icolls - Sustainable Tourism CRC

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ECOLOGY, THREATS AND MANAGEMENT OPTIONS FOR SMALL ESTUARIES AND ICOLLS Sampling Methods 38 Table 7: Physical properties recorded at the time of sampling for each location Belongil Ck. (High Tide) Belongil Ck (Low Tide) Tallows Ck Date/Time 10/12/03. 11am 10/12/03. 1.30pm 15/12/03. 11am pH 7.91 7.98 7.9 DO 6.74ppM 6.03ppM 5.21 Temp. 24.6 o C 28.6 o C 26.4 o C Cond. 47.9mS 44.6mS 10.2mS Salinity 29.0ppK 26.9ppK 5.62ppK Fish were collected using a 20m seine net with 6mm mesh trawled over predominately sandy substrates in close proximity to littoral vegetation and submerged structures and debris. Seine trawls were conducted between 9:00am and 2:00pm on each sampling occasion. Captured fish were removed from the net after each trawl and placed immediately into an ice-slurry for transportation back to the laboratory for processing. Sampling of potential prey items was also conducted using a dip net and a small seine net with a mesh size < 2mm. All components of the food web were sampled across a range of habitat types representing those available at each site. Sampling protocols were identical to those outlined in Chapters 2 and 3. Sample Processing The total length of each fish was measured and recorded in the laboratory to the nearest 5mm at the conclusion of each field collection. The gut contents of each fish were removed and preserved in 4% formalin. Gutted fish were frozen for proposed tissue sampling for stable isotope analyses of trophic relations (following the methods detailed in Chapters 2 and 3). Two days prior to microscopic investigation the preserved stomach contents were transferred to a solution of 70% ethanol. Before ingested items were removed, the gut cavity contents were dried with blotting paper and excess ethanol allowed to evaporate before being weighed in grams to two decimal places. The entire contents of the digestive tract were removed by dissection and placed in a Petri dish with 70% ethanol. The contents were then sorted taxonomically under a Wild M3Z dissecting microscope (6.5-40X). Prey items were recorded under nine broad dietary categories adapted from existing literature and personal observation of possible prey items found in the food web sampling. Dietary Analysis Gut contents analyses The dietary analyses utilised three main parameters proposed by Hyslop (1980). These methods have been widely adopted for the analysis of teleost diets (Edgar & Shaw 1995; Hyndes, Platell & Potter 1997; Sarre, Platell & Potter 2000; Schafer et al. 2002) and therefore facilitated comparisons between studies. The percent frequency of occurrence of each dietary item in the stomachs of all fish and the relative contributions of each dietary item to the number and volume in the stomach of each fish were calculated. Volumes of dietary categories were calculated indirectly, whereby the items within each diet category were pooled and their percentage contribution estimated against a standard 1mm grid. This method takes into account the stomach fullness of each individual analysed (Hyslop 1980). Gut content analyses were restricted to those individuals whose stomachs contained dietary items at the time of capture. However, individuals with no gut contents were still included in the isotopic analyses of diet composition, as this method uses muscle tissue as an integrated measure of diet over a longer period of time than instantaneous gut content methods (Jones & Waldron 2003). Since volumetric data best represents the relative importance of each dietary category, especially when a combination of both large and small prey items are eaten that may over- or under-emphasise their importance (Hyslop 1980), subsequent analyses were performed using only volumetric data for each dietary category. The use of the volumetric contribution also allows amorphous food items such as algae and detritus to be included for analysis (Hyslop 1980). Furthermore, comparison of diet reconstructions using gut contents and stable isotope methods is facilitated using this parameter (Jones & Waldron 2003). Indices of diversity (Shannon-Wiener, H`) and evenness (J) were calculated to examine the dietary breadth of each species at each location (Marshall & Elliot 1997). These indices have been widely used to assess teleost
ECOLOGY, THREATS AND MANAGEMENT OPTIONS FOR SMALL ESTUARIES AND ICOLLS diets and whilst it is impossible to compare values of H` across studies, this is possible for the standardised evenness value (J) (Hyndes, Platell & Potter 1997). Stable isotope analyses For food web analyses and diet reconstructions based on stable isotope signatures in fish muscle tissue, samples were processed as per the methods outlined in Chapters 2 and 3. Ground and dried sampled were analysed using a continuous flow-isotope ratio mass spectrometer (Micromass Isoprime EuroVector EA300, Manchester, UK) at Griffith University. Isotope ratios are expressed as either δ 13 C or δ 15 N and relate to the ratio of 13 C: 12 C and 14 N: 15 N respectively and values were calculated according to the following equation: δ 13 C or δ 15 N = [(Rsample / Rstandard) - 1] x 1000 where Rsample is the isotopic ratio for the sample and Rstandard is the isotopic ratio of the standard (PeeDee belemnite carbonate for δ 13 C and atmospheric N for δ 15 N). Food web and diet analyses were conducted using IsoSource mixing model software (Phillips & Gregg 2003). Trophic fractionations of carbon and nitrogen signatures were set at 0.2‰ and 1.5‰ respectively, based on values reported in the stable isotope literature (Peterson & Fry 1987; McCutchan et al. 2003; Hadwen & Bunn 2004). Results Catch Data A total of 93 fish, across the four target species, was collected for analyses from Belongil and Tallows Creeks (Table 8). Marked differences in catch rates between systems were evident, with 64 individuals recorded for Belongil Creek in contrast to 24 from Tallows Creek. The catch rates for S. ciliata and A. australis showed the greatest differences between each system, with the highest catches coming from Belongil Creek for both species. Only three individuals of S. ciliata were collected from Tallows compared to 36 for Belongil Creek. Similarly, 13 individuals of A. australis were captured from Belongil Creek and only one small individual from Tallows Creek. Comparable numbers were recorded for each system for the other two species (M. cephalus, P. fuscus). Overall S. ciliata was the most commonly caught species, comprising 41.9% of the teleosts captured at the two sites. Table 8: Catch data and individual characteristics for A. australis, P. fuscus, S. ciliata and M. cephalus in Belongil and Tallows Creek Species Sample Size Belongil Creek Tallows Creek Mean Stomach Weight (g) Mean Total Length (cm) Sample Size Mean Stomach Weight (g) Mean Total Length (cm) A. australis 13 5.53 (0.73) 18.08 (1.12) 1 1.1 (-) 9 (-) Empty 1 - 7.69% 2.54 (-) 17 (-) - - - 0-100mm - - - 1 1.1 (-) 9 (-) 101-150mm 4 2.585 (0.48) 14 (0.58) - - - 151-200mm 6 6.736 (0.62) 18.66 (0.61) - - - 201-300mm 2 7.84 (0.88) 24.5 (0.5) - - - 301-400mm - - - - - - 400mm+ - - - - - - P. fuscus 5 4.06 (1.56) 23.5 (3.27) 12 12.18 (2.2) 33.5 (1.43) Empty 2 - 40% 9.08 (0.66) 32 (0) 6 - 50% 8.06 (3.23) 29.83 (3.49) 0-100mm - - - - - - 101-150mm - - - - - - 151-200mm 1 1.31 (-) 17 (-) - - - 201-300mm 2 5.44 (1.28) 26.75 (0.75) 1 9.41 (-) 30 (-) 301-400mm - - - 5 12.73 (2.61) 34.2 (1.53) 400mm+ - - - - - - M. cephalus 10 21.39 (6.13) 21.25 (3.16) 13 22.38 (15.6) 18.08 (2.75) Empty 0 - 0% - - 0 - 0% - - 0-100mm - - - - - - 39
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