icolls - Sustainable Tourism CRC

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ECOLOGY, THREATS AND MANAGEMENT OPTIONS FOR SMALL ESTUARIES AND ICOLLS For A. australis individuals there was general agreement between the contributions of prey items to individual diets (Figure 6). However, mixing model analyses indicated that a greater contribution from teleost fish was to be expected on the basis of muscle carbon and nitrogen signatures (Figure 6). Furthermore, difficulties associated with identification of prey items in some guts (high miscellaneous contribution in Figure 6) makes comparisons between these two methods difficult. Figure 6: Comparison of gut contents and stable isotope analyses of percent contribution of prey types to the diet of A. australis individuals in Belongil Creek Given that the diet of M. cephalus is generally dominated by miscellaneous detritus and particulate matter (Table 9 and 10) and that similar catch sizes were recorded in Belongil and Tallows Creeks (Table 8), we used mixing model reconstructions of dietary compositions from primary carbon sources to compare the basis of M. cephalus nutrition in the two sites. Whilst epilithon contributed almost 100% of the carbon and nitrogen in M. cephalus muscles in Belongil Creek, equivalent contributions were made from epilithon, seston and riparian vegetation in Tallows Creek (Figure 7). 42 100% 80% 60% 40% 20% 0% 100% 80% 60% 40% 20% 0% Gut Contents Mixing Model Tallows Belongil Gastropods Polychaetes Decapods Figure 7: Mixing model analyses of sources of carbon contributing to M. cephalus diets from individuals caught in Tallows (n=12) and Belongil (n=13) Creeks Stable isotope analyses revealed that S. ciliata individuals fed on a broader range of prey taxa than was revealed on the basis of gut contents analyses (Figure 8). While the guts of many individuals were full of polychaete worms (Table 9 and 10), isotope data indicates that these prey items had contributed only a small proportion of the dietary carbon and nitrogen in their muscle tissues. Other teleost fish, decapod crustaceans and gastropod molluscs also contributed to S. ciliata diets despite the low abundance or complete absence from the guts of individuals in both sites. Fish Epilithon Seston Miscellaneous Riparian Vegetation
ECOLOGY, THREATS AND MANAGEMENT OPTIONS FOR SMALL ESTUARIES AND ICOLLS 100% 80% 60% 40% 20% 0% Figure 8: Comparison of gut contents and stable isotope analyses of percent contribution of prey types to the diet of S. ciliata individuals in Belongil Creek Dietary Breadth The dietary breadth index of each species was highly variable between locations and across each species (Figure 9). A. australis had the widest dietary breadth (particularly at Belongil Creek, N=13), and relatively high evenness indices for both locations. The greatest differences in dietary breadth and evenness between ICOLLs were evident for S. ciliata. S. ciliata from Belongil Creek had the lowest dietary breadth and evenness of any species from the two locations. In contrast S. ciliata from Tallows Creek had the second greatest dietary breadth across all species and locations with an evenness value (J) of 0.71 (Figure 9). 2.5 2 1.5 H' 1 0.5 0 P. fuscus (Belongil)*3 H' J P. fuscus (Tallows)*6 Gut Contents Mixing Model A. australis (Belongil)*12 A. australis (Tallows)*1 S. ciliata (Belongil)*27 S. ciliata (Tallows)*2 M. cephalus (Belongil)*10 Figure 9: Dietary breadth of P. fuscus, A. australis, S. ciliata and M. cephalus as determined by Shannon- Wiener (H`) and Evenness (J) indices * = No. individuals analysed with prey items) Gastropods Polychaetes Decapods Fish M. cephalus (Tallows)*13 Miscellaneous 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 J 43
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