Anders Mangor-Jensen

Design and Operation of an Incubator for Yolk-Sac Larvae of Atlantic Halibut

Abstract A system design and operational procedures for rearing yolk-sac larvae of Atlantic halibut (Hippoglossus hippoglossus) were developed from 1988 to 1992 at the Austevoll Aquaculture Research Station. The objective has been to develop a method for mass rearing of yolk-sac larvae. The method involves tanks with a cylindrical upper part and a conical lower part. Water inflow at the bottom and exit through overflow sieves provide continuous exchange of water and no dead volume. Over the 5-year period, detailed operating procedures have been established. The survival of larvae through the yolk-sac stage has improved each year, and survival rates from 60 to 80% were achieved in 1992.

Digestibility of dry matter, protein, starch and lipid by cod, Gadus morhua: comparison of sampling methods

Atlantic cod were fed 11 different extruded diets spanned by a multiple simplex design. The dietary ranges of protein, fat and starch were 36-66%, 8-28% and 8-18%, respectively. All diets contained 0.1% yttrium oxide as an inert marker. After several months of feeding, faeces were sampled, either by stripping or by dissecting the last part of the intestine, which in turn was frozen and faecal matter removed while still partly frozen. No significant differences were found in average digestibility results of dry matter, protein, starch or lipid when comparing the two methods of sampling. All groups showed high digestibility of dry matter, protein and lipid, with minor variations, and only extreme feed compositions resulted in changed digestibility of dry matter and protein. A larger variation was found in apparent starch digestibility, from 60% to 90%, however, independent of dietary level when this was below 17% of dry matter, in agreement with findings for this nutrient as concerns other carnivorous fish. No difference was detected for any of the compared digestibility coefficients when evaluating the four centre points of the multiple simplex design. In conclusion, no difference between methods gives the opportunity to freely choose sampling method, and either stripping or dissection of faecal matter can be used to determine apparent digestibility values in cod.

Copepods enhance nutritional status, growth and development in Atlantic cod (Gadus morhua L.) larvae — can we identify the underlying factors?

The current commercial production protocols for Atlantic cod depend on enriched rotifers and Artemia during first-feeding, but development and growth remain inferior to fish fed natural zooplankton. Two experiments were conducted in order to identify the underlying factors for this phenomenon. In the first experiment (Exp-1), groups of cod larvae were fed either (a) natural zooplankton, mainly copepods, increasing the size of prey as the larvae grew or (b) enriched rotifers followed by Artemia (the intensive group). In the second experiment (Exp-2), two groups of larvae were fed as in Exp-1, while a third group was fed copepod nauplii (approximately the size of rotifers) throughout the larval stage. In both experiments, growth was not significantly different between the groups during the first three weeks after hatching, but from the last part of the rotifer feeding period and onwards, the growth of the larvae fed copepods was higher than that of the intensive group. In Exp-2, the growth was similar between the two copepod groups during the expeimental period, indicating that nutrient composition, not prey size caused the better growth on copepods. Analyses of the prey showed that total fatty acid composition and the ratio of phospholipids to total lipids was slightly different in the prey organisms, and that protein, taurine, astaxanthin and zinc were lower on a dry weight basis in rotifers than in copepods. Other measured nutrients as DHA, all analysed vitamins, manganese, copper and selenium were similar or higher in the rotifers. When compared to the present knowledge on nutrient requirements, protein and taurine appeared to be the most likely limiting nutrients for growth in cod larvae fed rotifers and Artemia. Larvae fed rotifers/Artemia had a higher whole body lipid content than larvae fed copepods at the end of the experiment (stage 5) after the fish had been fed the same formulated diet for approximately 2 weeks.