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Fish and macrobenthic faunas of the upper Swan Estuary: Interrelationships and possible influence of a dinoflagellatae bloom

Kanandjembo, Angie-Riitta Nangula (1998) Fish and macrobenthic faunas of the upper Swan Estuary: Interrelationships and possible influence of a dinoflagellatae bloom. Masters by Research thesis, Murdoch University.

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Fish and macrobenthic invertebrates were sampled in the shallow (< 2.0 m) and deeper waters (> 2.5 m) of the upper Swan Estuary in each season between the winter of 1995 and the autumn of 1997. The diets of four abundant benthic teleosts in this region were also determined in each of those seasons. Further sampling was undertaken prior to, during and after large blooms of dinoflagellates were present in that part of the estuary to determine whether these blooms had a deleterious effect on the fish and macrobenthic faunas.

A total of 18351 fish, representing 31 species, was caught by seine netting in the shallow waters of three regions in the upper Swan Estuary. These regions were the upstream and downstream saline regions of the Swan River and the saline region of the Canning River. The most abundant fish species by far in the shallow waters were Engraulis australis and Nematalosa vlaminghi, which collectively contributed 55.6% of the total fish catch. The next seven most abundant teleosts included the benthic and estuarine-spawning species Leptatherina wallacei, Amniataba caudavittata, Pseudogobius olorum and Acanthopagrus butcheri, which were used for the dietary analyses. The most abundant species caught using gill nets in the deeper waters were N. vlaminghi, A. caudavittata and A. butcheri, these species collectively contributing 88.5% to the total fish catch in those waters.

The compositions of the ichthyofauna in the shallows of the two Swan River regions differed from that in the Canning River. The number of species, density and biomass in both shallow and deeper waters, were highest in summer and autumn, when salinities were highest, and were lowest in winter, when salinities were lowest. The compositions of the fish fauna in the shallow and deeper waters in summer and autumn also differed from those in winter, a difference that reflected in part the presence of greater numbers of N. vlaminghi and A. caudavittata which spawn and provide 0+ recruits in that area during those two seasons.

The 31 species recorded in the shallows of the upper Swan Estuary contained 14 marine species, which contributed oz 11% to the total number of fish caught. Although the number of estuarine-spawning species was the same as that of marine species, i.e. 14, this group made a far greater contribution, i.e. > 69%, to the total abundance of fish. The contribution made to the total abundance of fish by marine species was also lower than that of estuarine-spawning species in the deeper waters, i.e. 10.1 vs 41.2%. The semi-anadromous N. vlaminghi made relatively high contributions to the total numbers in both the shallow (19.6%) and deeper waters (48.6%).

Forty two species of macrobenthic invertebrates were collected using an Ekman grab in the shallow and deeper waters of the three regions in the upper Swan Estuary. The bivalve mollusc Arthritica semen contributed 25.6 and 27.7% to the total number of macrobenthic invertebrates in the shallow and deeper waters, respectively. The polychaetes Boccardiella limnicola (16.8%) and Ceratonereis aequisetis (25.2%) also made substantial contributions in the shallow waters, as did a sabellid species (24.6%) in the deeper waters. The number of species and number of individuals of macrobenthic invertebrates were both greater in the shallow than deeper waters. Furthermore, the compositions of the macrobenthic invertebrate fauna in the shallow and deeper waters differed, reflecting the relatively greater abundance of the bivalve mollusc Xenostrobus securis, the nereid polychaete Ceratonereis aequisetis and the spionid polychaete Boccardiella limnicola in the shallows. Although the composition of the invertebrate fauna did not differ between regions and did so only marginally between seasons, it did differ between the two years of the study.

The composition of the diets of the two small species L. wallacei and P. olorum, with maximum lengths of 73 and 54 mm, respectively, and the two larger species, A caudavittata and A. butcheri, with maximum lengths of 240 and 415 mm, respectively, were significantly different from each other. This feature is reflected both by the results of classification and ordination and by the very limited prevalence of significant dietary overlap between the diets of the different size classes of these four species.

Leptatherina wallacei consumed both epibenthic and benthic crustaceans, including copepods, amphipods and cladocerans, as well as prey found throughout the water column. In contrast, P. olorum ingested mainly diatoms and detritus and, to a lesser extent, epibenthic amphipods and nereid polychaetes, which suggests that this species feeds predominantly on and within the sediment. Amniataba caiidavittata fed on a wide variety of prey, including nereids, eunicids, spionids and a sabellid polychaete, epibenthic crustaceans, detritus and algae. This wide diversity of prey was reflected by far the greatest value for dietary breadth of any species. Acanthopagrus butcheri was the only species that fed mainly on molluscs, with the estuarine mussel Xenostrobus spp., which was mainly X. securis, constituting 46.8% of it's diet by volume. Leptatherina wallacei, P.olorum, A. caudavittata and A. butcheri underwent ontogenetic changes in diet. Furthermore, the dietary breadth of A. butcheri declined progressively with increasing body size as this teleost focused increasingly on Xenostrobus spp. as a food source.

The trends exhibited by the density of fish in the shallows and by catch rates of fish in deeper waters in the period prior to and during the bloom of dinoflagellates in the upper Swan Estuary, suggests that either a lowering of oxygen and/or the production of toxins may have had a detrimental influence on the abundance of fish. However, no such trends were observed with the macrobenthic fauna and there was no evidence that the blooms led to a change in the compositions of either the fish or macrobenthic faunas.

Item Type: Thesis (Masters by Research)
Murdoch Affiliation(s): School of Environmental Science
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Potter, Ian
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