The effects of stocking density on gonad growth, survival and feed intake of adult green sea urchin (Strongylocentrotus droebachiensis)
Introduction
Sea urchins are harvested in coastal areas around the world for their roe (gonads) (Keesing and Hall, 1998, Lawrence, 2001, Lawrence et al., 2001, Andrew et al., 2002). During the last three decades, the demand for high quality sea urchin roe has led to an extensive exploitation of sea urchin populations, with over-fishing and declining stocks as a result in many countries (Keesing and Hall, 1998, Andrew et al., 2002, Robinson, 2003, Botsford et al., 2004). This situation has created an interest in the development of sea urchin aquaculture (Grosjean et al., 1998, Le Gall, 1990, de Jong-Westman et al., 1995a, de Jong-Westman et al., 1995b, Lawrence et al., 1992, Lawrence et al., 1997, Kelly et al., 1998, Klinger et al., 1998, Robinson, 2003).
Farming of sea urchin is at a starting point in Norway and several other countries, and effort has been put into establishing small and medium sized enterprises using intensive culture systems for roe enhancement and full cycle cultivation of sea urchins (Aas, 2004). There is still a need to develop and improve the techniques for on-growing, long term storage and roe enhancement in order to improve gonad quality and to obtain higher value products (Aas, 2004, Motnikar et al., 1998, Aas, 2004). Determining the optimum stocking density during grow-out is important, as too densely populated cultures increase the risk of cannibalism, mortality and injuries (Jobling, 1994). The profitability in sea urchin culture depends on a number of factors. For obvious reasons gonad yield (growth) per unit cost (investments and running costs) is important. In a recent study, Valvåg (2003) showed that mortality was a crucial biological parameter for financial returns in gonad enhancement operations. Thus, stocking density in sea urchin culture is important both from economic and animal welfare reasons.
To our knowledge only few studies have addressed stocking densities in the cultivation of adult sea urchins (Le Gall, 1990, Scheibling and Hatcher, 2001, Aas, 2004, James, 2006). The present study was therefore undertaken in order to elucidate the effects of different stocking densities on gonad growth performance, feed intake and survival in adult green sea urchin (Strongylocentrotus droebachiensis) under intensive grow-out conditions.
Section snippets
Origin of sea urchins and experimental conditions
Sea urchins (S. droebachiensis) were collected by divers in Kvalsund in northern Norway, and brought to the Aquaculture Research Station in Tromsø (69°50′N, 18°55′E). Urchins were allowed to adjust to laboratory condition for 14 days prior to the experiments.
The experiments were carried out in 755 l plastic tanks; length: 1550, width: 1080, height: 690 mm (Sæplast, double-walled polyethylene container). The tanks were compartmentalised into eight chambers by vertical lamellae (length: 600,
Mortality and frequency of injuries
In experiment I, mortality was observed in all size groups and stocking densities, except for the control group, where no mortality occurred (Fig. 2, Fig. 3). The highest mortality was observed in the highest density groups (Fig. 2, Fig. 3). By the end of the experiment the accumulated mortality exceeded 60% (L) and 80% (S) in the 16 kg m− 2 density groups. Accumulated mortality was 25% (L) and 35% (S) in the 6 kg m− 2 density groups (Fig. 2, Fig. 3). The frequency of visual injuries ranged from
Discussion
Determining the optimum stocking density is one of the key factors for successful urchin culture. The present study provides knowledge about the effects of different stocking density on survival, gonad growth and feed consumption of adult green sea urchins, S. droebachiensis.
Acknowledgement
This study was supported by The Norwegian Research Council (NFR 147524/120). We would like to thank Ivar Nevermo, Hugo Tøllefsen and Oddvar Haugland for their technical assistance and advice on this work.
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