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Fishery Bulletin 107(3) 
Table 1 
Summary of mean (±standard deviation) key technical, environmental, and biological variables collected during the deployments 
of trawls to assess the discard mortality of school prawns ( Metapenaeus macleayi) in the Clarence and Hunter rivers during 2007 
and 2008. The number of replicate deployments is in parenthesis. 
Variable 
Clarence River 
Hunter River 
Technical 
Deployment 
Speed (m/s) 
1.4 ±0.2 (32) 
1.3 ±0.1 (16) 
Depth (m) 
7.3 ±1.8 (32) 
6.3 ±1.7 (16) 
Environmental 
River temperature (°C) 
Surface 
23.9 ±1.1 (32) 
22.5 ±0.8(16) 
Bottom 
23.8 ±1.1 (24) 
22.8 ±0.3 (16) 
River salinity (psu) 
Surface 
3.7 ±2.3 (32) 
4.3 ±1.8 (16) 
Bottom 
5.4 ±1.9 (24) 
10.2 ±5.8(16) 
Cloud cover (%) 
42.9 ±38.8 (32) 
3.8 ±6.2 (16) 
Air temperature (°C) 
22.7 ±3.3 (32) 
22.6 ±4.5(16) 
Water tray 
Dissolved oxygen (mg/L) 
5.8 ±1.0 (16) 
3.8 ±0.8 (8) 
Temperature (°C) 
24.4 ±1.6 (16) 
22.0 ±1.6(8) 
Salinity (psu) 
2.5 ±1.5 (16) 
4.6 ±2.3 (8) 
Catch air exposure (min) 
2-min conventional tray 
13.8 ±5.1 (8) 
6.3 ±1.9(4) 
15-min conventional tray 
23.5 ±6.0 (8) 
20.0 ±2.7 (4) 
2-min water tray 
2.0 ±1.0 (8) 
1.0 ±0.0 (4) 
15-min water tray 
1.6 ±0.7 (8) 
1.3 ±0.5 (4) 
Biological 
Weight of catch (kg) 
Total 
11.8 ±7.4 (32) 
19.4 ±16.1 (16) 
School prawns 
9.7 ±6.6 (32) 
18.0 ±15.7 (16) 
Carapace length (mm) of trawled-and-monitored school prawns: 
15.3 ±2.6 (32) 
15.1 ±3.8(16) 
tors: treatment of prawns (four trawling and one control 
group); sample times (T 0 , T 24 , T 120 and T 120 ); deploy- 
ments (n = 2); and cages (n= 3). The factors “sampling 
time” and “treatment of prawns” were considered or- 
thogonal to each other and fixed. Deployments were 
random and nested in the treatment of prawns, whereas 
cages were nested in all factors. Data were transformed 
as required and tested for homogeneity of variances 
by using Cochran’s test. Any missing replicates were 
replaced with the cell mean and the residual degrees of 
freedom were adjusted accordingly. Significant .F-ratios 
of interest were examined with Student-Newman-Keuls 
multiple comparisons of means tests. 
Results 
School prawns represented more than 82% of the total 
catch from the Clarence River and 93% of the total 
catch from the Hunter River, and although their mean 
catches per 30-min deployment were quite variable, 
were still within the ranges typically experienced in 
the fishery (Table 1). Bycatches were comparatively low 
in both experiments and, in addition to juvenile school 
prawns (approximately 9% of the total school prawn 
catch in each river), mostly comprised fish <200 mm 
total length, such as yellowfin bream (Acanthopagrus 
australis ), southern herring ( Herklotsichthys castel- 
naui), silver biddy (Gerres subfasciatus ), and narrow- 
banded sole ( Aseraggodes macleayanus). 
Many of the technical and environmental variables 
were comparable between experiments and except for 
salinity and cloud cover, remained fairly consistent 
among replicate deployments (Table 1). In both rivers, 
there was evidence of a salinity gradient between the 
surface and bottom (Table 1). Catches were subjected 
to similar mean durations of air exposure during the 
15-min conventional-tray (>20 min) and all water- 
tray (<2 min) treatments during both experiments. 
By contrast, and although highly variable, the mean 
duration of air exposure for catches handled during 
the 2-min conventional-tray treatment in the Clarence 
