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Dynamics of juvenile sea scallop (Placopecten magellanicus) and their predators in bottom seeding trials in Lunenburg Bay, Nova Scotia M.A. Barbeau, B.G. Hatcher, R.E. Scheibling, A.W. Hennigar, L.H. Taylor, and A.C. Risk
Abstract: Thousands of juvenile sea scallop (Placopecten magellanicus) were released on the sea bed in summer and winter at two sites (a topographically open and an enclosed site) along the southwestern coast of Nova Scotia, Canada. Seeded and wild scallops and their predators (sea stars Asterias vulgaris and A. forbesi and rock crab Cancer irroratus) were monitored by video and diver surveys over 17 mo. Following release, the density of seeded scallops rapidly decreased, and stabilized after 1–8 wk; variation in final densities was greater between sites than between seasons of seeding. Seasonal temperature affected the rate at which the final densities were attained. Seeding effectively doubled scallop density at each site. Survivorship was ,10% due to loss from crab predation and dispersion of seeded scallops at the open site and ,1% due to mainly crab predation at the enclosed site. Temporal variation in abundance and spatial distribution of predators was not correlated with that of seeded scallops, suggesting that predatory mortality of seeded scallops was due to a functional response, rather than an aggregative response of predators. Growth of seeded scallops was similar at both sites. The dynamics of the seeded scallop populations reflected the interaction of predation, dispersion, and growth. Résumé : Des milliers de juvéniles du pétoncle géant (Placopecten magellanicus) ont été lâchés en deux endroits sur le fond de la mer (un secteur à la topographie ouverte, un autre à la topographie fermée) le long de la côte sud-ouest de la NouvelleÉcosse, Canada, à deux saisons, soit en été et en hiver. Les pétoncles indigènes et introduits ainsi que leurs prédateurs (les étoiles de mer Asterias vulgaris et A. forbesi ainsi que le crabe Cancer irroratus) ont fait l’objet d’une surveillance vidéo et d’examens visuels par des plongeurs sur une période de 17 mo. La densité des pétoncles introduits a diminué rapidement après les lâchers et s’est stabilisée au bout d’une à 8 sem; à la fin du projet, l’écart de densité entre les stations était plus grand que celui entre les saisons d’ensemencement. La température saisonnière influe sur la vitesse à laquelle la densité finale est atteinte. L’ensemencement a permis de doubler effectivement la densité des pétoncles à chaque endroit. Le taux de survie a été d’environ 10 % sur le site ouvert, à cause de la prédation exercée par le crabe et de la dispersion des pétoncles introduits, et d’environ 1 % sur l’autre site, principalement à cause de la prédation par le crabe. Il n’existe pas de corrélation entre les variations dans le temps de l’abondance et de la distribution spatiale des prédateurs et celles du pétoncle introduit; cela suggère que la mortalité par prédation de ce dernier est attribuable à une réponse fonctionnelle plutôt qu’à une réponse par agrégation des prédateurs. Le développement des pétoncles introduits était sensiblement le même aux deux endroits. La dynamique des populations de pétoncles introduits reflétait les interactions entre la prédation, la dispersion et le développement. [Traduit par la Rédaction]
Introduction With increased interest in aquaculture of bivalves, two methods are presently being evaluated: suspended culture, in which bivalves are placed in cages or attached to lines suspended in the water column, and bottom culture, in which juvenile bivalves are released (seeded) onto the sea bed and
Received July 20, 1995. Accepted May 1, 1996. J13010 M.A. Barbeau,1 R.E. Scheibling, A.W. Hennigar, L.H. Taylor, and A.C. Risk. Department of Biology, Dalhousie University, Halifax, NS B3H 4J1, Canada. B.G. Hatcher. Department of Oceanography, Dalhousie University, Halifax, NS B3H 4J1, Canada. 1
Corresponding author. New address: Woods Hole Oceanographic Institution, Coastal Research Lab, Mail Stop 2, Woods Hole, MA 02543-1525, U.S.A. e-mail:
[email protected]
Can. J. Fish. Aquat. Sci. 53: 2494–2512 (1996).
harvested once they reach commercial size (Quayle and Newkirk 1989; Shumway 1991; Boghen 1995). Bottom culture is less labour- and cost-intensive than suspended culture, but produces lower yields due to increased rates of mortality and dispersion. Bottom culture also results in slower growth rates than suspended culture (MacDonald 1986; Wildish et al. 1988; but see Kleinman et al. 1996). Bottom culture preceded by a period of growth in suspended culture (intermediate culture) may prove to be the most economically viable culture method for scallops (Ventilla 1982; Wildish et al. 1988; Couturier et al. 1995). Bottom culture of scallops has had varying success in previous seeding trials, with survivorship ranging from 0 to 80% (Ventilla 1982; Minchin 1991; Tettelbach and Wenczel 1993; Bull 1994; Dao et al. 1994; Goshima and Fujiwara 1994; Halary et al. 1994). Low survivorship of seeded scallops may result from one or more causes of mortality which include predation (Volkov et al. 1983; Minchin 1991; Spencer 1992; Cliche et al. 1994; Halary et al. 1994; Hatcher et al. 1996), © 1996 NRC Canada
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Fig. 1. Lunenburg Bay, Nova Scotia, Canada, showing location of Sites 2 and 3 where bottom seeding trials with juvenile sea scallop (P. magellanicus) were conducted. Site 1 indicates the location of the pilot study detailed in Hatcher et al. (1996).
interspecific competition (Halary et al. 1994), and adverse environmental conditions, such as extreme temperatures, low salinity, heavy sedimentation (Tettelbach et al. 1990), strong wave action (Kalashnikov 1991; Tettelbach and Wenczel 1993), and toxic algal blooms (Tettelbach and Wenczel 1993). As well, loss of seeded scallops may be due to dispersion from sites and be related to high initial densities of seeded scallops (Volkov et al. 1983; Hatcher et al. 1996), inappropriate substratum type (Dupouy 1983; Picard and Vigneau 1992), local hydrodynamics (Morgan et al. 1980; Hatcher et al. 1996), and high water temperatures (Morgan et al. 1980; Carsen et al. 1996b; Hatcher et al. 1996). Predation is usually the main cause of loss of seeded scallops, especially when the scallops are of small size and (or) in poor condition (Morgan et al. 1980; Lake et al. 1987; Minchin 1991; Barbeau et al. 1994; Halary et al. 1994). However, the effect of predators is multifaceted. Predation rates may increase with increased scallop density due to greater consumption rates of individual predators (a functional response) and (or) enhanced local densities of predators (an aggregative numerical response) (Taylor 1984; Barbeau et al. 1994). The magnitude of the predator effect also depends on the size distribution of predators at a given site in relation to that of seeded scallops (Lake et al. 1987; Minchin 1991; Barbeau and Scheibling 1994a). In addition, predators trigger escape swimming in many scallop species, which causes scallops to disperse (Thomas and Gruffydd 1971; Peterson et al. 1982; Dadswell and Weihs 1990). The sea scallop (Placopecten magellanicus) is an important commercial species in fisheries and aquaculture in the northeastern United States and eastern Canada (Brand 1991; Couturier et al. 1995). In coastal environments most suitable for bottom culture, sea stars (Asterias vulgaris and A. forbesi) and rock crab (Cancer irroratus) are the major predators of sea scallop (Naidu and Cahill 1986; Barbeau et al. 1994; Cliche et al. 1994; Hatcher et al. 1996). This study builds on a previous pilot seeding trial in Nova Scotia, in which 10 000
juvenile sea scallops were released in autumn and followed for ,1 yr (Hatcher et al. 1996). The pilot study demonstrated that enhancement of wild scallop populations was possible, but it did not identify the reasons for successful seeding. Here, we report on four subsequent seeding trials, in which 13 000 – 21 000 juvenile sea scallops were seeded in summer and winter at two sites in the same locale. Our objectives were to determine the effects of site and season on mortality, movement, and growth of seeded scallops, and to estimate the relative contribution of predation and dispersion to losses of seeded scallops. As well, we suggest recommendations for enhanced success in future bottom seeding operations.
Material and methods Study sites The seeding experiments were conducted at two contrasting locations near Lunenburg, Nova Scotia, Canada (Fig. 1). The sites were selected to meet the following criteria based on a pilot study (Hatcher et al. 1996): a 1-ha area of relatively uniform cobble and silt sea bed, experiencing similar weather conditions and moderate currents (>0.1 and 0.05). Crab distribution varied greatly at all spatial scales and sur-
vey dates (–1.000 < SMI < 0.520), but on average was random at Site 2 (SMI = –0.218 6 0.606 (SD)) and Site 3 (SMI = –0.002 6 0.568; indices were not calculated for surveys 4, 5, 6, and 8 because of low abundance). Crab density (Fig. 4) showed significant temporal variation at both sites (Site 2: Friedman X2 = 19.03, df = 10, p < 0.05; Site 3: X2 = 114.84, df = 7, p < 0.001). Crab density did not correlate with seeded scallop density for the same survey dates (Site 2: r = –0.072, df = 7, p > 0.05; Site 3: r = –0.210, df = 5, p > 0.05), but correlated significantly for crab survey dates lagged by one behind seeded scallop survey dates at Site 3 only (Site 2: r = –0.007, df = 6, p > 0.05; Site 3: r = 0.864, df = 5, p < 0.05). At Site 3, 558 crabs were trapped and removed between 28 April and 16 October 1992 (days 245 and 416). Size frequency distributions In the survey prior to the summer seeding, only 4% of the wild scallops were within the size range of the seeded scallops at Site 2 and none was within this range at Site 3 (Fig. 10). At Site 2, cohorts of summer- and winter-seeded scallops could be followed until August 1992 when the two modes joined together. Growth of summer-seeded scallops as measured by modal progression was initially rapid at Site 2 and then slowed in winter (Fig. 11). Growth of winter-seeded scallops paralleled that of the summer-seeded scallops in winter. Overall at Site 2, the growth rate was 0.078 6 0.008 (SE) mm⋅d–1 (r2 = 0.948, df = 1, 5, p < 0.001) for summer-seeded © 1996 NRC Canada
2502 Fig. 6. Mean 6 SD displacement (squares) of seeded sea scallop (P. magellanicus) and average monthly water temperature (dotted line) at (a) Site 2 and (b) Site 3. Displacement of scallops on the seeding day was not available. Letters indicate the results of multiple mean comparisons (displacements with the same letter did not differ significantly). Arrows indicate days when the sites were seeded with juvenile scallop. n = 25–1118 for displacement. See Table 1 for survey number and date.
Can. J. Fish. Aquat. Sci. Vol. 53, 1996
was 73 6 24 mm CW (n = 70, range: 13–120 mm CW) at Site 2 and 64 6 18 mm CW (n = 81, range: 23–100 mm CW) at Site 3. Within each site, the proportion of crabs in the three different size classes (Figs. 12b and 12d) varied significantly over time (Site 2: G = 88.43, df = 22, p < 0.001; Site 3: G = 143.74, df = 10, p < 0.001 (sampling dates with
