Norman Hall

The information produced by this study will be of value to industry and managers for at least the following reasons. Firstly, the completion of this research satisfies part of condition 5(a) of the fisheries Wildlife Trade Operation accreditation under the Environment Protection and Biodiversity Conservation Act (1999) that allows for continued export from WA’s temperate demersal gillnet fisheries, i.e. “to undertake a study to estimate risk of interactions between (gillnet) fishers and Australian sea lions” in WA. The information generated by this study will be used by the Department of Fisheries, WA, to investigate the appropriateness and design of a future observer program for monitoring ASL/fishery interactions and potentially for developing strategies for mitigating any risks posed to individual ASL colonies by gillnetting. Secondly, the model and information developed during this project could assist WA’s commercial gillnet fisheries to pursue Marine Stewardship Council accreditation to demonstrate the ecological sustainability of this regionally important fishery. Lastly, the information produced by this study will be relevant to imminent discussions on marine park planning in WA.

The Swan-Canning Estuary is highly valued for its ecological, recreational, commercial and indigenous importance (e.g. Seddon 1972, Swan River Trust 2008, 2009). It supports a diverse range of fish species (several of which complete their life cycles in the system and/or are recreationally or commercially important, e.g. Loneragan et al. 1989, Kanadjembo et al. 2001, Hoeksema and Potter 2006), migratory and resident waterbirds (Bamford et al. 2003), submerged and fringing vegetation (e.g. Hillman et al. 1995, Astill and Lavery 2001, McMahon 2001) and a dolphin population (Lo 2009). The Swan-Canning Estuary and its large (ca 125 000 km2) catchment have been subjected to substantial anthropogenic change since European settlement in the early to mid 1800s, and the system is now classified as highly modified (Commonwealth of Australia 2002). These artificial modifications, combined with the ongoing effects of local population growth and climate change, continue to have a wide range of implications for the water quality of this system. For example, reduced river flow due to damming or diversion of the major tributaries and the effects of climate change, increased tidal exchange through widening and deepening of the estuary mouth and extensive clearing of catchment vegetation, have all contributed to rising salinity throughout this system (Hamilton et al. 2001, Thomson et al. 2001, Chan et al. 2002, CSIRO 2009). Changes in the volumes of marine vs riverine flow have also exacerbated the stratification of salinity and dissolved oxygen concentration within the water column, particularly in the upper estuarine reaches where bottom waters become hypoxic during drier periods of the year (Hamilton et al. 2001, Thomson et al. 2001, http://www.swanrivertrust.wa.gov.au/science/river/Content/plots.aspx). This lack of dissolved oxygen has become so extensive that remedial oxygenation of both the Swan and Canning rivers is now undertaken mechanically (http://www.swanrivertrust.wa.gov.au/ science/river/content/oxygenation.aspx). Widespread land clearing, shoreline modification and the growth of surrounding urban and agricultural activity have also resulted in increased surface runoff from the catchment, and thus also of the sediment, nutrient and pollutant loads entering the estuary. These loadings have also risen due to the vast network of drains servicing residential, farming and industrial areas that discharge into the system, and their impacts are further compounded by the reduced flushing of the estuary due to diminishing rainfall (Jakowyna et al. 2000, Swan River Trust 2003, 2009, Foulsham 2009). The system, and particularly its upper reaches, is now considered to be eutrophic to hypereutrophic (Swan River Trust 2009), and the levels of various non-nutrient contaminants in the sediment exceed ANZECC and ARMCANZ Interim Sediment Quality Guideline Trigger Values at several locations throughout the estuary (Nice 2009).

A total of 14,200 fishes was caught in the lower and middle regions of the Leschenault Estuary using a 21.5m seine net in each season between winter 2008 and autumn 2010. This total was only 3% less than the 14,601 fishes caught using the same seine net at the same sites twice seasonally in 1994, i.e. with the same amount of fishing effort. The numbers of species recorded in 2008-10 (36) and 1994 (33) were also similar. The above absence of a marked difference in the abundance of fish is consistent with the similarity in the mean densities of fishes per sample in the two periods. However, the mean number of species per sample and measures of diversity were greater in the current than earlier period. The eight most abundant species in 2008-10 ranked among the 11 most abundant species in 1994. Furthermore, the five most abundant species in 1994, which collectively accounted for ~ 90% of the total number of fish caught in that period, ranked amongst the top six species in 2008-10, recognising, however, that they contributed less, i.e. ~ 69%, to the total number of fish in that later period. These five species were the Elongate Hardyhead Atherinosoma elongata, the Sandy Sprat Hyperlophus vittatus, the Yelloweye Mullet Aldrichetta forsteri, the Silverfish Leptatherina presbyteroides and the Southern Longfin Goby Favonigobius lateralis. The far greater contributions of the most abundant species in 1994 than 2008-10 reflects the extreme dominance of the Southern Longfin Goby and Sandy Sprat in the earlier period. These two species thus contributed nearly 70% to the total number of fishes caught in the earlier period, compared with only 35% by the two most abundant species, i.e. Elongate Hardyhead and Sandy Sprat, in the later period. This helps account for the diversity of the fish fauna being less in the earlier period.

Managers, scientists and fishers now have an understanding of the implications of the age and size compositions, growth and total mortality of Black Bream and Estuary Cobbler and the current status of the stocks of those species in south coast estuaries. In particular, our fisheries-independent data has shown that the abundance of Estuary Cobbler in Wilson Inlet, which contributes by far the most of any estuary, to the commercial catches of this species, has declined markedly over the last 20 years. The outcomes of this project will assist in the development of plans aimed at sustaining the commercial and/or recreational fisheries for Black Bream and Estuary Cobbler and maintaining the environments of estuaries on the south coast of Western Australia. Such management plans can now be based on sound fisheries-independent data on the biology and contemporary status of those two species and knowledge of their relationships with the environment. Furthermore, managers can now be confident that the closure of certain areas within estuaries is an effective tool to protect the stocks of Estuary Cobbler in those estuaries. In addition, the implications of hypersalinity for the stocks of Black Bream within estuaries are now well understood by fishery managers and local communities. A wide understanding by fishers and members of local communities of the significance and benefits of the study has been created through their strong engagement with the research team during the course of the study. In addition, through their involvement in the study, two honours and a PhD student have been trained in contemporary techniques in fisheries science and population and community ecology.

This study was undertaken to estimate the biomasses, biologically sustainable catches and current average annual commercial catches of three fished species of abalone, Haliotis roei (Roei), Haliotis laevigata (Greenlip) and Haliotis conicopora (Brownlip) in the proposed sanctuaries of the Capes Marine Park, south-western Australia. The current annual, catch estimates represent the catches that would be foregone by commercial fishers if the sanctuaries are implemented and will be used to evaluate the potential compensation to fishers (not part of this study). The biomass and catch estimates for each species in the proposed sanctuaries were estimated from a combination of scientific survey data and commercial catch information (provided by fishers) for the proposed sanctuaries. It should be noted that the design, field surveys, analyses of data and writing of this report had to be completed in less than a year, which limited the scope of this study. Commercial abalone fishers who operate in the Capes area were consulted to identify areas where commercial quantities of abalone were known to occur within the proposed sanctuaries. Of the 12 proposed sanctuaries, three were identified as containing commercial stocks of Roei, with one of those zones, i.e. Cape Naturaliste, having two optional configurations. Four sanctuaries were identified with commercial stocks of both Greenlip and Brownlip. Roei, which occur over intertidal and shallow, subtidal reefs, were sampled using 0.5 m2 quadrats along 34 transect lines (136 quadrats) set perpendicular to the shore. Greenlip and Brownlip, which are found in deeper waters over reefs, were sampled using 30 m2 transects (2 transects per site) at 116 randomly selected sites (232 transects) within the areas identified by commercial fishers. The numbers and shell lengths of all abalone were recorded, and length-weight (total and bled meat weight) relationships were determined for each species from sub-samples taken from a range of sites, which thereby enabled estimation of the weights of all individual abalone recorded in the surveys. For all three abalone species of abalone, industry harvests abalone at lengths above the minimum legal length (MLL) for capture. The minimum size at which Roei is harvested commercially in the Capes region is 70 or 75 mm, depending on location within the region (cf. 60 mm MLL); minimum sizes for Greenlip and Brownlip in the region range from 150 to 153 mm (cf. 140 mm MLL for Greenlip and Brownlip). Between 15 and 39% of Roei measured in the proposed sanctuaries surveyed were above the respective minimum size at which it is harvested commercially in those areas. In comparison, nearly half of the Greenlip and Brownlip in the proposed sanctuaries were above the minimum size at which these species are harvested in those areas.

This study provides the sound quantitative data that are required by managers for developing plans for conserving the stocks of the Western Blue Groper Achoerodus gouldii, the Blue Morwong (previously Queen Snapper) Nemadactylus valenciennesi and the Yellowtail Flathead (previously Bar-tailed Flathead) Platycephalus endrachtensis in south-western Australian waters. The first two species are commercially and recreationally important in coastal waters and the third is one of the most important angling species in the Swan River Estuary. All three species have been identified by managers as requiring detailed studies of their biology, and Blue Morwong and Yellowtail Flathead are among a small suite of species selected as indicator species for the status of fish populations in marine and estuarine waters, respectively, in south-western Australia. As juveniles, Western Blue Groper typically occupy reef areas in protected inshore waters along the coast and around neighbouring islands. As the individuals of this species increase in size, they move offshore to deeper and more exposed waters over reefs. Spawning occurs in the latter environment, between early winter and mid-spring. The maximum length and age we recorded for Western Blue Groper were 1162 mm and 70 years, respectively, the latter age being the greatest by far yet recorded for any species of wrasse. However, most of the growth of this species occurs in the first 20 years of life. The Western Blue Groper is shown to be a monandric protogynous hermaphrodite, namely all of its individuals begin life as females and, after maturing, many subsequently change sex to males. Females typically first become mature at about 650 mm and 15-20 years and typically change to males at lengths of about 800-850 mm and ages of about 35- 39 years. As sex change takes place over a narrower range in lengths (650 to 900 mm) than in ages (15 to 49 years), that change is apparently related more to size than age. The fact that sex change is typically accompanied by a change in body colour from green to blue can be used to determine the approximate size at which females change to males, without having to cut open the fish to determine whether it possesses ovaries or testes. Growth curves fitted to the lengths at age of individuals of each sex of this hermaphroditic species using a novel technique demonstrated that, with increasing age, the lengths of males became increasingly greater than those of females. Thus, at ages 15, 30 and 60 years, the “average” lengths of females were approximately 600, 670 and 680 mm, respectively, those of males were approximately 695, 895 and 975 mm, respectively. As the Western Blue Groper is very long-lived and maturity and particularly sex change occur late, it is potentially very susceptible to overfishing. Thus, because the mortality estimates and per recruit analyses indicate that, at present, this species is close to or fully exploited, fisheries managers will need to take a precautionary and watchful approach to managing and thus conserving the stocks of this species. As with Western Blue Groper, the Blue Morwong moves to deeper, offshore waters as it increases in size and then matures and spawns in those waters. Although Blue Morwong has a maximum length of close to 1 m and thus, like Western Blue Groper, is a moderately large fish species, it has a far shorter life span, namely 21 years compared with 70 years. While female Blue Morwong do not grow to as large a size as their males (max. lengths = 846 and 984 mm, respectively), the maximum age of both sexes was 21 years. From the growth curves, the average lengths attained by ages 3, 6 and 10 years were 435, 587 and 662 mm, respectively, for females, compared with 446, 633 and 752 mm, respectively, for males. Both sexes exhibited little growth after 10 years of age. Juveniles of Blue Morwong less than 400 mm in total length were found exclusively in shallow, coastal waters on the south coast, whereas their adults were abundant in offshore waters of both the south and lower west coasts. The lengths and ages at which females and males typically mature in offshore waters of the south coast were about 600-800 mm and about 7-9 years. In contrast, the vast majority of females caught in offshore waters of the lower west coast (where they were of a similar length and age range to those in offshore waters on the south coast) became mature at lengths of 400-600 mm and 3-4 years of age. The attainment of maturity by Blue Morwong at far lesser lengths and ages on the lower west coast than south coast suggests that the former coast provides better environmental conditions for gonadal maturation and spawning. Furthermore, the contrast between the almost total absence of the juveniles of Blue Morwong in nearshore waters on the lower west coast and their substantial numbers in comparable waters on the south coast indicates that the larvae of this species produced on the lower west coast are transported southwards to the south coast, where they become juveniles. As spawning occurs between mid-summer and late autumn, the larvae, which spend a protracted period in the plankton, would be exposed, on the lower west coast, to the influence of the southwards-flowing Leeuwin Current at the time when that current is strongest. Although Blue Morwong is caught by recreational line fishing and commercial gillnet fishing when they are as young as 3-4 years, they do not become fully vulnerable to these fisheries until they are about 9 years old. Consequently, the individuals of this species can potentially breed over a number of years before they become particularly prone to capture by fishers. Mortality estimates and per recruit analyses suggest that the Blue Morwong in south-western Australia is currently not overfished. A greater resilience to fishing by Blue Morwong than Western Blue Groper reflects, in part, its shorter lifespan, gonochorism (namely, it is not hermaphroditic) and early maturity. The Yellowtail Flathead spawns in the Swan River Estuary between late spring and early autumn and completes the whole of its life cycle in this system. Although its females attain a far larger length (615 mm) than its males (374 mm), this species, unlike some of its relatives, is not a protandrous hermaphrodite, namely, it does not change from male to female with increasing body size. As the maximum age of both sexes is eight years, the far greater length attained by females is largely related to the far faster growth of that sex. Females outnumbered males in each age class in which the sample size exceeded 25, with the overall sex ratio being 2.7 females: 1 male. As the minimum legal length for retention of Yellowtail Flathead is 300 mm, and relatively few males exceed this length, the recreational fishery which targets this species is largely based on its females. The estimates of mortality and results of the per recruit analyses provided no evidence that the Yellowtail Flathead is currently overfished. From a management point of view, it is advantageous that the current size limit for Yellowtail Flathead exceeds the average length at which its females (259 mm) attain maturity. Furthermore, this species appears to be resilient to capture and release. The biological data provided in this study will be very useful for the ongoing development of management policies for three important commercial and/or recreational species in south-western Australian waters and will alert managers to the need to monitor closely the status of Western Blue Groper.

Data on the fish communities and environmental conditions in three normally-closed estuaries (Stokes, Culham and Hamersley inlets) on the central south coast of Western Australia have been obtained seasonally for three years. The sampling regime and analyses were designed so that the data and their implications would be of value to both fisheries and environmental managers. Salinities in all three estuaries rose as a result of a combination of salt loading through land clearing, dry winters and high evaporation rates during summer. These increases were most marked in the Culham and Hamersley inlets, eventually resulting in the salinities in these two estuaries exceeding by several times that of sea water. Massive mortalities of Black Bream occurred in these two estuaries when salinities were approximately twice that of sea water, a finding that has been published in an international journal. The development of extremely high salinities was accompanied by a reduction in the number of species and density of fish in Culham and Hamersley inlets, with only a small species of hardyhead surviving when salinities reached levels equivalent to four times sea water. Dietary data emphasise that Black Bream is a highly opportunistic omnivore and thus able to withstand major changes in potential food types. Survival by Black Bream over several years was greatest in Stokes Inlet, the most environmentally stable estuary. Growth of Black Bream varied greatly among estuaries, which appeared to reflect differences in density rather than diet. The results emphasise that (1) the stocks of Black Bream can only be sustained permanently in the basins of estuaries if the quality of environmental conditions in those systems is maintained at an appropriate level and (2) upstream pools can act as refugia for Black Bream when extreme conditions exist downstream.

The results of this study show that hatchery-reared Black Bream can be used to enhance the stock of the population of this commercially and recreationally important species in the Blackwood River Estuary in which it has become depleted. An initial trial of different stains demonstrated that alizarin complexone was particularly effective for staining the otoliths (ear bones) of Black Bream. The mark on the otoliths, produced by this stain following immersion of hatchery-reared juveniles, was still visible to the naked eye 3.5 years later. Substantial numbers of the stocked Black Bream, which were introduced into the Blackwood River Estuary, were still living at the end of 3.5 years. On average, these individuals did not grow as rapidly as those in the wild population, and unlike the wild fish, not all stocked Black Bream attained maturity by 4 years of age. However, they still grew at a rate that was greater than that in some other estuaries and many did reach maturity by 4 years of age. The Black Bream is thus a particularly good candidate for restocking an estuary as it completes its life cycle within these systems in south-western Australia and consequently any stocked fish are unlikely to move into other estuaries in this region. The ease and relatively low cost of culture of Black Bream and its hardiness and restriction to its natal estuary make the restocking of Black Bream a feasible and economically-viable proposition. This study shows that restocking provides managers with a further and viable option for countering the effects of a decline in a stock of Black Bream in an estuary.

The biological data required by fisheries managers to develop plans for conserving fish stocks have been obtained for Mulloway and Silver Trevally in Western Australian waters. The first species is commercially and recreationally important and the second is caught in large numbers by recreational fishers. We have produced, for managers in the Department of Fisheries Western Australia, data on crucial aspects of the biology of Mulloway and Silver Trevally. These include data on habitats, spawning periods, size and age compositions, growth and reproductive biology in the northern and southern parts of the distribution of these two species on the west coast of Australia. Although all of the results are relevant to developing management plans for conserving the stocks of these two important species, the following are of particular relevance. The females and males of Mulloway typically reach first maturity at lengths of about 930 and 880 mm, respectively. These lengths, which are usually attained at 5 - 6 years in age, are far greater than the minimum legal length (MLL) of 500 mm for the retention of this species. In the case of Silver Trevally, the average length of females at first maturity is 60 mm greater than the current MLL of 250 mm and thus the females are exposed to one year of fishing mortality before they are able to spawn. Thus, managers will need to assess whether the current MLLs for the above two species, and particularly for Mulloway, are appropriate for ensuring that the stocks of these heavily-fished species are conserved. Management policies will also need to take into account the schooling behaviour of Mulloway at spawning as this results in this species becoming an easy target for fishers at a crucial stage in its life cycle.

Data have been collected on those crucial aspects of the biology of Blue and King Threadfins, Estuary Rockcod, Malabar Grouper and Mangrove Jack that are required to develop appropriate management plans for conserving the stocks of these five commercially and recreationally important species. The following biological data have been obtained. (1) The size and age at which each species reaches sexual maturity. (2) The size and age at which the two species of threadfin change from male to female and the Estuary Rockcod and Malabar Grouper change from female to male. Note that, unlike the above four species, the Mangrove Jack is not hermaphroditic and thus does not change sex. (3) The habitats, size and age compositions, duration and location of spawning, and mortality of each species. As the two threadfin species are largely restricted to areas over bare substrate in nearshore waters, they are particularly accessible to fishers. Our results indicate that, currently, the Blue Threadfin is fully exploited and the King Threadfin is over-exploited, whereas the fisheries for the Estuary Rockcod, Malabar Grouper and Mangrove Jack are apparently sustainable at current fishing levels. However, our results emphasize that fishing mortality has a very marked adverse impact on the abundance of the ultimate sex of the four hermaphroditic species, and this needs to be considered when specifying legal lengths for retention. Managers also need to monitor the status of the stocks of Mangrove Jack which, because of its high value, is attracting an increasing amount of attention from the recreational, commercial and charter boat fishing sectors. Finally, the results of this study emphasize the pressing need to develop better methods for determining the natural mortality of fish species and thus being able to derive more robust estimates of fishing mortality.