Anthony J. Fowler

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

The present study investigated the stock structure of snapper (Pagrus auratus) in South Australia, and the extent to which this is influenced by adult movement. Fish from the 9+ age class were sampled from six different regions, encompassing >2000 km of coastline and different habitat types. The chemistry of transverse sections of otoliths was sampled using laser ablation inductively coupled plasma-mass spectrometry, providing elemental profiles that were related to age for the first nine years of the fish’s lives. The age-related annual averages for both 88Sr and 138Ba differed significantly between regions. They were, however, similar for the first three years, then diverged considerably between the ages of three to five years, and then remained consistently different through to the age of nine years. This suggests that all fish, regardless of where captured, originated from only one or two nursery areas, but dispersed throughout the different regions between the ages of three to five years, before becoming resident to their new regions of occupancy. Thus, this population of snapper represents a single, large, stock where the individuals have a common origin, but through age-related emigration ultimately disperse and supplement the low abundance populations in regional State waters.

Population characteristics of southern sea garfish (Hyporhamphus melanochir, Hemiramphidae) in South Australia

Population dynamics, demography and spatial distribution of an important species of ‘halfbeak’, the southern sea garfish (Hyporhamphus melanochir), were studied in South Australia, which accounts for most of the national catch. Commercial catch and effort data were used to indicate spatial and temporal abundance. Catch sampling provided fish measurements and estimates of age, sex and reproductive maturity. Catches were from throughout South Australia’s gulf region, but three areas separated by hundreds of kilometres produced the highest catches. These relatively protected areas support extensive beds of intertidal and subtidal seagrasses. Statewide commercial fishery catches demonstrated considerable inter-annual variation of 240–320 t year–1. Catches were dominated by 1+ and 2+ fish, with few individuals from the 3+ to 5+ age classes. Females dominated the sex ratios, were the largest individuals, and the largest from each age class. No running ripe fish were sampled. Seasonal variation in the catch and in the lack of spawning fish suggested that adults might move from fishing areas during the spawning season. Length/weight and length/age relationships did not differ between areas separated by hundreds of kilometres, suggesting large-scale movement. Possible population truncation related to fishery exploitation has prompted a stock rebuilding program for the South Australian stock.

Evidence for multiple year classes of the giant Australian cuttlefish Sepia apama in northern Spencer Gulf, South Australia

Giant Australian cuttlefish form a mass spawning aggregation at a single site in northern Spencer Gulf (NSG) in South Australia every austral winter. Samples of cuttlefish were collected from this region over three consecutive years. Analysis of regular growth increments in the cuttlebones of these individuals, revealed a polymorphism in growth pattern for both sexes. Three distinct “bone patterns” were identified based on the variation in increment widths over the lengths of the bones. All bones analysed conformed to one of the three bone patterns, and the increment width patterns were consistent between years. Interpretation of the patterns, suggested that Sepia apama have two alternative life cycles. The first involves rapid juvenile growth during the first summer after hatching, with maturity reached within 7–8 months. These individuals return to spawn in their first year as small individuals. The second life cycle involves much slower juvenile growth during the first summer, with maturity deferred until their second year, when they return to spawn as much larger individuals. Thus, the age compositions of populations of S. apama in the NSG appear to consist of two year classes for both sexes.

Age in Years from Otoliths of Adult Tropical Fish

Otoliths of fish are hard, calcified internal structures that assist in orientation and sound perception (Popper et al. 2005, Green et al., Chapter 1, this volume). As otoliths grow inside each fish’s head they record an extraordinary amount of information about the life of that fish and the environments that it experiences (Begg et al. 2005). The challenge to scientists is to retrospectively access this information, to interpret it accurately in terms of the biology and life history of the fish, and to use the data to appropriately understand and manage the natural resources of the fish population and the broader aquatic ecosystem. The greatest application of otoliths to date has been in providing information on the age and growth of fish in years, which has ultimately been used in the management of associated fisheries (Campana & Thorrold 2001, Campana 2005). Prior to the 1980s, and stretching back to the late 19th Century, most such ageing work was done in the temperate regions of the world and contributed to managing the enormous fisheries in the oceans of these regions (Beamish 1992, Beamish & McFarlane 1995).

Stock discrimination of Southern Garfish (Hyporhamphus melanochir) by stable isotope ratio analysis of otolith aragonite

Stable isotope analysis (δ18O and δ13C) of otolith aragonite was used to infer ontogenetic patterns of movement, delineate sub-population structure, and determine the mixing of Southern Garfish (Hyporhamphus melanochir) in South Australian waters. Adult garfish from the 2+ age class were collected from three sites within each of six regions along the South Australian coast: Northern Gulf St. Vincent, South West Gulf St. Vincent, Kangaroo Island, Northern Spencer Gulf, South West Spencer Gulf and the West Coast of Eyre Peninsula. Significant spatial variation was detected in mean δ18O and δ13C values among sites within regions and as a function of fish age. Salinity, rather than sea surface temperature, appeared to account for regional variation in δ18O, whereas variation in diet and metabolic processes were suggested to account for observed differences in δ13C. Otolith oxygen and carbon stable isotope values indicated that the South Australian Southern Garfish fishery is comprised of multiple, regional, population components that persist through time. Both the West Coast and Northern Spencer Gulf regions constitute distinct population components that exhibit little inter-regional mixing and therefore may be considered as discrete management units. The South West Spencer Gulf region may also be considered a separate population component, however, its level of connectivity with Gulf St. Vincent is uncertain. With the exception of juvenile garfish from Kangaroo Island, there was no clear regional separation within Gulf St. Vincent.

The use of food resources by 0+ snapper, Chrysophrys auratus, from northern Spencer Gulf, South Australia

Food availability is an important factor in survival and growth of juvenile fish and has been proposed as a major factor shaping the patterns of distribution and abundance of 0+ snapper, Chrysophrys auratus, in New Zealand and Japan. Thus, we tested the hypothesis that diet of 0+ snapper would exhibit spatial variation related to their abundance within a major nursery region. Therefore, we described the stomach contents of 0+ snapper collected from within a nursery region in areas of low and high abundance of 0+ snapper. The data also provided the opportunity to consider sized based differences in diet and temporal feeding patterns. 0+ snapper were generalist carnivores that preyed on a wide range of invertebrates and fish. No evidence of sized based dietary variation was identified but feeding either ceased or was much reduced during the hours of darkness. The major finding was that more polychaetes were present in the stomachs of 0+ snapper in the area of high 0+ snapper abundance than in the areas of low 0+ snapper abundance. The spatial differences in diet observed supports the hypothesis that prey availability is important as a driver of habitat selection by 0+ fish.

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

This study investigated the inter- and intra-annual variation in the spawning dynamics of snapper (Chrysophrys auratus). Reproductive samples from northern Spencer Gulf, South Australia were collected over three seasons (2005/06, 2006/07 and 2007/08) and were analysed macro and microscopically. Spawning activity was determined by calculating estimates of spawning fraction and batch fecundities. The onset of spawning occurred in November but varied between years and corresponded with times when water temperature was between 18 °C and 20 °C. The length of the spawning season also differed between years. In each year the peak spawning activity occurred during December when fish spawned almost daily. Spawning frequency and relative batch size did not differ between the first two spawning seasons but, in the third season, batch size was considerably greater and spawning fractionally lower. These results are contrasted with a known recruitment of 0+ over the three years of the study.

Temporal variation in feeding behaviour and trophic ecology of the temperate hemiramphid, Hyporhamphus melanochir

The feeding ecology of southern garfish, Hyporhamphus melanochir, in South Australia was investigated to determine the dietary composition and diurnal feeding patterns, especially in relation to a hypothesised relationship between prey availability and feeding patterns. Samples of H. melanochir were collected at different times of the day and night during autumn and winter, and assigned to 3-hourly intervals. The dietary composition and feeding patterns were determined from gut contents analysis. The main food items were seagrasses (Zosteraceae) and hyperbenthic crustaceans, mainly amphipods. Polychaetes and insects were also consumed. A clear diurnal trophic shift was evident: seagrass was consumed in large volumes during the day, whilst hyperbenthic invertebrates dominated the diet during the night. Plankton samples indicated that this trophic shift reflected the higher abundances of hyperbenthic invertebrates in the water column at night. Less time was spent consuming seagrass during winter, coinciding with shorter day-lengths. H. melanochir has a strong trophic association with seagrass beds, which may account for the high garfish abundance in northern Gulf St. Vincent where extensive Zosteracean seagrass beds occur. This may be useful fundamental information for future environmentally-based fishery management decisions to help ensure the sustainability of southern garfish population and the ecosystem of which it is a part.

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

Snapper is a significant fishery species in Australasia whose movement behaviour remains poorly understood. This was addressed in the present study at the within-region scale using acoustic telemetry in the Gulf St Vincent, South Australia. Over 3 years from May 2011, 54 snapper were monitored throughout ~160km2 using 41 acoustic receivers. The dispersion of >500000 detections varied in space and time, reflecting three types of space use, dependent on different types of movement behaviour. One station, near a large shipwreck, accounted for 67.8% of all detections, particularly during winter, when fish were sedentary and site attached. In spring, the fish dispersed throughout the study area to different habitats and, through summer, occupied different larger restricted areas than used in winter. Snapper were highly mobile and demonstrated systematic behaviour at several temporal scales. They moved linear distances of up to ~100km and could achieve tens of kilometres in a day in episodic movements. Through the year, their activity was distributed across areas of hundreds of square kilometres. The regional spatial management regimen was assessed against this enhanced understanding of movement behaviour. A new marine park sanctuary zone that encompassed the shipwreck was appropriately located, but possible benefits of a nearby spatial spawning closure area appear limited.

Evidence for a broad-scale decline in giant Australian cuttlefish (Sepia apama) abundance from non-targeted survey data

Little is known about the population trajectory and dynamics of many marine invertebrates because of a lack of robust observational data. The giant Australian cuttlefish (Sepia apama) is IUCN-listed as Near Threatened because the largest known breeding aggregation of this species in northern Spencer Gulf, South Australia, has declined markedly since the turn of the century. We used by-catch records from long-term trawl surveys to derive abundance data for S. apama and commercial cuttlefish harvest data as a measure of exploitation. Using Bayesian hierarchical models to account for zero-inflation and spatial dependence in these abundance counts, we demonstrated a high probability of broad-scale declines in the density of S. apama, particularly surrounding the primary aggregation site, which supports the recent closure of the entire S. apama fishery in northern Spencer Gulf. Historical harvest data were positively correlated with S. apama density estimated from the trawl surveys, suggesting that the commercial cuttlefish catch tracks the species abundance. Our results also indicated the possibility that the known S. apama breeding grounds might be supplemented by individuals that were spawned elsewhere in northern Spencer Gulf.