84 



J. E. SHELBOURNE 



a lethal trap; crevices in tank walls are a 

 particular menace — larvae swim into them 

 and seem unable to back out. One incuba- 

 tor inclusion is necessary, however, and 

 that is some sort of screen to prevent eggs, 

 larvae, and larval food from being flushed 

 away. The outlet system shown in figure 

 1 consists of a submerged vertical poly- 

 thene pipe, corked at its lower end and 

 perforated along part of its length. The 

 perforations are covered by 155-meshes- 

 per-inch nylon bag, held at a distance by 

 three polythene disks surrounding the 

 pipe. This screen is connected to a hori- 

 zontal overflow tube passing through a 

 watertight seal in the incubator wall. The 

 nylon mesh is fine enough to retain 

 Artemia salina nauplii, a convenient and 

 reliable larval food organism. Plaice 

 larvae are visual feeders; tanks require 

 overhead illumination at water surface in- 

 tensities around 500 lux during "first 

 feeding." Later on, dimmer lighting 

 helps to depress larval activity in over- 

 crowded conditions. It is also important 

 to black out the walls and bottoms of feed- 

 ing tanks, since food organisms are more 

 easily seen and captured by early feeders 

 against a dark background. 



To regulate water temperature in warm 

 surroundings, incubators are immersed in 

 a fresh-water bath cooled by a copper coil 

 linked to a thermostatically controlled 

 domestic refrigerator unit. The fresh 

 water can be slowly circulated around the 

 incubators by hard aeration. Vertical 

 temperature gradients are inevitable in 

 water-cooled tanks at high ambient tem- 

 peratures and slow irrigation rates; the 

 agitation required to break them down is 

 detrimental to delicate fish larvae. Con- 

 trolled air temperature systems are more 

 efficient than water baths. A slow salinity 

 increase occurs in any closed circulation, 

 owing to evaporation. This can be offset 

 by frequent routine additions of distilled 



water to the reservoir. Copper distilla- 

 tion is not recommended. 



In retrospect, the following improve- 

 ments could be made to the apparatus as 

 described : 



1. A substantial increase in reservoir volume. 



2. Water-bath cooling replaced by air-tempera- 

 ture control. 



3. Automatic pH adjustment, b.v linking a pH 

 meter control unit to a variable light source 

 above the header. 



4. Automatic salinity adjustment, by coupling 

 a salinometer and control to a solenoid valve 

 regulating the input of distilled water into the 

 reservoir. 



5. A glass-wool filter inserted at the point 

 where the common outlet pipe discharges into 

 the reservoir. 



6. Bacterial control in the reservoir by means 

 of a low-power ultraviolet unit (see later), and 

 in the incubators by occasional dosing with anti- 

 biotic mixtures. 



Results were encouraging, even without 

 the suggested improvements. They dem- 

 onstrated that a closed circulation could be 

 used to rear plaice from the Qgg stage 

 through and beyond metamorphosis — a 

 period of 3 to 4 months — at a minimum 

 ratio of 1 survivor per 2.5 liters of sea 

 water. 



The 3,500-gallon system 



Our expanded closed circulation (fig. 

 2), built in 1959, has two main com- 

 ponents; a sunken concrete reservoir (15 

 by 10 by 4 feet) and an adjacent brick 

 hatchery (22 by 13 by 8 feet). The res- 

 ervoir contains roughly 2,500 gallons of 

 offshore seawater. Enteromorpha is once 

 again used to control metabolites, being 

 submerged at a depth of 6 inches, on large 

 trays consisting of polythene film sewn 

 to a wooden framework with no metal 

 fastenings. TJie alga receives illumination 

 at night from fluorescent tubes arranged 

 in banks of four above the trays. Each 

 bank hangs by a nylon rope and pulley 

 (permitting vertical adjustment) from a 

 stout timber gallows spanning the tank. 



