Field studies of the depth distribution of recruit-sized prawns Penaeus merguiensis and P. monodon in the Gulf of Papua: implications for management.

Abstract

During 29* March to 11th April 1996, transects and commercial operations covering the fishing grounds, from 8 to 30 m depth indicated that catch per unit of fishing effort (CPUE) of recruit-sized banana prawns was highest in 8 to 12 m depth. Relative abundance decreased with increasing depth (p = 0.031, single-tail test for negative correlation). Similarly, the CPUE of recruit-sized black tiger prawns decreased with depth from 8 to 30 m (p - 0.046). The percentage catch (by weight) of recruit-sized banana prawns and blacktiger prawns were similarly each negatively correlated with depth (p = 0.018 andp =0.012 respectively). The permanent closure of the 3-mile zone, i.e. trawl-able areas extending seawards 3 miles from the mean low water mark, to industrial trawling, is supported by these findings. Extension of the eastern boundary of the area closure for 1997 (and subsequent years) eastward from Cape Cupola to the fishing grounds as far south-east as lokea village is recommended in order to include and protect the mangrove nursery areas and adjacent offshore recruitment areas located in 4 to 8 m depth in Freshwater Bay (Lesi Creek, Lakekamu Estuary and Mopu inlet) during the principal recruitment season, January through April. There is increasing interest from resource owners to develop the nearshore prawn resources within the 3-mile zone in a sustainable manner, by opening the 3-mile zone to small-scale, beam trawling, i.e. a limited number of relatively small beam trawls towed by dory. The nets used should be of relatively large mesh sizes in the net body and cod-end, because the results of the present study indicate that, particularly during January through June each year, a large proportion of the catch in the 3-mile zone will probably be new recruits which have not realised their growth potential. Studies to assess the impact of beam trawls should ideally be conducted prior to allowing this kind of fishery to develop. From the transect studies (29* March - 2nd April) alone, the average CPUE (the recruitment index) of banana prawns was 8.23 +/- SE 2.30 kg of tails/ trawl-hour. Similarly, for black tiger prawns the recruitment index was 1.53 +/- SE 0.34 kg (head-on)/trawlhour. The average depth trawled during the transect studies was 17.7 m (SD 6.39 m). Similar field transect studies to determine comparable recruitment indices in future years could be undertaken.