Biometric characteristics, age and growth, reproductive biology, size composition of the catch, stock assessment and fisheries management models were examined for king soldier bream (Argyrops spinifer) and spangled emperor (Lethrinus nebulosus) in the Arabian Sea off Oman.
The relationship between head length, standard length and fork length to total length and the length-weight relationships were established for both species and were independent of fish sex.Male A. spinifer were more common in the larger size classes, which may be evidence of protogyny.
A study of the age and growth of the two species showed that the fishery targets a wide range of ages (A. spinifer, 2-25 years old; L. nebulosus, 2-29 years old; :::::20-70 cm TL for both species). Smaller fish (0-1 years old) were absent in the hand-line fishery catches but were captured accidentally in trawling gear. Marginal increment analysis of monthly otolith samples
showed the annual formation of one opaque and one translucent ring each year. The timing of formation of the translucent ring was approximately 10 months (June - March) and the opaque ring formed between April and May with the cycle dependent on annual changes in seawater temperature in the Arabian Sea. Von Bertalanffy growth (VBG) constants were calculated for both species as follows: L00 = 64.6, K=0.142 and t0= -0.489 for A. spinifer and Loo = 72.2,
K=0.086 and t0= -3.041 for L. nebulosus. No differences in VBG were observed for male and female fish for both species. Since ageing fish using sectioned otoliths is time-consuming, the utility of using the linear relationship between otolith weight (OWT) and age to rapidly estimate
fish age was examined. For both species, it was found that this approach could be used to derive VBG curves that were not significantly different from the curve obtained using sectioned otoliths (>360 for both species) when the age of 200 fish was estimated from the OWT-age calibration graph: a minimum sample size of 100 fish was necessary for the OTW-age calibration line for A.
spinifer and 200 fish for L. nebulosus. Thus, the use of OWT-age relationships can be used successfully to predict age in these species providing that these minimum sample sizes are used. Argyrops spinifer and L. nebulosus are multiple asynchronous spawners, with males and females reaching maturity and spawning at the same time. Seasonal changes in gonad weight, Gonado-Somatic Index (GSI), Liver Somatic Index (LSI) and condition factor, indicated that the main spawning season was from September (after the monsoon season) to January for A. spinifer, and September to December for L. nebulosus with a peak of spawning between October and November for both species. The size and age at first maturity were calculated as 35.3 cm TL and 5.1 years old for A. spinifer and 33.7cm TL and 4.3 years old for L. nebulosus respectively.
 Average fecundity was calculated as 118,000 oocytes for Argyrops spinifer and 415,000 oocytes for L. nebulosus respectively.
Fisheries management models, utilizing Length Cohort Analysis (LCA), indicated that both A. spinifer and L. nebulosus fisheries are currently underexploited. This conclusion was supported by several points: 1) estimated rates of natural and fishing mortalities were low (F=0.02-0.03); 2) selective artisanal fishing gears targeting both species provide the bulk of the landings; 3) the occurrence of the monsoon season for five months of the year (May to September) reduces the fishing pressure on the two species; 4) the upwelling of nutrient-rich
waters during the monsoon season and the resultant high productivity may enhance fish production by increasing survival, growth and recruitment of these species in the Arabian Sea; and 5) juvenile fish tend not to be exposed to the fishery and a large number of juveniles are estimated to recruit into the stock each year in January and February. Taken together, these factors indicate that both species are currently in a healthy state in the Arabian Sea. The present
healthy status of this fishery in the Arabian Sea is reflected in the management models proposed for both species in the region; i.e. a 1.6 fold increase in fishing effort which will result in a 51 % increase in the total sustainable catch whilst the proportion of spawning stock biomass (SSB) remains at 90% of its maximum value. No mesh size regulation is recommended as most of the
fishing gears targetting these species are highly selective and a closed season for the trawl fishery is not recommended due to the negative impact that this will have on the artisanal fishery.