286 
Modified sorting technique to mitigate 
the collateral mortality of trawled school prawns 
(Metapenaeus macleayi) 
Matt K. Broadhurst (contact author ) 1 
Russell B. Millar 2 
Craig P. Brand 1 
Sebastian S. Uhl matin' 
Email address for contact author: mbroadhurst@nmsc.edu.au 
1 NSW Department of Primary Industries 
Fisheries Conservation Technology Unit 
2 Bay Drive 
Coffs Harbour, New South Wales 2450, Australia 
2 Department of Statistics 
The University of Auckland 
Princes St. 
Auckland 1142, New Zealand 
Abstract — The potential for changes 
to onboard handling practices in order 
to improve the fate of juvenile school 
prawns (Metapenaeus macleayi) dis- 
carded during trawling were inves- 
tigated in two Australian rivers 
(Clarence and Hunter) by comparing 
a purpose-built, water-filled sorting 
tray against a conventional dry tray 
across various conditions, including 
the range of typical delays before 
the start of sorting the catch (2 min 
vs. 15 min). Juvenile school prawns 
(n = 5760), caught during 32 and 16 
deployments in each river, were caged 
and sacrificed at four times: immedi- 
ately (T 0 ), and at 24 (T 24 ), 72 (T 72 ), 
and 120 (T 120 ) hours after having 
been discarded. In both rivers, most 
mortalities occurred between T 0 and 
T 24 and, after adjusting for control 
deaths (<12%), were greatest for the 
15-min conventional treatment (up to 
41% at T 120 ). Mixed-effects logistic 
models revealed that in addition to 
the sampling time, method of sorting, 
and delay in sorting, the weight of 
the catch, salinity, and percentage 
cloud cover were significant predic- 
tors of mortality. Although trawling 
caused some mortalities and compa- 
rable stress (measured as L-lactate) 
in all school prawns, use of the water 
tray lessened the negative impacts 
of some of the above factors across 
both the 2-min and 15-min delays in 
sorting so that the overall discard 
mortality was reduced by more than a 
third. When used in conjunction with 
selective trawls, widespread applica- 
tion of the water tray should help to 
improve the sustainability of trawling 
for school prawns. 
Manuscript submitted 18 September 2008. 
Manuscript accepted 20 February 2009. 
Fish. Bull. 107:286-297 (2009). 
The views and opinions expressed 
or implied in this article are those 
of the author and do not necessarily reflect 
the position of the National 
Marine Fisheries Service, NOAA. 
Mitigating the collateral mortality 
from trawl fisheries is a complex issue 
that requires multifaceted strategies 
(Davis, 2002; Broadhurst et al., 2006). 
One option is to spatially and tempo- 
rally restrict fishing to avoid known 
large assemblages of nontarget organ- 
isms (termed “bycatch”) (Andrew and 
Pepperell, 1992). A broader approach 
involves physical modifications to 
gears to improve their selection of spe- 
cies and size of species. Such modi- 
fications are assumed to indirectly 
reduce mortality on the premise that 
the survival of escapees during fish- 
ing is generally much greater than 
those of organisms brought to the 
surface and discarded (Broadhurst 
et al., 2006). However, few (if any) 
modifications to trawls are 100% effec- 
tive and therefore, despite their use in 
many fisheries, there remains at least 
some unwanted bycatch (Andrew and 
Pepperell, 1992; Broadhurst, 2000). 
A remaining ancillary strategy that 
has been rarely applied, but which 
may help to address this problem, is 
to refine operational and postcapture 
handling techniques to improve the 
survival of discarded bycatch (e.g., 
Gamito and Cabral, 2003; Macbeth 
et al., 2006). 
Similar to attempts in many of the 
world’s penaeid fisheries, attempts at 
resolving collateral mortality in the 
New South Wales river prawn-trawl 
fishery have mostly focused on the 
first two strategies described above. 
From the early 1940s until 2003, 
this fishery comprised over 300 small 
(mostly <10-m) trawlers predominant- 
ly targeting either school ( Metapenae- 
us macleayi ) or eastern king (Penaeus 
plebejus ) prawns in five rivers and 
estuaries. In addition to the targeted 
catches, these trawlers have tradition- 
ally caught, and then discarded, large 
quantities of bycatch (estimated at 
up to 177 t/yr in some rivers and es- 
tuaries; Liggins and Kennelly, 1996; 
Liggins et al., 1996), often comprising 
juveniles of economically important 
species, including penaeids too small 
for sale (<approximately 15-17 mm 
carapace length [CL]). During the last 
20 years, concerns over the mortality 
of these organisms have culminated 
in complete closures to fishing at 
some locations, and the development 
of, and legislation for, modifications 
to trawls, including bycatch reduction 
devices (BRDs) and square-mesh co- 
dends, for use throughout the remain- 
ing fleet (Broadhurst and Kennelly, 
1996; Macbeth et al., 2007). 
Effort in this fishery has now been 
reduced to 204 vessels distributed 
among three rivers; the Clarence (114 
vessels), Hawkesbury (61) and Hunt- 
er (29). Although the modified gears 
used by the operators of these ves- 
sels are more selective than histori- 
