Catch, discard and bycatch rates in the Western Gulf of Carpentaria Mud Crab Fishery: summary for 2017 and 2018

Catch, discard and bycatch rates in wire mesh mud crab pots set in two regions of the Western Gulf of Carpentaria are summarised for 47 fishery observer trips undertaken aboard commercial crab fishing vessels in the months of April and May in 2017 and 2018. Catch rates of the Giant Mud Crab (Scylla serrata) in both regions and both years were relatively high (i.e. 0.74–1.02 kg/pot-lift), as were discard rates (i.e. 59–69% of individual Mud Crabs captured). Between 33% and 57% of all discarded crabs had attained the minimum legal size, but were released on the basis that they were ‘commercially unsuitable’, having a soft shell, low vigour and low meat content. A total of 88 individuals from 12 different taxa were recorded as bycatch from 4337 potlifts. Catfishes (Arius spp.) were by far the most numerous bycatch, accounting for 64% of all individuals. There were no interactions between mud crab pots and threatened, endangered or protected species during any observer trips. The low incidence of bycatch in wire mesh mud crab pots is consistent with previous observations on the use of this type of gear both locally and interstate. Introduction Commercial mud crab fishery licensees in the Northern Territory, Australia, use baited wire mesh pots (approximately 60 x 70 x 20 cm; L x W x H) to capture mud crabs (Scylla spp.) in mangrove and mud-flat habitats. One of the primary indicators of the success of crabbing operations (and mud crab abundance) is catch rate, typically expressed as the quantity of crabs retained (in kg) per unit of fishing effort (as pot-lifts). Controls on the commercial harvest of mud crabs in the Northern Territory include a Minimum Legal Size (MLS) for each sex (i.e. 140 mm carapace width [CW] for males and 150 mm CW for females) and a prohibition on the take of recently moulted “Commercially Unsuitable Crabs” that exceed the MLS, but have a soft shell, low vigour and low meat content; the latter being an industry initiative to reduce transport mortality and increase the market price. These regulations often mean that a large proportion of the mud crab catch (i.e. >70%; Ward et al. 2008) has to be returned to the water, with such individuals referred to here as ‘discards’. The discard rate can vary in both space and time, depending on the size structure of the local population and the timing of moulting events. The bait used in mud crab pots also attracts a range of scavengers and predators to the gear, some of which get trapped and become ‘bycatch’. There are relatively few accounts of bycatch in Australian mud crab fisheries, but those that are available (e.g. Butcher et al. 2012; Grubert & Lee 2013) suggest that bycatch in rigid wire mesh pots is considerably lower than that in collapsible trawl mesh pots, primarily because of the smaller mesh used in the latter gear type. Management of the Northern Territory Mud Crab Fishery underwent significant reform in 2017, with two discrete regional management units implemented – the Arafura-West Mud Crab Fishery, which encompasses the Northern Territory coastline anti-clockwise from Cape Grey (13°0’ S, 136°39’ E), and the Western Gulf of Carpentaria Mud Crab Fishery which constitutes the remainder of the Northern Territory coast to the Queensland border (Figure 1). The performance of these management units is assessed each year through an analysis of catch rates over the indicator months of April and May, derived from compulsory fishery logbooks. The catch rate during this period is considered a reliable indicator of that for the remainder of the year. One of three seasonal closures (beginning in October) may then be imposed if the April–May catch rate is particularly low. A fishery Figure 1. Extent of mud crab fishery management units and study regions. Western Gulf of Carpentaria mud crab fishery Northern Territory Naturalist (2019) 29 89