condition indices at all temperatures. When no food was added, condition indices were significantly lower than with ‘low’ rations at 22.98C and 17.28C and lower, but not
significantly so, than the ‘full’ ration at 6.68C. Condition index of unfed spat was not Ž
. significantly different at these three temperatures F
s 2.78, P s 0.208 .
2, 3
4. Discussion
For nursery rearing of scallops, higher temperatures gave faster growth rates, but Ž
. required bigger rations to sustain the increase Fig. 1 . There is probably no commercial
advantage to be gained from rearing scallop spat in the nursery above 178C–188C. At these temperatures, scallops fed a ration of 0.15 g g
y1
week
y1
grew from 5 to 10 mm shell height in 23–24 days. At 208C and 238C, this was reduced to 21 days and 19 days,
respectively. The amount of food required to achieve this, however, was increased by 20 at 208C and 65 at 238C, and seawater heating costs could be greater. Also, the
Ž .
condition of the spat reared at these higher temperatures was lower Table 3 , which may affect their performance when initially planted out into the sea. The dry meat
Ž .
weight of 10 mm shell height scallops grown at 178C 5.4 mg was 16 and 21 Ž
. Ž
. greater than scallops grown at 208C 4.65 mg and 238C 4.45 mg , respectively. A
Ž .
y1
Ž .
y1
. ration of about 0.15 g organic weight of algae g
live weight of spat week was
required at 178C. Larger rations than this did not lead to greater food consumption Ž
. Ž
. Table 2 and did not give any increase in growth rate Fig. 1 .
At field sites, it is probable that low temperature is a critical factor affecting success of cultivation. Scallops in the laboratory did not feed or grow at 4.78C, although no
mortalities were observed within the timescale of the experiments. In field studies Ž
. Ž
. carried out in Norway by Brynjelsen and Strand 1996 all scallops P. maximus died
where the temperature fell to below 28C, and acceptable levels of survival required temperatures higher than 48C. Other species of scallops also appear to be sensitive to
Ž .
low temperatures. About 50 of hatchery-reared bay scallops Argopecten irradians
Ž .
died when mean seawater winter temperature fell below 58C Barber and Davis, 1997 . Ž
. Strand et al. 1993 have suggested that sensitivity to low temperatures is increased
when salinity falls below 26 psu. At this salinity, mortalities of P. maximus greater than 50 were observed at 58C. This may be related to a lower tolerance to salinity stress at
low temperatures, as a consequence of the lower condition index of the scallops at these
Ž .
temperatures Table 3 . Ž
. Ž
. Wilson 1987 and Chauvaud et al. 1998 , from observations on the growth of 1 and
2 year old spat in Birterbuy Bay. Galway, Ireland and the Bay of Brest, France, respectively, have suggested that growth at cultivation sites is mainly regulated by water
temperature, together with an effect of flow rate and salinity, rather than the amount of food available. Data from the laboratory studies, when compared with measurements of
Ž .
growth of hatchery-reared spat at UK cultivation sites Fig. 2 , support this idea. The Ž
y1
. data points on Fig. 2 are mean growth rates increase in shell height, mm day
calculated from monthly measurements, taken between April and November over 2 years, of 50 hatchery-reared scallop spat, planted out at 10–12 mm in pearl nets, at two
commercial cultivation sites. Temperature was measured every 5 h on automatic
Ž
y1
. Fig. 2. Growth rates increase in shell height, mm day
of 10–60 mm king scallops kept in pearl nets at Ž
three field sites and measured monthly, compared with mean seawater temperature during the month five .
Ž readings per day . The dashed line represents the regression of mean maximum growth rates with ‘full’
. Ž
rations in the laboratory experiments at the temperatures tested Growth rates 0.0199=Temperaturey0.081; .
F s 74.9, P - 0.0001 .
1, 8
recorders placed in the pearl nets, and mean monthly values calculated. The sites were geographically separated, one on the south coast of England and the other in a sea loch
on the west coast of Scotland. Salinity was not limiting, never falling below 30 psu at either site. The broken line on Fig. 2 represents the relationship between mean
Ž .
maximum growth rates with ‘full’ rations and temperature in the laboratory experi- ments. It can be seen that for any temperature, growth rates in the laboratory when food
was not limiting can generally be used to predict growth rates at the cultivation sites, although there are exceptions. Some faster growth rates were recorded at higher
temperatures. Scallop spat are capable of consuming much greater quantities of food
Ž .
than is normally required for growth at these temperatures Table 2 and it may be that under certain circumstances, the food value of the algae naturally available is exception-
ally high. The lower growth rates observed indicate that a factor other than temperature can at times be severely limiting. This could be food availability, although the fact that
growth rates were considerably lower than expected also suggests that this could be related to, for example, the effects of blooms of harmful dinoflagellates, which have
Ž .
been shown to inhibit growth of scallops Chauvaud et al., 1998 . However, there are no data to support this, and other factors recognised as being potentially limiting for growth
in the sea could equally be responsible. For example, it has been suggested that the flow Ž
. rate of the seawater current speed can limit the amount of suspended food available to
Ž .
cultivated animals Wilson, 1987; Wilson-Ormond et al., 1997 . The field studies from which the above results were obtained are ongoing and it is hoped to develop this work
in relation to the measured amount of food available in the water and filtration rates of king scallops.
There is very little information in the literature on the effect of temperature and ration on condition index of king scallop spat. The results obtained in this study are broadly
similar to those from work on other species of scallops. Sea scallops, Placopecten magellanicus, showed a reduced metabolic condition at temperatures less than 78C,
Ž .
manifest as significantly lower condition indices Kleiman et al., 1996 . Results from the present study also show that condition index is lower at temperatures that are higher
than the range at which the food source is utilised most efficiently. This is presumably related to higher metabolic requirements restricting growth of the scallop meats in
relation to increase in shell size. Food-limited growth gave lower condition indices at all
Ž .
temperatures and Rheault and Rice 1996 showed a similar effect in the bay scallop, A. irradians irradians in a field study. They concluded that growth rates and condition
index are linearly correlated with the average chlorophyll ration consumed in a commercial aquaculture setting.
5. Conclusions
