Broadhurst et al.: Modified sorting technique to mitigate the collateral mortality of Metapenaeus macleayi 
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other penaeids (e.g., western king prawns [ Penaeus lati- 
sulcatus\ [Sang and Fotedar, 2004] and fleshy prawns 
[P. chinensis ] [Chen et al., 1995]), Maguire and Allan 
(1985) observed that although juvenile school prawns 
tolerate a range of salinities (from 1 to 30 psu), the 
mortality of captive individuals (in grow-out ponds) 
was negatively associated with salinities between 10 
and 30 psu. On average, the salinities observed in 
the Clarence River were much lower than those of the 
Hunter River (5.4 vs. 10.2 psu) and possibly below 
the optimal range for osmoregulation. Although not 
necessarily fatal in the absence of other stressors, low 
salinities would require school prawns to maintain a 
comparatively greater metabolic rate to achieve osmotic 
balance, thereby increasing their susceptibility to other 
trawl-related impacts. Further, an abrupt reduction 
to even lower salinities during the retrieval of the 
trawl to the surface (as a consequence of the observed 
halocline) would require some readjustment of osmotic 
concentration to regulate tissue water. A similar transi- 
tion through haloclines has been identified as a factor 
contributing towards mortality in other species, includ- 
ing Norway lobster (Harris and Ulmestrand, 2004), and 
probably had a cumulative impact on the stress, and 
ultimately mortality, of some school prawns. 
It is difficult to accurately quantify the cumulative 
impact of all trawl-related stressors on total mortality, 
especially since the death of some control school prawns 
indicates the potential for natural mortality (which was 
assumed to be constant across all treatments). However, 
after monitoring the fate of individuals collected imme- 
diately after the codend was emptied (individuals that 
had minimal air exposure before being sorted), Macbeth 
et al. (2006) estimated a short-term (over three days) 
mortality of 16% directly attributable to 60-min trawls, 
which was only slightly greater than that determined 
for individuals escaping through codend meshes during 
trawling (11%, Broadhurst et al., 2002). The results 
from the present study support these estimates, with 
an adjusted, protracted mortality in the 2-min water 
tray (arguably the mildest treatment) of less than 21% 
for both rivers. 
Assuming comparable trawl-induced mortalities 
across treatments, the remaining differences in fatali- 
ties can be attributed to the use of the water tray for 
minimizing some of the negative impacts associated 
with onboard handling, and especially with air expo- 
sure. Most likely, by facilitating aerobic respiration, 
the water tray would have allowed some school prawns 
to recover and restore argine phosphate levels and 
acid-base regulation (Taylor and Spicer, 1988), which 
probably helped to limit further physiological damage 
and mortality. 
In addition to limiting air exposure, the water tray 
maintained temperature homeostasis. Although there 
was no significant effect of air temperature in the mod- 
els applied to the conventional-tray treatments in either 
experiment, there was a significant negative relation- 
ship between cloud cover and mortality in the Clarence 
River (P<0.05). This association probably reflects the 
greater convection, heating, and subsequent desiccation 
of school prawns in direct sunlight on the dry tray. 
Gamito and Cabral (2003) observed similar effects of 
heating on the mortality of brown shrimp (Crangon 
crangon) and suggested that this could be reduced by 
using light-colored sorting containers and avoiding 
trawling during the hottest time of the day. Sorting in 
the water tray is probably a more effective alternative 
in New South Wales, although the lack of any impacts 
of cloud cover in the Hunter River, combined with the 
similar range of temperatures between the two rivers 
also illustrates the utility of shading the conventional 
sorting tray with an appropriate cover. 
The only obvious limiting factor of the water tray 
was its size for the trawler working in the Hunter 
River. Vessels in the Hunter River typically catch fewer 
school prawns than those in the Clarence River, and 
therefore a smaller water tray was used in the Hunter 
River . It is possible that, in addition to the trawling- 
related impacts discussed above, large catches in the 
sorting tray on the Hunter River trawler contributed 
towards mortality as a consequence of their greater bio- 
logical load. Although there was no significant impact 
of dissolved oxygen on mortality in the water tray, the 
mean dissolved oxygen recorded during the Clarence 
River deployments was higher than that in the Hunter 
(5.8 mg/L vs. 3.8 mg/L). Further, during one catch of 
47 kg in the Hunter River, the dissolved oxygen in the 
water tray after 15 minutes of sorting was 2.4 mg/L. 
The potential for any negative effects associated with 
low dissolved oxygen could be simply reduced by in- 
creasing the water exchange or volume of the water 
tray, or both. 
Although the results from this study indicate a 
significant reduction in discard mortality associated 
with using the water tray, it is important to remember 
that the discarded school prawns were protected in 
cages and therefore other collateral mortalities were 
not quantified. The levels of L-lactate at T 0 indicated 
that alive, discarded school prawns were fatigued and, 
during conventional fishing, these individuals could 
be more susceptible to predation during their descent 
to the bottom (Lancaster and Frid, 2002). Further, 
because of the temporal increase in mortality, some 
school prawns may have maintained their vulnerability, 
particularly during the first 24 hours after having been 
discarded. The potential for these effects could be miti- 
gated by subtle modifications to the water tray. In par- 
ticular, because low (e.g., <15°C) water temperatures 
have been demonstrated to reduce metabolic activity in 
penaeids (Paterson, 1993), cooling the water tray may 
reduce some of the ancillary stressors identified above 
and help school prawns to recover more quickly. The 
concomitant use of a covered guiding panel that directs 
the discarded school prawns into the water behind the 
trawler could reduce predation. 
Notwithstanding the need for some refinements, it is 
clear that using the water tray would eliminate most 
of the short-term mortalities associated with onboard 
handling, which would translate to a total reduction 
