Grandcourt et a\ : Biology and assessment of Diagramma pictum and Lethnnus nebulosus in the southern Arabian Gulf 



83 



10 



20 



30 40 50 



Size (cm Lp) 



60 



70 



Figure 9 



Aggregated length-frequency distributions for (A) 

 Diagramma pictum and (B) Lethrinus nebulosus 

 showing the mean sizes at first capture (Lj,,). sexual 

 maturity iL^^^), and the size at first capture which 

 would maximize yield per recruit (i^ax'- 







B 







0.2 



0.4 



0.6 



0.8 



0.6 



0.4 



33 

 - 0.2 !L 



0.6 



0.4 



0.2 







0.2 0,4 0,6 0.8 1 



Exploitation rate (/yr) 



Figure 10 



Relative yield and respective biomass-per-recruit curves 

 (descending lines) for (A) Diagramma pictum and (B) 

 Lethrinus nebulosus showing the existing exploitation 

 rate (£) and the exploitation rate at which the slope of 

 the yield-per-recruit curve is 0.1 of its value at the origin 

 iEqj). Curves show the effect of increasing the existing 

 mean size at first capture (LjqI to the mean size at first 

 sexual maturity (imat* ^^^ ^^^ ^'^^ which would maximize 

 yield per recruit ih^^^). 



Estimates of precautionary target and limit exploita- 

 tion rates (E ^ and £|,j„,t) for D. pictum were 0.34 and 

 0.40, respectively. 



The exploitation rate for L. nebulosus (0.64/yr) was 

 marginally greater than that which would maximize 

 yield per recruit (0.63/yr) at the existing mean size at 

 first capture (Table 6). The yield-per-recruit function 

 indicated that an increase in the size at first capture 

 to that which would maximize yield per recruit would 

 be associated with an increase in yield at the current 

 level of exploitation. An increase in the size at first 

 capture to that at which sexual maturity occurs iL^^^) 

 was also predicted to be associated with an increase in 

 yield, although to a lesser degree (Fig. 10). 



The relative biomass per recruit for L. nebulosus at 

 the current exploitation rate was less than 20% of that 



Table 7 



Relative biomass per recruit at precautionary exploita- 

 tion rates (E ^ and E^^^^^) at the existing size at first cap- 

 ture (L5Q ), the size at first capture at sexual maturity (Lj^^^^j), 

 and the size at first capture at maximum yield per recruit 

 '•^max' '*"" Diagramma pictum and Lethrinus nebulosus. 



Relative biomass per recruit 



Exploitation 

 rate 



D. pictum 



L. nebulosus 



^50 



mat max 



^50 



mat max 



£i,m,t</yt 



0.52 0.55 0.60 

 0.44 0.48 0.53 



0.50 0.51 0.53 

 0.39 0.40 0.45 



