Growth and mortality of 



Lutjanus vittus (Quoy and Gaimard) 



from the North West Shelf of Australia 



Tim L.O. Davis 

 Grant J. West 



CSIRO Division of Fisheries, Marine Laboratories 

 GPO Box 1538, Hobart, Tasmania 7001, Australia 



The tropical waters of the North 

 West Shelf of Australia are highly 

 productive (Tranter 1962) and sup- 

 port a diverse fish fauna (Sainsbury 

 et al. 1985). A significant multispe- 

 cies traw^l fishery has developed in 

 the region, its total catch peaking 

 in 1973 at 37,000 t, although this 

 had decreased to 2700 t in 1989 

 (Jernakoff and Sainsbury 1990). 

 Lutjamis vittus is an important and 

 highly valued fish in this trawl fish- 

 ery, comprising about 4% of the 

 total catch (Jernakoff and Sains- 

 bury 1990). 



Assessment of fish yields of the 

 North West demersal trawl fishery 

 is based mainly on a Beverton and 

 Holt dynamic pool model (Sainsbury 

 1987), which requires estimates of 

 mortality and growrth parameters 



for each species. In this paper we in- 

 vestigate the age, growth, popula- 

 tion structure, and mortality of L. 

 vittus collected from random trawl 

 surveys of the North West Shelf 

 during 1982-83. 



Materials and methods 

 Field collection 



Material was obtained from the 

 CSIRO North West Shelf program 

 (Young and Sainsbury 1985), which 

 surveyed the shelf waters within 

 latitudes 116-119°E about every 

 two months between August 1982 

 and October 1983 (Fig. 1). Fish 

 were caught with a Frank and 

 Bryce trawl (30.5 m foot rope and a 

 20 mm cod-end hner) towed at 3.5- 



4.5 knots for 30 minutes during the 

 day. Demersal tows were made at 

 computer-generated random posi- 

 tions in 13 strata defined by water 

 depth (10-50 m, 50-120m, and 120- 

 210 m), sediment type (nominally 

 shelly sand, sand, sandy silt, and 

 silt), and two geographical zones in 

 which different fishing regimes 

 were planned in the future (Table 

 1). Sixty-two trawl positions were 

 produced for each sampling survey, 

 with effort being allocated accord- 

 ing to the mean and variance of 

 catches determined by preliminary 

 surveys and the area of each stra- 

 tum. On average, 58 trawls were 

 completed each survey. 



At each random station the total 

 weight of L. vittus and the fork 

 length by 10 mm classes of each L. 

 vittus was recorded. A subsample of 

 20-40 fish, approximately repre- 

 senting the size/frequency composi- 

 tion of the total catch (Kimura 1977) 

 was then selected from each station 

 for further analysis. Fork length 

 was measured to the nearest mm 

 and total weight to the nearest g, 

 and sex were recorded. Sagittal 

 otoliths and urohyals were collected 

 for age determination. 



Manuscript accepted 18 February 1992. 

 Fishery Bulletin, U.S. 90:395-404 (1992). 



18°S 



19° 



20° 



115°E 116° 



120° 



Figure 1 



Distribution of 407 random stations sampled during seven 

 cruises on the North West Shelf, September 1982-October 

 1983. The 20, 50, 120, and 200m depth contours used to 

 stratify sampling are shown. 



Table 1 



Stratified random trawl survey on the North West Shelf. The 

 13 strata sampled during each survey based on depth, geo- 

 graphical zones, and sediment type. Area (km') of each stratum 

 and number of random trawls made in each stratum (in paren- 

 thesis) on each survey are shown. 



Sand 



Shelly- 

 sand 



Silt 



Sandy- 

 silt 



20-49 m 



116°E-117°30'E 

 117°30'E-119°E 



50-119m 



116°E-117°30'E 

 117°30'E-119°E 



1 20-200 m 



116°E-117°30'E 

 117°30'E-119°E 



3278(6) 

 6309(6) 



6123(7) 

 9679(7) 



3123(4) 

 1732(4) 



2381(4) 

 2381(4) 



5505(5) 

 1423(5) 



4577(4) 

 4886(4) 2412(2) 



395 



