Repeatability of behaviour, often referred to as animal personality, is the degree of behavioural variation between-individuals in a population. Behaviours are often plastic in response to environmental change. Personality is thought to constrain behavioural plasticity as individuals express behaviours within their personality type. To date, no field studies have focused on the environmental conditions that determine how personality is exhibited. Here we use biotelemetry derived activity data to assess environmental factors influencing the repeatability of activity patterns (i.e., behaviours) in two euryhaline free-ranging elasmobranchs with differing life histories; the largetooth sawfish (Pristis pristis) and bull shark (Carcharhinus leucas) in the Fitzroy River. Individuals showed consistent individual differences in their repeatability, supporting personality. In response to increasing environmental temperatures and metabolism across the study period, between-individual repeatability varied in sawfish, but not bull sharks. Repeatability increased across the study as sawfish altered their behaviours presumably to mitigate competition. Repeatability decreased across the study with respect to diurnal activity rhythms, indicating sawfish became less diurnal in their activity use. This study demonstrates that repeatability can be studied in the field using acoustic telemetry and this approach can be used to understand the effects of environmental change on specialisation in other aquatic ectotherms.