289
16 Loligo vulgaris and Doryteuthis opalescens
and 15 Lx at midwater. Water was processed through the biofilter, a 15-W ultraviolet (UV) sterilizer, a 5-μm polyester filter and activated carbon. Mortality still was
high, but an improvement compared to earlier works. This was attributed to greater
reduction of skin and fin damage owing to a lower wall-surface-to-volume ratio;
improved prey capture with black sidewalls and adequate illumination; and higher
water quality from the efficient filtration. The larger tanks, however, required higher
current speeds for thorough water mixing. This was later found to decrease survival
because paralarvae still were subject to fin and skin damage, as explained below.
For the first 2 months of life in D. opalescens, Vidal et al. (2002a) used natural
seawater collected from offshore locations, which was maintained at about 16°C
and between 30 and 35 psu, and thus obtained high hatchling survival rates. Light
intensities were low (< 6.0 Lx at the surface, Vidal et al. 2002a) and evenly distributed to avoid patchy distribution of the paralarvae and their prey. Sudden light
changes should be avoided as they will set off escape reaction by backward jets
making the squids hit the walls of the tanks and causing skin damage (Boletzky and
Hanlon 1983). Thus, light conditions can greatly influence swimming and feeding
behaviour and consequently promote stress. Homogeneous flat black colour tanks
have improved feeding of paralarvae by reducing reflectance of light and increasing the visual contrast of prey organisms (Yang et al. 1983a, b; Vidal et al. 2002a).
The minimum water volume for rearing paralarvae or adults is not known, but a
safe practice is to use a larger volume than required (Boletzky and Hanlon 1983).
Rearing densities ranged substantially from 0.67 L
−1
for D. opalescens paralarvae
(closed system, Yang et al. 1986) to 25 L
−1
for L. vulgaris paralarvae (open system,
Villanueva 2000a) (Tables 16.1 and 16.2). Best survival has been found from 6.8 to
13 hatchlings L
−1
(Vidal et al. 2002a).
Vidal et al. (2002a) advanced rearing of D. opalescens by reproducing circulation
patterns in tanks in which paralarvae had higher survival and lower incidence of
fin damage. Tanks were of intermediate diameter (1 m) and smaller height (0.4 m).
This provided adequate swimming space. Fine-scale adjustment of the position and
intensity of water inflow allowed optimizing current speed to achieve circulation
of low speeds (1.0–1.4 cm s
−1
) and gentle mixing. Higher speeds (> 2.0 cm s
−1
) and
greater turbulence were directly responsible for the lowest survival rates of 30-dayold paralarvae. Circulation thus is a crucial design consideration. Properly implemented, it noticeably reduces mortality caused by contact of the fragile paralarvae
with the tank walls and also produces a uniform distribution of hatchlings and prey,
and this enhances their interaction.
If circulation is too slow, the negatively buoyant paralarvae (Martins et al. 2010)
tend to remain near the tank bottom while prey items, such as Artemia nauplii
and zooplankton, have a patchy distribution near the surface. This segregation of
paralarvae and their food reduces feeding interactions and, as a consequence, both
survival and growth. In contrast, the stronger circulation needed for adequate mixing in large-diameter tanks (Yang et al. 1980, 1983a) increases abrasive damage of
fins, thus lowering survival.
Optimal circulation and mixing greatly reduce the mechanical stress on the delicate paralarvae and can be achieved in several ways, as for example: (1) placing
spray bars near the surface at intermediate angles (40–50°) and (2) introducing
16 Loligo vulgaris and Doryteuthis opalescens
and 15 Lx at midwater. Water was processed through the biofilter, a 15-W ultraviolet (UV) sterilizer, a 5-μm polyester filter and activated carbon. Mortality still was
high, but an improvement compared to earlier works. This was attributed to greater
reduction of skin and fin damage owing to a lower wall-surface-to-volume ratio;
improved prey capture with black sidewalls and adequate illumination; and higher
water quality from the efficient filtration. The larger tanks, however, required higher
current speeds for thorough water mixing. This was later found to decrease survival
because paralarvae still were subject to fin and skin damage, as explained below.
For the first 2 months of life in D. opalescens, Vidal et al. (2002a) used natural
seawater collected from offshore locations, which was maintained at about 16°C
and between 30 and 35 psu, and thus obtained high hatchling survival rates. Light
intensities were low (< 6.0 Lx at the surface, Vidal et al. 2002a) and evenly distributed to avoid patchy distribution of the paralarvae and their prey. Sudden light
changes should be avoided as they will set off escape reaction by backward jets
making the squids hit the walls of the tanks and causing skin damage (Boletzky and
Hanlon 1983). Thus, light conditions can greatly influence swimming and feeding
behaviour and consequently promote stress. Homogeneous flat black colour tanks
have improved feeding of paralarvae by reducing reflectance of light and increasing the visual contrast of prey organisms (Yang et al. 1983a, b; Vidal et al. 2002a).
The minimum water volume for rearing paralarvae or adults is not known, but a
safe practice is to use a larger volume than required (Boletzky and Hanlon 1983).
Rearing densities ranged substantially from 0.67 L
−1
for D. opalescens paralarvae
(closed system, Yang et al. 1986) to 25 L
−1
for L. vulgaris paralarvae (open system,
Villanueva 2000a) (Tables 16.1 and 16.2). Best survival has been found from 6.8 to
13 hatchlings L
−1
(Vidal et al. 2002a).
Vidal et al. (2002a) advanced rearing of D. opalescens by reproducing circulation
patterns in tanks in which paralarvae had higher survival and lower incidence of
fin damage. Tanks were of intermediate diameter (1 m) and smaller height (0.4 m).
This provided adequate swimming space. Fine-scale adjustment of the position and
intensity of water inflow allowed optimizing current speed to achieve circulation
of low speeds (1.0–1.4 cm s
−1
) and gentle mixing. Higher speeds (> 2.0 cm s
−1
) and
greater turbulence were directly responsible for the lowest survival rates of 30-dayold paralarvae. Circulation thus is a crucial design consideration. Properly implemented, it noticeably reduces mortality caused by contact of the fragile paralarvae
with the tank walls and also produces a uniform distribution of hatchlings and prey,
and this enhances their interaction.
If circulation is too slow, the negatively buoyant paralarvae (Martins et al. 2010)
tend to remain near the tank bottom while prey items, such as Artemia nauplii
and zooplankton, have a patchy distribution near the surface. This segregation of
paralarvae and their food reduces feeding interactions and, as a consequence, both
survival and growth. In contrast, the stronger circulation needed for adequate mixing in large-diameter tanks (Yang et al. 1980, 1983a) increases abrasive damage of
fins, thus lowering survival.
Optimal circulation and mixing greatly reduce the mechanical stress on the delicate paralarvae and can be achieved in several ways, as for example: (1) placing
spray bars near the surface at intermediate angles (40–50°) and (2) introducing