Factors affecting the distribution of the introduced goby Tridentiger trigonocephalus Gill (Gobiidae) in south eastern Australia
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NO FULL TEXT AVAILABLE. Access is restricted indefinitely. ----- Studies of biological invasion have often noted the tendency of invaders to accumulate in disturbed habitats. While this pattern has been observed repeatedly it has been difficult to establish a mechanism: do these habitats possess attributes, such as fewer native species, vacant niches and lower biotic resistance, which make them more susceptible to invasion? The presence of introduced species that persist in such habitats but fail to establish outside these regions suggests that there are factors characteristic of disturbed habitats' that allow the initial invasion of these species, while outside these regions invasion is suppressed. The introduced gobiid T. trigonocephalus (Gill 1858; Family: Gobiidae) was introduced to SE Australia, to Port Jackson in Sydney Harbour (33°51' S) and Port Phillip Bay in Melbourne (37°52' S) in the early 1970's and while populations have persisted since that time the species remains restricted within these estuaries to the . Populations of T. trigonocephalus reach their greatest density around the commercial regions of Port Jackson and Port Phillip Bay and densities are positively associated with physical properties of these regions, such as relief and sediment size. While recruitment to regions outside these core areas does occur, adult populations apparently fail to establish. In Port Phillip Bay, the population of T. trigonocephalus reaches significantly greater densities and is distributed over a broader area than the population in Port J ackson. The positive association in densities of this species with physical and biological indicators (low visibility, deep water, fine sediment grain size and a fish assemblage typical of these regions), suggests that predation may prevent establishment of this species outside the regions where these factors are present. However, pre-settlement factors such as poor reproductive output may also limit the distribution of this species. Field and laboratory studies demonstrated that adult populations in both locations completed a normal reproductive cycle up to the point of gamete production (ovulation in females). Reproduction occurs from early Spring until mid-Summer at both locations, when water temperatures increasing above 22°C appear to trigger gonadal regression. Temperature in Port Jackson reaches this point earlier in the year than Port Phillip Bay and as a result, the reproductive season of T. trigonocephalus is significantly shorter in Port Phillip Bay. Similar temperatures (about 22°C) trigger gonadal regression in both locations and while regression in the Port Jackson population is triggered at a slightly higher temperature, this is not enough to compensate for the more rapid increase and higher maximum and regression occurs approximately one month earlier than in Port Phillip Bay. In females, temperature affects reproduction by inhibiting ovulation, resulting in oocyte over-ripening and subsequent oocyte atresia and resorption. Males have a slightly higher tolerance to temperature in that testicular regression occurs at temperature slightly higher than those triggering ovarian regression, however the mechanism is less apparent. Temperature also appears to have substantial flow-on effects on the abundance/output of egg and larval stages of T. trigonocephalus. Ichthyoplankton samples from Port Jackson taken in the region supporting the highest population densities of T. trigonocephalus showed a diverse gobiid fauna but, except for a single specimen, a complete absence of T. trigonocephalus. Recruits of this species were observed in Port Jackson indicating that some degree of larval settlement, however this was significantly lower than that recorded in Port Phillip Bay. The success of the invasion of T. trigonocephalus into south-eastern Australia appears to depend on several factors. Disturbed habitats do appear to possess attributes that make them more easily invaded by T. trigonocephalus than other regions; however these appear to be physical attributes rather than biological ones. Thus, there is the potential for T. trigonocephalus to invade any habitat with these attributes, most notably newly dredged regions of harbours or regions of freshwater and sediment input. Outside these habitats biotic resistance does appear to play a role in limiting the invasion success of this species.
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