Brine shrimp nauplii (Artemia spp.) are still commonly used as live feed, but without complex algal enrichment they lack essential nutrients and are not suitable for small fish larvae, such as those of pike-perch.
Potential live feeds, which have to fulfill the nutritional requirements of sensitive larvae, are determined by their nutritional composition as well as their size. Thus, a range of live feeds and appropriate feeding regimes are required to improve the outcome of larval culture.
At the EAS conference Aquaculture Europe 2015 held in Rotterdam, Netherlands, scientists from the chair of Aquaculture and Sea-Ranching at the University of Rostock, Germany, described a series of experiments that screened suitable live feed organisms and evaluated their potential.
Mix of treatments
The treatments combined microalgae with different zooplankton organisms. Control groups were either exclusively fed with Artemia sp. and/or dry feed. All experiments were conducted in recirculating aquaculture systems, which varied in size and design.
Based on the results of these laboratory-scale experiments a semi-commercial technical device for improved production was designed, built and applied for pike-perch larviculture.
The experiments tested commonly used diets, such as Artemia and dry feeds as well as diets of rotifers (Brachionus sp.), either exclusively or in combinations with other zooplankton organisms such as the cladocerans Chydorus sp., Moina sp. and/or the copepod Eurytemora sp.
Some of the experiments were conducted under green water conditions. Abiotic parameters as well as dissolved nitrogen and phosphorus compounds were examined regularly. Survival, growth rates and fatty acid compositions of the live feed and of the cultured pike-perch larvae were recorded. Culture conditions were in the range described as ideal for pike-perch larvae.
Artemia vs dry feeds
Common feeding practices, such as non-fortified Artemia or dry feed diets resulted in poor survival rates of Sander larvae, and somatic growth was only detectable while internal larval nutrient reserves (yolk sack and oil droplet) remained. Once these were depleted the only detectable physiological development of the larvae was of the eyes and mouth but not growth in length. In contrast, all live feed organisms, including Artemia, yielded superior survival and growth compared to exclusively dry feed diets. Combining alternative live feed sources such as microalgae and different zooplankton organisms resulted in higher survival rates.
Growth of pike-perch larvae recorded for successive experiments increased and a specific growth rate (SGR) of 5.6% per day was recorded for the last experiment performed under semi-commercial conditions.
Survival rates also increased for successive experiments and values up to 94% were reached under green water conditions and continuous live feed supply.
The amount and composition of fatty acids changed during the early larval stages. When somatic growth was detectable the absolute amounts of fatty acids increased. This was due to an increase of individual cells and each cell was equipped with the least amount of fatty acids required for normal development. Nevertheless, the amount of fatty acids on a dry matter basis stabilized at a certain level.
What was learned
The size, biochemical composition, and concentration of live-feed organisms are crucial for the first feed of pike-perch larvae. Pike-perch larvae start feeding at day two post-hatch, even though their yolk sacks had not been depleted. Therefore the larvae should be presented with live feed as soon as possible after hatching.
Newly hatched larvae were transferred into green water containing large numbers of zooplankton organisms immediately after hatching. In this manner, the larvae could get accustomed to the swimming behaviour of their feed organisms, essential conditioning for future feeding success. Given a sufficiently high density of zooplankton organisms, larval survival rates significantly exceeded typical recorded values.
The authors maintain that exclusive Artemia diets foster only the survival of larger larvae; smaller larvae are unable to swallow the Artemia nauplii. Dry feeds also cause low survival – and growth rates – probably because the larvae are unable to digest the feeds, plus their high nutrient content leads to rapid lowering of water quality and larval survival rates.
Fatty acid profiles
The fatty acid profiles of larvae cultured during the experiments revealed some interesting facts. When feed levels were insufficient larval fatty acid profiles showed two remarkable features: these were dramatic reductions in the amount of fatty acids coincident with the depletion of the yolk sack and oil droplet. Physiologically, these incidents caused other events, such as the cessation of somatic growth after the consumption of the yolk sack, even though growth of eyes and mouth could still be detected. Shortly after the depletion of the oil droplet the larvae died.
Supplying pike-perch larvae with enough feed of the right size and biochemical composition under green water conditions leads to increased survival rates and satisfactory growth rates.
— Adrian Bischoff