Many species of the Pteriidae family have been used in pearl mariculture and valuable pearls are produced in many parts of the world (Beer & Southgate, 2000; Norton et al. 2000). Since ancient times, they have contributed to the natural pearl industry worldwide and played an important role with their meat, shells and pearls (Almatar 1992; Nagai 2013). The pearl oyster
Spat collection is a method of collecting juveniles and requires lower costs compared to larvae production in hatcheries (Friedman & Bell 1996). Due to the negative effects of collecting mature oysters from the wild, the spat harvesting procedure has become much more important for enterprises (Knuckey 1995). Furthermore, spat collectors should be placed in areas where the existence of the species is essential to achieve high spat efficiency (Monteforte & Garcia-Gasca 1994).
Artificial collectors collect the required amount of pearl oyster spat in specific seasons. Spat collectors are designed to create a suitable environment for settlement and safe growth (Su et al. 2007). Larvae, including the metamorphosis stage, generally prefer to take refuge on safe materials to protect themselves (Beer & Southgate 2006). Materials such as plastic strips, coconut branches, bushes or commercial polyethylene mesh bags are used for spat collection (Victor et al. 1987; Gervis & Sims 1992; Friedman et al. 1998; Urban 2000). While spat harvesting efficiency is related to the material and shape of a collector, the area, depth, water current, seasonality and water conditions are also important factors (Coeroli et al. 1984; Sims 1990; Monteforte & Garcia-Garca 1994; Knuckey 1995; Friedman et al. 1998; Yigitkurt et al. 2017). The correct timing is also crucial for the collection of target species and high efficiency, otherwise the required amount of spat cannot be harvested because of undesired (by-catch) species (Brand et al. 1980; Cabral et al. 1985; Yigitkurt et al. 2017).
This study aimed to determine the difference in spat collection efficiency between surface and bottom water. The study was based on monthly spat collection and growing activity of spat, using collectors with commercial polyethylene mesh bags at Karantina Island (Izmir/Turkey).
The study was conducted at 30 km west of Izmir (38°22’44N and 26°47’12E) between July 2007 and August 2008 on the west coast of Urla Karantina Island (Fig. 1).
The depth of the water column was between 10 and 11 m, with sandy and muddy, partially stony sediment. Collectors were placed at the surface (1 m depth) and at the bottom (8 m depth) in July.
Seawater samples were collected from both depths. Temperature, pH and salinity were measured using a mercury thermometer, a pH meter (Hanna HI 8314) and a refractometer, respectively. Water samples were collected in a sterilized glass tube and transported to the laboratory. In the laboratory at the Ege University, the content of chlorophyll
The collectors were made of polyethylene mesh bags according to Beer & Southgate (2000). They were prepared by placing polyethylene material with the same surface area into the main collector body. The length and width of the mesh bags were 100 and 29 cm (0.29 m2), respectively. Mesh bags consisted of two parts, which were an outer mesh bag and an inner mesh bag for the surface and bottom. The mesh size of the outer mesh bag was 5×4 mm and of the inner mesh bag – 4×2 mm. A collector system was designed as “placed surface collectors” (PSC) and “placed bottom collectors” (PBC) and is presented in Figure 2. In this study, three collector systems were deployed in the area. Each collector system consisted of 24 mesh bags, 12 of which were placed at the surface and 12 at the bottom so as to triple the sets. These mesh bags were tied with a plastic rope to PVC pipes at 15 cm intervals. Two PVC pipes were fixed on the main rope at a depth of 1 and 8 m, with the surface and bottom collectors on both sides. The system was completed by connecting a buoy to the top part and an anchor to the bottom.
Every month, six mesh bags were taken from each collector system for spat efficiency. Length groups were defined as follows: ≤ 10 mm, 11–20 mm, 21–30 mm, 31–40 mm, 41–50 mm and >50 mm. In this study, individuals with a diameter of 10 mm or less were considered as new spat according to Yigitkurt et al.
(2017). The length (dorsoventral) of the target species
The Kolmogorov–Smirnov test was used to test the normal distribution. Pearson correlation analysis was employed to determine the relationship between spat attachment and environmental parameters (temperature, salinity, chlorophyll
The highest temperatures were measured in July and August and were 30.0°C at the surface and 28.5°C at the seabed, respectively. The minimum values were recorded in February and were 13.2°C and 13.0°C for the surface and bottom water, respectively (Fig. 3a). Mean salinity values were determined as 36.86 ± 0.2 PSU and 36.40 ± 0.16 PSU for the surface and the seabed, respectively (Fig. 3b). The lowest chlorophyll
The total number of spats harvested during 12 months was 2059±64.08 spat year−1 on both collectors (PSC and PBC). In total, 1401 ± 46.18 spat PSC−1 were collected on PSC (surface outer mesh bag: 597±18.47 spat; surface inner mesh bag: 804 ± 27.71 spat). The harvested number in PBC was 658 ±17.89 spat PBC−1, including the bottom outer mesh bag – 226 ± 14.43 spat and the bottom inner mesh bag – 432±3.46 spat. The total spat attachment values of each collector were significantly different between the two depths and between the inner and outer mesh bags at both depths (
The maximum spat harvest from both collectors was 235 ±6.92 spat (PSC +PBC)−1 in October and the minimum was 123 ± 2.30 spat (PSC + PBC)−1 in April. The total spat efficiency was significantly different in each month (
spat PSC−1 in May. On average 116.75 ± 25.16 spat PSC−1 were harvested per month from PSC during the study period. The highest and the lowest number of collected spats from PBC were 94±1.15 spat PBC−1 and 25±1.73 spat PBC−1 in August and July, respectively. The average number of spats collected from PBC per month was 54.83 ± 21.81 (Fig. 4). Individual values of monthly spat attachments were significantly different on PSC and PBC (
New spat (≤ 10 mm) attachments were determined throughout the year, except April and May. The maximum number of newly attached spats was observed in August on both collector groups (151 ± 4.61 spat (PSC + PBC)−1). The number of spats with shell length less than 10 mm was determined to be 44 ± 3.46 in the outer mesh bag of PSC and 26 ± 2.81 spats were found in inner mesh bags. The number of attached spats smaller than 10 mm was 33±5.77 and 48±6.92 in the outer and inner mesh bags of PBC, respectively (Table 1). There was a significant difference in the total number of new spat attachments in particular months (
The number of
Months/Collectors | < 10 mm | 11–20 mm | 21–30 mm | 31–40 mm | 41–50 mm | > 50 mm | ||
---|---|---|---|---|---|---|---|---|
August | SO | 49 | 89.8 | 10.2 | 0.0 | 0.0 | 0.0 | 0.0 |
SI | 48 | 54.2 | 45.8 | 0.0 | 0.0 | 0.0 | 0.0 | |
BO | 35 | 94.3 | 5.7 | 0.0 | 0.0 | 0.0 | 0.0 | |
BI | 59 | 81.4 | 18.7 | 0.0 | 0.0 | 0.0 | 0.0 | |
Sampling failed in September | ||||||||
October | SO | 58 | 5.2 | 50.0 | 44.8 | 0.0 | 0.0 | 0.0 |
SI | 92 | 1.1 | 28.3 | 68.5 | 2.2 | 0.0 | 0.0 | |
BO | 25 | 16.0 | 56.0 | 28.0 | 0.0 | 0.0 | 0.0 | |
BI | 60 | 5.0 | 38.3 | 56.7 | 0.0 | 0.0 | 0.0 | |
November | SO | 55 | 5.5 | 14.6 | 72.7 | 7.3 | 0.0 | 0.0 |
SI | 94 | 3.2 | 30.9 | 48.9 | 17.0 | 0.0 | 0.0 | |
BO | 33 | 18.2 | 24.2 | 51.5 | 6.1 | 0.0 | 0.0 | |
BI | 44 | 11.4 | 15.9 | 59.1 | 13.6 | 0.0 | 0.0 | |
December | SO | 52 | 7.7 | 19.2 | 46.2 | 26.9 | 0.0 | 0.0 |
SI | 73 | 0.0 | 16.4 | 56.2 | 27.4 | 0.0 | 0.0 | |
BO | 10 | 20.0 | 10.0 | 40.0 | 30.0 | 0.0 | 0.0 | |
BI | 44 | 4.6 | 18.2 | 65.9 | 11.4 | 0.0 | 0.0 | |
January | SO | 40 | 2.5 | 12.5 | 40.0 | 45.0 | 0.0 | 0.0 |
SI | 90 | 2.2 | 18.9 | 55.6 | 22.2 | 1.1 | 0.0 | |
BO | 18 | 5.6 | 33.3 | 44.4 | 16.7 | 0.0 | 0.0 | |
BI | 36 | 11.1 | 27.8 | 44.4 | 16.7 | 0.0 | 0.0 | |
February | SO | 74 | 1.4 | 16.2 | 47.3 | 35.1 | 0.0 | 0.0 |
SI | 81 | 1.2 | 16.1 | 44.5 | 38.3 | 0.0 | 0.0 | |
BO | 8 | 0.0 | 12.5 | 50.0 | 37.5 | 0.0 | 0.0 | |
BI | 31 | 3.2 | 16.1 | 51.6 | 29.0 | 0.0 | 0.0 | |
March | SO | 45 | 2.2 | 15.6 | 53.3 | 28.9 | 0.0 | 0.0 |
SI | 71 | 4.2 | 29.6 | 47.9 | 16.9 | 1.4 | 0.0 | |
BO | 34 | 11.8 | 35.3 | 32.4 | 20.6 | 0.0 | 0.0 | |
BI | 32 | 0.0 | 28.1 | 65.6 | 6.3 | 0.0 | 0.0 | |
April | SO | 42 | 0.0 | 14.3 | 50.0 | 35.7 | 0.0 | 0.0 |
SI | 48 | 0.0 | 18.8 | 50.0 | 25.0 | 6.3 | 0.0 | |
BO | 16 | 0.0 | 6.3 | 62.5 | 31.3 | 0.0 | 0.0 | |
BI | 17 | 0.0 | 17.7 | 35.3 | 47.1 | 0.0 | 0.0 | |
May | SO | 38 | 0.0 | 13.2 | 50.0 | 31.6 | 5.3 | 0.0 |
SI | 44 | 0.0 | 18.2 | 47.7 | 22.7 | 11.4 | 0.0 | |
BO | 9 | 0.0 | 11.1 | 33.3 | 44.5 | 11.1 | 0.0 | |
BI | 35 | 0.0 | 17.2 | 25.7 | 42.9 | 14.3 | 0.0 | |
June | SO | 46 | 4.3 | 10.9 | 37.0 | 21.7 | 17.4 | 8.7 |
SI | 49 | 4.1 | 14.3 | 36.7 | 20.4 | 18.4 | 6.1 | |
BO | 22 | 4.5 | 18.2 | 27.3 | 13.6 | 27.3 | 9.1 | |
BI | 31 | 3.2 | 9.7 | 35.5 | 29.0 | 19.4 | 3.2 | |
July | SO | 49 | 8.2 | 18.4 | 30.6 | 16.3 | 12.2 | 14.3 |
SI | 50 | 12.0 | 14.0 | 26.0 | 16.0 | 14.0 | 18.0 | |
BO | 6 | 16.7 | 0.0 | 0.0 | 16.7 | 16.7 | 50.0 | |
BI | 19 | 0.0 | 10.5 | 5.3 | 31.6 | 31.6 | 21.1 | |
August | SO | 49 | 4.1 | 16.3 | 14.3 | 14.3 | 26.5 | 24.5 |
SI | 64 | 10.9 | 9.4 | 17.2 | 10.9 | 25.0 | 26.6 | |
BO | 10 | 0.0 | 20.0 | 10.0 | 20.0 | 10.0 | 40.0 | |
BI | 24 | 0.0 | 16.7 | 20.8 | 20.8 | 37.5 | 4.2 |
In August, the shell length distribution of newly attached spats (≤10 mm) was 89.79% in the outer PSC mesh bag and 51.46% in the inner PSC mesh bag. These abundance ratios in the same month were calculated as 94.28% and 81.35% in the outer and inner mesh bags of PBC (Table 1). Oysters with a size of 21–30 and 31–40 mm indicated a steady growth. The number of oysters in the 41–50 mm size class increased in May. New spat attachments were observed in June and it was found that previously attached individuals grew up to 50 mm. In June, all length groups of pearl oysters were observed on the collectors. Adult specimens of
The percentage distribution of bivalve families on PSC and PBC during the year was determined (Fig. 5). The total number of bivalve spats in the collector groups during the year was 3316 ind./year. Ten bivalve families were identified, the majority of which were species of Pteriidae – 62.23%. The percentage of families
Twelve bivalve species were identified:
We also encountered crabs:
The collection of pearl oyster spats is affected by many factors: (1) availability of metamorphosing larvae, (2) timing of collector deployment, (3) type of collectors and mesh, (4) placement depth of collectors, (5) environmental parameters. The formation of pearl oyster spat after fertilization takes between 16 and 30 days, depending on the water temperature, food availability and the presence of a suitable substrate (Gervis & Sims 1992; Kanjanachatree et al. 2005). This reproduction period is also the time when collectors should be placed in water, because they should be placed in the area where the spat is planned to be collected approximately 30 days before the metamorphosis. In this study, we deployed spat collectors in July, before spawning, taking into account the reproduction cycle of the species. This period, selected on the basis of metamorphosis, also shows the importance of correct timing for collector deployment. The time of deployment of spat collectors is very important due to the availability of larvae at the early metamorphosis stage in sufficient quantity. Crossland (1957) reported that a timing error in the deployment of spat collectors would cause the presence of undesired species on collectors. In this study, the timing for settling and immersion of collectors was also observed. July was the best month for collector deployment in this region. The Pteriidae family accounted for 62.23% of all organisms collected. The obtained results show the accuracy of timing for the deployment of collectors in water.
One of the most important environmental parameters triggering reproduction is temperature and chlorophyll
The largest total spat harvest in both collectors was in October (235 ± 6.92 spat (PSC + PBC)−1) and the lowest in April (123 ±2.30 spat (PSC +PBC) −1). As mentioned earlier, there was no newly attached spat in April. The whole spat harvested in April was larger than 10 mm as it attached in the previous months. Although there was a difference in the number of spat attachments between PSC and PBC, the values of environmental parameters such as temperature, salinity, chlorophyll
In addition, the number of <10 mm individuals in the outer mesh bags of PSC was 44 ± 3.46 spats and in the inner mesh bag – 26±2.81 spats, resulting in a total of 70±4.61 spat PSC−1 in August. The comparison of the results obtained by Yigitkurt et al. (2017) with the present study shows an increase from 35 spat PSC−1 to 70 ± 4.61 spat PSC−1 in the
Knuckey (1995) reported that the attachment of Pteriidae family members in Australia was 198 spat m−2 throughout the year. The author also stated that the annual average of
The retention time of spat collectors in water may vary depending on the desired spat size and quantity, because as the residence time increases, the collection systems start to function like growth systems. In our study, we harvested 20 mm individuals from the collectors during one or two months and individuals larger than 50 mm during 11–12 months. Approximately the same results as ours were published for French Polynesia and the Cook Islands, as 55–60 mm dorsoventral length individuals were harvested from spat collectors (Coeroli et al. 1984; Sims 1993; Pouvreau & Prasil 2001). During the study, all length groups of
A large number of bivalve species were determined in the collector groups. It was found that these species were the cause of different attachment ratios of
Natural predators are attached to collectors in their larval stages and they develop within the collectors, destroying the living organisms (Friedman & Bell 1996; Urban 2000). Crabs harmful to spat, such as
Moreover, spat was harvested from sheltered and high water flow parts of the collectors. This attachment behavior can be aimed at protection against predators. At the same time, it was observed that spat of pearl oysters preferred dark areas for settlement (Coeroli et al. 1984; Sims 1990). Yigitkurt et al. (2017) collected
Eventually, if the spat is harvested from a collector to increase the stock, surface collectors should be preferred and the collectors should remain in the sea for 6–7 months, which will allow us to collect the desired amount of individuals. However, if the spats below 10 mm are needed for scientific studies, the collectors should be removed from water in the following month and the spat should be harvested. The number of similar studies such as the present one, which are the primary scientific source on pearl farming, should be increased and more comprehensive studies are required. This study aimed to determine the efficiency of spat collectors at the surface/bottom of the water and findings were presented in relation to this objective. It is believed that the results of this work will lead to further research.