Clark and Tracey: Population changes of Hoplostethus atlanticus on the Challenger Plateau 



239 



EEZ boundary 



Figure 2 



The Challenger Plateau survey area, showing bathymetry (depth contour in 

 m) and details of survey stratification. 



ment of the fishery through 

 to maximum exploitation. 

 It is felt that the fishery 

 was not constrained much 

 by the TAC over this time. 

 CPUE in winter months 

 from 1983 to 1991. This in- 

 cludes data from 1990 and 

 1991, following a substantial 

 reduction in TAC and effort. 

 CPUE in non-winter months 

 from 1983 to 1991. 

 Trawl survey indices from 

 1987 to 1989. These sur- 

 veys covered the same area, 

 had the same design, and 

 used the same vessel. 

 Trawl survey indices from 

 1984 and 1987 to 1990. 

 This series incorporated 

 data from a smaller area 

 surveyed in 1984 and from 

 the 1990 survey, both of 

 which used a different ves- 

 sel from 1987 to 89. 



The coefficient of variation (CV) is a measure of 

 the precision of the biomass estimate, and was cal- 

 culated by 



CV = 5 B /Bxl00. 



Stock reduction analysis 



A stock reduction technique was used to estimate 

 virgin biomass based on the method of Francis 

 (1990, 1992). This incorporated a complete catch 

 history for the stock, a time series of abundance 

 indices, and life history parameters used in a deter- 

 ministic age-structured population model (see 

 Clark, 1992). The latter were the von Bertalanffy 

 growth parameters (L m =39.5 cm, &=0.059yr _1 , t = 

 -0.3 yr), natural mortality=0.04yr _1 , weight-length 

 parameters (a=0.0963, 6=2.68), age at maturity (24 

 yr), age at entry to the fishery (24 yr), and Beverton- 

 Holt recruitment steepness of 0.75. 



Five sets of abundance indices were used from 

 trawl surveys between 1984 and 1990, and commer- 

 cial catch per unit of effort (CPUE) data (unstand- 

 ardized mean catch per tow by monthly groupings): 



1 CPUE in winter months (June-September) from 

 1983 to 1989. This covered the period of develop- 



The maximum likelihood 

 method was used to estimate 

 virgin biomass. Ninety-five 

 percent confidence intervals 

 were estimated by using bootstrapping techniques 

 with the coefficient of variation fixed at 20%. The 

 best estimate of virgin biomass was then used in an 

 age-structured model (detailed in Francis, 1992) to es- 

 timate current biomass. 



Biological data 



Standard procedure during trawl surveys was to 

 take a random sample of about 200 fish from each 

 tow. These were measured (standard length rounded 

 down to the nearest whole cm [standard MAF Fish- 

 eries procedure]) and sexed. Twenty of these fish 

 were randomly selected, their otoliths extracted, and 

 more detailed data collected: standard length (rounded 

 down to the nearest whole mm), weight (rounded down 

 to the nearest gm), sex, stage of gonad maturity (see 

 below), gonad weight (rounded down to the nearest 

 gm), fullness of stomach, state of digestion of contents, 

 and stomach contents (to species level where possible). 



Size Length-frequency distributions have been con- 

 structed to represent the total population where 

 possible. In the years 1984 and 1987-90, data have 



