The construction of canal developments in Australia has attracted widespread criticism as such developments are often considered to adversely affect the fisheries and ecological value of estuaries. However, few studies of fish communities using the habitats that canal developments commonly replace (saltmarshes and upper littoral mangroves), or of canals themselves, have been conducted in Australia and most data relates to North America. The present study examines fish communities of saltmarshes, mangroves and canal developments in the Moreton Region (Noosa Heads to Coolangatta); an area which contains the greatest concentration of canal developments in Australia.
The fishes occurring in the main tidal inlet and upper littoral pools of a saltmarsh situated at Coomera Island (southern Moreton Bay) were studied from January to December 1984 inclusive. A total of 19 species were recorded from the inlet, of which 11 are of economic importance and six species were commonly captured. The diversity of the fish assemblage from the inlet was low when compared with findings of other studies conducted in Australian subtidal mangrove areas. Recapture rates from two mark-recapture programmes conducted at the inlet, indicated that some fish, including species of economic value, regularly utilise the tidal inlets to saltmarsh. It is unlikely that these species rely heavily upon the tidal inlet habitat. Wide variations in pool water levels and salinity were recorded. Eight species of fish were collected from the pools, four of which are of economic importance to fisheries. Three taxa, Gambusia affinis. Pseudomugil signifer. and gobiids, predominated in terms of abundance and occurrence. The abundance of fishes fluctuated markedly on a seasonal basis with few fish being taken in winter and spring (June-November) when pool water levels were low. Results from the present study indicate that fish important to commercial fisheries rarely utilise the pools habitat within saltmarsh.
The fishes occurring in waters adjacent to and within a mangrove forest in southern Moreton Bay, were studied from November 1987 to November 1988 inclusive. Fishes within the mangroves were sampled using a block net, whilst those in adjacent waters were sampled using seine and gill nets. Forty six percent of the species, 75% of the number of fishes and 94% of the biomass (all methods combined) were of direct importance to regional fisheries. The fish community utilising the habitat within the mangrove forest differed from that occurring in adjacent waters in terms of standing crop, density, species composition and diversity index-values. Intermediary carnivores (feeding on macrobenthos) and herbivores were major components of the fish community taken within mangroves but were rarely taken in seine samples of adjacent waters. The importance of mangrove stands to fish of economic value may have been underestimated in previous studies of mangrove areas in the Moreton Region. Planktivores were more abundant in waters adjacent to mangroves than within the mangrove forest. The species composition and dominance of trophic levels in waters adjacent to mangroves was similar to that recorded in studies of subtidal mangrove-lined creeks in subtropical Eastern Australia. Standing crop estimates for the fishes within the mangroves (study period mean ± SD = 25.3 ± 20.4 gm-2) were amongst the highest recorded values for estuarine areas whilst those for adjacent waters (2.9 ± 2.3 gm-2) were comparable to other estuarine studies.
The hydrological conditions and fish fauna in canals and their modified parent estuaries (Nerang River and Tallebudgera Creek) on the Gold Coast were studied from December 1985 to February 1987 inclusive. Sediments analyses indicated that the Nerang River canals, situated in the middle reaches of an estuary, acted as silt traps. In contrast, the Tallebudgera Creek canals (at the mouth of an estuary) did not entrap sediments. Minimal silt deposition occurred only in those canals most isolated from the creek. Stratification of the water column, particularly with respect to oxygen saturation values, was commonly observed in deep (> 2.5 m) dead-end canals but rarely recorded in flow-through canals or in the parent estuaries. 0xygen saturation values of bottom water in deep dead-end canals were often less than 50%, particularly during summer. Canals supported an ichthyofauna similar to the parent estuaries although of lesser abundance, Macrobenthic carnivores occurred less frequently in the canals than in the parent estuaries. Two planktivorous species, Gerres ovatus and Harengula abbreviata. numerically dominated the fish fauna of all sites. The canals generally contained a comparable number of species and exhibited only slightly lower diversity index values than nearby undisturbed mangrove-dominated estuaries. Some canals supported substantial numbers of fish of direct value to recreational fishermen.
Comparisons between the fish communities in the canals and undisturbed saltmarsh/ mangrove habitats (similar to those destroyed in canal development), indicated that canals supported a more abundant fish community that was less diverse and of different trophic composition to that in undisturbed estuarine areas. Fish communities in canals were numerically dominated by planktivores/microbenthic carnivores of no direct importance to fisheries whilst mangrove and saltmarsh habitats were numerically dominated by macrobenthic carnivores/detritivores generally of direct fisheries value.
Future management of saltmarshes should include protection of tidal inlets because they are regularly used as feeding areas by species of fisheries value. The pools of the upper littoral saltmarsh in the Moreton Region are utilised by few species and not of critical direct value to fisheries. The value of mangrove habitat to fisheries may have been underestimated by other fish community studies conducted in the Moreton Region. Mangroves function both as a nursery area for juvenile fishes and as a feeding area for adult fishes, most of which are of direct economic importance. Canals constructed in mangrove or saltmarsh areas result in a loss of fisheries resources. Most canals contain few fish of economic value although some canals may support substantial fish populations. Future management of estuaries should ensure that canals are constructed in non-tidal lands, are adequately flushed to ensure good water quality, preferably contain a physically diverse substrate and/or macrophytic vegetation, and approvals are considered in a regional context.