The use of acoustic accelerometer tags to determine seasonal changes in activity and catchability of a recreationally caught marine teleost

Understanding the way catchability of exploited fish species varies due to changes in individual fish behaviour is a seldom addressed but important requisite for extracting accurate information on fish abundance from catch per unit effort (cpue) data, particularly from baited gears. In this study, cpue data analysis was combined with analysis of movement and activity data from acoustic telemetry tags to determine the nature of seasonal changes in cpue of the recreationally caught sand flathead (Platycephalus bassensis) in a coastal bay and estuary in southeast Tasmania, Australia. Water temperature had a significant influence on cpue of sand flathead with lower catches at lower temperatures. Yet, even at the relatively small scale of this study (10 s of km), the mechanisms in which temperature affects cpue were highly area specific. In the shallow, estuarine part of the study area, changes in cpue were driven by changes in availability, due to seasonal movements of fish in and out of the area. In the deeper bay at the mouth of the estuary, on the other hand, changes in cpue with temperature were most likely driven by temperature-related changes in activity. At lower temperatures, fish were less active, indicating that fish will have a lower probability of encountering bait as well as lower feeding motivation due to lower metabolic debt. This shows the importance of the inclusion of an environmentally influenced catchability parameter in stock assessment models that utilize cpue data from baited gear types, which is often only done implicitly on a coarse temporal scale by accounting for changes in cpue with season. Our study furthermore highlights the usefulness of acoustic telemetry in a fisheries context beyond the basic study of fish movement, allowing the monitoring of activity levels of exploited fish in relation to environmental parameters in the field.