Abstract
We sampled the demersal fish community of the Bonney Canyon, South Australia at depths (100–1,500 m) and locations that are poorly known. Seventy-eight species of demersal fish were obtained from 12 depth-stratified trawls along, and to either side, of the central canyon axis. Distributional patterns in species richness and biomass were highly correlated. Three fish assemblage groupings, characterised by small suites of species with narrow depth distributions, were identified on the shelf, upper slope and mid slope. The assemblage groupings were largely explained by depth (ρw = 0.78). Compared to the depth gradient, canyon-related effects are weak or occur at spatial or temporal scales not sampled in this study. A conceptual physical model displayed features consistent with the depth zonational patterns in fish, and also indicated that canyon upwelling can occur. The depth zonation of the fish assemblage was associated with the depth distribution of water masses in the area. Notably, the mid-slope community (1,000 m) coincided with a layer of Antarctic Intermediate Water, the upper slope community (500 m) resided within the core of the Flinders Current, and the shelf community was located in a well-mixed layer of surface water (<450 m depth).
Highlights
Abrupt submarine topographies such as canyons, seamounts and shelf-breaks are becoming increasingly recognised as key hotspots of productivity in the oceans [1,2], often vitally important to sustaining fish production
We conclude that variables other than depth failed to provide an improved explanation for the biological pattern. This voyage was scheduled to coincide with the most favourable period for upwelling, and fortuitously our sampling coincided with one of the most significant upwelling events recorded off South Australia
Satellite sea surface temperature (SST) imagery of this event confirmed that the upwelling extended along most of the Bonney Coast, and at least for 50 km either side of the Bonney Canyon (Figure 8)
Summary
Abrupt submarine topographies such as canyons, seamounts and shelf-breaks are becoming increasingly recognised as key hotspots of productivity in the oceans [1,2], often vitally important to sustaining fish production. While smaller zooplankton and phytoplankton may be advected offshore by temporally variable flows generated around canyons, swimming and vertically migrating micronekton such as krill and mesopelagic fish can maintain position within canyons by behavioural interaction with the flow field [4,5,7]. These aggregations of micronekton are preyed upon by commercial species, such as Sebastes on the North American west coast [8], and provide a rich food source for cetaceans [9]. Similar processes may enhance fisheries around canyons off South Australia, but have yet to be investigated
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