Mangrove fish communities in tropical Queensland, Australia: spatial and temporal patterns in densities, biomass and community structure
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A.I. Robertson & N.C. Duke


Regular daylight sampling over 13 mo (February 1985-February 1986) in and adjacent to intertidal forested areas, in small creeks and over accreting mudbanks in the mainstream of a small mangrove-lined estuary in tropical northeastern Queensland, Australia, yielded 112 481 fish from 128 species and 43 families. Species of the families Engraulidae, Ambassidae, Leiognathidae, Clupeidae and Atherinidae were numerically dominant in the community. The same species with the addition of Lates calcarifer (Latidae). Aeanthopagrus berda (Sparidae) and Lutjanus agentimaculatus (Lutjanidae) dominated total community biomass. During high-tide periods, intertidal forested areas were important habitats for juvenile and adult fish, with grand mean (+/-SE) density and biomass of 3.5+/-2.4 fish m-3 and 10.9 +/- 4.5 g m-3, respectively. There was evidence of lower densities and less fish species using intertidal forests in the dry season (August, October), but high variances in catches masked any significant seasonality in mean fish biomass in this habitat. On ebb tides, most fish species (major families; Ambassidae, Leiognathidae, Atherinidae, Melanotaeniidae) moved to small shallow creeks, where mean (+/-1 SE) low-tide density and biomass were 31.3+/-12.4 fish m -2 and 29.0+/-12.1 g m -z, respectively. Large variances in catch data masked any seasonality in densities and biomasses, but the mean number of species captured per netting in small creeks was lowest in the dry season (July, August). Species of Engraulidae and Clupeidae, which dominated high tide catches in the forested areas during the wet season, appeared to move into the mainstream of the estuary on ebbing tides and were captured over accreting banks at low tide. Accreting banks supported a mean (+/- 1 SE) density and biomass of 0.4 +/- 0.1 fish m- z and 1.7 + 0.3 g m- 2, respectively, at low tide. There were marked seasonal shifts in fish community composition in the estuary, and catches in succeeding wet seasons were highly dissimilar. Comparison of fish species composition in this and three other mangrove estuaries in the region revealed significant geographic and temporal (seasonal) variation in fish community structure. Modifications and removal of wetlands proposed for north Queensland may have a devastating effect on the valuable inshore fisheries of this region, because mangrove forests and creeks support high densities of fish, many of which are linked directly, or indirectly (via food chains) to existing commercial fisheries.

Main Results and Conclusions:
  • The conclusions of the study are as follows, indicating a need for a total understanding of individual mangrove ecosystems rather than making a generalization for all mangrove forests:
    • “Our work indicates that large-scale destruction of mangrove forests (deforestation), in addition to substantially reducing coastal primary production, which is available for export to other near-shore habitats (Boto et al. 1984, Robertson et al.1989), will have the direct effect of removing a major habitat for fish. (Coastal) Developments which do not remove mangroves, but which increase water-column organic-matter concentrations or dissolved nutrient loads via runoff, are also likely to have significant negative effects on the dense aggregations of fish and zooplankton (Robertson et al. 1988) which inhabit shallow creeks at low tide, since water exchange in these creeks is poor (Wolanski and Ridd 1986) and mangrove-creek waters in Australia are often naturally characterized by low oxygen concentrations, particularly in their upper reaches (Boto and Bunt 1981). Increased biological oxygen demand in these important fish habitats will lead to greatly reduced densities of juvenile fish.
    • “Too often, developers and governments consider these systems” - mangrove swamps in tropical Queensland - “to be homogeneous over large scales with respect to fish-community structure, and that removal of mangroves in a particular catchment will not affect the general fish community in that region. The data we have presented here indicates that this view needs to be modified, to acknowledge that the overall mangrove fish fauna of an area like north Queensland is composed of different fish communities inhabiting a number of estuaries”(378).
Works Cited:

Boto, K. G., Bunt, J. S. (1981). Dissolved oxygen and pH relationships in northern Australian mangrove waterways. Limnol. Oceanogr. 26:1176 1178.

Boto, K., Bunt, K. S., Wellington, J. T. (1984). Variations in mangrove forest productivity in northern Australia and Papua New Guinea. Estuar., cstl Shelf Sci. 19:321-329.

Robertson, A. I., Alongi, D. M., Daniel, P. A., Boto, K. G. (1989). How much mangrove detritus reaches the Great Barrier Reef lagoon? Proc. 6th int. Symp. coral Reefs 2:601 606 [Choat, J. H., et al. (eds.) Sixth International Coral Reef Symposium Executive Committee, Sydney].

Robertson, A. I., Dixon, P., Daniel, P. A. (1988). Zooplankton dynamics in mangrove and other nearshore habitats in tropical Australia.  Mar. Ecol. Prog. Ser. 43:139-150

Wolanski, E., Ridd, P. (1986). Tidal mixing and trapping in mangrove swamps. Estuar., cstl Shelf Sci. 23:759-771.