Broadhurst et at: Modified sorting technique to mitigate the collateral mortality of Metapenaeus macleayi 
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cal configurations, even under optimal conditions the 
relative reductions of many bycatch individuals are only 
between approximately 50% and 70% and therefore, at 
times, large numbers are still caught and discarded 
(Broadhurst and Kennelly, 1996). Recent work with 
trawlers in the Clarence River indicates that simply 
deploying trawls for shorter durations (30 rather than 
60 min) and sorting catches in water can minimize the 
negative impacts to some of these discards (Uhlmann 
and Broadhurst, 2007; Broadhurst et al., 2008). 
In particular, Broadhurst et al. (2008) described a 
purpose-built, onboard water-sorting system (termed 
a “water tray”) designed to facilitate the separation 
of prawns and fish, and then prawns into retained 
and discarded categories, while minimizing their ex- 
posure to air. This system was examined for its utility 
in reducing the mortality of fish after both immediate 
and delayed discarding from trawls deployed for the 
shortest commercially viable period (30 min). Although 
the mechanical interactions associated with trawling 
meant that the mortalities of many fish remained high, 
there were significant reductions in fatalities when the 
water tray was used during longer delays in starting 
sorting. 
No work has been done to assess whether the water 
tray similarly reduces the mortality of discarded juve- 
nile prawns, although two relevant pilot studies support 
the application of this modification (Macbeth et al., 
2006; Broadhurst and Uhlmann, 2007). Specifically, 
during four deployments at one location in the Clarence 
River, Macbeth et al. (2006) observed that the short- 
term (three days) mortality of juvenile school prawns 
was reduced from approximately 35% during conven- 
tional sorting to about 16%, simply by holding them in 
water-filled containers. However, irrespective of their 
handling, all surviving school prawns showed similar 
elevated stress responses (measured as L -lactate) over 
the monitoring period, which may have increased their 
susceptibility to other types of mortality (e.g., through 
infection or predation). By contrast, using comparable 
replication, Broadhurst and Uhlmann (2007) observed 
that regardless of their handling (including maximum 
and minimum gear deployments and subsequent air ex- 
posure), school prawns appeared resilient to both seine- 
and trawl-induced impacts, which manifested as total 
mortalities of <15%. The lack of impacts was further 
reflected by a return of elevated L -lactate concentrations 
immediately to baseline levels within 24 hours after the 
prawns had been discarded. 
At least some of the observed discrepancies between 
the two studies above probably reflect their limited 
replication in space (one location) and time (one day of 
fishing). Other studies have demonstrated that a range 
of technical (e.g., gear design, deployment duration, 
and speed), biological (e.g., species, physiology, size, 
and catch volume and composition) and environmental 
(e.g., temperature, hypoxia, sea state, and light) factors 
can have complex interacting effects on the fate of dis- 
cards during trawling (Davis, 2002; Broadhurst et al., 
2006). It seems appropriate, therefore, that during any 
study that seeks to assess the utility of modifications 
to reduce collateral mortality, adequate information is 
collected on these factors across a range of commercial 
conditions. We sought to use this approach to assess 
the water tray for reducing the mortality of discarded 
school prawns in the Clarence and Hunter rivers. 
Materials and methods 
Trawlers and the water tray 
Separate experiments were undertaken in the Clarence 
River (29°27'S, 153°09'E) between March and April 2007 
and in the Hunter River (32°53'S, 151°45'E) between 
February and April 2008. A commercial prawn trawler 
(<14 m in length) rigged with standard twin trawls was 
used in the Clarence River and a commercial prawn 
trawler (<14 m) rigged with a single trawl was used in 
the Hunter River. All trawls were attached to square- 
mesh codends (27-mm mesh). The Hunter River trawler 
was equipped with a large horizontal canvas awning 
that covered most of the back deck (and catch sorting 
area). The Clarence River trawler had a much smaller, 
obliquely orientated, and less effective cover made from 
loose-weave polyvinyl chloride (PVC). 
Before starting the experiments, the conventional 
sorting trays were removed from both vessels and re- 
placed with a modified design (comparable in price to 
conventional trays), termed the “water tray” which was 
scaled to fit, and measured 1800x1500x300 mm (Clar- 
ence River trawler) and 1200x1190x300 mm (Hunter 
River trawler) (Fig. 1 and Broadhurst et al., 2008). 
Irrespective of overall dimensions, the design of the 
water tray remained similar and included a V-shaped 
bottom, designed to concentrate the catch towards its 
center and below a vertically orientated, variable open- 
ing (Fig. 1). A pump provided flow-through water from 
the river at 30 L/min to the water tray through a 50- 
mm diameter inlet located in one side. For the Clarence 
River trawler, a metal screen (50-mm mesh size) was 
positioned horizontally above the bottom of the water 
tray to allow school prawns to filter through onto the 
V-shaped bottom, separating them from any larger fish 
(Fig. 1). This metal screen was removed from the water 
tray that was used on the Hunter River trawler. Dur- 
ing the sorting process in each water tray, nonpenaeid 
bycatch was either removed by hand, or directed out 
of the variable opening along with water and collected 
in a 35-L container. Once the bycatch was removed, 
school prawns were washed out with the remaining 
water and passed over a riddler (made from hexagonal 
mesh; Fig. 1), which separated unwanted and retained 
individuals into two 35-L containers (see also Macbeth 
et al., 2006 for details). 
Monitoring sites 
Two 3000-L polyethylene holding tanks were positioned 
at one location on the banks of each river. All tanks 
