290
E. A. G. Vidal and S. von Boletzky
water at the tank bottom to generate a gentle upwelling. In either case, one indication of ideal circulation is a relatively smooth water surface (see Vidal et al. 2002a)
and long horizontal and vertical displacements of squids away from the walls. A
tank-side window permits observation of distribution, feeding and swimming behaviour without disturbing the paralarvae.
The effect of small-scale turbulence on swimming performance and prey capture
yet is an unexplored field of paralarvae research. As noted above, current speed and
mixing directly affect survival rates (Vidal et al. 2002a). Further, studies of fish larvae demonstrate the negative effect of turbulence in removing prey from a larva’s
effective pursuit field, thereby reducing capture success (Mackenzie and Kiørboe
2000). As a consequence, the effect of turbulence on encounter rates has been suggested as an explanation for the lack of a straightforward relationship between larval feeding and prey density. Turbulence is also believed to be a critical stressor
that severely increases energy expenditure, which also reduces survival. Understanding how small-scale turbulence affects swimming and feeding behaviour is a
fundamental step towards improving paralarvae growth and survival. Future efforts
should also focus on the influence of tank design on feeding behaviour.
16.6.2 Water Quality
Water of excellent quality is a primary requirement for successful rearing of loliginid paralarvae. Both natural and artificial seawater have been used, but more
research is needed on the efficacy of artificial seawater in supplying essential elements for paralarval development (Boletzky and Hanlon 1983).
Water quality in open (or flow-through) systems is managed by continual replacement of culture water with fresh seawater. Though effective, this may afford little control over specific water-quality parameters; and this, in turn, can make it more difficult
to resolve low quality, contamination and pollution problems that jeopardize rearing.
In closed (or recirculation) systems, water quality is managed by implementing
mechanical, physical adsorption, disinfection and biological processes. The main
parameters that require control are particulate organic matter (POM), nitrogenous
wastes, dissolved gases (oxygen, carbon dioxide and nitrogen), pH, alkalinity and
pathogens. Each is discussed briefly below. Detailed discussions on the theory and
application of these processes are found in Spotte (1979) and Losordo et al. (2001).
16.6.2.1 Particulate Organic Matter
The POM load is expected to be very low for paralarvae rearing. Yang et al. (1983a)
estimated POM at < 1.0 g m
−3
, consisting mainly of dead paralarvae and prey. Denser
POM sinks to the bottom of the rearing tanks and is removed easily by siphoning.
Suspended POM is removed by mechanical filtration, such as employing a finemeshed (50 µm) screen in the biofilter inflow stream. Vidal et al. (2002a) found that
E. A. G. Vidal and S. von Boletzky
water at the tank bottom to generate a gentle upwelling. In either case, one indication of ideal circulation is a relatively smooth water surface (see Vidal et al. 2002a)
and long horizontal and vertical displacements of squids away from the walls. A
tank-side window permits observation of distribution, feeding and swimming behaviour without disturbing the paralarvae.
The effect of small-scale turbulence on swimming performance and prey capture
yet is an unexplored field of paralarvae research. As noted above, current speed and
mixing directly affect survival rates (Vidal et al. 2002a). Further, studies of fish larvae demonstrate the negative effect of turbulence in removing prey from a larva’s
effective pursuit field, thereby reducing capture success (Mackenzie and Kiørboe
2000). As a consequence, the effect of turbulence on encounter rates has been suggested as an explanation for the lack of a straightforward relationship between larval feeding and prey density. Turbulence is also believed to be a critical stressor
that severely increases energy expenditure, which also reduces survival. Understanding how small-scale turbulence affects swimming and feeding behaviour is a
fundamental step towards improving paralarvae growth and survival. Future efforts
should also focus on the influence of tank design on feeding behaviour.
16.6.2 Water Quality
Water of excellent quality is a primary requirement for successful rearing of loliginid paralarvae. Both natural and artificial seawater have been used, but more
research is needed on the efficacy of artificial seawater in supplying essential elements for paralarval development (Boletzky and Hanlon 1983).
Water quality in open (or flow-through) systems is managed by continual replacement of culture water with fresh seawater. Though effective, this may afford little control over specific water-quality parameters; and this, in turn, can make it more difficult
to resolve low quality, contamination and pollution problems that jeopardize rearing.
In closed (or recirculation) systems, water quality is managed by implementing
mechanical, physical adsorption, disinfection and biological processes. The main
parameters that require control are particulate organic matter (POM), nitrogenous
wastes, dissolved gases (oxygen, carbon dioxide and nitrogen), pH, alkalinity and
pathogens. Each is discussed briefly below. Detailed discussions on the theory and
application of these processes are found in Spotte (1979) and Losordo et al. (2001).
16.6.2.1 Particulate Organic Matter
The POM load is expected to be very low for paralarvae rearing. Yang et al. (1983a)
estimated POM at < 1.0 g m
−3
, consisting mainly of dead paralarvae and prey. Denser
POM sinks to the bottom of the rearing tanks and is removed easily by siphoning.
Suspended POM is removed by mechanical filtration, such as employing a finemeshed (50 µm) screen in the biofilter inflow stream. Vidal et al. (2002a) found that