The effects of stocking density on gonad growth, survival and feed intake of adult green sea urchin (Strongylocentrotus droebachiensis)

Abstract

In this study the effects of different stocking densities on survival, gonad growth and feed intake in adult green sea urchin (Strongylocentrotus droebachiensis) were examined. Two size groups of sea urchins with respective initial mean weight of 45 g (S) and 70 g (L) were used. The experiments were carried out in 755 l plastic tanks divided into chambers by vertical fibreglass lamellae. In the two experiments stocking densities of 6, 12, 14 and 16 kg m^− 2 (Exp I) and 3, 6 and 8 kg m^− 2 (Exp II) were used. Sea urchins reared individually in compartmentalised raceways were used as controls in both experiments. All groups were fed a formulated moist feed ad libitum for 60 days at 8 °C (± 0.5). Mortality was observed in all groups, except for the control group and the lowest density group of large sea urchins (3 kg m^− 2, L). Both mortality and occurrence of injuries increased significantly with increasing stocking density in both experiments and for both size groups. In the highest density groups mortality was 60% (L) and 80% (S). There was a significant increase in gonad index during the experimental period in both experiments. Increased stocking density had a significantly negative effect on gonad growth in both size groups while feed intake was unaffected. The results show that adult S. droebachiensis can maintain high survival rate and high gonad growth at stocking densities up 6 kg m^− 2 in holding facilities of the design used in the present study.

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.