Title: Biological characteristics of commercially exploited Penaeidae shrimp, P. semisulcatus in the north-western part of the Persian Gulf

In this study the results of 40 months of sampling on Penaeus semisulcatus, De Hann 1848, in the Bushehr area at the north-western part of the Persian Gulf, is discussed. The spatial and temporal distribution of shrimp is highly variable by month and year. Higher shrimp densities are often found in early summer between the latitudes of 28° and 29°N and extend north and south in August. In later months (September to December) one can find a relatively high-density zone in the northern most part of the distribution area and in the autumn smaller aggregations are found at lower latitudes.

The main recruitment, which the fishery is dependent upon, is from July to August and a second recruitment period occurs from December to February. The timing and strength of both recruitments vary inter-annually. The proportion of pre-recruits in the southern area is always higher and the modes more prolonged than in the northern area. The period of major recruitment starts earlier and lasts longer in the south than the north. A consistent pattern is not seen for the secondary recruitment, which usually occurs in from December to January. Maturity is highly seasonal and exhibits two peaks in the autumn and again the following spring, although the exact timings may vary between years. The spatial distribution of the mature female shrimps in the spawning months (October to June) indicates that the northern part of the study area is an aggregation ground. The size at mass maturity L50% was found to be variable by year in the range of 13.3 to 15.9 cm. The length-frequency distribution of shrimps by sex reveals one to three modes depending on the month and year. In the southern part a more complicated and less consistent pattern of recruitment and growth between years was observed. There is multiple or/and continuous recruitment especially during the summer months. The von Bertalanffy growth parameters were estimated. It was found that both male and female shrimps could attain higher lengths in the southern area with inter-annual consistency throughout the study period. There are two generations of fast and slow growing shrimps, which contribute to summer and autumn recruitment respectively. On the basis of growth parameters there is a possibility of interlocking, by means of contribution of the spring generation to the autumn spawning season, but the autumn generations are too small to take part in the spring spawning season. In this study, the timing and geographical location of recruitment and spawning of P. semisulcatus in Iranian waters of the Persian Gulf were identified. The findings will be used to fine-tune the ongoing sampling programme and design new sampling schemes to cover the whole life cycle of this species enabling one to explain the variations in their response to the environment and fishery. The results of this study assist us in building appropriate assessment and management models.