Grandcourt: Effect of intensive line fistiing on biomass of Pnstipomoides rilamentosus 



307 



addition, the weather condition and current strength were 

 recorded on a subjective scale from 1 (good) to 5 (poor). 



Data analysis 



Daily catch and effort data were analyzed by fishing trip 

 duration, depth, gear type, bait type, and climatic condi- 

 tions to determine the effect on catch rates. The catch- 

 ability coefficient {q) of the Leslie constant catchability 

 model (Leslie and Davis, 1939) and intercept parameter 

 (a) were determined by using least squares linear regres- 

 sion, where "(7" is the value of the regression coefficient. 

 The adjusted cumulative catch (x) — the cumulative catch 

 to interval / plus one half of the catch during interval i — 

 was used as the independent variable; this adjustment 

 proposed by Chapman ( 1961) compensates for the decline 

 in catchability during each time interval (King, 1995). The 

 daily catch per unit of effort was used as the dependent 

 variable. Data from the first day of fishing were excluded 

 from the analyses because P. filamentosus was not being 

 fully targeted. The Leslie constant catchability model was 

 used to derive an estimate of the initial population biomass 

 of P. filamentosus accessible to a line fishery within the 

 study area. Polovina (1986) described the Leslie model as 

 the catch per unit of effort during a time interval it), and 

 because (CPUE^f^) is defined as the product of catchability 

 (q) and the mean population biomass present during the 

 period t (B,,,), the model can be expressed as 



CPUE,„ = qB,, 



(1) 



Suppose that up to the beginning of period <, Z^,,, fish have 

 been caught and removed. If the period t is relatively short, 

 the population of fish closed or isolated, and the fishing 

 pressure heavy enough so that it can be assumed that 

 mortality from other factors is negligible, then B,^) can be 

 expressed as 



was calculated by applying the initial biomass estimate 

 to the length and surface area of the grounds fished. The 

 annual maximum sustainable yield ( MS Y ) was determined 

 by using the results of Mees (1993), where the MSY was 

 estimated as 24% of the initial virgin biomass off! filamen- 

 tosus on the edge of the Seychelles Bank, situated 400 km 

 north west of the study site. Potential yield estimates were 

 calculated per km and km- on a yearly and daily basis by 

 applying the maximum sustainable yield as a proportion 

 to the respective biomass densities. 



B„ 



'■ "(0) "^{l)  



Results 



Where B,,,, is the initial population biomass at the begin- 

 ning of the experiment (t=Q), inserting this expression for 

 B|,| in Equation 1 produces the Leslie constant catchability 

 model: 



CPUE,„ = qB,,,,-qK„, 



For the initial population biomass estimate, bootstrapping 

 was used to determine 95% confidence intervals. 100 runs 

 were made. Daily estimates of the remaining biomass size 

 were calculated by subtracting the cumulative catch from 



-K^i^, where B,, 



the initial biomass estimate i.e: B^,, = B^^ 

 is the biomass present at the beginning of time (t); Bm, is 

 the initial virgin biomass prior to fishing at / = 0; and /f,,, is 

 the cumulative catch to the beginning of time period U). 



The length of the 100-m contour, mean width of the 

 55-130 m depth band, and surface area of the fishing 

 ground were determined from chart data. Stock density 



A total of 173 dory fishing trips with a mean trip length of 

 7.7 hours were made over a period of 13 days. There were 

 two fishermen in each of the 12 dories; all used electric fish- 

 ing reels, the hooks of which were baited with skipjack tuna 

 (Katsuwonus pelamis). The mean catch rate for all species 

 was 28.5 kg/man hour, equivalent to 429.4 kg per dory trip. 

 The crimson jobfish represented 80.17( of the total catch 

 of 74.3 t. The Serranidae were the second most abundant 

 family, and of this family Epinephelus multiiiotatus formed 

 7.2%, Epinephelus chlorostigma 2.5'/f, and Epinephelus 

 morrhua 2.5% of the total catch from the dropoff at Saya de 

 Malha Bank (Table 1). The mean depth fished was 88.8 m 

 and ranged from 55 m to 130 m. The length of the 100-m 

 contour was 60.2 km, mean width of the 55-130 m depth 

 band was 510 m, and the surface area of the grounds fished 

 was 30.7 km^. 



There was an overall reduction of the catch rate over 

 the 13-day period (Fig. 2 and Table 2). This could not be 



