305 



Abstract— An intensive commercial 

 hook-and-line fishing operation targeted 

 the demersal fisheries resources at Saya 

 de Malha Bank in the Southwest Indian 

 Ocean. Fishing was conducted with 12 

 dories that were equipped with echo 

 sounders and electric fishing reels and 

 supported by a refrigerated mothership. 

 Over a 13-day period in the 55-130 m 

 depth range, a total of 74.3 metric tons 

 (t) of fish were caught, of which the 

 crimsonjohfishiPristipomoidesfilamen- 

 tosus) represented 80%. Catch rates 

 decreased with time and could not 

 be attributed to changes in location, 

 climatic conditions, fishing depth, 

 fishing method, or bait type. The ini- 

 tial virgin biomass of P. filamentosus 

 available to a line fishery at the North 

 Western promontory of Saya de Malha 

 Bank was estimated at 72.6 t through 

 application of the Leslie model to daily 

 catch and effort data. Biomass densi- 

 ties of 2364 kg/km- and 1206 kg/km 

 were obtained by applying the initial 

 biomass estimates to the surface area 

 and to the length of the dropoff that was 

 fished. The potential sustainable yield 

 prior to exploitation was estimated at 

 567 kg/km^ per year. The quantity of 

 P. filamentosus caught by the mother- 

 ship-dory fishing operation represented 

 82% of the initial biomass available to 

 a hook-and-line fishery, equivalent to 

 more that three times the estimated 

 maximum sustainable yield. The results 

 of the study are important to fisheries 

 managers because they demonstrate 

 that intensive line fishing operations 

 have the potential to rapidly deplete 

 demersal fisheries resources. 



The effect of intensive line fishing on the 

 virgin biomass of a tropical deepwater snapper, 

 the crimson jobfish iPristipomoides filamentosus) 



Edwin M. Grandcourt 



Marine Environmental Research Centre 



Environmental Research and Wildlife Development Agency 



Corniche Road 



PO Box 45553 



Abu Dhabi, United Arab Emirates 



E-mail address egrandcourtigierwda gov.ae 



Manuscript accepted 23 October 2002. 



Manuscript received 9 January 2003 

 at NMFS Scientific Publications Office. 



Fish. Bull. 101:305-311 (2003). 



The crimson jobfish, Pristipomoides fil- 

 amentosus (Valenciennes, 1830), occurs 

 throughout the tropical Indo-Pacific 

 from the Red Sea in the west to Hawaii 

 in the east and has a latitudinal distri- 

 bution in the western Pacific ranging 

 from southern Japan to New Caledonia 

 (Randall et al., 1997). It is discontinu- 

 ously distributed in the western Indian 

 Ocean and has been recorded from 

 Madagascar, Reunion, the east coast 

 of Africa, the west coast of India, and 

 the Chagos archipelago (Allen, 1985), 

 and tends to aggregate in shoals in up- 

 current localities and near underwater 

 promontories and headlands (Ralston 

 et al., 1986). The habitat occupied is 

 characterized by deep waters from 90 

 to 360 m over rocky bottoms, along 

 the edge of the continental shelf, and 

 around isolated oceanic islands and 

 banks (Randall et al., 1997). 



Juveniles inhabit flat, featureless 

 shallow banks and sediment bottoms 

 close to sources of drainage, moving into 

 deeper waters as they mature (Haight 

 et al., 1993a; Parrish et al., 1997). Nev- 

 ertheless, for adult fish there has been 

 no correlation found between size and 

 depth (Ralston and Wilhams, 1988). 



Studies from Hawaii showed that P. 

 filamentosus is primarily a zooplankti- 

 vore, although fish, crustaceans, and 

 moUusks also feature in the diet (Ral- 

 ston et al., 1986; Haight et al, 1993b). 

 Reproductive studies off! filamentosus 

 in the Seychelles suggest that spawning 

 is protracted and peaks between Febru- 

 ary and April, and in November (Mees, 

 1993). The size at which 50% of females 

 reach sexual maturity (Lm^,,) on banks 

 in the South West Indian Ocean is ap- 



proximately 52 cm fork length (Mees, 

 1993). 



Pristipomoides filamentosus is a com- 

 mercially important tropical snapper 

 that is caught with handlines, electric 

 fishing reels, and deepwater gill nets 

 (Hardman-Mountford et al., 1996). 

 Because lutjanid species are favored 

 for consumption or sale, they are com- 

 monly targeted by fishermen (Munro, 

 1983; Koslow et al., 1988) and their ag- 

 gressive nature and relatively large size 

 makes them more vulnerable to fishing 

 gears (Munro and Williams, 1985). 

 Furthermore, low rates of growth, 

 recruitment, and natural mortality, 

 combined with a prolongation in the 

 attainment of sexual maturity, make 

 lutjanids particularly vulnerable to 

 overfishing (Russ, 1991). Owing to the 

 steeply shelving substratum over which 

 P. filamentosus is found, the stock den- 

 sity for this fish has been estimated to 

 be 10 times greater than that of other 

 lutjanids in adjacent shelf areas (Mees, 

 1993). The concentration of the stock in 

 a narrow depth band makes targeting 

 easy and consequently the potential for 

 overfishing is great (Mees, 1993). 



Intensive fishing, over a period that 

 is sufficiently short to permit the as- 

 sumption that a population is closed, 

 can produce data suitable for estimat- 

 ing the initial population size (e.g. Mees, 

 1993; Polovina, 1986). Where a popula- 

 tion is exploited for the first time, know- 

 ing the initial biomass is often useful in 

 determining whether overexploitation 

 has occurred during the development 

 of the fishery (Hilborn and Walters, 

 1992). The practicality of using daily 

 catch-and-effort data from commercial 



