R.C.J. Lenanton

Comparisons between the roles played by estuaries in the life cycles of fishes in temperate Western Australia and Southern Africa

SynopsisThe rivers of south-western Australia and south-eastern Africa lie at similar latitudes, open into the Indian Ocean and frequently have estuaries that are periodically closed off from the sea by sand bars at their mouths. The present study has compared the species, genera and families represented in the fish assemblages of estuaries in south-western Australia and temperate southern Africa, i.e. below 31°S, and the way in which these estuaries are used by fish. The Clupeidae, Mugilidae, Atherinidae and Gobiidae were important families in both regions. However, the Terapontidae and Tetraodontidae, and the tropical families Apogonidae and Gerreidae, were represented by large numbers of individuals only in the estuaries of south-western Australia. Although 45 out of a total of 112 families and 32 of 233 genera occurred in both south-western Australia and temperate southern Africa, only 15 of the 326 species were found in both regions. The contributions made by the number of marine species which regularly enter estuaries in large numbers (marine estuarine-opportunists) to the total number of species recorded in the estuaries of south-western Australia and temperate southern Africa were similar (13.4 and 12.2% respectively) and the same was also true of species capable of completing their life cycles in estuaries (8.8 and 8.2%). The number of fresh water and diadromous species recorded in both regions was small. By contrast, the species of marine stragglers contributed approximately 70% to the total number of species in both regions. The adaptations of marine estuarine-opportunists and estuarine spawners to life in estuaries, and particularly to the effects of the closure of estuary mouths, is discussed. Although only one marine species was restricted to estuaries at any particular interval of its life cycle in south-western Australia, the juveniles of a number of marine species were confined to estuaries in temperate southern Africa. It is suggested that this difference can be attributed to the presence of a greater area and quality of alternative nursery habitats in the inshore marine environments in south-western Australia than in southern Africa.

Contribution of estuaries to commercial fisheries in temperate Western Australia and the concept of estuarine dependence

Fisheries catch statistics for temperate Western Australia are considered in conjunction with life cycle data to elucidate the importance of estuaries to the commercial and recreational fisheries in this region. The data are used to discuss whether the term estuarine-dependent is strictly applicable to all species of finfish found in abundance in estuaries. Between 1976 and 1984, 96 species of finfish, 7 species of crustaceans and 12 species of mollusks contributed to the large commercial fishery in estuaries, protected coastal areas and open marine waters of temperate Western Australia. The mean annual weight and monetary value (in 1984 terms) of this fishery was 21,355 t and

Influence of site, season and year on contributions made by marine, estuarine, diadromous and freshwater species to the fish fauna of a temperate Australian estuary

A151.3×106. The contribution of the weight (4,340 t) and value (

Stock enhancement as a fisheries management tool

A3.7×106) of the estuarine-dependent species to the total fishery was 20.3 and 2.4%, respectively. Estuarine-dependent marine species frequently use protected inshore waters in temperate Western Australia, and have to do so when they occur in subtropical regions in Western Australia where there are no permanent estuaries. Even the semi-anadromous Perth herring and some species which are estuarinesensu stricto in south-western Australia complete their life cycle within the marine waters of this latter more northern region. Since virtually none of the commercially important marine species in temperate Western Australia can be considered to be entirely dependent on estuaries, and a similar conclusion is valid for many species of marine teleosts found in abundance in estuaries in temperate waters elsewhere in the world, these marine species would be best regarded as estuarine opportunists rather than estuarine dependents.

Spatial and seasonal differences in the fish fauna in the shallows of a large australian estuary

Catches obtained at regular intervals by beach seining, gill netting and otter trawling at ten, four and six sites, respectively, have been used to determine the contribution of the different species and life-cycle categories of fish to the ichthyofauna of the large Swan Estuary in temperate south-western Australia between February 1977 and December 1981. These data were also examined to investigate the influence of site, season and year on the densities of the more abundant species. A total of 630 803 fish, representing 36 families and 71 species, were caught in the shallows using beach seines during this 5 yr study. Although the majority of these species were marine teleosts that were caught infrequently (marine stragglers), representatives of 7 of the 15 most abundant species were marine teleosts which entered the estuary regularly, and in large numbers (marine estuarine-opportunists). Of the remaining 8 most abundant species in the shallows, 7 completed their life cycle within the estuary (estuarine species) and 1 (Nematalosa vlaminghi) was anadromous, feeding for a period at sea and spawning in the upper reaches of the estuary. The contribution of individuals of the marine estuarine-opportunist category to catches in the shallows declined from nearly 95% in the lower estuary, to 17% in the middle estuary and 6% in the upper estuary. The estuarine and anadromous groups made a considerable contribution to the catches in both the middle and upper estuaries. By contrast, the contribution of freshwater species was small and even in the upper estuary accounted for only 0.2% of the catch. Site within the estuary generally influenced the catches of individual species to a greater extent than either season or year, or the interactions between these factors. When seasonal effects were strong, they could be related to summer spawning migrations into the upper estuary (Nematalosa vlaminghi, Amniataba caudavittatus), spring immigrations into the lower estuary (Mugil cephalus), or winter movements into deeper and more saline waters (Apogon rueppellii). Annual variations in the density of Torquigener pleurogramma were related to marked annual differences in the recruitment of the 0+age class.

Influence of environmental variables on the fish fauna of the deeper waters of a large Australian estuary

Stock enhancement has been viewed as a positive fisheries management tool for over 100 years. However, decisions to undertake such activities in the past have often been technology-based, i.e., driven by the ability to produce fishes, with most stock enhancement projects having limited or no demonstrated success. The reasons for this have been due to an inability to identify and/or control the underlying reasons why a fishery is under-performing or not meeting management objectives. Further, stock enhancement has often been applied in isolation from other fisheries management tools (e.g., effort control). To address these issues and consider stock enhancement in a broader ecosystem perspective, a new approach for stock enhancement is proposed. The proposed model comprises four major steps; a review of all information about an ecosystem/fishery/stock and the setting of clear management targets; a comparison of all relevant fisheries management tools with the potential to meet the management targets; the instigation of a scientifically based, pilot-scale, stock enhancement program with clear objectives, targets, and evaluations; and a full-scale stock enhancement program if the pilot project meets the objectives. The model uses a flow-chart that highlights a broad range of scientific and other information, and the decisions that need to be made in relation to stock enhancement and fisheries management in general. In this way all steps are transparent and all stakeholders (managers, scientists, extractive and non-extractive users, and the general public) can contribute to the information collection and decision making processes. If stock enhancement is subsequently identified as the most-appropriate tool, then the stepwise progression will provide the best possible chance of a positive outcome for a stock enhancement project, while minimizing risks and costs. In this way, stock enhancement may advance as a science and develop as a useful fisheries management tool in appropriate situations.

Habitat partitioning by whiting species (Sillaginidae) in coastal waters

Samples of fish were collected by beach seine throughout the shallow waters of the large Peel-Harvey estuarine system (south-western Australia) in the wet (June to November) and dry periods (December to May) between August 1979 and July 1981. The number of species, density and biomass declined with distance from the estuary mouth and rose with increasing temperature and salinity. Both classification and ordination distinguished the faunal composition of the saline reaches of the rivers from that of the narrow Entrance Channel and two large basins (Peel Inlet and Harvey Estuary). Classification also separated the fauna of the riverine group into wet- and dry-period components, and divided samples taken in the Entrance Channel from those in the basins. Differences between the faunal composition of the Peel Inlet and its tributary rivers were related to differences in salinity regime. The riverine fauna was subjected to much more variable and lower minimum salinities. Species characteristic of the rivers included teleosts such as Atherinosoma wallacei and Amniataba caudavittatus, which are estuarine sensu stricto in southwestern Australia, the semi-anadromous Nematalosa vlaminghi and juveniles of the marine Mugil cephalus. The species diagnostic of the wet periods in the rivers were the estuarine species A. wallacei and Favonigobius suppositus, while the dry periods were characterised by the marine species Atherinomorus ogilbyi and Sillago schomburgkii. Marine species also characterised the Entrance Channel (Favonigobius lateralis, Sillago bassensis), whereas the indicators in Peel Inlet and Harvey Estuary were Hyporhamphus regularis and Apogon rueppellii, both of which can pass through the whole of their life cycle in estuarine as well as marine environments.

Blue-green algae and fish population changes in a eutrophic estuary

Fish were collected by gill nets from the deeper waters of the Entrance Channel, basins and rivers of the large Peel-Harvey estuarine system (south-western Australia) in the wet (June to November) and dry (December to May) periods between August 1979 and July 1981. Simple-regression analysis showed that the number of species, abundance and biomass of fish in the rivers rose with increases in the salinity and temperature of both the surface and bottom of the water column. No such significant correlations were found in the Entrance Channel andbasins (Peel Inlet and Harvey Estuary), where salinity changes were far less marked. The number of species at sites throughout the estuary was inversely correlated with distance from the estuary mouth. Multiple-regression equations showed that, compared with the other environmental variables tested, bottom salinity had a greater influence on the nunber of species and abundance both in the rivers and in the system as a whole. These results indicate that salinity has a greater effect on the fauna in the deeper waters than in the shallows (cf. Loneragan et al., 1986). The larger fish which characterise the deeper waters may thus be less tolerant to low salinities than the smaller fish typically found in the shallows. Both classification and ordination separated the faunal composition of the rivers from those of the Entrance Channel and basins. The fauna of the two narrow and deeper sites in the rivers separated into wet- and dry-period components. Differences between the faunal composition of the riverine regions and those of the Entrance Channel and basins have been related to the much more variable and lower minimum salinities in the rivers. Species characteristic of the rivers included Amniataba caudavittatus, which is estuarine sensu stricto in south-western Australia, the semianadromous Nematalosa vlaminghi and the highly euryhaline Mugil cephalus. The indicator species for the Entrance Channel and basins were all marine species (Cnidoglanis macrocephalus, Hyporhamphus melanochir, Gerres subfasciatus and Pomatomus saltator).

Blue-green algal blooms and the commercial fishery of a large Australian estuary

SynopsisThe ways in which the distributions of six species of whiting (Sillaginidae) in the coastal marine waters of south-western Australia are related to the type of substrate (bare sand vs. seagrass), degree of exposure of habitat, water depth and body size have been investigated. Whiting in near shore waters (< 1.5 m) were sampled using a fine-meshed seine net, while those in ‘shallow’ (5–15 m) and ‘deep’ (20–35 m) waters of the inner continental shelf were sampled with a trawl net. Shallow nearshore waters are shown to provide nursery habitats for five of the six whiting species. In these waters, Sillaginodes punctata, Sillago burrus, Sillago schomburgkii and Sillago vittata mainly occur in protected areas, while Sillago bassensis predominantly occupies areas that are more exposed to wave and swell activity. The first three of these species also use estuaries as nursery areas. In nearshore waters, whiting were captured almost exclusively over bare sand, rather than in interpersed beds of the seagrass Posidonia spp., presumably reflecting the fact that the dense canopies produced by the wide blades of Posidonia spp. must inhibit penetration by the benthic whiting species. As 0 + S. punctata increase in size, they tend to move offshore during the day and inshore at night. Many mature representatives of S. schomburgkii are present in nearshore areas, whereas the other four species move offshore into inner-shelf waters as they increase in length. Sillago burrus and S. vittata remain in shallow inner-shelf waters, whereas the larger S. bassensis subsequently migrate into deeper inner-shelf waters. Large Sillago bassensis thus co-occurs with Sillago robusta, which is mainly found in those deeper waters, but does not reach as large a size. The larger S. punctata occupy areas near reefs which could not be sampled by trawl netting. There are thus interspecific differences in (i) the times of recruitment of the 0 + age class into nearshore areas, (ii) the types of habitat occupied during juvenile and adult life, and (iii) the degree to which fish move into more offshore waters as they increase in length, and one species is restricted to deeper waters. The resultant partial segregation among habitats of the coastal waters of south-western Australia by different size groups of these relatively abundant whiting species presumably reduces the potential for intra- and interspecific competition amongst these species.

The use of stable isotope ratios in whitebait otolith carbonate to identify the source of prey for Western Australian penguins

Evidence is presented which indicates that dense blooms of the blue-green alga Nodularia spumigena have affected fish and crab populations in a large estuarine system in south-western Australia. For example, the numbers of fish were generally very low at sites in which the chlorophyll a level, an excellent indicator of Nodularia density during the late spring and summer, was above 100 μgl−1. Moreover, commercial fishermen have recorded greatly reduced catches in Nodularia-affected areas and dead fish and crabs were found in regions where Nodularia was very dense. While the effects of this blue-green alga apparently led to death in the case of some bottom-living species in the most affected parts of the system, more active species moved into regions where Nodularia was virtually absent.