- Title
- Life history characteristics of glassfish, Ambassis jacksoniensis, adjacent to saltmarsh within a large and permanently-open estuary
- Creator
- McPhee, Jack J.
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2017
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Saltmarsh vegetation, which typically occurs in intertidal areas within estuaries globally, provides an important habitat and feeding ground for estuarine organisms such as crustaceans, gastropods, birds and fish (some of which are of economic importance). Within south-east Australian estuaries, saltmarsh vegetation is both typically bordered by mangroves and tidally inundated three or four times per month during the high tide of the spring tidal cycle (during the day high tide in summer and during the night high tide in winter). In recent decades, saltmarsh vegetation has declined globally due to anthropogenic influence, and in Australia, ‘Coastal Saltmarsh’ is now listed as an Endangered Ecological Community under the Threatened Species Conservation Act 1995. This study was conducted within a representative and relatively “unmodified” saltmarsh habitat (Empire Bay Wetland) in a large and permanently open estuary, Brisbane Water Estuary, located in south-eastern Australia. This study, which was conducted at two markedly different times of the year during 2012, examined the general “response” of the estuarine fish (using seine nets) and zooplankton (using plankton nets) assemblages to tidal inundation, with further emphasis being placed on selected biological and ecological characteristics of the abundant estuarine ambassid, Ambassis jacksoniensis. Abundances of A. jacksoniensis (mean standard length=37.3 mm, ±0.021 (SE)) and overall fish diversity were greater in nightly winter catches than daily summer catches, which is consistent with previous evidence of important feeding times for estuarine fish (including A. jacksoniensis) upon saltmarsh derived zooplankton (e.g. crab zoeae released by saltmarsh-dwelling grapsid crabs), during ebb tides that drain saltmarsh following its inundation. Indeed, zooplankton assemblages were dominated by crab zoeae during ebb tides following saltmarsh inundation, while calanoid copepods dominated these assemblages at other times. Moreover, stomach content analyses of A. jacksoniensis showed that crab zoeae were heavily preyed upon during such times, with dietary “switching” to caridean decapods being evident when crab zoeae were not abundantly present within the water column (i.e. during flood tides and during ebb tides that did not follow saltmarsh inundation; as shown within zooplankton assemblages). Despite their high abundance within zooplankton assemblages, calanoid copepods were not preyed upon by A. jacksoniensis, which is likely to reflect the relatively fast escape responses of calanoids to predators. Further, stomach fullnesses of A. jacksoniensis were generally highest during ebb tides on days of saltmarsh inundation, implying that feeding was most marked at these times. Trophic relay is an ecological model that involves the movement of biomass and energy from vegetation, such as saltmarshes, within estuaries to the open sea via a series of predator-prey relationships. Therefore, the trophic relationship between saltmarsh-dwelling grapsid crabs (which feed on saltmarsh-derived detritus and microphytobenthos), A. jacksoniensis and their predators (which include economically important fish, such as Acanthopagrus australis, Platycephalus fuscus and Argyrosomus japonicus, provides evidence of partial trophic relay within this system, and thus highlights the ecological and economic importance of saltmarsh within this system. The trophic relationship between A. jacksoniensis and its zooplanktonic prey (e.g. crab zoeae, which is of a red/orange colour) was further investigated, for the first time, by comparisons of the calorimetric contribution of its potential prey (i.e. crab zoeae, and the far paler caridean decapods and calanoid copepods), which found no difference in the energetic densities among such potential prey, suggesting that prey (i.e. zooplankton) abundance and/or prey visibility (due to colour) has a stronger relationship than prey energetic density to the diets of A. jacksoniensis. The feeding ecology of A. jacksonsiensis was also explored, for the first time, in light of its various life history characteristics (e.g. the seasonality of sex ratios, sexual maturity and somatic/reproductive growth), with links being made between saltmarsh-derived tropic relay and energetic requirements for reproductive purposes. Thus, the gonads of A. jacksoniensis were found to be generally maturing and ripe during summer, while juvenile/inactive and spent gonads were prevalent during winter, consistent with previous evidence that A. jacksoniensis spawn during summer with a lull during winter. The sex ratios of A. jacksoniensis were also heavily female-biased during summer before equalising (to approximately 1:1) during winter, suggesting that male A. jacksoniensis may avoid the shallow sampling locations (seagrass adjacent to the saltmarsh/mangroves) in a strategy to counteract visual predation from fish and birds during daytime (summer) before returning to these waters during the night winter, during a lull in spawning, for important feeding opportunities. Female A. jacksoniensis, alternatively, may remain in such vulnerable locations due to increased energetic requirements for reproductive purposes (as demonstrated in male vs female somatic/gonadal growth analyses). These findings therefore suggest that the seasonal timing of spawning for A. jacksoniensis may be linked to their feeding behaviours (i.e. upon saltmarsh-derived zooplankton), the latter of which is governed by the tidal inundation of saltmarsh vegetation. As there is a global ecological and economic reliance by fish (particularly A. jacksoniensis) on saltmarshes, which facilitate trophic relay within these systems, it is imperative that management of Australian estuaries is employed in a manner that appropriately incorporates the conservation of saltmarsh vegetation and thus protects its ecological function within these estuaries.
- Subject
- dietary compositions; coastal wetlands; life history; mangroves; estuaries; Australia; calorimetry; glassfish; Ambassis jacksoniensis; zooplankton; Brisbane water estuary
- Identifier
- http://hdl.handle.net/1959.13/1343123
- Identifier
- uon:29095
- Rights
- Copyright 2017 Jack J. McPhee
- Language
- eng
- Full Text
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