288 
Fishery Bulletin 107(3) 
Mesh screen 
4 4 
1800 mm 
Figure 1 
Schematic diagram of the water tray and mechanism by which school 
prawns ( Metapenaeus macleayi) were riddled into discarded and retained 
categories in the Clarence and Hunter rivers during 2007 and 2008. 
were supplied with local flow-through seawater (at a 
maximum rate of 63 L/min) and aerated by stone diffus- 
ers. At two sites (termed “monitoring sites”) within the 
limits of trawling in each river, several 10-mm diameter 
ropes (50 m in length) were attached horizontally at a 
level corresponding with the average low tide to 2.5 m 
stanchions or existing pylons (fixed to the river bed). 
The rope configurations were designed to secure up to 
90 portable cylindrical cages (0.3-m diameter x 0.4-m 
depth), each made from a modified 35-L bucket and 
comprising one top and three lateral openings that were 
covered by 6-mm PVC mesh (each <230 cm 2 ) and a solid 
base filled with locally collected sediment to a depth of 
about 6 cm (see also Broadhurst and Uhlmann, 2007 for 
details). Clips were attached to the tops of each cage so 
that they could be suspended along the 10-mm diameter 
rope. The cages were designed so that two could fit into 
in 75-L aerated water-filled PVC containers located on 
a dory, enabling them to be transported between the 
fishing and monitoring sites. 
Experimental design 
In both rivers, between seven and five days before start- 
ing each experiment, approximately 1000 school prawns 
were collected in <5-min trawls rigged with a fine- 
meshed knotless polyamide, 10-mm mesh codend towed 
slowly in shallow water. At the end of each deployment, 
the codend was emptied into a water-filled container. 
Live and active juvenile school prawns were quickly 
removed, placed in tanks supplied with oxygen and 
transported to the two holding tanks on the river bank. 
The captive school prawns were fed chopped pilchard 
( Sardinops neopilch ardus) at a rate of 5% biomass/24 
hr and left to recover for at least five days, after which 
surviving, intact individuals were used as controls in 
the experiments described below. 
Four treatment groups associated with trawling and 
discarding and one control group were examined in 
each river. The four treatments were chosen to repre- 
sent the temporal limits of the conventional (i.e., dry 
tray) and modified (i.e., water tray) onboard handling 
of catches. All treatments comprised a 30-min deploy- 
ment of the trawl, followed by the sorting of unwanted 
school prawns that was started after one of the follow- 
ing treatments: 1) a 2-min delay in a dry tray (termed 
the “2-min conventional-tray” treatment); 2) a 2-min 
delay in the water tray (“2-min water-tray” treatment); 
3) a 15-min delay in the dry tray (“15-min conventional- 
tray” treatment); or 4) a 15-min delay in the water tray 
