Mervi Kangas

Management adaptation of invertebrate fisheries to an extreme marine heat wave event at a global warming hot spot

Abstract An extreme marine heat wave which affected 2000 km of the midwest coast of Australia occurred in the 2010/11 austral summer, with sea‐surface temperature (SST) anomalies of 2–5°C above normal climatology. The heat wave was influenced by a strong Leeuwin Current during an extreme La Niña event at a global warming hot spot in the Indian Ocean. This event had a significant effect on the marine ecosystem with changes to seagrass/algae and coral habitats, as well as fish kills and southern extension of the range of some tropical species. The effect has been exacerbated by above‐average SST in the following two summers, 2011/12 and 2012/13. This study examined the major impact the event had on invertebrate fisheries and the management adaption applied. A 99% mortality of Roei abalone (Haliotis roei) and major reductions in recruitment of scallops (Amusium balloti), king (Penaeus latisulcatus) and tiger (P. esculentus) prawns, and blue swimmer crabs were detected with management adapting with effort reductions or spatial/temporal closures to protect the spawning stock and restocking being evaluated. This study illustrates that fisheries management under extreme temperature events requires an early identification of temperature hot spots, early detection of abundance changes (preferably using pre‐recruit surveys), and flexible harvest strategies which allow a quick response to minimize the effect of heavy fishing on poor recruitment to enable protection of the spawning stock. This has required researchers, managers, and industry to adapt to fish stocks affected by an extreme environmental event that may become more frequent due to climate change.

Catch Predictions in Stock Assessment and Management of Invertebrate Fisheries Using Pre-Recruit Abundance—Case Studies from Western Australia

Catch predictions based on pre-recruit abundance have proven valuable in the stock assessment and management of major invertebrate fisheries in Western Australia for western rock lobster (Panulirus cygnus), pearl oyster (Pinctada maxima), abalone (Haliotis roei), prawn (Penaeus esculentus), scallop (Amusium balloti), and blue swimmer crab (Portunus armatus). Predictions are based on puerulus stage (post-larval) rock lobster from artificial collectors; 0+ and 1+ pearl oyster spat attached to commercial shell; dive survey of Roes abalone; and trawl surveys of prawn (shrimp), scallop, and crab. These reliable predictions (R2 = 0.67–0.97) are used in stock assessment and management harvest strategies to ensure that fishing effort or catch quotas are set so that adequate breeding stocks are maintained. This pro-active management based on predicted abundance avoids the pitfall of heavy fishing on poor recruit classes, which is a common cause of recruitment overfishing. The catch-pre-recruit relationship provides valuable information on density-dependent effects between the two life-history stages, with high levels evident in rock lobster and little evidence for prawns and crabs, which reflects the stage at which pre-recruits are measured. The review emphasizes the advantages of catch predictions based on pre-recruit abundance relative to environmental variables and the need for cost-effective pre-recruit monitoring to ensure long-term data. The pre-recruit abundance is valuable in investigating factors affecting year-class strength, including environmental factors and overfishing. The pre-recruit abundance is probably the most valuable information under climate change situations as it provides firm evidence of a change in abundance to justify management action before the change is reflected in the fishery.

Variability in the spatial and temporal distribution of the saucer scallop, Amusium balloti, in Shark Bay – management implications

The present paper is the first description using geostatistical modelling of recruitment (0+) and residual (1+) scallop variability from an annual survey in a semi-tropical embayment. Geostatistical modelling provides a useful tool to explore and interpret distribution patterns and can provide information to determine potential behaviour of fishers. It may also aid in determining the time it will take from the beginning of the season to reach a catch-rate threshold, which is the management strategy implemented in the Shark Bay scallop fishery since 2004. High variability in recruit abundance and spatial distribution was observed among years, whereas patterns of residual abundance and distribution were less variable because of the fishing patterns of both the scallop and prawn fleets. Comparisons of commercial catch patterns indicated that high survey-abundance areas correlate with higher catches, validating that survey results are a good tool for fishers to utilise to target their fishing practices to optimise and maximise catch efficiencies. The study highlighted the inherent annual variability of scallop recruitment abundance and distribution that are primarily considered to be environmentally driven. However, both recruits and residual scallops contribute to the whole catch, so retaining residual scallops from year to year is important.

Biology and management of a multi-sector blue swimmer crab fishery in a subtropical embayment – Shark Bay, Western Australia

Stock assessments to support sustainable management in data-limited fisheries present a challenge to fisheries scientists and managers. This is the case with the Shark Bay Crab Fishery, which has expanded rapidly in the past 10 years, to become Australia’s highest-producing blue swimmer crab fishery. The resource is harvested commercially by two sectors, the Shark Bay crab trap and trawl fisheries (combined catch of ~800 t), as well as supporting a small but important recreational fishery. Commercial catch and effort data have been collected for the fishery since the early 1980s, commercial trap-monitoring data since 1999, and fishery-independent trawl-survey data since 2001. There is conflicting evidence on the impact that significant increases in catch and effort over the past decade has made on this fishery, such as legal catch rates remaining relatively constant, but declines occurring in the abundance of large crabs. There has also been concern over the level of latent effort in the fishery, with the trap sector currently operating at 70–80% of its potential effort and the capacity for further increases in crab landings by the trawl fleet. Since July 2011, the relative abundance of all size classes of crabs declined significantly. The reasons for this unexpected decline are yet to be understood, but are likely to be linked to adverse environmental extremes (flooding and very warm water temperatures) during the summer of 2010–2011, associated with a very strong La Niňa event. Preliminary assessment indicated that the spawning stock that led to the low recruitment was within historic ranges. The current challenge for the research and management of this fishery is to clarify the causes for this recent decline, and establish suitable biological indicators that will determine the appropriate level of catch and harvest strategy to ensure the future sustainability of the stock.

Performance of square-mesh codends in reducing discards and by-catch in the Shark Bay scallop fishery

The performance of square-mesh codends (as opposed to the conventional diamond mesh) was examined as a discard and by-catch reduction device for the Shark Bay saucer-scallop trawl fishery. Of the three different-sized square-mesh codends (50, 55, 60 mm) tested against the standard 100-mm diamond-mesh codend, the 50-mm square-mesh codend performed poorly with relatively high retention of small (<85 mm shell height) scallops, whereas the 55- and 60-mm square-mesh codends retained 22–33% less smaller scallops than did the diamond-mesh codend. Overall, a mean of 5% loss in commercial-sized scallops across all three square-mesh codends and significant by-catch reductions of up to 95% were achieved compared with the diamond mesh. Catch rate of prawns by the square-mesh codend was less than 2% of that of the standard prawn net. The performance of the 60-mm square-mesh codend presents a good basis for its use in commercial trials in the Shark Bay scallop trawl fishery. A move to square-mesh codends could result in a significant reduction of small scallops and by-catch and may potentially increase the catches of commercial-sized scallops as a result in improvements in water flow and net efficiency.

Seasonal, Spatial, and Postharvest Variability in the Survival of Repeatedly Discarded Saucer Scallops in Shark Bay, Western Australia

ABSTRACT Saucer scallops, Amusium balloti, are targeted by Shark Bay prawn and scallop trawl fisheries where regulatory discarding can occur during summer and winter months that may adversely impact the recruitment of this resource. Survival of repeatedly discarded saucer scallops was thus estimated for the Shark Bay trawl fisheries using short-term tag—recapture experiments under various fishing and environmental conditions. Estimates of apparent survival of discarded scallops was significantly higher during the postspawning winter period (>90%) than during the prespawning summer period (20–90%), but no differences in survival between fishing grounds or between postcapture treatment groups (air exposed vs. those kept in a holding tank) were observed. This suggests that thermal stress from large differences in seasonal temperatures was more critical to scallop survival than differences in scallop reproductive condition. Thus past (pre-2004) management regulations that imposed regulatory discarding during the warmer summer months would have incurred high discard mortalities. Under current management measures, however, regulatory discarding is predominantly during winter months, when scallops exhibit higher resilience to trawlinduced stress. The results support the current management strategy of fishing during the warmer prespawning summer months when the amount of discards is less. Regulatory discarding during the winter spawning period may generate a higher discard rate but the associated discard mortality is at its minimum.

Trawl impacts and biodiversity management in Shark Bay, Western Australia

Trawl by-catch species composition and impact of trawling on soft bottom habitats was investigated in Shark Bay over four seasons. Spatial and temporal variability in faunal abundance and assemblages were investigated for sites with varied levels of trawl intensity, including areas permanently closed to trawling. Environmental conditions were found to be more important in determining differences in faunal assemblages between sites than trawl intensity. In total, 241 fish and 360 invertebrate species were recorded in Shark Bay during this study. The 20 most abundant fish species contributed to 80% of the total number of fish caught overall and these occurred at 73 to 100% of the sites sampled whilst the 20 most abundant invertebrate species contributed to 88% of the total number of invertebrates caught and these occurred at 62 to 100% of sites sampled. Depletion experiments during two time periods were undertaken to determine the catchability (vulnerability) of fish and invertebrate species within these assemblages to better understand trawl impacts. These results, combined with the spatial and temporal distribution patterns of individual species, indicate that long-term trawl impacts can be mitigated by restricting trawling within current trawl areas that constitute 20–40% of the fishery area and for daily monitoring of the trawl fleet with the Vessel Monitoring System for adherence to permanent, spatial and temporal closures.

King Prawn Catch by Grade Category from an Economic and a Stock Management Perspective

We give a geostatistical analysis of western king prawn logbook data collected from the Shark Bay prawn fishing fleet in Western Australia for the 2000 and the 2004 fishing seasons, aggregated into total catch, together with three weight sub-classes and grouped into lunar months. For each of the two years we discuss both the spatial correlation between the weight classes and the spatial correlation for corresponding months in the two years under consideration. Finally, we use a cost function that takes account of the different weight classes to compare the financial return by location between 2000 and 2004.