Fate of juvenile school prawns, Metapenaeus macleayi, after simulated capture and escape from trawls

Abstract

Two laboratory experiments were done to assess the fate of juvenile school prawns, Metapenaeus macleayi, after simulated multiple capture and escape from trawls. In the first experiment, prawns that were trawled and escaped one, five or 10 times, sustained some physical damage (mostly limited to the loss of antennae), but this was not significantly different from that sustained by control prawns that had not been trawled. Similarly, there were no significant differences between the different treatments and control prawns in their stress levels (as measured by changes in concentrations of L-lactate). Levels of L-lactate were greatest in all prawns immediately after the experiment started and then significantly reduced after 24 and 48 h. In the second experiment, treated prawns were trawled and escaped 10 times and then monitored for mortalities over 2 weeks. Compared with control prawns (that were not trawled), significantly more treated prawns died at the end of the 2 weeks, but the overall post-trawl survival rate was >89%. It is concluded that the multiple contact and escape of juvenile school prawns from trawls had minimal effect on their overall condition.

Atypical size selection of captive school prawns,Metapenaeus macleayi, by three recreational fishing gears in south‐eastern Australia

“…Mean L-lactate concentrations (± SE) ranged between 0.5 ± 0.21 and 2.22 ± 0.38 µmol g-1 (Fig. 5) and approached baseline levels (3.72 ± 0.38 µmol g-1 ) previously recorded for this species (e.g., Broadhurst et al 2002). Such low levels of the metabolite in active school prawns indicate that they were not overly stressed during the experimental procedure.…”

“…Although the fate of school prawns escaping from recreational gears was not examined here, a previous study by Broadhurst et al (2002) demonstrated minimal stress, physical damage and mortality (<11% over 2 weeks post-escape) by the same size cohort after their simulated multiple capture and escape from trawls. During this earlier work, the mean L-lactate concentration recorded from school prawns at 2 min post-escape (7.78 ± 0.5 µmol g-1 ) was considerably greater than that observed here (e.g., between 0.58 ± 0.22 and 1.63 ± 0.61 µmol g-1 - Fig.…”

“…Approximately 6500 prawns were selected, removed and placed in groups of c. 180 into each of the 35 plastic 70-litre holding tanks. Because there was no evidence of sexual dimorphism in the morphology of these sizes of school prawns (see Results section and Broadhurst et al 2002), individuals were placed in tanks irrespective of their sex. These prawns were left to acclimatise for 15 h. On the fourth morning of each experiment, at least three of the small holding tanks were designated as stock tanks and used to replace mortalities in the remaining tanks, which were subsequently used as replicates in the experiments described below.…”

“…The six prawns were immediately secured in labelled aluminium satchels and frozen in liquid nitrogen. Analyses of L-lactate were done for four frozen prawns (and not six-because of the cost involved) at each period following the methods described by Broadhurst et al (2002).…”

“…A concern with this approach, however, is the possible confounding effects of experimentally induced stress in school prawns, which could alter their activity and/or escape response. The potential for such effects can be determined by quantifying the concentrations of lactic acid in the haemolymph of experimental individuals and then comparing these to routine or base levels (Broadhurst et al 2002).…”