In general, the patterns of live prey consumption observed in the
three rearing mesocosms were found to be very similar for all the
preys. The pattern of live prey consumption recorded using the Ivlev's
index (Fig. 7) showed an initial preference for copepod nauplii in all
tanks, reaching a maximum around 11 dph (3.0–4.0 mm in TL), and
remained higher than 0 until larvae reached a size between 8.0 and
12.0 mm in TL. In M3, an absolute maximum preference for this item
was recorded at 26 dph (mean size 9.5 mm in TL). Larval stages of
bivalve (trochophores) show high values of the index within 8–20
(2.0–5.0 mm in TL) dph. In contrast, the value of the index for B.
plicatilis was about 0 throughout all the experimental rearing in all the
mesocosms. Larval and adult stages of T. holoturiae always represented
one of the most actively selected species of copepods after 20 dph. The
Ivlev's index value for this prey became positive at about 7.0 mm in TL.
On the other hand, planktonic copepods were ingested only at the end
of the experiment, although with a negative or near 0 index value.
Table 2 reports the mean value of the Ivlev's index, computed
throughout the experiment, for each food items. The mean values
computed in the three rearing parts are coincident in the mesocosms
for all food items. In the first part (3–14 dph or 4–4.5 mm in TL) the
food items which show the highest values of the Index are copepod
nauplii and rotifers, but in both cases the value is 0. In the second part
(14–23 dph or 4.5–8.0 mm in TL), the value of the index for copepod
nauplii reach 0.6, indicating that this item is actively selected by dusky
grouper larvae. In the last part (14–23 dph or TLN8.0 mm), Artemia
nauplii show a value of 0.4, and then represent the preferred item for
larvae.