86 



Fishery Bulletin 104(1) 



recruit (0.63/yr), given that E^^^ was probably overes- 

 timated, the results indicate that growth overfishing is 

 also occurring for this species. 



The specified precautionary target (F„p, = 0.5.M) and 

 limit (F,|^jj,=2/3.M) values are considered to be more 

 appropriate biological reference points in light of the 

 constraints of the yield-per-recruit model. The range 

 of fishing mortality rates estimated for D. pictum 

 (0.37-0.62/yr) was substantially greater than both the 

 target (F„pj=0.07/yr) and limit (F,„„„ = 0.09/yr) biologi- 

 cal reference points, and the existing exploitation rate 

 (0.79/yr) was more than double the optimum level (0.34/ 

 yr). The range of fishing mortality rates for L. nebulo- 

 sus (0.15/yr to 0.57/yr) were also in excess of the biologi- 

 cal reference points for this species {Fppj=0.10/yr and 

 F,,^,t=0.13/yr) and the exploitation rate (0.64/yr) was 

 approximately double the optimum target level (0.33/ 

 yr). This result clearly indicates growth overfishing for 

 both species and, in combination with the results of the 

 yield-per-recruit analyses, demonstrates that effort re- 

 ductions are also required in the fishery because target 

 reference points cannot be achieved by modification of 

 the gear-selectivity characteristics alone. 



A critical limitation of the yield-per-recruit model is 

 the assumption that there is no relationship between 

 the size of the spawning stock biomass and recruit- 

 ment (Buxton, 1992). Even if the size at first capture 

 is less than the size at first sexual maturity, the stock 

 size may approach zero at high levels of fishing mor- 

 tality in spite of predictions of high levels of yield per 

 recruit. It is therefore important to consider the size of 

 the spawning stock biomass across the range of fishing 

 mortality rates when interpreting results. The relative 

 biomass per recruit of D. pictum and L. nebulosus at 

 the estimated fishing mortality rates was particularly 

 low at less than 10% and 20%, respectively, of unex- 

 ploited levels. If the critical spawning stock biomass 

 is between 20% and 50% of the unexploited levels, as 

 suggested by King (1995), recruitment overfishing is 

 likely to be occurring for both species. This is most 

 clearly seen in the age structure for D. pictum, for 

 which only 13.8% of the total number offish were above 

 the age at which fish were fully exploited by the gear 

 (ages 4-13 years). For this species, the majority of the 

 yield was derived from the newly recruited age class 

 representing fish that had just become fully vulnerable 

 to the gear. 



The relative biomass per recruit at the exploitation 

 rates that were equivalent to F^^^ corresponded to 52% 

 and 51% of the theoretical unexploited biomass levels 

 for D. pictum and L. nebulosus, respectively. The associ- 

 ated levels of fishing mortality are therefore considered 

 appropriate target reference points, given the present 

 fisheries policy which is aimed at resource conserva- 

 tion and stock rebuilding. Accordingly, the results of 

 our study are important to fisheries management au- 

 thorities in the region because they indicate that both 

 a substantial reduction in fishing effort and an increase 

 in mesh-size of traps are currently necessary for the 

 previously unregulated demersal trap fishery. 



Acknowledgments 



We are very grateful to Sultan Al Ali, Khaled Al AH, 

 John Hoolihan, and Kunath Gopalan for their assis- 

 tance with data collection and Abdulla Aboubaker for 

 helping with data entry. Howard Choat and Will Rob- 

 bins assisted in the training of technicians in otolith 

 processing. The comments of the anonymous reviewer 

 helped considerably in improving the manuscript. This 

 study was funded by the Government of the United 

 Arab Emirates through the Environmental Research 

 and Wildlife Development Agency's Marine Environment 

 Research Centre. 



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