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Fishery Bulletin 107(3) 
When they initially contact the anterior sections of 
a trawl, prawns and other crustaceans with extended 
abdomens (e.g., Norway lobster Nephrops norvegicus) 
typically respond by contracting their tail muscle ven- 
trally, effectively propelling themselves backwards 
into the top netting panels (Watson, 1976; Newland 
and Chapman, 1989). This behavior may be repeated 
several times, until an individual becomes fatigued, 
after which they often attempt to orientate towards 
the seabed using their swimmerets (Watson, 1989). 
Such activity is ineffective against the speed of the 
trawl, and therefore prawns are quickly forced against 
the meshes into the posterior trawl body and directed 
into the codend where they accumulate along with the 
rest of the catch. 
The physiological responses supporting the above 
activity include anaerobic respiration and the rapid 
depletion of arginine phosphate in the tail muscle (On- 
nen and Zebe, 1983; Paterson et al., 1995). An end 
product of this exertion is lactic acid, which is removed 
during aerobic metabolism (Head and Balwin, 1986). 
In the present study, the concentrations of L -lactate in 
live school prawns immediately sampled (i.e., at T 0 ) af- 
ter being discarded from the various treatments in the 
Clarence River were elevated to mean levels between 
14.2 ±2.0 and 15.6 ±1.5 pmol/g, that were greater than 
those previously observed for this species after exercise 
(typically <12 pmol/g), indicating that these individu- 
als had been heavily exercising. Although some of the 
observed lactate accumulation could be attributed to 
onboard handling and associated anoxia (Hill et al., 
1991), especially during the conventional procedures, 
the similar rate of accumulation across all treatments 
supports a strong, uniform, negative impact of the 
trawling process. 
For some individuals, the physiological damage de- 
scribed above may have been sufficient to result in 
their immediate death, while for others the cumulative 
impacts of significant covariates, such as the weight of 
catch and salinity, would have contributed toward their 
more protracted (over 24 hours) trawl-related mortali- 
ties. In particular, as the catch accumulated, at least 
some school prawns probably sustained fatal injuries, 
including wounding and blood loss, due to pressure 
and abrasion. Also, the ongoing stimuli associated with 
repetitive contact among conspecifics and other organ- 
isms may have triggered additional bursts of anaerobic 
exercise, and further compromised their physiological 
condition. These density-dependant effects were more 
obvious in the Hunter than in the Clarence River, prob- 
ably reflecting the considerably greater mean size of 
catch (19.4 kg [Hunter River]; 11.8 kg [Clarence Riv- 
er]), but also perhaps the slightly smaller mean size of 
school prawns (Table 1) and the presence of some soft 
individuals (i.e., 13% at T 0 ). Both of these latter fac- 
tors have previously been demonstrated to increase the 
vulnerability of organisms to the impacts associated 
with discarding (Broadhurst et al., 2006). 
Salinity also appeared to contribute towards mor- 
tality during trawling in the Clarence River. As with 
