Comparative Analysis of Bivalves Diversity on Experimental Spat Collectors

Ines Peraš1, Slađana Gvozdenović1, Slavica Petović1 and Milica Mandić1

 

1 Institute of Marine Biology, Dobrota bb, 85330 Kotor, Montenegro; E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

 

Abstract

This paper presents the results of bivalves diversity on different types of experimental spat collectors. Samples were collected from two sites in the Boka Kotorska Bay, Orahovac and Kamenari, where the shellfish farms are located. Two types of collectors were set up in June 2017 and processed after 6 months of immersion following which 9 families were determinated with a total of 15 different species of bivalves. Physical and chemical parameters at the depth where placed different types of collectors proved to be significant for the individual development and development of young shells.

Keywords: diversity, bivalves, collectors, Boka Kotorska Bay.

Introduction

 

Boka Kotorska Bay is situated on the southeastern part of the Adriatic Sea and represents a specific area with relief formed by the combined effects of tectonic processes and river erosions (Milojević, 1953). The bay is characterized by specific hydrography and dynamics considering that it represents a semi enclosed basin (Lepetić, 1965). Boka Kotorska Bay is formed from four smaller bays, the Bay of Kotor, Tivat, Risan and Herceg Novi. The Bay of Kotor and Risan form the inner part of Boka Kotorska Bay creating the submerged bay cavity (Stjepčević & Žunić, 1964). The Strait of Verige (340m wide) connects the inner part of the bay with the outer part, Tivat and Herceg Novi Bay.

Streaming of sea currents in Boka Kotorska Bay are almost irregular and depend on sea periods and free oscillations so called 'sesh' (Mandić et al., 2001). Winds, pressure changes and also the mixing of sea and freshwater have a big influence on the direction and strength of the sea currents in Boka Kotorska Bay. Weaker sea currents are present in summer months while stronger currents are specific for autumn, winter and spring months. During summer months, inflow currents are stronger and have a northwestern direction. From October to May vertical streaming is present which is mainly expressed in the inner parts of Boka Kotorska Bay. It causes the mixing of water layers and the movement of nutritious salt deposits and detritus from the bottom, thereby significantly contributing to the increase in the amount of nutrients and productivity in general (ibid). The Bay of Kotor and Risan is characterized by strong turbulent currents, that are stronger during freshwater inflow, as in the area of Morinj and Orahovac (Stjepčević & Parenzan, 1980). During the months when surface freshwater inflow is small in the Kotor-Risan Bay, streaming has an outflow direction with the smallest speeds in the peripheral parts of the bay. The middle layer of the water column, at a depth of 5 to 10 m, sea currents still have an outflow direction but with lower intensity. Streaming of sea water in the Strait of Verige is about 0,66 knots (34 cm/sec). The dynamics of streaming become more intensive if there are higher inflows of atmospheric and freshwater springs. Peripheral parts of the bay then have less intense streaming while circular currents periodically appear in the central parts due to the configuration of the basin. The velocity of the currents range from 0,1 to 0,5 knots (5 to 26 cm/sec). In the Strait of Verige, intense outflow currents reach velocities of 0,9 to 1,1 knots (46 to 56 cm/sec) (Mandić et al., 2001).

Inflows of freshwater in Boka Kotorska Bay vary. There are two rivers flowing into the Bay of Kotor - Škurda, which are active throughout the entire year and Ljuta which dries up during the summer months. In addition, there are a number of streams, small springs and submarine springs that also represent a significant source of fresh water. Situated near Kotor in the Kotor-Risan Bay) is an area with the highest amount of precipitation in Europe – Crkvice, from which by way of different springs, submarine springs and by erosion large amounts of freshwater inflow into Boka Kotorska Bay (Magaš, 2002). This water carries large amounts of suspension that affect the ecological conditions of sea water - color, transparency, salinity, density, etc. The Risan Bay receives a considerable amount of fresh water from the Sopot spring. Besides Škurda, The Široka River that flows into the Bay of Tivat, is also active throughout the entire year, as river Gradiošnica. The Sutorina River, which flows into Herceg Novi Bay, is active in the autumn, winter and spring months like most rivers in the bay (Stjepčević, 1974). By the influence of freshwater inflow, also winds, leads to surface streaming outflow from Boka Kotorska Bay to the open sea.

The first survey on benthic Mollusca populations in the area of the inner part of Boka Kotorska Bay was conducted between 1970-1972. (Stjepčević et al., 1982). The survey data revealed 284 different species of Mollusca, of which 127 represent different species of bivalves. Composition of population changes under the influence of different types of pollutants that are present in the inner part of Boka Kotorska Bay, considering that most of the marine organisms are sessile, hemi-sessile and mobile. Recent research on bivalves diversity revealed 123 different species in Boka Kotorska Bay (Petović et al., 2017).

Based on the results of bivalves diversity in different types of experimental collectors, it is possible to conclude which type would be the most appropriate for collecting shell larvae and commercial farming. The materials used for making the experimental collectors is also very important for better catching and successful water streaming which brings nutrients that are important for growth, development and the survival of individuals.

 

Materials and Methods

The experimental collectors were set up on two locations, bivalves farms, Orahovac (inner part of the bay) and Kamenari (outer part of the bay). The locations of the collectors are shown in Figure 1.

 

fig01
Figure 1: Location of farms and experimental collectors.

 

Collectors are structures that are installed in commercial farms with the aim of collecting young bivalves for the purpose of increasing the production of cultivated species. Two types of collectors were used: Type 1 - made of vegetables bag (45x45) filled with same vegetable bag, plastic nets in the form of sleeves of different diameters (2-3cm and 4-5cm) which are used for mussel farming; and Type 2 - made of vegetables bags (dimension 45x45 cm) filled with fishing nylon. The collector types are shown in Figure 2.

 

fig02

           a)                                                   b)                                                         c)
Figure 2: Type of experimental collectors a)Type 1, b) Type 2, 3) way of setting.

 

Collectors were installed at both shellfish farms in June 2017. The average depth of the areas is 25 m, and both types of collectors were set vertically on the same rope at different depths with 1-2 m spacing between them. The top of the collector rope was attached to a floating park and the bottom was anchored to a concrete block which was used to hold the rope vertically in order to avoid shifting by sea currents that could tear it. Eleven collectors were installed at the Orahovac shellfish farm - 5 Type 2 and 6 Type 1, while 12 collectors were installed at the Kamenari farm - 5 Type 2 and 7 Type 1. After immersion from June to December 2017 the collectors were set out and individuals were determined to species or genus. The collectors were preserved in 70% ethanol prior to determination.

Furthermore, salinity values and temperature were measured at different depths at both locations using MultiLine 4, WTW and CRISON, CM 35+ automated probes, in order to determine the depth at which the values of the examined parameters are most favorable and provide the best reception of bivalves spat and uninterrupted growth and developing.

 

Results and Discussion

Results of determination from both shellfish farms are presented in Tables 1 and 2.

 

tab01

 

tab02

 

During this experiment 9 families with 15 species of bivalvia were determined in both areas using different types of collectors. From the results we could see that there were no differences in bivalves diversity between the two types of collectors in Orahovac, but in Kamenari, the Tapes decussatus and Pinctada radiata species were not present in either type. So it could be concluded that environmental conditions have a greater influence on diversity than the collector type.

At the Kamenari shellfish farm, the highest number of individuals was found at a depth of 13m (107 individuals), the dominant species were Mytilus galloprovincialis, Chlamys multistriata, Lima hians and Modiolarca subpicta, while at the Orahovac shellfish farm, the highest number of individuals was found at a depth of 19.5m (95 individuals), with the dominant species Mytilus galloprovincialis, Chlamys multistriata and Lima hians. The lowest number of individuals at Kamenari was counted at a depth of 2m with only 28 individuals while at Orahovac 39 individuals were counted at a depth of 6m.

A small number of collected individuals at lower depths is due to the stronger streaming of sea currents that have a negative influence on individuals catching on collectors, as opposed to greater depths where the streaming is weaker. The streaming of sea currents in the bay and their influence on individuals catching in the Tivat Bay area – Kamenari, as follow surface currents has output character in winter that can be linked to lower number of individuals on collectors posted closer to surface, while streaming on depths from 5-10 m and closer to bottom is in the state of "relative standstill". Streaming of currents is the same for locality in Orahovac (Bay of Kotor) (Mandić et al., 2001).

During the diversity survey on experimental collectors, special attention was paid to two bivalvia species from two families, Pectinidae and Mitilidae. The species was counted and its length and width was measured. Tracked species, Chlamys multistriata (Pectinidae) and Mytilus galloprovincialis (Mitilidae) known as most numerous species on different types of fishing tools.

At the Kamenari locality, the Pectinidae, Chlamys multistriata species family was tracked and showed an increase of dimension which was dependent upon the depth at which the collector was installed. From our results, the number of individuals increased from the bottom to the top of the water column, where the lowest mean length value of 7,27 mm was noted at 19 m, and the highest of 12,12 mm was noted at 2 m. However, other species, Mytilus galloprovincialis, did not exhibit the same changes of dimensions, the values of which irregularly varied with depth, the lowest being at 17 m and the highest at 2 m. The presence of species at different depths at the Kamenari locality and their mean value of dimension showed in Table 3.

 

tab03

 

At the Orahovac locality, the presence of the same species from the Pectinidae family - Chlamys multistriata, and Mytilus galloprovincialis from Mitilidae was monitored. These species did not show regular changes of dimensions with depth. Chlamys multistriata exhibited the lowest mean length values of 4.29 mm at 7.5m and the highest of 13.07 mm at 18 m. For the Mytilus galloprovincialis species the lowest mean length values were 26.29 mm at 10.5m, and the highest of 38.86 mm at 15 m. The presence of species at different depths and their mean value of dimension at the Kamenari locality is presented in Table 4.

 

tab04

 

Amount of species on collectors could depends on material used for making the collectors, enviromental conditions that were during the immersion and durability of immersion. Similar types of collectors were first used in Japan in the 70s (Motoda,1977; Pena et al., 1996) as also in addition to some Mediterranean countries such as Italy, Spain and the neighboring Croatia where they proved to be an excellent substrate for the reception of young bivalve individuals (Marguš i Teskeredžić, 2005).

Collectors were set from June to December, a part of year that offers good conditions for undisturbed growth, development and survival of individuals due to the large amount of nutrients and suitable temperature conditions. Our results revealed that depths with the highest amounts of species from collectors were in middle of the water column between 10-20 m where the best physico-chemical and biological conditions for growth exist. The material from which the collectors were made also had some influence on the bivalvia amount collected. Vegetables bags contributed to the streaming of water with nutrition, gave protection from predators at same time, while the nylon and plastic sleeves increased the volume of collectors (Gosling, 2003).

In Croatia (eastern Adriatic), estuary of the Krka River, Marguš (1994) used that type of collector for collecting shell spat which proved vulnerable substrate for juveniles from the Pectinidae family, noted 4 species, small scallop Chlamys varia as most counted, Great Mediteranean scallop Pecten jacobeus, Flexopecten flexuosus (Chlamys flexuosus) and queen scallop Aequipecten opercularis (Chlamys opercularis). Comparing with our results, species from the family Pectinidae noted in the estuary of the Krka River, match with the noted species on the experimental collectors set in Boka Kotorska Bay, where the most prevalent species was Chlamys multistriata.

Brand et al. (1980) in similar collectors noted the species Modiolus modiolus, Hiatella arctica, Pecten maximus, Aequipecten opercularis, Chlamys varia and Chlamys tigerina, that showed the possibility for using this type of experimental collector for collecting species from different families. Our experimental collectors identified the same species from the family Pectinidae, except for Chlamys tigerina which was not observed in Boka Kotorska Bay. Genus Modiolus, was identified along with species Modiolus barbatus.

Diversity on collectors in paper Pena i sar. (1996), noted 9 different bivalvia species from which 5 species were from the family Pectinidae - Flexopecten flexuosus, Chlamys varia, Aequipecten opercularis, Pecten jacobaeus and Palliolum incomparabile, then Mytilus galloprovincialis, Hiatella artica, Plagiocardium papillosum and Musculus costulatus. Comparing with our results, species Palliolum incomparabile from family Pectinidae was not noted in the experimental collectors and neither were Plagiocardium papillosum and Musculus costulatus.

Literature data that gave the best points on the efficiency of using experimental collectors for gathering young shells, either for the purpose of breeding or determining diversity, is a study carried out in the Timor Sea in Australia, where the use of such collectors noted as many as 47 different species from 14 families of shells (Knuckey, 1995).

The presence of the tracked species, Chlamys multistriata and Mytilus galloprovincialis, in great numbers can be associated with the period in which collectors were set with the period of shells spawning. Species Chlamys multistriata has two intense spawning periods, in spring (April - June) and autumn (September - October), according to this collectors were immersed from June to December, so that both spawning seasons and settlement were covered. A similar spawning period is linked to the species Mytilus galloprovincialis.

In research conducted by Stjepčević (1967), Stjepčević and Parenzan, (1980) as well as Mandić et al. (2001) Boka Kotorska Bay is identified as an area with a strong freshwater influence. The freshwater ingress from many sources present in the bay can strongly modify temperature, salinity and similar parameters. Figures 3 and 4 represent results from measuring temperature and salinity values from the researched areas.

Juvenile stages of shellfish after fertilization spend up to three weeks as planktonic organisms, followed by metamorphosis and their attachment to plants, rocks, etc. According to the literature data, the optimal temperature for the normal development of young individuals is around 15-20 °C. In addition, metamorphosis is also affected by salinity and the most favorable is in the range of 30-35 ‰ (Gosling, 2003, Stjepčević, 1974). According to our results the middle of the water column 10-20 m provides the best values of the parameters mentioned for temperature 20-21˚C and salinity up to 37 ‰ (Figure 3 and 4). During this period fluctuations were insignificant in the middle of the water column where the highest number of individuals was observed in the collectors (Figure 5).

 

fig03
Figure 3: Temperature and salinity in Orahovac.

 

fig04
Figure 4: Temperature and salinity in Kamenari.

 

fig05
Figure 5: Number of individuals on different depths.

 

Conclusion

Literature data and results from our research showed that different types of experimental collectors present a simple method for qualitative and quantitative evaluation of the attached juvenile shells, with the aim of evaluation the possibilities importing new native species in the farming process, increasing the diversification of production in mariculture. Analysing recipience of young shells from different depths provide information for the best position for farming installations in sea water columns, in regards to their positioning, with the best conditions for individual growth and development. This research concludes that very good conditions for growing and developing seashells exist in Boka Kotorska Bay, therefore the species Chlamys multistriata, which was one of the most prevalent species in the experimental collectors, requires further research in order to determine the possibility of including it in the farming process, in addition to improving the farming production of Mytilus galloprovincialis, which has been cultivated in the Bay of Kotor for many years now.

 

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