Bio-Dynamics and Life Cycle of Shrimp (Penaeus Semisulcatus De Haan), in Bushehr Coastal Waters of the Persian Gulf

Shrimp fisheries in the Persian Gulf have a long history. The trawl fishery for penaeid prawn developed along the mid-northern coasts of the Persian Gulf (Bushehr waters) beginning in 1960. From 1998 to 2004 shrimp landings have fluctuated between 480t and 2700t per year. The major species of penaeid prawn, in terms of catch (>80%) and economic importance, in the Persian Gulf (including Bushehr waters) is Penaeus semisulcatus, commonly known as the green tiger prawn. More than 2000 vessels and 33000 fishermen from the Bushehr study area have participated in the shrimp fishery each season (August-September) over the last decade. In the Arabian side of the Persian Gulf, P. semisulcatus has been now become the most important single component species. Despite of commercial importance, there has been very little published research in the biology this species. In the study area, Bushehr waters, shrimp fishery is only controlled during open season. Because of lack of any data, there is no any prohibition and restricted areas in the nursery and spawning grounds and also the fluctuations of shrimp resources are unknown. The present study was carried out in Bushehr coastal waters during 2003-2005 and describes the bio-dynamics and life history of P. semisulcatus. Monthly sampling of shrimp was collected by R/V Lavar II for 15 months from January 2003 to March 2004 from 50 fixed stations. Carapace length (CL) frequency was used to estimate growth, mortality and exploitation rates for males and females using FiSAT statistical analyses software. The Von Bertalanffy growth parameters were estimated as K = 1.6 Year-1 and 2.2 Year-1 and L¥ = 38 mm and 50.4 mm for males and females, respectively. Estimated annual mortality rates for males were Z = 6.4, F= 4.3, and M = 2.11. While females were Z = 8.2, F = 5.8, and M = 2.41. Exploitation rates for males and females were E = 0.67 and E = 0.70, respectively. Relatively high values of total mortality and exploitation rates were obtained, suggesting this species is overexploited. The maximum ages calculated were 20 months for males and 15 months for females. Gonad maturity results indicated that the major spawning activity for P. semisulcatus within the study area occurs during winter and early spring, with an additional minor peak evident in autumn. These periods coincided with seasonal rainfall in the Bushehr study area. Size at 50% maturity (L50%) was 40.6 mm carapace length (CL). Less than 5% of females were smaller than 24 mm CL. Mature shrimps aggregated in the middle and southern regions of study area at depths of 20-30 m. The areas of Imam Hassan, Genaveh and Helleh represented the most important spawning grounds in Bushehr waters. A sex ratio of 0.84 (male/female) occurred over the study period. A total of 2571 prawn were tagged and released using a green color polyethylene steamer tag during the 15 month survey. From these, 68 recaptures were reported during August-September of 2003-04, of which four were omitted from analysis due to lack of information. Maximum recapture depths were 16-20 m and 26-30 m for males and females, respectively. The majority of recaptures indicated that these prawn moved at speeds of < 2 km/day. Maximum speed was recorded 3-4 km/day for males and 2 km/day for females. Around 70% of the tagged shrimp were recaptured within 20 km of their release locations, while approximately 10% of them traveled a linear distance > 50 km. The maximum recorded distance was 127 km after 95 days of release for a female shrimp. The maximum time at-liberty was 381 days for a female that moved 20 km. Tag recapture data indicated shrimp generally moved to the northern area (Bahrekan) where waters were greater in depth. In the shallow study areas, post-spawning sampling of juvenile P. semisulcatus was carried out during July, November and December 2003 and January, February, April, June, July 2004 and February and March 2005. Some stations that recorded a depth of less than 10 m were trawled using a small vessel equipped with a small beam trawl. A total of 748 juvenile P. semisulcatus of less than 15 mm CL were collected. These juveniles were observed in July and November 2003 and April and June 2004. The CL of juveniles ranged from 3 mm to 15 mm and minimum of mean of CL were 4.5 mm in November and maximum 9.6 mm in July. Observed locations of juveniles were around vegetated sites at Mottaf (southern region) and Helaileh (middle region), although a few juveniles were also collected in the non-vegetated areas. Within these nursery grounds, 47% of marine fauna included other marine species such as fishes of Sparidae and Platycephalidae family, crabs, shells and other penaeid shrimp species such as Metapenaeus affinis and M. stebbingi. Seasonal abundance of juveniles was greatest during the warmer months (from April to November) and very few were captured during the colder seasons (from December to May). The study also used Random Amplified Polymorphic DNA (RAPD) analysis to compare the genetic diversity of the population of prawn, P. semisulcatus. The results showed that the genetic diversity is not significantly different and it was confirmed that this species should be identified as one population in the Bushehr waters. In the present study mature shrimp were dominant from December through March, while juvenile stages were observed generally from April through July. Exploitation rates of P. semisulcatus suggest that fishing activities should be regulated during spawning periods and within the nursery grounds.

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