Daniel D. Johnson

Influences of hanging ratio, fishing height, twine diameter and material of bottom-set gillnets on catches of dusky flathead Platycephalus fuscus and non-target species in New South Wales, Australia

Three experiments were done to test for the influences of different (i) hanging ratios (E=0.5, 0.65 and 0.8); (ii) fishing heights (25 and 12 meshes); and (iii) twine diameters (0.41, 0.56 and 0.62 mm) and materials (multifilament nylon and multimonofilament nylon polyamide) on catches and by-catches in the estuarine gillnet fishery for Platycephalus fuscus in New South Wales, Australia. In each experiment, the various 100-m treatment panels comprising 80-mm (nominal) mesh rigged according to the different configurations being examined were configured in a single gang between 1090 and 1310 m in length, and fished according to commercial practices. The results showed no significant differences between different hanging ratios or twine diameters for the numbers, weights and size compositions of catches and by-catches. Twine material had an effect on only one key by-catch species (Acanthopagrus australis), with fewer caught in panels made from multifilament nylon compared with multimonofilament nylon. In contrast to the above modifications, lowering the fishing height of the floatline significantly reduced total by-catch by up to 46% and the individuals of key by-catch species (Mugil cephalus, A. australis and Girella tricuspidata) by between 60 and 85% with no effect on catches of targeted P. fuscus, or legally retained byproduct, Portunus pelagicus. The results are used to provide directions for the future management of this fishery and have relevance to other similar bottom-set gillnet fisheries.

Utility and efficiency of multi-mesh gill nets and trammel nets for sampling assemblages and populations of estuarine fish

Two replicate multi-mesh gill and trammel nets, each comprising five 30 m long panels made from different-sized mesh (38, 54, 70, 90 and 100 mm stretched mesh openings) were fished in a south-east Australian barrier estuary over seven nights to evaluate their potential as sampling gears for fishery-independent surveys of estuarine fish assemblages. There were no differences in composition and structure of assemblages, mean abundance, or diversity of catches between the two types of net. The composition and structure of catches differed between mesh sizes, with the panels made from 38 and 54 mm mesh retaining significantly more fish and species than the larger-sized meshes. The two smallest mesh sizes were important for capturing sub-adults and juveniles of some species. Based on a greater precision of catch per unit effort (CPUE) estimates, less sampling effort and greater ease of use, the multi-mesh gill net was a better sampling unit than the trammel net for assessments of estuarine fish populations.

Using length data in the Schnute Model to describe growth in a metapenaeid from waters off Australia

This paper presents a novel approach for fitting length data to the Schnute growth model. Cohorts were fitted manually to a time series of length distributions from two stocks (Clarence and Hunter Rivers) of Metapenaeus macleayi and considered analogous to individuals from tag–recapture data, in order to estimate growth parameters. Data for Clarence males best fitted the three-parameter Schnute Model (L∞ = 21.3 mm CL, κ = 0.025 day–1 and γ = –1.35), whereas those for Hunter males were best fitted to a two-parameter version of the model (L∞ = 33.5 mm CL, κ = 0.009 day–1 and γ = 0 fixed). The equivalent to the von Bertalanffy growth function was the best fit to female data from both stocks (L∞ = 36.6 and 40.2 mm CL, κ = 0.004 and 0.005 day–1 and γ = 1 fixed for Clarence and Hunter respectively). Females grew larger than males and took longer to achieve their maximum size. No significant differences in female growth were found between stocks; however, males from the Hunter grew to a longer mean maximum length but at a slower rate than those from the Clarence. This study shows how the Schnute Model can be fitted to length based data and thus include the flexibility of comparing fits between asymptotic and non-asymptotic growth functions.

Preliminary investigation of perfluoroalkyl substances in exploited fishes of two contaminated estuaries

Perfluoroalkyl substances (PFASs) are being increasingly detected in a range of aquatic and terrestrial ecosystems, often resulting from the use of legacy fire-fighting foams. This study conducted an initial investigation of the concentrations of PFASs in the commercially and recreationally exploited species Dusky Flathead, Mud Crab, School Prawn, Sea Mullet, Yellowfin Bream, Eastern King Prawn and Sand Whiting, across two contaminated estuaries. All samples contained perfluoro-n-octane sulfonate (PFOS) except four Yellowfin Bream samples (two from each estuary). Perfluoro-n-octanoic acid (PFOA) was detected only in School Prawn samples from Fullerton Cove, while perfluoro-n-hexane sulfonate (PFHxS) was detected in prawn muscle and in fish liver samples from both estuaries. This study presents one of the first surveys of PFAS in a range of edible saltwater fish and crustaceans in Australia, and these baseline levels of contamination will prove useful for informing future surveys of these emerging contaminants.

Reproductive Biology of Portunus pelagicus in a South-East Australian Estuary

Abstract Portunus pelagicus (blue swimmer crab) is widely distributed and fished throughout the indo-west Pacific, but little is known of its biology and ecology in southeast Australia. In this study, the reproductive biology of P. pelagicus inhabiting Wallis Lake, a large coastal lake that supports the largest regional commercial fishery for the species was investigated. Males and females with mature gonads occur throughout the fishing season (November to July), however, ovigerous females are most prevalent in November and December. The mean size at which 50% of females reached maturity is estimated at 46 mm carapace length (CL). Fecundity increased linearly with CL and females are capable of producing up to 3 broods of eggs within the observed spawning season, with crabs of 60-69 mm CL producing approximately 76% of the estimated total egg production. Increasing the current minimum carapace length above 60 mm CL would potentially provide greater protection to mature females and increase total egg production, but this would reduce total retained catches in the fisheries.

Depuration of perfluoroalkyl substances from the edible tissues of wild-caught invertebrate species.

Detection and quantification of poly- and perfluoroalkyl substances (PFASs) in aquatic organisms is increasing, particularly for saltwater species. Depuration can remove PFASs from the tissues of some species once they are removed from the contaminant source, but it is not known if this process occurs for saltwater crustaceans. Such information is important for managing human health risks for exploited migratory species following exposure. We present the results of a depuration trial for School Prawn (Metapenaeus macleayi) and Mud Crab (Scylla serrata), two commercially important crustaceans in Australia. Perfluorooctane sulfonate (PFOS), perfluorohexane sulfonate (PFHxS), and perfluorooctanoic acid (PFOA) were present in samples of both species collected following exposure under natural conditions in contaminated estuaries. Depuration was tested in uncontaminated water for 33days. PFOA was present at levels close to LOR in both species, and was not detected after 4.5h and 72h in School Prawn and Mud Crab respectively. PFHxS was rapidly depurated by School Prawn, and had a depuration half-life of 5.7h. PFOS was also depurated by School Prawn, with a depuration half-life of 158.5h. PFHxS and PFOS concentrations were highly variable in Mud Crab both at the start, and during the depuration experiment, and a depuration model could not be fitted to the data. For School Prawn, depuration of total PFASs to the relevant screening value for protection of human health (9.1μgkg-1) occurred within 7.1h. Rapid depuration of PFASs in School Prawn indicates that human health risks associated with consumption may decrease as this species migrates away from the contamination source. Further research is required to better understand the relationships between contaminant load and life-history characteristics (such as growth, reproduction, and moult cycle) in Mud Crab, and future work should target broader time frames for depuration in this species.

Small-scale variation in reproduction and abundance of greentail prawn, Metapenaeus bennettae Racek and Dall, 1965

Sampling on a single night or day within a month is common in studies of reproduction and abundance of aquatic invertebrates, but patterns from month-to-month may be confounded by variability at smaller time-scales. We used hierarchical sampling in two estuaries to test the hypothesis that temporal variability in reproductive condition, relative abundance and size-structures of populations of Metapenaeus bennettae Racek and Dall, 1965 would be greater at the larger scales of months and seasons than at smaller scales of weeks and nights. In both estuaries, variation in the proportion of mature/ripe females and mean abundance of M. bennettae was often largest at the temporal scales of nights and weeks, but variation in the size-frequency distributions was generally greater at the scale of months and seasons. Our results demonstrate that future studies on any species with the potential for high temporal variability in reproduction and population structure, like penaeids, should incorporate or examine the need for small-scale temporal sampling.

Bioaccumulation of perfluoroalkyl substances in exploited fish and crustaceans: Spatial trends across two estuarine systems

Spatial patterns in perfluoroalkyl substances were quantified for exploited fish and crustaceans across two contrasting Australian estuaries (Port Stephens and Hunter River). Perfluorooctane sulfonate (PFOS) was detected in 77% of composites from Port Stephens and 100% of composites from Hunter River. Most species from Port Stephens showed a clear trend with distance to source. In contrast, only a subset of species showed this trend in the Hunter River, potentially due to species movement patterns and differing hydrology. Spatial modelling showed that PFOS concentrations were expected to exceed the relevant trigger value up to ~13,500 m from the main point source for Port Stephens and ~9000 m for the Hunter River. These results represent the first major investigation of bioaccumulation of PFASs in exploited species in Australian estuaries, and highlight various factors that can contribute to spatial patterns in bioaccumulation.

Don’t Blame It on the Moonlight: Abiotic Drivers of Reproductive Development in an Estuarine-Dependent Prawn

Lunar phase is regarded as a key driver of reproductive development and spawning activity in prawns, but across smaller-temporal scales, other processes may have a significant influence. Here, we examined the effects of lunar phase and other abiotic variables on the reproductive development of an estuarine-dependent prawn, Metapenaeus bennettae Racek & Dall. We found that turbidity, salinity, water temperature and lunar phase each had a significant independent influence on the likelihood of female prawns being ripe throughout the spawning period. The likelihood of catching a ripe female increased on the new moon and first quarter of the moon and also during conditions of low turbidity, high salinity and high water temperature. For male prawns, however, significant interactions between salinity and lunar phase and also between turbidity and lunar phase were detected. These interactions indicated that salinity and turbidity have variable effects among the different lunar phases. Such complex relationships of reproductive timing and spawning success of M. bennettae, and potentially other aquatic species, are most likely adaptive responses to the highly variable conditions of estuarine environments.