Harry Gorfine

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.

Abundance estimation of blacklip abalone (Haliotis rubra) I. An analysis of diver-survey methods used for large-scale monitoring

Abstract A comparative evaluation of radial transect and timed collection methods used in large-scale monitoring of abalone stocks in Victoria was undertaken. Methods were assessed according to their accuracy, precision, robustness and sensitivity (ability to detect change). Sample size required to detect a 50% decrease was estimated using data from a pilot study, and three replicate stock depletion experiments were undertaken in a simple before/after design testing the null hypothesis of no change at the impacted sites. Harvesting was undertaken by commercial abalone divers. On all occasions, both methods were sensitive enough to detect the specified change and robust to both a significant effect of observer and violation of the assumption of equality of variances. No significant differences in precision occurred, however the 30 m 2 transect estimated the actual number of abalone harvested to within 10% of the true value. Only a weak linear relationship (pre-recruits r 2 = 0.12; post-recruits r 2 = 0.39) was observed between relative abundance (timed collection data) and absolute abundance (30 m 2 transect data). Importantly, the fishing behaviour (targeting of aggregations) of commercial abalone fishers resulted in much reduced variances for both transect estimates and timed collections, and consequently, increased power to detect a decrease in population size. In the long term however, this phenomenon may be offset by re-aggregation of populations, the extent of which needs to be throughly determined. Overall, the 30 m 2 transect was considered the safer, more robust method for long-term monitoring of abalone stocks.

Exploring Spatiotemporal Trends in Commercial Fishing Effort of an Abalone Fishing Zone: A GIS-Based Hotspot Model

Assessing patterns of fisheries activity at a scale related to resource exploitation has received particular attention in recent times. However, acquiring data about the distribution and spatiotemporal allocation of catch and fishing effort in small scale benthic fisheries remains challenging. Here, we used GIS-based spatio-statistical models to investigate the footprint of commercial diving events on blacklip abalone (Haliotis rubra) stocks along the south-west coast of Victoria, Australia from 2008 to 2011. Using abalone catch data matched with GPS location we found catch per unit of fishing effort (CPUE) was not uniformly spatially and temporally distributed across the study area. Spatial autocorrelation and hotspot analysis revealed significant spatiotemporal clusters of CPUE (with distance thresholds of 100’s of meters) among years, indicating the presence of CPUE hotspots focused on specific reefs. Cumulative hotspot maps indicated that certain reef complexes were consistently targeted across years but with varying intensity, however often a relatively small proportion of the full reef extent was targeted. Integrating CPUE with remotely-sensed light detection and ranging (LiDAR) derived bathymetry data using generalized additive mixed model corroborated that fishing pressure primarily coincided with shallow, rugose and complex components of reef structures. This study demonstrates that a geospatial approach is efficient in detecting patterns and trends in commercial fishing effort and its association with seafloor characteristics.

Contrasting patterns of population connectivity between regions in a commercially important mollusc Haliotis rubra: integrating population genetics, genomics and marine LiDAR data

Estimating contemporary genetic structure and population connectivity in marine species is challenging, often compromised by genetic markers that lack adequate sensitivity, and unstructured sampling regimes. We show how these limitations can be overcome via the integration of modern genotyping methods and sampling designs guided by LiDAR and SONAR data sets. Here we explore patterns of gene flow and local genetic structure in a commercially harvested abalone species (Haliotis rubra) from southeastern Australia, where the viability of fishing stocks is believed to be dictated by recruitment from local sources. Using a panel of microsatellite and genomewide SNP markers, we compare allele frequencies across a replicated hierarchical sampling area guided by bathymetric LiDAR imagery. Results indicate high levels of gene flow and no significant genetic structure within or between benthic reef habitats across 1400 km of coastline. These findings differ to those reported for other regions of the fishery indicating that larval supply is likely to be spatially variable, with implications for management and long‐term recovery from stock depletion. The study highlights the utility of suitably designed genetic markers and spatially informed sampling strategies for gaining insights into recruitment patterns in benthic marine species, assisting in conservation planning and sustainable management of fisheries.

Abundance estimation of blacklip abalone (haliotis rubra) II. A comparative evaluation of catch-effort, change-in-ratio, mark-recapture and diver-survey methods

Abstract Assessments of three replicate populations of blacklip abalone (Haliotis rubra) were made utilising a range of techniques, enabling critical assumptions to be tested in an empirical manner. Methods included catch-effort (Leslie), change-in-ratio (CIR), mark-recapture (MR) and diver-survey techniques. Population assessments were carried out under the same experimental protocol and within adjacent, closed areas of 5 ha. To establish a Leslie estimate, a 50% stock reduction of each population was carried out using commercial abalone divers. The principal objective was to make a comparative evaluation of methods with respect to their accuracy, precision, robustness and data requirements. A total of 8 tonnes (35729 individuals) of abalone (≥ 10 cm) was harvested. The diver-survey (30 m2 transect), MR and CIR (using 30 m2 transect data) methods were given the highest overall rating. In general, the CIR (using timed collection data) did not perform as well as the CIR (using transect data). Leslie estimators were inaccurate and imprecise for two out of the three populations. Furthermore, the correction of unit effort from total dive time into searching and handling time for both the Leslie and timed collection population estimators did not improve the reliability of the estimate. The timed collection diver-surveys, while precise, were highly inaccurate and their use for abundance estimation in abalone is not recommended.

Parameterization of European perch Perca fluviatilis length-at-age data using stochastic Gompertz growth models

Three stochastic versions of the Gompertz growth model were used to parameterize total length (L(T) )-at-age data for perch Perca fluviatilis, an important target species for commercial and recreational fishers and a food species for predatory fishes and aquatic birds. Each model addresses growth heterogeneity by incorporating random parameters from a specific positive distribution: Weibull, gamma or log-normal. The modelling outputs for each version of the model provide L(T) distributions for selected ages and percentiles of L(T) at age for both males and females. The results highlight the importance of using a stochastic approach and the logistic-like growth pattern for analysing growth data for P. fluviatilis in Curonian Lagoon (Lithuania). Outputs from this modelling can be extended to a stochastic analysis of fish cohort dynamics, incorporating all length-based biological relationships, and the selectivity-related interactions between fish cohorts and fishing gear.