291
16 Loligo vulgaris and Doryteuthis opalescens
this had a considerable impact on reducing pH and nitrogenous compounds variations
and maintaining a stable and high-quality rearing environment for D. opalescens
paralarvae. Dissolved organic matter (DOM) is not a problem when POM is removed
quickly. If, however, DOM increases owing to some malfunction in the system, it
can be reduced effectively by protein skimmers operated outside of the rearing tanks.
16.6.2.2 Nitrogenous Wastes
Potentially deleterious inorganic nitrogen compounds are un-ionized ammonia
(NH 3 -N), nitrite (NO 2 -N) and nitrate (NO 3 -N). Ammonia management deserves special attention in closed systems because its un-ionized form (NH 3 -N) is particularly
toxic to many marine species, even at very low concentrations. Together with its ionized form, ammonium (NH 4
+
-N), the two are referred to as total ammonia nitrogen
(TAN) and exist in an equilibrium determined by water temperature, salinity, pH and
pressure. Experiments at high temperature and (especially) high pH must thus monitor
NH 3 -N closely. Ammonia is produced by heterotrophic excretion and so increases, although not linearly, with stock ingestion rate and feed protein content. Thus, at higher
stocking densities, more feed will be ingested and more ammonia will be produced.
Excess feed, faeces and dead animals not removed from the rearing unit will be mineralized by heterotrophic microorganisms, which will add to the ammonia load.
Tolerance of loliginid embryos and paralarvae to nitrogen species is largely
unknown, so current guidelines follow those for most marine invertebrates: Unionized ammonia should not exceed 0.05 mg L
−1
(Timmons and Ebeling 2007);
nitrite-N, < 0.10 mg L
−1
; and nitrate-N, < 20 mg L
−1
(Spotte 1979; Yang et al. 1989).
It is also very important to consider the system’s filter capability, which should
have a surface-area-to-volume ratio as large as possible to offset total animal biomass.
Recommendations for loliginids suggest a ratio > 10 for small biomass loads, as in
the case of paralarvae (Yang et al. 1989). In a robust biofilter, ammonia is converted
to NO 2 -N that is oxidized to NO 3 -N. This NO 3 -N may accumulate to relatively high
levels, but it eventually must be removed by water exchange. Care also must be taken with oxygen levels, as a low oxygen concentration leads to insufficient biological
oxidation and subsequent build-up of toxic NO 2 -N. Even with good filtration, water
exchange must be done periodically to ensure proper types and concentrations of trace
elements, as these have important metabolic functions in growth and survival of squid
hatchlings (Boletzky and Hanlon 1983; Yang et al. 1983a).
16.6.2.3 Dissolved Gases
In loliginids, oxygen demand increases considerably after hatching because of
the highly active paralarvae as shown for L. vulgaris (Rosa et al. 2012). Feeding,
respiration and excretion of paralarvae and their prey all impact water quality. Dissolved oxygen levels thus should be maintained at saturation at all times (Boletzky
and Hanlon 1983). Oxygen solubility depends on temperature, salinity and the
16 Loligo vulgaris and Doryteuthis opalescens
this had a considerable impact on reducing pH and nitrogenous compounds variations
and maintaining a stable and high-quality rearing environment for D. opalescens
paralarvae. Dissolved organic matter (DOM) is not a problem when POM is removed
quickly. If, however, DOM increases owing to some malfunction in the system, it
can be reduced effectively by protein skimmers operated outside of the rearing tanks.
16.6.2.2 Nitrogenous Wastes
Potentially deleterious inorganic nitrogen compounds are un-ionized ammonia
(NH 3 -N), nitrite (NO 2 -N) and nitrate (NO 3 -N). Ammonia management deserves special attention in closed systems because its un-ionized form (NH 3 -N) is particularly
toxic to many marine species, even at very low concentrations. Together with its ionized form, ammonium (NH 4
+
-N), the two are referred to as total ammonia nitrogen
(TAN) and exist in an equilibrium determined by water temperature, salinity, pH and
pressure. Experiments at high temperature and (especially) high pH must thus monitor
NH 3 -N closely. Ammonia is produced by heterotrophic excretion and so increases, although not linearly, with stock ingestion rate and feed protein content. Thus, at higher
stocking densities, more feed will be ingested and more ammonia will be produced.
Excess feed, faeces and dead animals not removed from the rearing unit will be mineralized by heterotrophic microorganisms, which will add to the ammonia load.
Tolerance of loliginid embryos and paralarvae to nitrogen species is largely
unknown, so current guidelines follow those for most marine invertebrates: Unionized ammonia should not exceed 0.05 mg L
−1
(Timmons and Ebeling 2007);
nitrite-N, < 0.10 mg L
−1
; and nitrate-N, < 20 mg L
−1
(Spotte 1979; Yang et al. 1989).
It is also very important to consider the system’s filter capability, which should
have a surface-area-to-volume ratio as large as possible to offset total animal biomass.
Recommendations for loliginids suggest a ratio > 10 for small biomass loads, as in
the case of paralarvae (Yang et al. 1989). In a robust biofilter, ammonia is converted
to NO 2 -N that is oxidized to NO 3 -N. This NO 3 -N may accumulate to relatively high
levels, but it eventually must be removed by water exchange. Care also must be taken with oxygen levels, as a low oxygen concentration leads to insufficient biological
oxidation and subsequent build-up of toxic NO 2 -N. Even with good filtration, water
exchange must be done periodically to ensure proper types and concentrations of trace
elements, as these have important metabolic functions in growth and survival of squid
hatchlings (Boletzky and Hanlon 1983; Yang et al. 1983a).
16.6.2.3 Dissolved Gases
In loliginids, oxygen demand increases considerably after hatching because of
the highly active paralarvae as shown for L. vulgaris (Rosa et al. 2012). Feeding,
respiration and excretion of paralarvae and their prey all impact water quality. Dissolved oxygen levels thus should be maintained at saturation at all times (Boletzky
and Hanlon 1983). Oxygen solubility depends on temperature, salinity and the