Project number: 1999-217
Project Status:
Budget expenditure: $90,100.00
Principal Investigator: Rod Connolly
Organisation: Griffith University Nathan Campus
Project start/end date: 12 Jul 1999 - 30 Jul 2003


An examination of which fisheries species are sustained by seagrass plant production has been highlighted as a major research priority in the recent reviews of fisheries habitat research gaps by Cappo et al. (1997) and Butler & Jernakoff (draft report to FRDC). The recommended method in Butler & Jernakoff for tracing seagrass production to fisheries species is stable isotope analysis. Coastal and fisheries managers currently consider seagrass to be valuable, nevertheless there are many seagrass meadows under threat and still being lost. An argument can be developed, supported by current scientific evidence, that many important fisheries species are not reliant on seagrass and that their numbers actually increase upon the decline of seagrass. Estuarine and offshore fisheries species that do not appear to be dependent on seagrass might actually be so, but indirectly; they may be deriving their food from animals in a trophic web that is sustained by energy (carbon) and nutrients (e.g. nitrogen) transported from seagrass meadows. Another estuarine habitat, mangrove forest, has previously been touted as generating plant production that drives food webs elsewhere in estuaries and offshore. Recent evidence from Australia and Asia suggests this is not so; mangroves seem to sustain only species living in mangrove areas. The question whether seagrass production is the major source of primary production sustaining fisheries production needs answering. The best method for tracing where fisheries species gain their nutrition is stable isotope analysis.

The proposed research will be done in Moreton Bay and Hervey Bay. These bays are of extraordinary importance to Queensland fisheries, with Moreton Bay alone comprising up to 30% of the total Queensland catch of inshore recreational and commercial species (Tibbetts & Connolly 1998). There are also important fisheries in deeper waters adjacent to these bays. Both bays have extensive areas of seagrass, but also mangroves, saltmarsh and occasional reefs offshore. They are also suffering ongoing seagrass loss.


1. Determine the ultimate source of primary (plant) production sustaining fisheries production of several key species of fish and crustaceans in subtropical Australian waters.
2. Quantify the contribution of seagrass meadows to fisheries species found outside seagrass areas, either elsewhere in estuaries or offshore.
3. Ensure that information about the relative importance of seagrass to production in different fisheries is taken to fisheries and other coastal managers to influence future management decisions.

Final report

ISBN: 0-909291-73-X
Author: Rod Connolly
Final Report • 2003-07-16 • 1.33 MB


Results from this project affect the relative importance coastal managers will place on different estuarine habitats.  Until now primary production from mangrove forests has been ranked highly for its presumed contribution to fisheries species occurring seaward of mangroves.  This project has shown, however, that in subtropical Australian estuaries and bays, fish and crustaceans caught over shallow mudflats are much more likely to obtain substantial nutrition from seagrass meadows and in situ production of microalgae.  Mudflats lacking conspicuous vegetation not only provide habitat for certain key fish and crustacean species but also seem to play an important trophic role.  The project also developed quantitative techniques for analysing stable isotope data.  These have already been taken up by other scientists, and will help them answer big picture questions about fisheries foodwebs that have appeared intractable.

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Department of Primary Industries and Regions South Australia (PIRSA)