticus through a multi-level investigation spanning behavioural thermoregulation, cardiovascular physiology, metabolic enzyme activity, and gene expression responses to thermal challenge. Critical thermal maximum and thermal preference experiments were conducted at the National Institute for Freshwater Fisheries Research using juveniles from four geographically distinct broodstock populations. RNA-sequencing of liver, gill, and muscle tissue from fish exposed to acute thermal stress (from 30 to 38 degrees Celsius over 4 hours) identified 2,847 differentially expressed genes in catfish and 1,934 in tilapia, with heat shock protein, antioxidant pathway, and hypoxia-inducible factor genes showing the most consistent upregulation across populations. An original theoretical model is proposed linking gill surface area, haemoglobin oxygen affinity, and cardiac output to aerobic thermal scope, predicting the population-specific critical thermal maximum as a function of measurable physiological parameters. Catfish showed significantly greater thermal tolerance (critical thermal maximum 40.2 degrees Celsius) than tilapia (38.7 degrees Celsius), with between-population variation of 1.3 degrees Celsius within each species. The study provides the mechanistic foundation for climate-adaptive aquaculture breed development in Nigeria.

Keywords: thermal tolerance, climate change, Clarias gariepinus, Oreochromis niloticus, gene expression