Abstract
As pressure on coastal marine resources is increasing globally, the need to quantitatively assess vulnerable fish stocks is crucial in order to avoid the ecological consequences of stock depletions. Species of Sciaenidae (croakers, drums) are important components of tropical and temperate fisheries and are especially vulnerable to exploitation. The black‐spotted croaker, Protonibea diacanthus, is the only large sciaenid in coastal waters of northern Australia where it is targeted by commercial, recreational and indigenous fishers due to its food value and predictable aggregating behaviour. Localized declines in the abundance of this species have been observed, highlighting the urgent requirement by managers for information on fine‐ and broad‐scale population connectivity. This study examined the population structure of P. diacanthus across north‐western Australia using three complementary methods: genetic variation in microsatellite markers, otolith elemental composition and parasite assemblage composition. The genetic analyses demonstrated that there were at least five genetically distinct populations across the study region, with gene flow most likely restricted by inshore biogeographic barriers such as the Dampier Peninsula. The otolith chemistry and parasite analyses also revealed strong spatial variation among locations within broad‐scale regions, suggesting fine‐scale location fidelity within the lifetimes of individual fish. The complementarity of the three techniques elucidated patterns of connectivity over a range of spatial and temporal scales. We conclude that fisheries stock assessments and management are required at fine scales (100 s of km) to account for the restricted exchange among populations (stocks) and to prevent localized extirpations of this species. Realistic management arrangements may involve the successive closure and opening of fishing areas to reduce fishing pressure.
Highlights
Nearshore coastal ecosystems provide an important source of food for human populations, supporting 90% of the global wild fish harvest whilst accounting for only 7% of the ocean worldwide (Pauly et al, 2002)
Management practices to support the sustainable harvest of P. diacanthus in Western Australia, Queensland and the Northern Territory are currently hindered by a lack of knowledge of the species’ population structure
The otolith and parasite data were broken down into three geographic regions for statistical analyses (i) because these ecological markers are thought to operate over smaller spatial and temporal scales compared to the genetics; they require a smaller scale of analysis and (ii) to test of the current jurisdictional management boundary between Western Australia and the Northern Territory and the regional management units across the Northern Territory
Summary
Nearshore coastal ecosystems provide an important source of food for human populations, supporting 90% of the global wild fish harvest whilst accounting for only 7% of the ocean worldwide (Pauly et al, 2002). Fishes of the family Sciaenidae, commonly known as croakers, are important components of commercial, recreational and indigenous fisheries in tropical and temperate regions worldwide (Lenanton & Potter, 1987). They are targeted for their flesh and, increasingly, for their swim bladders, which are sold fresh or dried in South-East Asia (Ghosh et al, 2009; Sadovy & Cheung, 2003; Tuuli, 2010). Management practices to support the sustainable harvest of P. diacanthus in Western Australia, Queensland and the Northern Territory are currently hindered by a lack of knowledge of the species’ population structure.
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