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The fish fauna of two south-western Australian estuaries: Influence of an artificial entrance channel and of hypersalinity and prolonged closure

Young, Glen Christopher (2000) The fish fauna of two south-western Australian estuaries: Influence of an artificial entrance channel and of hypersalinity and prolonged closure. PhD thesis, Murdoch University.

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The work undertaken for this thesis had the following two broad aims. The first was to determine the characteristics of the fish fauna of the Peel-Harvey Estuary in the mid 1990s, soon after the construction of a large artificial channel, the Dawesville Channel, into this estuary, and to compare these characteristics with those recorded in the early 1980s and thus prior to the construction of that channel. The second was to determine the characteristics of the ichthyofauna of the normally closed Wellstead Estuary, focusing in particular on elucidating the influence of both the extremely high salinities that are found in this system and the opening of the estuary mouth which occurs when freshwater discharge is sufficiently strong to breach the large sand bar at that mouth.

The fish larvae on flood and ebb tides in the artificial Dawesville channel and the original Mandurah Channel of the Peel-Harvey Estuary were sampled at monthly intervals during 1997 using bongo nets. Postlarval, juvenile and adult fish in the Mandurah Channel and two regions in each of the large basins (Peel Inlet and Harvey Estuary) were sampled at regular intervals between the end of 1995 and end of 1997 using seine nets that were 5.5, 21.5 and 102.5 m in length. Nearshore, shallow and offshore, deeper waters of Wellstead Estuary were sampled at eight-weekly intervals between July 1996 and May 1998 using a 41.5 m seine net and composite gill nets, respectively.

The number of fish species caught as larvae was far greater in the Dawesville Channel (49) than in the Mandurah Channel (34), a difference that was mainly due to a far greater number of marine stragglers, and often reef-associated species, in the former channel. The difference in number of species is presumably related to the presence of a better developed reef system just outside the Dawesville Channel and the fact that the velocity and volume of tidal water that is exchanged through this channel is much greater than through the Mandurah Channel. The number of fish larvae caught on flood tides was greater than on ebb tides in both channels, which suggests some individuals of marine species settle in the shallow and slower flowing regions in the channels or just inside the basins. Favonigobius lateralis and Atherinosoma elongata, which spawn in the estuary, tend to be transported out as preflexion larvae on ebb tides and inwards as postflexion larvae on flood tides. The species composition of the ichthyoplankton on both the flood and ebb tides in both channels underwent pronounced and consistent cyclical changes throughout the year, which was clearly related to differences in the spawning times of the various species within and outside the estuary.

The catches of postlarval and early 0+ juvenile fish provide very strong evidence that the majority of marine species, that were caught as larvae on flood and ebb tides in the Mandurah and Dawesville channels, do not become established in the basins of the Peel-Harvey Estuary. However, the catch and length-frequency data for Hyperlophas vittatus indicated that large numbers of this clupeid enter this estuary as postlarvae, settle soon after they enter the estuary and then subsequently slowly penetrate into the basins.

The use of Classification and Multidimensional scaling ordination demonstrated that the species composition of the fish fauna of nearshore, shallow waters in the large basins of the Peel-Harvey Estuary during the 1980s was influenced more by region within the estuary than by time of year, whereas the reverse pertained in the mid-1990s. The shift to a strong seasonal influence on species composition in the latter period is presumably related to the far greater tidal water movement that now occurs in the basin regions as a result of the constmction of the Dawesville Channel. This seasonality in the basins is attributable, in part, to differences in the patterns of immigration and emigration exhibited by the juveniles of marine species. The number of species and overall density of fish were positively correlated with salinity and inversely correlated with distance from estuary mouth. This trend was followed by all species except Leptatherina wallacei, which is known typically to occupy the upper reaches of estuaries where salinities are reduced.

Since the construction of the Dawesville Channel, the density of fish in Peel Inlet has declined, presumably due to the reduction in the amount of macroalgae. Prior to the construction of the Dawesville Channel, when the basins, and in particular Peel Inlet, contained massive growths of macroalgae, the fauna of these regions was dominated by three weed-associated species, namely Relates sexlineatus, Apogon rueppellii and Genes sub fascial us. Although these species thus contributed nearly 60% to the total number of fish caught in the early 1980s, they contributed only about 8% in the mid 1990s after the construction of the Dawesville Channel, when, as a result of increased Hushing, there was a reduction in macroalgae.

The construction of the Dawesville Channel has resulted in the Harvey Estuary becoming (1) directly connected with the ocean at its northern and seaward end, (2) better flushed and (3) far less susceptible to the development of blue-green algal blooms in the spring and early summer, which had previously been shown to have a deleterious effect on fish abundance. These changes account for the greater number of fish species and greater densities of fish that were recorded in the post- Dawesville period than pre- Dawesville period.

Wellstead Estuary remained closed between October 1993 and September 1997 and then opened for just under a month, before once again becoming closed until the end of April 1998 when it opened and remained so until the end of the 24 month sampling period in May 1998. During the study period, mean monthly salinities underwent massive seasonal variations and sometimes varied markedly between regions. For example, mean monthly salinities in the lower estuary rose from 52.9%c in July 1996 to 111.7%c in the following March, before declining precipitously to 14.3%c in September 1997. In March 1997, they ranged greatly from 63.0%c in the upper estuary to 111.7%o in the lower estuary. A total of only 20 species were caught using seine and gill nets throughout this estuary. Not surprisingly, the fish fauna in nearshore, shallow waters was dominated by species which spawn in the estuary, with these species contributing over 98% of all fish caught. In contrast, marine species contributed approximately 58% of the fish caught in offshore, deeper waters. These species were larger teleosts which live for a number of years and moved in and out of the estuary when the bar at the estuary mouth was breached. The species composition in nearshore, shallow waters changed progressively and significantly between the lower, middle and upper estuary, reflecting in part the greater densities of Atherinosoma elongata and Aldrichetta forsteri towards the lower end of the estuary and of Acanthopagrus butcheri, Leptatherina presbyteroides. Pseudogobius olorum and Leptatherina wallacei towards the upper end. Since the fish typically found in nearshore, shallow waters represented species that complete their life cycles within estuaries, it is not surprising that the species composition did not change when the estuary mouth opened. In contrast, the species composition in offshore, deeper waters did not vary among regions but did change after the estuary mouth became open. This change in composition was due to the emigration of Mugil cephalus and Aldrichetta forsteri and the immigration of Arripis georgiana, Arripis truttaceus and Pomatomus saltatrix.

Item Type: Thesis (PhD)
Murdoch Affiliation: Division of Science and Engineering
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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