Introduction Materials and methods

Ž . Aquaculture 183 2000 325–334 www.elsevier.nlrlocateraqua-online Effect of temperature and ration on growth and ž condition of king scallop Pecten maximus spat Ian Laing The Centre for EnÕironment, Fisheries and Aquaculture Science, CEFAS Conwy Laboratory, Benarth Road, Conwy, North Wales, LL32 8UB, UK Accepted 7 July 1999 Abstract Ž . Ž . Hatchery-reared king scallop Pecten maximus L. spat 5–14 mm shell height were held in mesh-based Perspex cylinders suspended in 2.5 l beakers of seawater. This seawater was Ž . Ž maintained at eight temperatures 58C–238C inclusive , and various rations 0.012–0.492 g Ž . y1 Ž . y1 . organic weight of algae g live weight of spat week of a diet consisting of a mixture of Ž . PaÕloÕa lutheri Droop Green and Chaetoceros ceratosporum Ostenfeld were fed. Size of the animals was estimated weekly by computer analysis of video images taken of the scallops in the cylinders while in seawater. The relationship between temperature and food ration was evaluated. As temperature increased, the scallops consumed a bigger ration, and the highest ration that they consumed efficiently determined their maximum growth rate. This ration, and the corresponding growth rate, both increased with temperature. At the end of each 3–4 week experiment, the Ž . condition ratio of dry meat weight to dry shell weight of the spat was estimated. Condition was greatest between 108C–178C and least below 8.08C. Condition decreased with lower rations at all temperatures above 108C. Comparison of results with preliminary observations made at commer- cial scallop cultivation sites suggest that temperature rather than food is the factor most often limiting for growth in the sea. Crown Copyright q 2000 Published by Elsevier Science B.V. Keywords: King scallop spat; Pecten maximus; Ration; Temperature

1. Introduction

Ž . Scallops Pecten maximus Linnaeus are a valuable seafood product with large established markets both within Europe and worldwide. There is a significant retail Tel.: q44-1492-593883; fax: q44-1492-592123; E-mail: i.laingcefas.co.uk 0044-8486r00r - see front matter Crown Copyright q 2000 Published by Elsevier Science B.V. Ž . PII: S 0 0 4 4 - 8 4 8 6 9 9 0 0 2 6 2 - 8 demand for good quality scallops which at present cannot be satisfied, and any increase in production within Europe to satisfy this demand is likely to come from cultivation on Ž . Ž . seabed sites Ansell et al., 1991; Chataigner, 1996 . Seed spat to supply this emerging industry will be from hatcheries and from collectors deployed in the wild. The success of seabed cultivation of scallops will depend on selection of appropriate sites. Some information is already available from other studies on certain aspects of site selection, Ž . Ž . such as substrate Dare et al., 1994 , exposure Duncan et al., 1995 and seabed area Ž requirement, in relation to dispersal and density Buestel and Dao, 1979; Minchin, 1992; . Dao et al., 1994; Fleury et al., 1996; Grall et al., 1996 . However, other environmental requirements of scallop spat, particularly temperature, food availability, water flow and salinity, must be determined to more fully establish the criteria for suitable sites. This information will also be useful for defining optimum conditions for rearing spat in the nursery, prior to planting out. Some empirical information is available from field studies Ž . Brynjelsen and Strand, 1996; Fleury et al., 1996; Chauvaud et al., 1998 , but closely controlled laboratory experiments are also needed. For this study, spat were held at a range of temperatures and rations and the effect on growth and condition noted. Growth rates were compared with some observations from commercial scallop cultivation sites.

2. Materials and methods

Ž . King scallop P. maximus spat were reared in the hatchery at the CEFAS Conwy Ž . laboratory Millican, 1997 at 178C. For each treatment, 8–16 spat of a range of initial Ž . sizes 5–14 mm shell height were placed into 1.5 mm nylon mesh-based, 11 cm diameter, 12 cm high Perspex containers. These containers had 1 cm holes, covered by mesh, spaced at 2 cm intervals around the base of the side wall, to allow for free exchange of seawater containing the diet. Scallops of this size range were used as it has Ž . been shown that for smaller animals less than 5 mm shell height growth rate, measured Ž . as increase in shell height, is lower in smaller spat Laing and Psimopoulous, 1998 . For Ž each temperature treatment, six containers, each stocked to give 0.10–1.30 g initial live . weight biomass of spat, were placed in 2.5 l glass beakers containing seawater at a salinity of greater than 30 psu. Live weight biomass of spat was estimated from Ž . individual shell height measurements, using the relationship: Live weight mg s 0.0732 Ž . 2.89 Ž . = shell height mm Laing and Psimopoulous, 1998 . Scallops were acclimated Ž . from the rearing temperature 178C to the experimental temperatures at rates of no greater than 18C per day. All six beakers were eventually maintained at one of the following nominal temperatures; 5.08C, 6.58C, 8.08C, 10.08C, 15.08C, 17.08C, 20.08C or 23.08C, by either control of the ambient temperature or by holding them in a water bath. Temperature of the seawater in the beakers was recorded daily. Four or five tempera- tures were tested in each of three experiments, with the 178C treatment always included Ž . to ensure consistency of results between experimental batches of spat Table 1 . Ž . Experiments were conducted at low 300–500 lx light levels in 12 h light:12 h dark cycles. Unfiltered seawater was used and was gently aerated to ensure an even suspension of food particles and an adequate supply of oxygen to the spat. It was exchanged three times each week, when the containers with the scallop spat were Table 1 Experimental design: range and number of rations tested in the three experiments. A total of 125 combinations of temperature and ration were used Ž . Temperature 8C Range of rations tested Number of rations tested in this range Minimum Maximum Experiment 1 Experiment 2 Experiment 3 5.0 0.012 0.021 4 – – 6.5 0.012 0.021 8 – – 8.0 0.020 0.110 9 – – 10.0 0.030 0.200 – 11 10 15.0 0.030 0.200 – 11 10 17.0 0.030 0.250 9 11 10 20.0 0.120 0.492 – 11 10 23.0 0.120 0.492 – 11 – immediately transferred to clean seawater at the appropriate temperature in another beaker to minimise stress from handling and desiccation. The scallops were fed a mixed diet of the intensively cultured algae species Ž . Ž Chaetoceros ceratosporum Ostenfeld and PaÕloÕa lutheri Droop Green in a 1:1 by . cell weight ratio. This diet has been shown to give good growth rates, equivalent to Ž those achieved with a natural assemblage of algae species Delaunay et al., 1993; Laing . and Psimopoulous, 1998 . The algae were grown in 3 l batch or semi-continuous Ž . cultures in a standard medium Walne, 1974 prepared from autoclaved seawater. All cultures were harvested in the exponential growth phase. Ž The diet was added daily to give rations in the range 0.012–0.492 g organic weight . y1 Ž . y1 of algae g live weight of spat week . The range and number of rations tested at each temperature is given in Table 1. The upper values were based on the amount of Ž algae consumed by scallop spat in previous experiments Laing and Psimopoulous, . 1998 and preliminary trials. Each ration was tested in duplicate beakers. The initial Ž . biomass of spat i.e., the size and number of animals held in the beaker was adjusted so that the algae cell concentration added to the beakers was always within the range 25–400 cells ml y1 . An estimate of the number of algal cells in suspension in the experimental systems immediately after feeding, and then after 24 h, was obtained using an electronic particle Ž . counter Coulter counter, Model Multisizer II with a 70 mm aperture, set to detect particles in the size range 2.0–5.0 mm. From these observations, the proportion of the Ž . ration consumed i.e., removed from suspension by filtration was calculated for each treatment. An additional experiment was carried out at 6.58C, 17.08C and 23.08C in which the spat, in duplicate beakers, were not fed any cultured algae. The unfiltered seawater contained some naturally occurring phytoplankton, equivalent to a ration of 0.004–0.008 Ž . y1 Ž . y1 g organic weight of algae g live weight of spat week . The shell height of the spat was measured at weekly intervals. The cylinders were transferred to a dish containing seawater. A video image of them, together with a scale Ž . for calibration, was captured with a RGB camera Model XC 711P, Sony UK and Ž further processed using AEQUITAS Image Analysis software Dynamic Data Links, . Ž Cambridge, England for individual shell height measurement perpendicular to the . hinge . This method was chosen in order to ensure the minimum of spat handling, since previous studies have indicated that P. maximus may be sensitive in this respect Ž . Ž . Millican, 1997 . At this stage, the new initial biomass was calculated as above and the ration fed was adjusted, as required. Each experiment was continued for 3–4 weeks. For each week of each experiment, at each temperature and ration treatment, the Ž y1 . mean growth rate mm day was calculated from the measurements on the 8–16 individual scallops in duplicate containers. The data set for temperatures in the range Ž . 6.58C–23.08C 121 combinations of temperature and ration, Table 1 was processed by Ž . the G3GRID and GCONTOUR procedures SAS Institute, Cary, NC, USA . The G3GRID procedure interpolates data sets of irregularly spaced observations to complete a rectangular grid that is displayed by GCONTOUR, in this case to show growth rates over the full range of temperatures and rations tested. The results for 5.08C were not included, as there was no growth at this temperature. Samples of five to six scallops were taken initially and from each of the beakers at Ž . the end of the experiments. Dry and organic ash-free dry weight estimations were made on individual spat. For these estimations, spat were washed three to four times in fresh water and transferred to porcelain crucibles previously dried in a heated cabinet at 608C for 4 h and weighed, to the nearest 1 mg, on an automatic electrobalance. The crucibles containing the individual spat were returned to the drying cabinet for 48 h at 608C and then re-weighed to give the total dry weight. The dried spat were then combusted at 4508C for 24 h in a muffle furnace and then re-weighed. Organic weight Ž . weight loss on combustion was determined as a percentage of the total dry weight. Further spat from each beaker were taken and the meats and shells were separated. The Ž . percentage organic weight of each component was then determined, as described above. These values were used to calculate, by simultaneous equations, the dry meat and dry shell weights of the spat in the other samples, from the total dry weight and total organic weight values obtained. Condition index of the scallops was then calculated, as Ž . the ratio of dry meat weight to dry shell weight Beninger and Lucas, 1984 . For Ž . comparison of condition index, an analysis of variance GLM procedure, SAS Institute was carried out on mean values from each beaker. Where a significant difference in condition was found, individual temperature and ration treatments were compared by least significant difference between means.

3. Results