Abstract: The seasonal pattern of recruitment of the mussel, Mytilus galloprovincialis, on artificial collectors, situated at four different depth (0.5, 4, 8, 12 m) in the Dardanelles. The presence of pediveliger mussels throughout the year and their abundance in march reflects the presence of spawning mussels throughout the year and the occurence of a major spawning period during beginning of spring. Different densities of mussel settlement were detected at the different depths. Mussel seed density was higher at 0.5 m (surface) and 4 m than 8 and 12 m throughout the year. Artificial collectors should be hung within the first 8 m (especially within 4 m) of the water column in order to exploit the highest settlement in the upper parts.
Harun Yildiz and Selcuk Berber, 2010. Depth and Seasonal Effects on the Settlement Density of Mytilus galloprovincialis L. 1819 in the Dardanelles. Journal of Animal and Veterinary Advances, 9: 756-759.
One of the most important bottlenecks in aquaculture sector is the difficulty in obtaining seed. Obtaining desired amounts of seed without the need for any hatchery production (Gosling, 1992; Acosta et al., 2009), has enabled the rapid development of mussel farms all over the world. The fact that approximately half of the total expenses of fish culture is due to the feed costs whereas, there is no such problem for mussels that feed by filtering the water (Bilecik, 1989; Lok et al., 2007) is another important factor that enabled the rapid development of this sector. The mussel culture studies in the country are yet very new. Whereas in 2006, 94.5% of the worlds mussel production of 1,998,000 tons was made by culturing, only 14% of the 10.779 tons of mussels produced were grown by aquaculture (FAO, 2007). Therefore, for the interest of our country, mussel culture studies should be accelerated.
Colonization of natural and artifical substrates by mussels, Mytilus sp. may occur by settlement of competent pediveliger larvae or by settlement of drifting post-larvae (Davies, 1974; King et al., 1989; Yildiz, 2004). It is presumed that initial settlement would prefentially occur at a depth chosen by the larvae upon their attainment of competence, assuming the larvae had immediate accessibility to a suitable substrate at that depth (Freeman et al., 2002). In designing this experimental, we were concerned with mussel settlement season, quantities of seed which could be collected, the best depths for setting out seed collectors in Dardanelles and space in order to generate advice to the mussel industry.
MATERIALS AND METHODS
Field studies were carried out on Mytilus galloprovincialis L. in Dardanelles of Turkey from February, 2002-December, 2002 (Fig. 1).
Collectors of net rope material were tied in February, May, August, November months onto main body made than polyprophylen rope. Each net collector prepared as 25 cm long and 4 cm wide (100 cm2) on average.
|Fig. 1:||The study area|
Main bodies suspended from beneath buoys in such a way that collectors were situated at four depths (0.5, 4, 8 and 12 m). After one month of submersion, the main bodies were retrieved. These collectors were untied from the main bodies and two small pieces of 8 cm2 (lenght: 2 cm, weight: 4 cm) each were randomly cut from each collector. Mussel seeds were removed from the net rope by vigorous agitation in water. They were drained on to a 100 μm nylon-mesh fitler. Larvaes were then carefully taken with a Pasteur pipette and transferred to a Petri dish. Density of settlement (number of mussels per 8 cm2 ) was determined by counting the pediveliger and postlarvae mussels contained in each sieve under a binocular microscope of Dolphus camera. Lenght measuring was carried out on randomly selected mussels with a micrometer. The identification of pediveliger larvae (<0.470 mm) was possible to genus (Mytilus) using the keys and observations from Rees (1950), Lutz and Hidu (1979) and Le Pennec (1980). All Mytilus pediveliger larvae were considered Mytilus galloprovincialis since this species is the only Mytilus species present in this area. Post-larvae (>0.470 mm) were easily identified as Mytilus galloprovincialis. Total longevity of sample collector parts of each collectors (4 cm2x2 collector parts = 8 cm2) rated to the whole collector lenght (100 cm2). Total mussel number was calculated and lenght groups were determined.
Friedman test was used in order to analyze the effects of the different seasons and depths on the number of mussel seeds. The Z ratio test was applied to compare the depths for the number of mussel seeds during each season.
Pediveliger mussel larvaes have been observed in all the samples taken from the research area, thus it has been determined that spawning continues throughout the year. It has been determined that main spawning period is between February-March and that among the individuals attachment onto the collectors at the surface (0.5 m), 4, 8, 12 m; 88, 88, 91 and 85% were at pediveliger stage, respectively. Whereas, during the months of june, september and december, it has been observed that of the mussel seed that attach onto the collectors at different depths, an average of 75, 69% and 61, 25% were pediveliger mussels, respectively (Table 1-4).
As a result of the Friedman test performed in order to analyze the effect of different seasons and depths on the number of mussel seeds, it has been determined that seasons greatly affect the number of mussel seeds (p = 0.007) and that the greatest number of mussel settlements were observed in march and that the lowest number was observed in december.
|Table 1:||Amount of mussel attachments at the different depths in March 2002|
|Table 2:||Amount of mussel attachments at the different depths in June 2002|
|Table 3:||Amount of mussel attachments at the different depths in September 2002|
|Table 4:||Amount of mussel attachments at the different depths in December 2002|
Friedman test was reapplied in order to analyze the effects of depth on the number of mussel seeds and it was determined that depth also greatly affects the number of mussel seeds (p = 0.019). As a result of the Z ratio test carried out in order to compare the depths with regards to the number of mussel seeds during each period, it was determined that the percentage in total of the number of mussel seeds at a depth of 4 m was greater. It was also determined that the percentage in total of the number of mussel seeds at the surface was greater during the June period.
In the first stages of their life, cycles mussels move as pelagic and search for a suitable substrate to pass onto sessile life (McGrath et al., 1988; Hunt and Scheibling, 1997; Yildiz et al., 2005). It was observed both in their natural environment and at laboratory conditions that mussel seed settle on filament surfaces such as bryozoa, hydroid and filiform alg species (Eyster and Pechenik, 1987; Folino-Rorem et al., 2006). It has been proved that especially artificial collectors with filamentous structure are effective in the settlement of mussel seeds (Davies, 1974; Petersen, 1984; Lekang et al., 2003). In the study, it has been observed that the net collectors that are used due to this principle are very effective. When it is considered that Çanakkale is a fishing city, we can suggest using old fish nets, which can be purchased for low prices to be used as mussel collectors.
It is known that in populations of mussel from different regions of the world, after a main spawning season, minor spawning during the year may occur (Seed, 1976; Ferran, 1991; Villalba, 1995). In this study, the presence of pediveliger mussels throughout the year and their abundance in March reflects the presence of spawning mussels throughout the year and the occurence of a major spawning period during beginning of spring. It has been observed that the results are in accordance with the results of the study made by Karayucel et al. (2002) at the Black Sea region. Again the fact that 75% of the mussel seeds obtained in June, 69% of those in September and 61, 25% of those obtained in December are mussels with pediveliger competence, whereas 88% of the mussels obtained in March are pediveliger mussels shows that the main spawning period starts close to the period in which sampling was made.
Vertical distribution of bivalve larvae prior to settlement depends on various factors, both active, in response to environmental stimuli (Bayne, 1976) and passive, as a consequence of physical forces (Mann, 1986). Mussel seeds begin to settle on the ropes near the surface because of the high temperature, abundance of light and available food (Karayucel et al., 2002). Fuentes and Molares (1994) found that settlement of the mussel Mytilus galloprovincialis was high at shallower dephts in a seaward site of the Ria de Arousa Bay in northeastern Spain. In this study, different densities of mussel settlement were detected at the different depths. Mussel seed density was higher at surface and 4 m than 8 and 12 m throughout the year. A similar pattern of vertical variation was obtained by Kautsky (1982) in a Baltic Sea population of M. edulis, Margus and Teskeredzic (1986) in the River Krka Estuary of Adriatic Sea, Joschko et al. (2008) German Bight coastal areas of the North sea.
In this study, the management of mussel spat collection in the Dardanelles are as follows. Mussel collectors should be hung from the rafts or long line systems before February in order to make full use of the main peak of settlement. Ropes should be hung within the first 8 m (especially within 4 m) of the water column in order to exploit the highest settlement in the upper parts. Such productive sites will be extremely useful in supplying seed for mussel farms.