icolls - Sustainable Tourism CRC

ECOLOGY, THREATS AND MANAGEMENT OPTIONS FOR SMALL ESTUARIES AND ICOLLS
Sampling Methods
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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