Broadhurst et al.: Modified sorting technique to mitigate the collateral mortality of Metapenaeus macleayi 
293 
Figure 3 
Differences in mean (± standard error) predicted percent mortality of 
school prawns ( Metapenaeus macleayi ) between the onboard sorting treat- 
ments and the controls for each of the sampling times in the Hunter 
River during 2008. 
As with the Clarence River analyses, 
there was a significant effect of sam- 
pling time detected in all four mixed- 
effects models that was largely due to 
proportionally more deaths to school 
prawns during the first 24 hours, ir- 
respective of their treatment (Table 3, 
Fig. 3, P<0.01). The method of sorting 
was also significant in the first model 
applied across all treatment deploy- 
ments, with relatively greater deaths 
after discarding from the conventional 
tray (overall unadjusted predicted mean 
±SE of 29.3 ±2.0%), than from the wa- 
ter tray (17.6 ±1.6%) (Table 3, Fig. 3, 
PcO.Ol). Sorting delay had no impact 
on the mortality of school prawns dis- 
carded across all deployments, or from 
the water tray, but was significant for 
those that were conventionally sorted 
(mortalities of 25.8 ±2.5% for the 2- 
min delay, and 32.9 ±2.8% for the 15- 
min delay; Table 3, P<0.01). 
The only remaining significant main 
effect in any of the models was the to- 
tal weight of catch, identified as having 
a positive relationship with mortality across all deploy- 
ments, and those restricted to sorting in the water tray 
(Table 3, PcO.Ol). For the water-tray deployments, indi- 
viduals caught during the largest catch weights (47 kg) 
were more than 2.5 times as likely to be dead at each 
sampling time than those from the smallest catches 
(4 kg, Fig. 4). For both the T 72 and T 120 sampling times, 
the absolute probabilities of mortality during the larg- 
est and smallest catch weights were approximately 43% 
vs. 16% (Fig. 4). 
Discussion 
The mortalities of school prawns after being trawled, 
conventionally discarded, and adjusted for control fatali- 
ties were consistent between experiments and encom- 
passed the range of earlier estimates by Macbeth et 
al. (2006). Further, the unadjusted mortalities were 
comparable to those recorded for several other similarly 
treated decapods monitored without controls, including 
caridians (1-30%, Wassenberg and Hill, 1989; Cabral et 
al., 2002), brachyurans (0-50%, Hill and Wassenberg, 
1990; Wassenberg and Hill, 1993; Kaiser and Spencer, 
1995) and anomurans (0-19%, Kaiser and Spencer, 
1995). These rates of death are considerably lower than 
those typically observed for many trawled-and-discarded 
fish and mollusks and further support the resilience of 
school prawns, and crustaceans in general, for with- 
standing a range of trawl-induced impacts (Broadhurst 
et al., 2006). 
Irrespective of apparent broad phyla-specific differ- 
ences among collateral trawl mortalities, as is the case 
for several locally caught fish (Broadhurst et al., 2008), 
Table 4 
Summary of model terms, F-ratios, and degrees of free- 
dom (df) for ANOVA used to investigate L-lactate among 
trawled-and-discarded (2- and 15-min conventional- and 
water-tray treatments) and control school prawns (Metap- 
enaeus macleayi) in the Clarence River during one day 
of fishing in 2007. Data were ln(x+l) transformed. Three 
replicates were missing, and therefore the cell means 
were used and the df was adjusted accordingly. **P<0.01. 
Variable 
F- ratio 
df 
Treatment 
of prawns 
14.09** 
4, 5 
Deployments 
(treatment of prawns) 
0.58 
5, 30 
Sampling times 
93.41** 
2, 10 
Treatment of 
prawns x sampling times 
0.73 
8, 10 
Sampling times 
x deployment 
(treatment of prawns) 
1.55 
10, 30 
Cages (treatment of 
prawns x sampling times 
x deployments) 
0.95 
30, 57 
use of the water tray significantly reduced the short- 
term mortality of school prawns in both experiments. 
The extent of these reductions can be explained by at- 
tempting to partition mortality, along with the various 
significant predictors, into those fatalities caused 1) by 
the trawling process, and 2) after onboard handling. 
