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Aspects of the ecology and zoogeography of fishes from soft-bottom habitats of the Tasmanian shore zone
thesisposted on 2023-05-26, 02:35 authored by Last, PR
The fish faunas of the shore zone of coastal lakes, estuaries and beaches of Tasmania were sampled seasonally over a two year period. Environmental characteristics of this zone were described and a classification of habitats was proposed. Three broadly defined soft bottom environments were identified. The first, a closed and semi-closed esturine environment, contained a complex of habitats including coastal lakes, bar-dammed lagoons and rivers and beach-dammed lagoons. A second type of environment consisted of a range of estuarine systems, such as open lagoons, bay estuaries, tidal rivers and tidal creeks, that are permanently open to the sea. Beach habitats comprised the final environment type and these varied in their exposure to wave action from sheltered to exposed. In general, most habitat types were found to be geographically variable and highly complex which stemmed mainly from regionally differing climatic factors, tidal regimes and degrees of wave exposure. Soft-bottom habitats did not represent clearly defined biotopes for fish. Instead the 125 species collected in daytime samples were assembled into 4 community types: a coastal freshwater assemblage; an estuarine assemblage; a sheltered beach assemblage; and an exposed beach assemblage. Some species occurred in more than one of these and 7 species were ubiquitous to all four. Compositional variations within each community type and their interactions with changes in substrate, tidal state and salinity were also examined. The community structures and habitat usages of fishes from two estuaries were examined in detail. Fishes living over a sandflat in the Derwent Estuary exhibited diel and seasonal changes in abundance and diversity. More species and individuals and larger fish occurred on the beach at night than during the day. Juveniles of several offshore species were also present within the fauna at night and their occurrences were probably jointly related to the closeness of the sampling site to deep water and a predator evasion response. Seasonal changes in species composition were evident and these were better demonstrated through the use of multivariate analyses than by diversity indices. The 50 fish species sampled, of which 56% were benthopelagic, 40% were benthic and 4% were pelagic, exhibited a high degree of family diversity. More than half of these species were resident in this habitat throughout the year, although some diel transients occurred only on the sandflat in the night or during the day. Seasonal transients consisted of fresh/brackish water invaders during winter and spring and marine invaders during summer and autumn. Transients consisted of several marine species and a few estuarine, migratory euryhaline freshwater and anadromous species. The distributions and habitat usages of the Great Swanport Estuary and adjacent marine beach by 54 fish species were also investigated during a 2 year sampling programme. Environmental characteristics of the estuary were also studied in detail. Fish were sampled monthly at 7 sites which extended from the semi-exposed beach adjacent to the estuary to a brackish part of the upper estuary. Three assemblages were recognised: a beach assemblage; a lower estuary assemblage; and a middle and upper estuary assemblage. Aspects af reproduction, size structure and trophic relationships of species in these assemblages are discussed. Ten of the 17 major species appeared to spawn in estuaries whilst only one species spawned off the beach. The main spawning period was late spring- early summer and was followed by an influx of juveniles into the estuary during summer and autumn. Most fishes were opportunistic feeders but amphipods were the major prey items consumed. A checklist is provided for the 216 fish species, more than a third of the fishes occurring in the Tasmanian region, that have been recorded from Tasmanian estuaries. Apart from 12 estuarine species, 9 diadromous species and 4 euryhaline freshwater species, this fauna consists of coastal marine fishes which venture into estuaries with dramatically varying degrees of penetration and frequencies of occurrences. The freshwater component in estuaries, which was shown to be large in the classical curve of Remane, was an insignificant part of the fauna in Tasmanian estuaries. About three-quarters of the Tasmanian estuarine fish fauna is endemic to Australian waters. A Maugean element is dominant within the the fauna although relationships to Flindersian, Peronian and New Zealand faunas are evident. Antarctic and tropical elements are poorly represented. At the family level, the Tasmanian estuarine fauna exhibits a close association with other temperate regions of the Southern Hemisphere. Temperate estuarine fish faunas of Northern and Southern Hemispheres are distinct whilst those in the North Pacific differ from those of the North Atlantic. Tasmanian estuaries and sheltered bays are nurseries for several fishes, however, unlike the situation in most other temperate regions, few adult commercial species live in these habitats. Current legislation relating to the management of fish resources in local estuaries is inappropriate and requires a rationalistic review.
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