HUNTER: CULTURE METHODS 



25 



larvae in rearing containers although constant illumination is 

 occasionally used. Typically fluorescent lamps are used which 

 provide 2,000-3,000 lux at the water surface (Houde, 1978; 

 Hunter, 1976). Night light levels vary; we provide no light at 

 night whereas Houde (1978) provides a dim light of 40-90 lux 

 at night, which is substantially above the visual threshold for 

 feeding for larval E. morda.x (6 mm larvae 50% feeding thresh- 

 old = 6 lux, and 10-15 mm larvae 50% threshold = 0.6 lux, 

 Bagarinao and Hunter, 1983). Clearly, longer periods for visual 

 feeding will probably enhance growth if food is limited. Rearing 

 at high light intensities such as natural sunlight may greatly 

 increase production of algae and zooplankton in the culture tank 

 and thereby increase larval survival (Kraul, 1983). On the other 

 hand, solar UV radiation is clearly lethal to younger larvae 

 (Hunter etal., 1 982) and use of deep tanks, or shaded or covered 

 tanks (screen cloth, acrylic plastic, glass or mylar film) is rec- 

 ommended for the first 1-2 weeks of larval life if tanks are to 

 be exposed to solar radiation. 



Water qualily.—C\osed, non-circulating systems are typically 

 used to rear marine fish larvae at least during the younger stages, 

 because in an open system planktonic larvae and their foods 

 are easily lost. Older (nektonic) larvae are able to resist the 

 current and to consume a daily ration in a short period so a 

 partially open system can be used. We fill our rearing containers 

 with UV treated sea water that is passed through three, in line, 

 cartridge filters (5, 3 and 1 ^m pore).' Although not a common 

 practice in small scale rearing work, the addition to rearing tanks 

 of antibiotics (sodium penicillin G at 50 i.u./ml plus strepto- 

 mycin sulphate at 0.05 g/ml) slightly improved survival of Pleu- 

 ronectes platessa eggs through hatching, but surprisingly this 

 single treatment greatly improved survival of larvae through 

 metamorphosis (Shelboume, 1975). 



Use of a closed system requires attention to water quality, a 

 problem which may be intensified at higher rearing tempera- 

 tures. In the most complete study of water quality in rearing 

 tanks for marine fish larvae, Brownell (1980a, b) considered 

 seven variables (pH, dissolved oxygen, carbon dioxide, am- 

 monia, nitrite and nitrate), but only high pH, low dissolved 

 oxygen and un-ionized ammonia had effects at levels likely to 

 be encountered in rearing tanks. First feeding incidence declined 

 by 50% in all species he studied when dissolved oxygen con- 

 centrations were between 4 and 4.75 mg/1 (49-58% saturation). 

 Dissolved oxygen in our rearing containers usually is not sat- 

 urated after planktonic foods are added, and typically it is about 

 80% saturation even with aeration. Clearly water quality is im- 

 proved by aeration and frequent water changes and lank clean- 

 ing. Werner and Blaxler (1980) exchanged 20% of the water in 

 Clupea harengus cultures (9° C) 3 times per week but at high 

 temperatures greater replacement rates are required. For ex- 

 ample Houde (1977) replaced 20% of the tank sea water on 

 alternate days while culturing Anchoa mitchilli and Achirus lin- 

 eatus at 26-28° C. Frequent tank cleaning is important as heavy 

 mortalities may result from toxins produced by debris on the 

 container bottom (Kraul, 1983). Aeration, unless very gentle, 

 can cause heavy mortalities among delicate eggs and newly 

 hatched larvae. In fact, Shelboume (1964) recommends no aer- 



' Aqua-Pure model APIO. AMP Cuno Division, Inc., Meriden. Con- 

 necticut USA. 



ation for Pleuronectes platessa larvae. I recommend very gentle 

 aeration but not until a week or so beyond the first feeding stage. 

 The mortality of cultured fish larvae often increases during 

 the period of initial swim bladder inflation in physoclistous 

 fishes (Doroshev et al., 1981; Kuhlmann et al., 1981) and this 

 could be related to water quality. Symptoms include delay or 

 complete failure of inflation or excessive inflation; in either case 

 normal swimming patterns are disrupted and death frequently 

 results. The causes of abnormal inflation are not clear; preven- 

 tion of larvae from reaching the water surface prevented excess 

 inflation in M. cephalus larvae (Nash et al., 1977), whereas the 

 same treatment in Atractoscion nobilis larvae had no effect. In 

 A. nobilis excess inflation was associated with abnormal devel- 

 opment of gas secretory tissue suggesting a more complex etiol- 

 ogy (SWFC. unpubl. data). Failure to inflate the swim bladder 

 is a common problem in Morone saxatilus culture and turbulent 

 aeration may reduce the incidence of this disease (Doroshev and 

 Comacchia, 1979) but it now appears that reduction in salinity 

 from 17 ppt to 4 ppt has a much greater eflect in reducing the 

 incidence of swim bladder malfunction (S. Doroshev and J. 

 Merritt, U. Cal. Davis, pers. comm.). 



Food 



The most critical aspect of rearing marine larvae is manage- 

 ment of their food. Food must be the correct density, size, 

 nutritionally adequate and must remain suspended in the water 

 column which usually requires the use of living pelagic organ- 

 isms. 



Food size.— Typ\c&\ pelagic fish larvae are 2.5-4.0 mm when 

 they begin feeding and acceptable prey are 20-1 50 /um in breadth 

 (Houde and Taniguchi, 1979). Some large larvae, e.g.. larval C. 

 harengiis (B\di\\.QT. 1965). Pleuronectes platessa {Riley. 1966) or 

 small larvae with large mouths, e.g., Merluccius productus {Sum- 

 ida and Moser, 1980), can begin feeding on prey 300 Mm or 

 larger in breadth. The optimal food size increases as larvae grow 

 (Hunter, 1981), so any culture technique should provide a stead- 

 ily increasing range of food sizes, because if the food is too small 

 growth slows and mortality occurs (Hunter, 1981). Food size 

 requirements can be expressed in terms of the ratio of prey width 

 to mouth width. The 50% threshold for feeding on a prey of a 

 particular width occurs when this ratio is about 0.75, although 

 occasionally larvae consume prey as wide as the width of their 

 mouth (ratio = 1) (Hunter, 1981). At the onset of first feeding 

 a small prey of about 'A the mouth width seems to be preferable 

 as capture success is low at this time but within a few days larvae 

 are able to consume food of about V2 the mouth width. 



Wild zooplankton— V/i\d zooplankton, primarily the naupliar 

 and copepodite stages of marine copepods but also mollusc 

 veligers, tintinnids, cladocera, and appendicularia larvae, are 

 the natural foods of most marine fish larvae and probably also 

 the best source of food for rearing a larval taxonomic series. 

 Wild zooplankton provide a wide range of sizes and types and 

 are probably nutritionally superior to cultured rotifers and Ar- 

 lemia nauplii (Kuhlmann et al., 1981). Collection of wild zoo- 

 plankton may require less effort than production of cultured 

 food except for brine shrimp nauplii (see below). Zooplankton 

 is collected in nets of about 50 ^m, and is graded by size in the 

 laboratory using various nylon nets (Houde, 1977, 1978), This 

 eliminates the larger zooplankton which larvae would be unable 



