Scientific manuscripts

Generalizing the Parker equation of DEPM: Incorporating the size dependence of population number and reproductive inputs to estimate spawning biomass and female population by size.

This study extends the daily egg production method (DEPM) to model fish populations by size.

Spawning biomass and female population numbers were estimated by weight class. Standard error for all abundance estimates were derived using the delta method. Fecundity versus weight and its residual error were modelled as allometric.

The methods are available in an R package DEPM.

Using otolith chronologies to understand long-term trends and extrinsic drivers of growth in fisheries.

A growth chronology, developed from otolith increments, was used to build a dataset spanning 37 years of Snapper growth across 4 regions in South Australia. Growth was then related to:

• environmental factors – sea surface temperature, chlorophyll-a, and Southern Oscillation Index
• population performance indicators.

Chlorophyll-a was found to be the best predictor of growth across all regions. Correlations were found for temperature and Southern Oscillation Index, indicating Snapper are sensitive to thermal stress and significant climatic events.

Results showed that under future ocean warming and increased frequency of extreme climatic events, fish growth and fisheries productivity are likely to be affected.

A stage-based estimator of daily egg production.

The traditional daily egg production method:

• assumes a fixed daily spawning hour
• divides eggs into daily age cohorts
• uses regression to estimate spawning egg density and egg mortality rate.

The proposed new method estimates daily egg production without the assumption of a fixed spawning hour. It also corrects for egg mortality using a range of estimates from previous studies.

Estimating daily egg production for each stage of eggs individually, rather than aggregating into daily cohorts, avoids the need for regression-based methods.

Age-related otolith chemistry profiles help resolve demographics and meta-population structure of a widely-dispersed, coastal fishery species.

This study used otolith chemistry to investigate the movement and stock structure of Snapper in south-eastern Australia.

Variations in the chemistry inferred that the episodic fishery along the southeast coastal region of SA is linked to periods of stock range expansion from a major nursery area – Port Phillip Bay.

These periods also coincided with exceptionally strong year-classes originating from spawning in Port Phillip Bay. The 2 gulfs in SA appear to be:

• separate and self-recruiting
• independent of inter-regional movement.

The results support SA’s Snapper fishery being divided into 3 stocks, each dependent on a primary nursery area.

Using in situ hybridisation to expand the daily egg production method to new fish species.

Reliably identifying fish eggs is critical in applying the daily egg production method to estimate biomass of commercially important species.

Researchers designed and validated an in-situ hybridisation approach for identifying whole eggs from Snapper. The colorimetric assay allowed specific detection of Snapper eggs from mixed-species samples comprising closely related taxa.

Insights into movement behaviour of Snapper (Chrysophrys auratus, Sparidae) from a large acoustic array.

Snapper movement behaviour was investigated using acoustic telemetry in northern Gulf St Vincent, South Australia.

Snapper were found to be highly mobile within the region, moving linear distances of up to ~100 km and achieving tens of kilometres in a day. Throughout the year, their activity was distributed across areas of hundreds of square kilometres.

The spawning dynamics of Snapper (Chrysophrys auratus) in northern Spencer Gulf, South Australia.

Snapper reproductive samples from northern Spencer Gulf were collected over 3 spawning seasons (2005–06, 2006–07, and 2007–08). These were analysed macroscopically and microscopically for variations in spawning dynamics.

Spawning began in November, though there were variations between years that coincided with water temperature of 18°C to 20°C. Peak spawning occurred during December, when fish spawned almost daily.

Variation in productivity of a key Snapper (Chrysophrys auratus) fishery related to recruitment and fleet dynamics.

Snapper fishery production varied in northern Spencer Gulf between 1984 and 2009, as estimated using fishery statistics and biological information.

Before the early 2000s, changes in fishery production were related to changes in fishable biomass, caused by periods of strong and weak juvenile recruitment. From the mid 2000s onwards, catches decreased considerably and effort was low, meaning catch per unit effort (CPUE) remained high.

The data indicates hyperstability, where CPUE remains high despite declining biomass. This is a result of:

• aggregating behaviour of Snapper
• improved efficiency of fishers.

Relationship between elemental concentration and age from otoliths of adult Snapper (Pagrus auratus, Sparidae): implications for movement and stock structure.

This study investigated the extent that Snapper stock structure is influenced by adult movement. Fish from the 9+ age class were sampled throughout SA to compare chemistry of transverse sections of otoliths.

Chemistry between regions varied, although similarities were observed for the first 3 years, indicating that all fish originated from up to 2 nursery areas.

Chemistry diverged significantly between ages 3 and 5, then remained consistent to age 9. This showed that fish dispersed through regions from nursery areas and then became resident.

Dynamics in 0+ recruitment and early life history for Snapper (Pagrus auratus, Sparidae) in South Australia.

Recruitment variability of Snapper was investigated in northern Spencer Gulf. Juveniles were sampled annually in 2000, 2001, and 2002, and early life history characteristics were determined from otolith microstructure examination.

The density of age-0 juvenile Snapper varied between years. Recruits were consistently sampled at the same locations in each year, which were characterised by soft, muddy benthic substrate.

Successful recruitment resulted from periods in the reproductive season with a specific water temperature regimen.

Ageing, catch-at-age, and relative year-class strength for Snapper (Pagrus auratus) in northern Spencer Gulf, South Australia.

Commercial landings of Snapper declined to 25-year lows in 1994, and sustainability of the SA fishery was questioned. This study investigated 2 potential explanations for the decline:

• changes in fishing effort
• recruitment variability.

Data for 1983–97 from northern Spencer Gulf, which contributes most Snapper catches in South Australia, showed that 74% of the variance in targeted catch was explained by catch per unit effort, and 20% by fishing effort.

An ageing procedure was established by examining sectioned sagittal otoliths of 1,046 Snapper caught in 1991 and 1994–95.

Recruitment variability was examined by analysing age structures from commercial catches. Interpretation of the age structures suggests that year-class strength is highly variable and has a strong influence on fishery production.