I. C. Potter

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.

SEASONAL CHANGES IN MOVEMENTS, ABUNDANCE, SIZE COMPOSITION AND DIVERSITY OF THE FISH FAUNA OF THE SEVERN ESTUARY

Estuaries perform a crucial role in the life-cycle of many fish (Cronin & Mansueti, 1971; Day, Blaber & Wallace, 1981; Dando, 1984). They provide a migratory route for anadromous and catadromous species and an environment in which the limited number of true estuarine teleosts spend the whole of their life cycle. The upper reaches are colonized by certain freshwater species while the lower regions are penetrated by some marine fish during high tide. However, many marine teleosts enter and remain within estuaries for a period of time, often in very large numbers and particularly during the early part of life (see e.g. Gunter, 1938, 1961, 1967; Huddart & Arthur, 1971; van den Broek, 1980; Blaber & Blaber, 1980; Potter et al. 1983; Whitfield, 1983). The recognition that estuaries act as important nursery areas for certain teleosts by providing such features as a rich food source and protection from predation (see e.g. Pearcy & Richards, 1962] Jacquaz, Able & Leggett, 1977; Blaber & Blaber, 1980) has led to such species being referred to as estuarine-dependent (see e.g. McHugh, 1976; Beal, 1980; Fortier & Leggett, 1982).

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

Partitioning of food resources amongst 18 abundant benthic carnivorous fish species in marine waters on the lower west coast of Australia

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

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

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.

Ancient colour vision: multiple opsin genes in the ancestral vertebrates.

The volumetric contributions made by prey and plant material to the diets of 4 elasmobranch and 14 teleost species, collected seasonally by trawling from waters along ca. 200 km of the lower west coast of Australia, have been compared. These benthic carnivores, which were all abundant and collectively contributed 83% to the total number of fish caught, represented nine families (Urolophidae, Scorpaenidae, Triglidae, Platycephalidae, Sillaginidae, Carangidae, Gerreidae, Mullidae and Pempherididae). Some species were numerous in both shallow (5-15 m) and deeper (20-35 m) waters and in both northern and southern regions, whereas others were largely confined to one of these water depths or regions. Comparisons between the diets of the different species, which utilised data collected from individuals throughout the study area, demonstrated that the dietary composition of any given species was almost invariably significantly different from that of every other species. This partly reflected the fact that, while errant polychaetes, gammarid amphipods and tanaids were ingested by all species, their contributions to the diets of the different species varied. Furthermore, echinoderms contributed to the diets of just nine species, and this was substantial only in the case of two sillaginid species, while teleosts were never consumed by six species and only made a marked contribution to the diets of the single species of platycephalid. The diet of each species underwent size-related changes, reflecting a shift from the consumption by smaller fish of prey such as amphipods, mysids and copepods, to the ingestion by larger fish of prey such as polychaetes, carid decapods, isopods and small teleosts. The interspecific and intraspecific differences in dietary compositions would spread the food resources amongst and within species, thereby reducing the potential for competition for those resources within the fish community. Non-metric multi-dimensional scaling (MDS) ordination plots emphasised that the dietary compositions of species within each family possessed some obvious similarities, reflecting similarities in body and mouth morphology and feeding behaviour. However, the extent to which the dietary compositions of the different families were similar or different was often not related to the phylogenetic relationships amongst those families. Furthermore, while differences in mouth size and morphology could sometimes be used to account for differences amongst the diets of the full suite of species, this was not always the case. Thus, the ways in which species feed and use their ancillary feeding structures were also employed to help elucidate the basis for variations in diets.

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.

Seasonal and spatial changes in the larval fish fauna within a large temperate Australian estuary

Molecular investigation of the origin of colour vision has discovered five visual pigment (opsin) genes, all of which are expressed in an agnathan (jawless) fish, the lamprey Geotria australis. Lampreys are extant representatives of an ancient group of vertebrates whose origins are thought to date back to at least the early Cambrian, approximately 540 million years ago [1.]. Phylogenetic analysis has identified the visual pigment opsin genes of G. australis as orthologues of the major classes of vertebrate opsin genes. Therefore, multiple opsin genes must have originated very early in vertebrate evolution, prior to the separation of the jawed and jawless vertebrate lineages, and thereby provided the genetic basis for colour vision in all vertebrate species. The southern hemisphere lamprey Geotria australis (Figure 1A,B) possesses a predominantly cone-based visual system designed for photopic (bright light) vision [2. S.P. Collin, I.C. Potter and C.R. Braekevelt, The ocular morphology of the southern hemisphere lamprey Geotria australis Gray, with special reference to optical specializations and the characterisation and phylogeny of photoreceptor types. Brain Behav. Evol. 54 (1999), pp. 96–111.2. and 3.]. Previous work identified multiple cone types suggesting that the potential for colour vision may have been present in the earliest members of this group. In order to trace the molecular evolution and origins of vertebrate colour vision, we have examined the genetic complement of visual pigment opsins in G. australis.

Factors influencing community structure and distribution of different life-cycle categories of fishes in shallow waters of a large Australian estuary

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.

The biology of the blue manna crab Portunus pelagicus in an Australian estuary

A total of 66814 fish larvae, representing 37 families and 74 species, were collected in samples taken monthly between January 1986 and April 1987 from 13 sites located at frequent intervals throughout the large Swan Estuary in south-western Australia. The Gobiidae was the most abundant family, comprising 88.2% of the total number of larvae, followed by the Clupeidae (3.4%), Engraulididae (2.9%) and Blenniidae (1.0%). The most abundant species were Pseudogobius olorum (53.3%), Arenigobius bifrenatus (31.2%) and Engraulis australis (2.9%). Abundance of fish larvae in the lower, middle and upper regions of the estuary each reached a maximum between mid-spring and early summer, 2 to 4 mo before the attainment of maximum temperatures. Larvae of species such as Nematalosa vlaminghi and Apogon rueppellii were collected only between November and February, whereas those of others such as P. olorum, E. australis and Leptatherina wallacei were present over many months. The times and locations of capture of larvae have been related to the distribution and breeding periods of the adults of these species. The mean monthly number of species was far greater in the lower than upper estuary (14.7 vs 2.7), whereas the reverse was true for mean monthly concentration (42 vs 197 larvae per 100 m3). Classification, using the abundance of each of the 74 species recorded at the different sites, showed that the composition of the larval fish fauna in the lower, middle and upper estuary differed markedly from each other. Most larvae caught in the lower estuary belonged to marine species, whereas those in the upper estuary almost exclusively represented species that spawn within the estuary. The fact that the larvae of the 59 species of marine teleosts recorded during this study were restricted mainly to the lower estuary, and yet contributed only 6.2% to the total numbers for the whole estuary, helps to account for the relatively high species diversity in this region. The lack of penetration of many of these larvae beyond the first 12.5 km of the estuary presumably reflects the weak tidal effect in the wide basins of the middle estuary and saline regions of the tributary rivers. The larvae of the 13 teleosts that typically spawn within the estuary contributed 93.8% to the total numbers of larvae. Most of these estuarine-spawned larvae belong to teleosts that deposit demersal eggs and/or exhibit parental care (egg-guarding and oral and pouch-brooding), characteristics which would maximize their chances of retention within the estuary.