Implementation of a harvest slot for Murray Cod: Initial impacts on the recreational harvest in a manmade reservoir and comparison to riverine fisheries

Freshwater reservoirs support valuable commercial and recreational fisheries resources worldwide. Recreational fisheries in reservoirs are commonly managed using length-based harvest regulations, although empirical data is required to ensure these regulations are fit-for-purpose and that any changes do not result in negative population responses. Following a change from a 600 mm minimum legal length to a 550-750 mm harvest slot (HS) limit for both reservoir and river based recreational fisheries for Murray cod, Maccullochella peelii, we used a stratified random sampling design to assess the summer recreational fishery in Lake Mulwala, an important reservoir fishery. Specifically, we assessed the immediate effects of the HS on angler effort, catch and harvest in this fishery. Standardised parameters based on data from existing angler surveys were also used to investigate variability among fisheries for Murray cod. We found that (1) in Lake Mulwala, more than 50% of Murray cod harvested after the introduction of the HS were between 550 and 600 mm; (2) boat-based anglers almost exclusively targeted Murray cod and harvested more and larger fish than shore-based anglers; (3) the Murray cod population was severely truncated with high mandatory discard rates (93.7% for boat fisheries and 99.8% for shore fisheries) of fish below the lower HS limit; (4) standardised parameters varied among waterbodies, and comparatively higher effort, discard and harvest densities were observed in the reservoir fishery. The reliance on newly harvestable Murray cod in Lake Mulwala highlights the need for early and ongoing monitoring of regulatory changes across riverine and reservoir environments, encompassing both social and biological aspects of the fishery.