Effects of Rearing Density on Growth of the Polychaete Rockworm Marphysa sanguinea

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  • ABSTRACT

    Effects of rearing density on growth and survival of the polychaete rockworm Marphysa sanguinea have been investigated in order to develop rearing techniques for this species. This study was examined over a nine-month period in the Fisheries Science and Technology Center of Pukyong National University. Three rockworm densities, 500, 1,000 and 2,000 worms∙m-2 with weight ranges of < 0.5 g, 0.6-1.5 g, and 1.6-2.5 g, and the no feed control treatment, were stocked in triplicate 0.10 m2 boxes with sand bottoms. Growth rates were checked with 15 randomly sampled rockworms from each box at months 3, 6 and 9. Results showed that SGRs in all treatments were higher during the first period (0-3 months) than the second (3-6 months) and third periods (6-9 months) for all treatment densities, while SGRs decreased with increasing density. However, survival and growth of worms at high density was not better than low density, but daily biomass production in medium and high density groups was 6.28 g m-2day-1 for the rockworms of 0.6-1.5 g with 2,000 inds∙m-2, and 12.6 g m-2day-1 for group between 1.6-2.5 g with 2,000 inds∙m-2, and 14.7 g m-2day-1 for the group of individuals <0.5 g with 1,000 inds∙m-2. Results showed that M. sanguinea can be one of the most suitable species to commercially exploit in a farming system. In particular, specified densities permit elevated pure production.


  • KEYWORD

    Polychaete , Marphysa sanguinea , Density , Growth

  • Introduction

    The ecological role of polychaetes in marine benthic communities is very important (Giangrande et al., 2005). They are known to be good indicators of species richness (Olsgart and Somerfield, 2000) and as bio-indicators of the marine environment (Pocklington and Wells, 1992; Giangrande et al., 2005).

    The rockworm Marphysa sanguinea (Montagu) that belongs to the Eunicidae family has been important bait for fisheries and sport fishing in Korea. There are several other families such as Arenicolidae, Nereidae, Glyceridae and Nephtyidae also exploited worldwide for sport and commercial fishing. Moreover, the demand for these species has increased rapidly in recent years because these worms are also very important nutrient sources for stimulating gonad maturation and spawning of fish and crustaceans in hatcheries for aquaculture purposes (Olive, 1999). The other main point of the culture and commercial use of this bait is to reduce the substrate harvesting disturbance, and the great biogeochemical and benthic community impact (Gambi et al., 1994).

    The production costs of polychaete worms in an intensive worm aquaculture system should be efficient enough to make profits (Nesto et al., 2012). Therefore, one of the most important factors for culturing these species is production density (Olive, 1999; Prevedelli, 1994; Safarik et al., 2006). The role of density dependent processes in the regulation of animal populations has also been a subject of interest for many ecologists, and density-dependent factors, including competition for space and food, are also important structuring agents for populations of a number of sediment dwelling polychaete species (Scaps et al., 1998; Buekema et al., 2000; Omena and Zacagnini, 2000; Reise et al., 2001).

    Preliminary information on the life cycle of these populations and the influence of several environmental variables was acquired from Prevedelli’s laboratory study (1994). It showed a positive correlation between the growth and developmental rate of juveniles and water temperature was observed. The present study examined the effects of density on growth and mortality of the rockworm Marphysa sanguinea.

    Materials and Methods

    A nine month long on-site controlled experiment was conducted with 3 density groups and 1 no feed group for 3 different body size groups (< 0.5 g, 0.6 to 1.5 g and 1.6 to 2.5 g) of the rockworm M. sanguinea in the Fisheries Science and Technology Center, Pukyong National University, Goseong, South Korea. The three different rearing density groups were: 500 inds∙m-2 (low, T1), 1,000 inds∙m-2 (medium, T2) and 2,000 inds∙m-2 (high, T3). The number of animals per boxes was set at 50, 100 and 200, in order to obtain densities of 500 inds∙m−2 (T1), 1,000 inds∙m−2 (T2) and 2,000 inds∙m−2 (T3), respectively. Along with the 3 density treatments, a no feed treatment (TC) was set with the density of 1,000 inds∙m-2. Therefore each body size group was subject to 4 different treatments: 3 density groups (T1, T2, and T3) and 1 no feed control (TC).

    The worms were reared in PVC boxes (0.1 m2), measuring 40 cm (L) × 25 cm (W) × 20 cm (D). The boxes were half-filled with sand (150-500 μm), which was sun dried and then rinsed several times with freshwater. For each different body size group (< 0.5 g, 0.6 to 1.5 g and 1.6 to 2.5 g), three boxes were allocated per density group (T1, T2, and T3) and the no feed control (TC). This totaled twelve boxes per weight group, and 36 boxes total for the experiment. All boxes were connected to a seawater flow through system and the water was pumped from Jaran Bay in front of the Center. In order to examine the growth rates, 15 worms were randomly sampled from each box for every 3 month period. Water temperature was maintained at 20 ± 2℃ using a Thermo Control Unit. Water quality (temperature, salinity, dissolved oxygen and pH) was measured three times per week by the Hydrolab Surveyor 4a device.

    M. sanguinea in each treatment box were fed with commercial high protein fish food pellets (Aquanet Co. Ltd. Korea; 54% protein), with the amount of 3.5% of body weight three times a week, except the TC group. Mortality of the worms was assessed at the completion of the nine months study. Growth was evaluated as follows;

    Specific growth rate (SGR) (%) = 100 [ln(Wf−Wi)] t−1, Weight gain (WG) (%) = [100 × (Wf−Wi)/Wi],

    where Wf is the final weight, Wi is the initial weight and t is the time interval in days. The survival rate was calculated with the number of surviving organisms at each time with respect to their initial number. The number of surviving organisms at the end of the experiment was used to calculate the final density (inds∙m−2), final biomass production, i.e. product of final density and individual mean wet weight (g m−2) and daily biomass production (g m−2 day−1).

      >  Statistical Analysis

    Confirming normality and homogeneity of variance of data were tested using the Kolmogorov-Smirnov test. Statistically significant differences among measured parameters were computed using a one-way ANOVA by SPSS 15 software for Windows (SPSS Inc., Chicago, IL, USA). Significant differences between treatments (P < 0.05) were evaluated using the Duncan’s Multiple Range Test.

    Results

    During the nine-month study, there was little variation in salinity (34-35.5 psu) and pH (7.5-8.2) in the treatment boxes. Dissolved oxygen concentrations within the treatment boxes ranged from 5.8-6.9. As shown in Table 1, significant differences (P < 0.05) were detected among the three density groups of rockworms with different treatments in final weight (FW), specific growth rate (SGR), weight gain (WG) and survival rate (SR).