The survey plan for Penaeus semisulcatus nursery grounds was designed to encompass the main coastal prawn catching in Bushehr waters, Persian Gulf, from July 2003 to March 2005.
Sampling stations were selected in the shallow waters < 10 m deep and collections of juveniles were made from a small vessel powered by a 150 hp outboard engine that was equipped with a small beam trawl net with 10 mm stretch mesh. Prawns less than 15 mm carapace length were classified as juvenile.
The catches of P. semisulcatus juvenile were abundant at only a few sites in the shallow waters around southern (Motaf) and middle (Helaileh) regions of the study area. The maximum number of juveniles occurred in June and November 2003 and April and June 2004. Juvenile abundance was higher in vegetated sites as compared to non-vegetated sites during this study.
Finding from the present study support the facts that the extensive shallow reef of flat and open coastline sea grass and algae communities are therefore likely to be critically important for the fishery and should therefore be afforded protection from pollution, fishing gear damage and industrial development.
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Keyword: Nursery grounds; prawn; Penaeus semisulcatus; coastal waters; Bushehr; Persian Gulf
Knowledge on the early life history characteristics of commercially important species is vital to fisheries managers. Many marine invertebrates, particularly decapod crustaceans including penaeid prawns, have a pelagic planktonic larval phase, after which they settle and adopt a benthic mode of life (Dall et al., 1990).
Many authors have noted that the preferential habitat of juvenile P. semisulcatus is seagrass beds (Busson et al., 1977; Al-Attar, 1984; Sumito et al., 1996 and Loneragan et al., 1994). It is presumed that P. semisulcatus juveniles dwell the shallow depths along coastal areas. Juvenile penaeids are mainly found in shallow beds of seagrass and algae (Dall et al., 1990; Haywood et al., 1995). Moreover, these areas have been distinguished as the main nursery grounds of P. semisulcatus.
In the life cycle of short-lived species such as prawn, the juvenile stage is very important and the protection of this resource has a positive effect on the total harvest levels of matured specimens in offshore waters. Nursery grounds have special characteristics and any changes in this ecosystem can affect the regional prawn resources. Therefore, the protection of nursery grounds of prawn throughout the year is one of the most important objectives in prawn management procedures. This part of the study was carried out from July, 2003 to March, 2005 in shallow waters of Bushehr, Persian Gulf. The objectives of the study are to describe the seasonal abundance of juvenile prawns, P. semisulcatus in the Bushehr coastal waters, size composition of juveniles and species composition in nursery areas of P. semisulcatus. The results of this study will provide baseline information on the biology and population dynamics of P. semisulcatus in the Persian Gulf.
2. Materials and Methods
2.1. Study Area
The survey plan for P. semisulcatus nursery grounds was designed to encompass the main coastal prawn catching in Bushehr waters (Fig. 1 and Table 1) from July 2003 to March 2005. The survey was conducted after the known spawning period of P. semisulcatus in the study area. The samplings were carried out in July, November, December 2003 and January, February, April, June, July 2004 and February and March 2005. These periods were after spawning seasons of P. semisulcatus in the study area. In some areas, the samplings were not completed and repeated in the following year.
2.2. Sampling Methods
Sampling stations were selected in the shallow waters <10 m deep and collections of juvenile shrimps were made from a small vessel powered by a 150 hp outboard engine. The vessel was equipped with a small beam trawl net with 10 mm stretch mesh. Sampling at each station was conduced by towing for 15-20 minutes at a speed of 3-4 km/hr. GPS instruments were used to maintain boat positioning and speed .
Each trawl sample was preserved in 3% formalin and subsequently subjected to laboratory analyses. Specimens of P. semisulcatus were identified, counted, and measured to the nearest 1 mm CL under a dissecting microscope.
To improve the identification, some samples of post larval and juveniles (from PL5 to 30 mm) of P. semisulcatus were obtained from shrimp hatcheries for comparison. The identification keys developed by Al-Yamani et al. (1995), Fischer and Bianchi (1984) and Carpenter et al. (1997) were also used to identify shrimps and other marine organisms. Seaweed species were identified using the keys developed by Kalioperumal et al. (1995) and Gharanjic et al. (2000).
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In the present study, prawns <15 mm CL were classified as juvenile. The mean CL obtained from the nursery grounds was calculated and the percent age size distributions were plotted for each month. The percentage of penaeid prawns and other marine organisms and also percentage of green tiger prawn and penaeid prawns and other aquatic organisms were calculated (by numbers) in the study areas during the nursery months of P. semisulcatus.
3.1. Seasonal Distribution
A total of 748 P. semisulcatus juveniles were collected during the study period in Bushehr coastal waters (Table 2). Sampling, were carried out to coincide with the P. semisulcatus spawning seasons and its maximum abundance in the nursery grounds. The maximum number of juveniles captured in bottom trawls occurred in July 2003 (table 2). Following this peak, the juveniles were caught in November 2003, April and June 2004. In some periods (March and May 2003-04), poor weather conditions have prevented sampling in the nursery grounds.
3.2. Size Distribution
Seasonal variation in P. semisulcatus juveniles CL was evident in the study area. Juvenile CL ranged from 3 mm to 15 mm (Fig. 2). In July 2003, the CL of juveniles ranged from 3 mm to 15mm with modes of 8-9 mm and 15 mm. In November 2003, the CL ranged from 3 mm to 8 mm with a mode of 3-4 mm. In April 2004, the CL ranged from 3 mm to 13 mm with a frequency (mode) in 7 mm and 9 mm. Finally, in June 2004 the CL ranged from 3 mm to 15 mm with modes of 6 mm, 10 mm and 15 mm.
The mean CL of P. semisulcatus juveniles during the periods of maximum abundance were as follows: in July 2003, the mean CL was 6.9 mm (SD = 3.2), in November it reduced to 4.5 mm (SD = 1.7 mm), and in April, 2004, the mean carapace increase to 8.4 mm (SD = 2.7 mm) and again increased to 9.3 mm (SD = 3.4 mm) in June, 2004 ( Table 2).
The mean lengths and modes of CL were relatively similar to each other, but the percentage of modes for size distribution varied across the study period. In all size distribution there were two or three modes and the size distribution covered a wide range in July 2003 and June 2004. however, in November 2003, the size distributions covered a limited range.
3.3. Nursery Areas
The catches of P. semisulcatus juvenile were abundant at only a few sites around the southern (Mottaf) and middle (Helaileh) regions of the study area(Fig. 3). Other major nursery grounds are located in the Tangestan and Daylam regions. However, the highest catch of juveniles was recorded from the Mottaf and Helileh regions, suggesting these locations are the most important P. semisulcatus nursery grounds in Bushehr coastal waters.
It was apparent that catches of P. semisulcatus juveniles peaked four times per year with the highest peaks in April and June. Another peak was evident in July in the nursery grounds mentioned above.
Juvenile abundance was higher in vegetated sites as compared to non-vegetated sites during this study. The seaweeds in this region vary in their morphology and are found in waters ranging <1 m to 10 m in depth, although some species may have extended into the deeper waters, outside the study area, at densities too low to be observed. The most abundant species identified in the vegetated habitat were Bryopsis pennata, Sargassum piluliferum, Padina boeregesenii, Gracilaria corticata, Gracilaria pygmaeu and Ahnfeltopsis pygmaea. Bryopsis pennata ( Chlorophyta) was observed in shallow waters and tidal zones, Sargassum piluliferum (Phaeophyta) was observed in all the study areas and this species was the most abundant algae in the region. Padina boeregesenii (Phaeophyta) was one of the most important species in the nursery areas. Gracilaria corticata and Gracilaria pygmaea (Rhodophyta) were observed throughout the study period and Ahnfeltopsis pigmaea (Rhodophyta) was dominant in the spring. Other seaweed species were identified in the study area, however the juveniles of P. semisulcatus sampled in the distribution zones were associated only with the aforementioned species. A few juveniles were observed in the non-vegatated areas, although these areas cannot be considered as P. semisulcatus nursery grounds. Aquatic vegetation provides critical nursery habitat for the juveniles of P. semisulcatus and as such, is highly important for the commercial shrimp fishery in Bushehr waters.
3.4. Non Prawns Organisms
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Five taxa groups dominated the total fauna in the nursery grounds of P. semisulcatus. They include 22% Sparidae, 18% platycephalidae, 7% crabs, 3% bivalves, 2% seastar and 1% other species (Table 3). In total, 47% of marine organisms were found in the nursery grounds of shrimp species. In addition to P. semisulcatus, M. stebbingi, M. affinis, P. stylifera and Acetes spp. were also recorded.
Other prawn species were found not in high abundant in the nursery grounds when, compared to P. semisulcatus. The only exception was in November 2003 when they formed 32% of the total marine organisms (Table 4). M. affinis and M. stebbingi shared the same nursery grounds, but Parapeneopsis stylifera was observed in other areas. Acetes sp. was highly dominant in the study areas. In contrast to P. semisulcatus, other shrimp species were observed in nonvegetated areas. The percentages of P. semisulcatus and other marine organisms in the study area were different during juvenile stages of P. semisulcatus. In July and November 2003 other marine species were found to be more dominant in the nursery grounds of P. semisulcatus, totalling 66% and 52% of the marine species in the nursery grounds (Table 4). In April and June 2004 P. semisulcatus was the dominant species in the nursery grounds, comprising of 65% in April and 90% in June of the total species. The high percentage of other marine species in July can be attributed to the use of the steel bottom trawler. For this sampling, trawling was hauled for an hour and the catch composition was very different from the other sampling areas. Therefore, this nursery ground cannot be compared with the other areas where the sampling was carried out by 15 minute beam trawls from a small vessel. Very few juveniles of P. semisulcatus were caught during the colder months (November 2003 and April 2004) and the seasonal abundance of juveniles were observed in the warmer months (July 2003 and June 2004).
3.5. Environmental Parameters
Water temperature and salinity were measured and the mean values were calculated during the study period. The inshore water temperature and salinity fluctuated greatly. On average, the minimum water temperature was 17.5°C in February and reached a maximum of 34°C in July (Fig. 4). Fluctuations in the mean salinity did not correspond to the water temperature. For instance, the minimum mean salinity was 22.3 in December and the maximum was 44.5 in January. Cold periods exhibiting minimum temperatures range from December to March. In shallow inshore areas, the rainfall runoff from rivers and estuaries acts to decrease the salinity level. Samplings in November and December were carried out after the rainfall to determine the effects on decreasing salinity.
In April, June, July and November, when juveniles of P. semisulcatus were most abundant, these parameters fluctuated. However the mean minimum water temperature was 18.5°C and the salinity was at 24.6, recorded in November. Other periods did not exhibit a great fluctuation as in November within these periods the mean water temperature ranged from 26.2°C to 34°C and the salinity ranged from 39.3 to 40.3. In general, the inshore waters exhibited similar trends in salinity and temperature during the study period.
The general importance of vegetation to juveniles of P. semisulcatus is well known, but little is known about how the abundance of juveniles varies between different types of vegetation beds. In the present study, different types of habitats (e.g. vegetation, sand) were investigated. The vegetated areas were recorded as the most important nursery grounds for P. semisulcatus in Bushehr coastal waters. However, the species of marine plants were different in the present study, besides Sargassum sp. none of the marine plant species in the present study were observed in the other areas. This suggests that seaweed species are not important for P. semisulcatus, although they use the vegetation as a shelter to hide from the predators. Despite the appearance of vegetation area in the other surveyed stations, juveniles of this species was not observed there and this suggests that other ecosystem parameters are important for aggregation of juveniles of green tiger prawn in the study area. But the existance of vegetation is the most important factor for P. semisulcatus colonization in any area.
Despite the importance of vegetated habitat to P. semisulcatus, these areas are rapidly declining due to continued degradation of the habitat by small trawlers and industrial development. During the open prawn season there are more than one thousand small vessels trawl along the shallow waters of less than 10 m depth. The negative effects on prawn catch in the sea resulting from habitat degradation is a vital concern. A closure on trawling activities along these shallow water areas is necessary for maintaining sustainable levels of prawn harvest in Bushehr waters. Allowing continued damage by trawling and industrial development activities in shallow water areas will impede the postlarval and juvenile life cycle periods of P. semisulcatus, contributing to its decline.
Unfortunately in some areas, understanding the biodynamics of nurseries has been impaired (rocky areas) by the inherent sampling difficulties in marine vegetation. It suggests that some of non-investigated areas (rocky areas) due to the mentioned problems can be recorded as the nursery grounds of this species. It was apparent that P. semisulcatus recruits the fishery occurred in the warmer months of the year (July-September), although some juveniles were caught in the colder months (November and April).
A review of major results from more than 200 relevant papers on seagrass meadows indicated that the abundance, growth and survival of P. semisulcatus juveniles were greater in seagrass compared to other ecosystems, such as sand or muddy bottoms that devoid of vegetation (Beck et al., 2003). Intertidal and shallow sub-tidal seagrasses are the critical settlement and juvenile habitat for P. semisulcatus (Lonergan et al., 1994).
The habitat requirements of the juvenile stages of the four prawn species important in the commercial fishery in the western Gulf of Carpentaria (P. esculentus, p. semisulcatus, M. endevouri and M. ensis) were studied by Staples et al. (1985). Three of the species (M. ensis was the exception) have restricted habitat requirements and most juveniles were found associated with the seagrass communities. In another study, the population dynamics of two tiger prawn species, P. esculentus and P. semisulcatus were examined from several seagrass sites in the Gulf of Carpentaria (Staples, 1987). Noticeable differences in the abundance of juvenile prawns in the four seagrass communities were observed. The lowest abundance of juveniles were in H. ovalis and H. spinulosa and the highest in E. acoroides. Coles et al. (1987) reported that H. uninervis was the most important of seagrass for P. semisulcatus in the nursery areas of Queensland waters. In Bahrain waters, the most important nursery areas for P. semisulcatus is known to contain vast seagrass beds dominated by H. uninervis (Abdulqader and Naylor, 1995). This area is infuenced strongly by the inshore tidal currents (Atkins, 1985) which evidently transport larvae and postlarvae from the northern part of the Bahrain fishing grounds where the spawning takes place. The occurrence of P. semisulcatus in Tubli Bay (Bahrain) suggests the importance of the Bay as a nursery ground for this species (Abdulqader, 1999).
In a series of enclosure experiments, Loneragan et al. (1996) found that the growth rates and stock density of P. semisulcatus can be maintained better in seagrass beds of E. acoroides, as compared to H. uninervis.
In the laboratory experiment, Liu and Loneragan (1996) examined the response of postlarvae and juveniles of P. semisulcatus in different types of habitat. They reported that five size classes of postlarvae were offered a choice between Zostera capricorni and base sand. Small size classes of postlarvae either did not response to Z. capricorni, or were more abundant on bare substrate than the Z. capricorni. In contrast, the larger postlarvae size classes were more abundant on Z. capricorni during the day, but not at night. Juveniles were both abundant on artificial sea grass than the bare ground during the day but not at night, indicating that they respond to the structured habitats. In this study, they reported that the laboratory behavior of P. semisulcatus postlarvae changes during the growth period. For example, postlarvae spend more time swimming in the water column than on the substrate and showed no preference between seagrass and bare substrate, while large postlarvae exhibit a preference for seagrass during the day. In this study the highest mean density was recorded at the intertidal site where H. ovalis and H. uninervis were the dominant species. Low shoot densities were recorded where either E. acoroides or C. serrulata was the dominant species.
Most nursery grounds and seagrass areas were located in the shallow waters. In the Gulf of Carpentaria, 64% of seagrasses were observed in the shallow waters of less than 2.5 m deep (Poiner et al., 1987).
For the Persian Gulf and Oman Sea, reports indicate that shallow waters and vegetated areas are the nursery grounds for P. semisulcatus and other penaeid species. Kuwait Bay is one of the important nursery areas on the west coast and seems to be shared by two species, M. affinis and P. semisulcatus, at different times of the year (Van Zalinge, 1984). Mohammad et al. (1981) and Jones and Al-Attar (1982) reported that the seaweed (Sargassum sp.) affixed to the bottom as well as floating forms an important habitat for juveniles of P. semisulcatus. Mohan and Siddeek (1996) reported the importance of mangrove and seagrass habitat for postlarvae and juveniles of P. indicus in the Gulf of Masira, Sea of Oman.
Zimmerman et al. (1984) investigated the relationship between the vegetation and the predators. Seagrass habitats may be particularly important to the postlarvae and juvenile stages that do not bury themselves in the substratum and are, thus, more vulnerable to predators than the older prawns (Kenyon et al., 1995). This study reported that a visual fish predator was three times more successful in catching small juvenile tiger prawns on bare substrate than in the seagrass dominated by C. serrulata, and twice as successful in the much smaller seagrass H. uninervis and H. ovalis. Predation rates on prawns were lower in seagrass with tall and wide leaves than in those with short and narrow leaves.
In the beam trawl catches of postlarvae of p. semisulcatus and P. esculentus around the Groote Eylondt in the western Gulf, small postlarvae were found on a greater range of habitats than bigger postlarvae. Large postlarvae were rarely found on unregulated substratum, which suggests that smaller postlarvae may still have been migrating into the water column, while the larger postlarvae had become epigenetic.
In the Gulf of Carpentaria, Haywood et al. (1995) reported that juveniles of P. semisulcatus are found in coarser sediments on beds of both sea grass and algae. Based on the abundance of juveniles on intertidal sea grass beds, Staples (1984) suggested that summer and winter recruitment peaks are of equally important for P. semisulcatus and p. esculentus in the north-eastern Gulf of Carpentaria.
The relationship between vegetated areas and the catch of prawn has been reported by various authors. A positive correlation between the annual prawn catch and the existing mangrove areas has been reported for the Philippines (Camacho and Bagarinao, 1987). Twilley (1989) reported that the loss of mangroves have been translated into a direct loss of habitat and species diversity of an unknown magnitude, which could be one of the causes for the decline in the abundance of wild prawn juveniles in Ecuadorian estuaries. Primavera (1988) suggested that the mangrove loss will lead to declining of penaeid populations in the wild. Tagging studies of P. semisulcatus have demonstrated the recruitment from the nursery areas into the adjacent offshore fishing (Somers and Kirkwood, 1984; Somers, 1987).
Findings from the present study support the facts that some vegetation areas of the Persian Gulf serve as habitats for juveniles of P. semisulcatus. This stage of life cycle is very important for the survivel of the species. The extensive shallow reef of flat and open coastline seagrass and algae communities are therefore likely to be critically important for the fishery and should therefore be afforded protection from pollution, fishing gear damage and industrial development.
The authors appreciate to the asistance of Department of Biology, Faculty of Science staffs during this study in Universiti Putra Malaysia. This project was founded by the Iranian Fisheries Research Organization (IFRO). The authors wish to thank the cooperation of IFRO and assistance of stock assessment division of Iran Shrimp Research Center (ISRC) in all phases of data collection and also the captain and crew of R/V Lavar II who helped with the field sampling. We thank Dr John Hoolihan for reviewing the manuscript and for his comments.