Stephen Mayfield

Survey estimates of fishable biomass following a mass mortality in an Australian molluscan fishery

Mass mortality events are relatively uncommon in commercially fished populations, but when they occur, they reduce production and degrade ecosystems. Observing and documenting mass mortalities is simpler than quantifying the impact on stocks, monitoring or predicting recovery, and re-establishing commercial fishing. Direct survey measures of abundance, distribution and harvestable biomass provide the most tenable approach to informing decisions about future harvests in cases where stock collapses have occurred because conventional methods have been disrupted and are less applicable. Abalone viral ganglioneuritis (AVG) has resulted in high levels of mortality across all length classes of blacklip abalone, Haliotis rubra Leach, off western Victoria, Australia, since May 2006. Commercial catches in this previously valuable fishery were reduced substantially. This paper describes the integration of research surveys with commercial fishermens knowledge to estimate the biomass of abalone on AVG-impacted reefs. Experienced commercial abalone divers provided credible information on the precise locations of historical fishing grounds within which fishery-independent surveys were undertaken. Abalone density estimates remained low relative to pre-AVG levels, and total biomass estimates were similar to historical annual catch levels, indicating that the abalone populations have yet to adequately recover. Survey biomass estimates were incorporated into harvest decision tables and used with prior accumulated knowledge of the populations to determine a conservative harvest strategy for the fishery.

Invertebrate Dive Fisheries Can Be Sustainable: Forty Years of Production from a Greenlip Abalone Fishery off Southern Australia

The South Australian greenlip abalone (Haliotis laevigata) fishery was established in 1964, and commercial catches have been stable at approximately 350 t.yr−1 for over 20 years. This contrasts the status of fisheries for abalone outside Australasia, where rapid over-exploitation and stock collapse, without recovery, have been widespread. This study of the sustainability of the South Australian greenlip abalone fishery is based on a range of fishery-dependent and fishery-independent data, and it contrasts with a previous assessment based on fewer data that suggested these stocks were overfished. This fisherys success demonstrates that prolonged harvests can be obtained from wild abalone stocks. Key elements of the management system critical in preventing over-exploitation include early limitations on access to the resource, establishment of appropriate minimum legal sizes, and effective controls on total catches. A biological research program that focused explicitly on stock assessment and underpinned timely adjustments to management arrangements based on changes in stock status has also been critical. Recommendations by fisheries management committees involving resource managers, scientists, industry members, and other stakeholders have been underpinned by formal management plans. Sustainability of benthic invertebrate fisheries vulnerable to overfishing could be enhanced by adopting elements of the management system underpinning the South Australian abalone fishery since 1968.

Factors influencing the growth rate of juvenile rock lobsters, Jasus lalandii

Environmental conditions for juvenile spiny lobsters can vary spatially, interannually, and seasonally. We examined both field and aquarium data to elucidate some of the most important factors influencing somatic growth rates of juvenile Jasus lalandii. Growth increments of juveniles were determined at two sites in Table Bay, South Africa. Premoult individuals were captured and held in aquaria until they moulted. In addition, growth rates of small, medium, and large juvenile lobsters were monitored in aquaria under approximately ambient conditions for nine months. A separate aquarium experiment addressed effects of temperature. Moult increments at the two field sites varied significantly with season (highest in winter, lowest in spring), and month and site interacted. Temperature and diet affected grow-out significantly. Tagging and limb regeneration slowed grow-out. The 10˚C and the 15˚C experimental groups did not differ significantly in mean moult increment, but intermoult period at 15˚C was 50% shorter, so lobsters at 15˚C grew faster. Temperature affects intermoult period but, provided food is not limiting, does not appear to affect moult increment (within normal temperature range). In the wild, however, moult increments can vary up to twofold with season, probably at least in part because of seasonal temperature variation.

Estimating Species-Specific Catch Rates in a Mixed-Species Dive Fishery

ABSTRACT Catch-per-unit-effort (CPUE) is frequently used as a measure of relative abundance in fisheries stock assessment. Determining reliable estimates of species-specific CPUE is more challenging in multispecies, rather than single-species, fisheries because identification of appropriate effort data for each species is often difficult. Divers in the South Australian abalone fishery can harvest blacklip (Haliotis rubra) and greenlip (Haliotis laevigata) abalone simultaneously, but report only a single value for daily fishing effort. This is problematic because total allowable commercial catches are set for each species following species-specific stock assessments in which CPUE is a key index of relative abalone abundance. To provide an evidence-based approach to the identification of the most appropriate CPUE estimation method for ongoing assessment of the fishery, we assessed six diverse CPUE estimation methods for estimating annual, species-specific CPUEs using 30 y of data. The candidate CPUE estimation methods yielded relative CPUE time series with similar temporal trends throughout the 30-y period. These relative CPUE estimates each had low coefficients of variation and were highly correlated with one another, requiring consideration of other factors to determine a preferred method. Using a catch-weighted estimate of CPUE (CPUEwt) overcomes many of the problems associated with using the other five methods tested. Specifically, CPUEwt (1) weights each daily catch and effort objectively; (2) removes the need to “subset” the data subjectively, which ensures that data availability and representation are not reduced by arbitrary rules; and (3) is relatively simple to explain to stakeholders and can be applied consistently to greenlip and blacklip abalone at multiple spatial scales across the fishery. Although the requirement to estimate species-specific catch rates in mixed-species dive fisheries is rare, our analyses demonstrate that CPUEwt could provide a robust measure of species-specific CPUEs across other diverse multispecies fisheries.

A diver survey method to quantify the clustering of sedentary invertebrates by the scale of spatial autocorrelation

Sedentary benthic invertebrates exhibit clustering at a range of spatial scales. Animal clustering reduces the precision of diver surveys and can accelerate overexploitation in dive fisheries. Dive harvesters target the densest aggregations of males and females that produce the highest rates of egg fertilisation during mass spawning events. By quantifying these effects of harvesting on fertilisation success, measuring animal clustering can inform stock management for reproductive sustainability. We present a method to measure the spatial extent of density aggregations down to 1 m, extending a previously described leaded-line survey design. Applying this method to abalone, research divers counted individuals in successive 1 × 2 m2 quadrats lying along adjoining pairs of 1 × 100 m2 transects. Clusters were observed as neighbouring quadrats of high animal density. Spatial autocorrelations at inter-quadrat distances of 1 to 100 m were calculated for four surveys, with eight pairs of transects swum in each survey. For all four surveys, inside two survey regions, spatial autocorrelation declined to non-significant levels at a distance of ~20 m. Quantified by the distance within which density counts are correlated, this quadrat-within-transect method provides a diver survey measure of the scale of spatial aggregation for sedentary invertebrates such as abalone, sea cucumbers and urchins.

Influence of Wind and Swelloncatch Rates in a Dive Fishery: A Case Study from the South Australian Abalone Fishery

ABSTRACT Catch per unit effort (CPUE) is widely used as an index of abundance in the assessment of abalone fisheries even though it has often been considered unreliable. This is because, it is susceptible to hyperstability and influenced by factors other than stock abundance such as increased fishing efficiency, market demand for particular product, weather, diver habits, and putative rotation of fishing grounds. These factors introduce uncertainty to the use of CPUE as an index of relative stock abundance, with some of these factors causing hyperdepletion of the index. In the western and southern zones (WZ and SZ) of South Australia, commercial fishers recently suggested that declining CPUE in 2014 was attributed to a fishing season with poor weather, and in particular higher than average swell, rather than to declines in stock abundance. To evaluate the effect of weather on CPUE in the SZ and WZ daily, logbook catch and effort data were linked with swell and wind observations. Analysis demonstrates that, although the observations of extrinsic factors were correct, fishers avoided diving on days with unsuitable weather conditions. Consequently, the observed decrease in CPUE is not likely to have been affected by weather-related hyperdepletion, and therefore managers should not rule out interpreting recent declines in CPUE as reflecting decreases in stock abundance. This highlights the independence of CPUE to the effects of the extrinsic factors evaluated and challenges some of the reasoning provided for it not being a reliable index of abundance. Further exploration of the effects of other factors that may affect CPUE, as well as the link between this index and fishery-independent estimates of abundance, are needed to determine the weighting it should receive in the stock assessment process.

Setting quotas using provisional data: a case study from the South Australian abalone fisheries

ABSTRACT Stock assessments of quota or effort managed fisheries in which the duration of the fishing season is 12 months are invariably delivered well into the subsequent fishing season. As a result, quotas are frequently based on year-old data. This delay is often unavoidable because it may take months to collect, collate and analyse data necessary to assess fishery performance. The South Australian fisheries for blacklip (Haliotis rubra Leach, 1814) and greenlip abalone (H. laevigata Donovan, 1808) have addressed this issue by using provisional data on current stock status to inform application of the harvest strategy decision rules that set the quota for the next year. The primary index of relative abundance for these fisheries is catch per unit effort (CPUE). Our study uses 25 years (1988–2012) of CPUE data to quantify the differences between the provisional and complete-season CPUE estimates at the spatial scales used to assess the fisheries. We demonstrate that, in most cases, there was a strong relationship between the provisional and complete-season CPUE estimates for both species, with little evidence of bias. As the provisional CPUE estimates were a reliable and accurate predictor of the complete-season CPUE estimates, this provides a high degree of confidence in using provisional CPUE estimates to set quotas, thereby overcoming the difficulty of basing decisions on aged data. These findings are likely to be applicable to other fisheries, particularly those where much of the annual catch is obtained (or effort expended) in a short time period at the commencement of the fishing season.