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Spatial management of reef fisheries and ecosystems: understanding the importance of movement
In the Northern Territory, the primary outcome was an improved understanding of the temporal and spatial dynamics of Protonibea diacanthus aggregations. Most importantly, the study demonstrated that the aggregations are likely to be separate adult populations. This has significant implications for stock assessment and management of the resource as such populations are likely to be highly vulnerable to localised depletion.
In Tasmania, the primary outcome was an improved understanding of the temporal and spatial movement patterns of Cheilodactylus spectabilis and Latridopsis forsteri. C spectabilis only moved to depth during the spawning season, suggesting that the deep water stocks that fishers believe act as a refuge population are in fact temporary residents during the spawning season, and the fishery may in fact target a major component of the stock. The result of C. spectabilis being highly site attached and occupying very small core areas of reef suggests that fishing has the potential to cause localised and serial depletion of this species. Despite being a mobile species, some L. forsteri individuals were site attached, suggesting that closed areas may be of some benefit for the sustainable management of this species.
Movement information is essential to understanding many aspects of exploited populations such as replenishment of fished reefs (from local, adjacent or deep reefs), aggregation behaviour, critical habitat requirements and cyclic variation in catch rates.It is particularly important to the understanding and application of spatial fisheries management techniques, including MPAs.
This project focused on species of commercial interest in both Tasmania and the Northern Territory in order to evaluate the effectiveness of spatial management for fishes with a range of life histories.
The Northern Territory component focused on the black jewfish Protonibea diacanthus, a large tropical sciaenid that forms a substantial component of both commercial and recreational inshore fisheries in the region. Fishers target spatially and temporally predictable aggregations, and catches have increased substantially over the past decade.
Repeated fishing of aggregations is not considered to be sustainable, particularly given the decline of P. diacanthus at other localities in Australia and the Asian-Pacific region. Acoustically tagged fish were monitored at two of the three known major aggregation sites, Channel Point and Chambers Bay, to determine whether the aggregations were separate populations and whether management measures such as spatial and temporal closures might be effective.
No evidence of movements between aggregations within the time frame of the study (~1 year) was observed although there was evidence for different behavioural types in the aggregations, with ‘movers’, which were highly mobile and ‘stayers’, which were site attached to varying degrees. Fish monitored for ≥ 1 year showed a decreased presence during cooler months, and an increased presence during warmer months, when peak spawning occurs. The tidal cycle significantly influenced the detection of tagged fish, with detections peaking on running tides. This coincided with the peak period for catching P. diacanthus, evidence that suggests this is when they are most active.The existence of separate adult populations at each aggregation site has significant implications for assessment and management of the P. diacanthus resource in the NT, with the potential for each population to be highly vulnerable to localised depletion.
Area closures during the peak summer spawning period may be a practical way to manage the resource, and would protect fish moving in and out of the aggregation sites to spawn. However, given P. diacanthus appears to form resident spawning aggregations, with fish present and caught at the sites year round, the fish would remain highly vulnerable during other periods of the year, potentially negating, or at least reducing the benefits of seasonal closures. As such, other management measures may need to be looked at in combination with seasonal closures, such as reducing catches in the different sectors of the fishery.
The Tasmanian component focused on two key large temperate reef species found in inshore south-eastern Australian and New Zealand waters, banded morwong Cheilodactylus spectabilis and bastard trumpeter, Latridopsis forsteri. Banded morwong are commercially gill netted in Tasmania and Victoria, and sold live for the Asian restaurant market. In Tasmania the biomass has been significantly fished down and as a result the population is mostly younger fish that are growing faster and maturing earlier.
The fish down of the biomass has been compensated to some extent by increasing productivity and recruitment, which means that the stocks have become increasingly reliant on recruitment events. As it is a live-fish fishery, there is very little fishing at depths greater than 30 m, to avoid barotrauma. Fishers believe that stocks in deep water habitats may buffer against overall stock decline, acting as a ‘refuge’ population.
Bastard trumpeter is a coastal schooling fish that resides on inshore reefs as juveniles, moving offshore after maturing. The fishery exhibits strong recruitment variability, is targeted by both commercial and recreational fishers and is based almost entirely on juveniles. All make them vulnerable to overexploitation.
Large- and fine-scale acoustic monitoring was used to examine the temporal/spatial movement patterns of both species on rocky reefs on the Tasman Peninsula. Banded morwong were highly resident, occupying very small core areas of reef which were maintained over the study period. Bastard trumpeters were more mobile, but a third of the monitored individuals were site attached at the scale of the detection range of a single receiver (~200 m). Both species were not detected on receivers separated by large areas of sand (embayments and offshore reef), suggesting that these act as natural barriers to movement.
Both species demonstrated clear diurnal activity patterns. Banded morwong fitted with depth tags moved to depths > 20 m and up to 45 m each morning, returning to depths <20 m in the afternoon. This movement was only observed during the spawning season which suggests that the so called deep water stock that fishers believe acts as a refuge is more likely to be temporary residents during the spawning season. If this is the case the fishery is probably targeting a major component or all of the stock. Combined with the observation that they are highly site attached and occupying core areas as small as 175 m2, fishing has the potential to cause localised depletion with the additional threat of serial depletion. Given that morwong populations are partially structured by size it also supports the hypothesis that the removal of biomass has led to reduced competition for space, with smaller fish replacing larger fish removed by fishing.Spatial protection, even at a small scale (< 1 km2), is likely to provide protection to morwong because they are highly site attached and are shown to share small patches of reef. Trumpeter, although relatively more mobile, are also likely to benefit from spatial protection because some individuals were shown to be site attached and were not detected moving across sand boundaries between reefs.
PublisherUniversity of Tasmania