Grandcourt et al : Biology and assessment of Diagiamma pictum and Lethnnus nebulosus in the southern Arabian Gulf 



85 



Reproduction 



Simulation models and evidence of the effects of fish- 

 ing have shown that protogynous species are far more 

 vulnerable to fishing pressure than comparable gono- 

 choristic stocks (Huntsman and Schaaf, 1994). For 

 protogynous species, in which males tend to be larger 

 than females on average, there are indications that 

 size-selective fishing mortality may result in the dif- 

 ferential loss of larger males (Sadovy, 1996) and the 

 possibility that insufficient males remain in the repro- 

 ductive population to fertilize eggs from all females 

 (Koenig et al., 1996). In this context, L. nebulosus may 

 be particularly vulnerable to such effects because the 

 female-biased sex ratios were consistent throughout all 

 the age categories and size classes. The overall female 

 bias and removal of the bias in the oldest and largest 

 age category for D. pictum is generally representative 

 of the sexual structure of a protogynous population. 

 Given these characteristics, histological confirmation 

 of the reproductive mode of this species should be 

 considered. 



The well-defined spawning period of D. pictum and 

 L. nebulosus between April and May supports the view 

 that seasonal reproductive cycles are common among 

 tropical fishes (Robertson, 1990; Montgomery and Gal- 

 zin, 1993; Sadovy, 1996). There were high levels of ju- 

 venile retention for D. pictum (35.1%) because fish were 

 recruited to the fishery before the mean size at which 

 sexual maturity occurred, indicating a need to increase 

 the mesh size of traps. 



Mortality and selectivity 



Estimates of natural mortality derived from other stud- 

 ies of Z). pictum range from 0.43/yr (Edwards et al., 

 1985) to 0.67/yr (Baillon and Kulbicki, 1988). The com- 

 paratively low value of M obtained in our study (0.13/yr) 

 can be attributed to the difference in methods used, but 

 errors in other estimates from the empirically derived 

 formula of Pauly (1980) may have occurred because the 

 relationship has tended to overestimate M, especially 

 for slow growing species (e.g., Ralston, 1987; Russ et 

 al., 1998). Similarly, our value of the instantaneous 

 natural mortality rate for L. nebulosus (0.20/yr) was 

 lower than other estimates that range from 0.279/ 

 yr (Edwards et al., 1985) to 1.18/yr (Baillon, 1991). 

 Although estimates of M derived from the Hoenig ( 1983) 

 relationship have been shown to provide a reasonable 

 approximation of M in tropical demersal fishes (Hart 

 and Russ, 1996; Newman et al., 1996), errors in this 

 parameter were potentially the greatest source of error 

 in our assessment. 



Upward bias in estimates of the total mortality rate 

 (Z) may have occurred if larger fish were less vulnera- 

 ble to the fishing gear or if adult fish underwent migra- 

 tions, for example. A survey of the biomass of demersal 

 species in the Arabian Gulf waters of the United Arab 

 Emirates showed that there were no seasonal changes 

 in the abundance of L. nebulosus and the haemulid 



Plectorhinchus sordidus (Shallard-). This finding indi- 

 cates that ontogenetic or spawning-associated migra- 

 tions would unlikely be altering the size and age com- 

 position and subsequent estimates of Z for the species 

 investigated. Although size-specific selectivity cannot 

 be discarded as a possible explanation for the small 

 proportion of larger and older fish in size-frequency and 

 biological samples, the impact of fishing on the size and 

 age structure of the respective populations is considered 

 a more likely reason for these observations and is likely 

 to be the probable cause given the historic absence of 

 regulation in the trap fishery. 



Because the size at first capture (21.1 cm Lp) was 

 considerably smaller than the size at which yield per 

 recruit would be maximized (44.4 cm L-p) for D. pictum, 

 an increase in the mesh size for the trap fishery should 

 be considered by management authorities especially 

 given the high rate of juvenile retention for this species. 

 The same is applicable for L. nebulosus with a mean 

 size at first capture of 26.4 cm Lp and size at maximum 

 yield per recruit of 36.9 cm Lp. 



Assessment of the fishery 



The use of yield-per-recruit models may be particularly 

 restrictive for fast growing tropical species with high 

 rates of natural mortality because the curves may not 

 reach a maximum within a reasonable range of fishing 

 mortality values (Gayanilo and Pauly, 1997). Although 

 the species examined in our study were relatively slow 

 growing and had low rates of natural mortality, failure 

 of the yield-per-recruit model may still have occurred at 

 the upper end of the fishing mortality range. 



Gulland (1970) suggested that in an optimally ex- 

 ploited stock, fishing mortality should be about equal 

 to natural mortality, resulting in an exploitation rate 

 of 0.5/yr. However, exploitation rates should be very 

 conservative for relatively long-lived reef fish (Newman 

 and Dunk, 2003), especially given that potential yields 

 may be over estimated by a factor of 3-4 where F=M 

 (Beddington and Cooke, 1983). With a range from 0.5 

 to 0.86/yr, the exploitation rates derived from yield-per- 

 recruit analyses (£„ j and S,,,.,^) are considered to have 

 overestimated the associated fishing mortality rates. 



The relative yield-per-recruit analyses indicated that 

 an increase in the size at first capture of D. pictum and 

 L. nebulosus to that which would maximize yield per re- 

 cruit would be associated with increases in yields at the 

 existing exploitation rates. However, the existing exploi- 

 tation rate for D. pictum (0.79/yr) was greater than that 

 which would maximize yield per recruit {E^^^^=0.57/yr). 

 Although the exploitation rate for L. nebulosus (0.64/yr) 

 was comparable to that which would maximize yield per 



- Shallard, B. 2003. Distribution and abundance of demersal 

 fish stocks in the UAE, 211 p. Technical Report 1. Fish 

 Resources Assessment Survey Project of Abu Dhabi and UAE 

 Waters. Bruce Shallard and Associates and Environmental 

 Research and Wildlife Development Agency, P.O. Box 45553, 

 Government of Abu Dhabi, United Arab Emirates. 



