Climate change has revived interest in context-dependent species interactions as abiotic environments shift rapidly. An integrative approach, focusing on physiological mechanisms influencing environmental tolerance and performance, provides valuable insights into this research. Using the invasive mosquitofish (Gambusia holbrooki) and the endangered Spanish toothcarp (Aphanius iberus) as a case study, we explored salinity impact on metabolism and behaviour. While it is well-documented that salinity influences mosquitofish capacity, the metabolic mechanisms and their impact on behaviour remain unexplored. Our goal is to examine metabolic and behavioural responses in different salinity conditions (freshwater and 15 psu) and the correlation between these variables. Using intermittent-flow respirometry, we measured standard metabolic rates (SMR), maximum metabolic rates (MMR), and absolute aerobic scope (AAS). Behavioural variables, like the proportion of time out of cover, were derived from speed, acceleration, and heading data using Raspberry Pi cameras. Our findings show that, with rising salinity, mosquitofish experience decreased metabolic rates, while toothcarp's rates remain stable, suggesting lower salinity tolerance in mosquitofish. In terms of behaviour, mosquitofish exhibit higher activity and trajectory count, while toothcarps cover a larger area with increased speed variability. Correlations between metabolism and behaviour are species-specific, with toothcarps' MMR significantly correlating with overall behaviour, particularly time out of cover, and mosquitofish's SMR correlating with the total distance moved. These insights highlight challenges in mosquitofish metabolism with higher salinities compared to endemic species. Furthermore, it emphasises the importance of considering intricate physiological mechanisms and their interactions with behaviour in the context-dependent competition of this globally expanding species.