Fishing mortality can exert strong, directional selection on wild fish populations, with the potential to drive evolutionary change in growth, maturation, behaviour, and physiology. In aquatic systems, where harvest rates often exceed natural mortality, such selective pressures may alter trait distributions over relatively short timescales. In Lutruwita (Tasmania), southern sand flathead (Platycephalus bassensis) represent a valuable case study for examining physiological responses to exploitation. This ambush predator dominates recreational flathead catches in the state, with southern population exposed to intense fishing pressure and northern populations experiencing comparatively less exploitation. These regions show pronounced differences life history traits such as size at age and size at maturation, but the underlying mechanisms, whether environmental, genetic or anthropogenic remain unresolved.
This study investigates regional differences in digestive performance, energy allocation, and growth under controlled laboratory conditions to better understand the physiological pathways that may contribute to observed size disparities between fish from each region. Size matched sand flathead were collected via hook and line from a heavily exploited southern population (North-West Bay, n = 44) and from lightly fished northern populations (Flinders Island, Musselroe Bay, Eddystone Point, n = 44). Fish were maintained in common conditions and assessed for specific growth rate (SGR), feed conversion ratio (FCR), and postprandial metabolic oxygen consumption (specific dynamic action, SDA) using intermittent flow through respirometry. In vitro gut sac experiments were performed to quantify nutrient and ion fluxes across the intestinal epithelium (Na, Cl, K glucose and water) in fed and fasted states. Fish from the high fishing pressure region exhibited reduced growth rates, a trend toward lower feed efficiency, and a shorter SDA duration, indicating lower energetic investment in digestion. These findings suggest reduced digestive efficiency and altered energy allocation in fish from heavily exploited populations. Such patterns are consistent with the potential for long-term directional selection acting on physiological traits under size- and mortality-selective harvest regimes. While fishing pressure cannot be isolated from other regional factors, these results support the hypothesis that exploitation may drive shifts in metabolic and digestive traits with implications for individual performance, population productivity, and the recovery potential of fished stocks.