The combined effect of temperature and photoperiod on the growth of juvenile Atlantic halibut Hippoglossus hippoglossus (initial weight 11.6 g) was investigated under different temperature (6 °C and 12 °C) and photoperiod (continuous light and simulated natural photoperiod) combinations by studying the effects on fish weight, specific growth rate (SGR), feed consumption (F%), feed conversion (FCE) and oxygen consumption and ammonia excretion under routine and fasting conditions. SGR was greater at the higher temperature, and growth was faster under continuous light than under natural photoperiod, irrespective of temperature, SGRs being 0.83% and 0.98%, and 1.61% and 1.71% at 6 °C and 12 °C respectively. The weight data revealed a significant interaction between temperature and light, and provided evidence of a stronger growth‐enhancing effect of continuous light at the lower temperature. F% increased with temperature but was independent of photoperiod, whereas FCE was higher among fish exposed to continuous light, irrespective of temperature. Independently of photoperiod, oxygen consumption and ammonia excretion increased with temperature, but the differences in these rates in relation to photoperiod became more pronounced at the lower temperature, where continuous light gave the highest rates. Higher oxygen consumption during darkness suggested that light may suppress activity in halibut. It was proposed that reduced activity and anabolic effects of photoperiod contribute to explain the increased growth and growth efficiency in fish subjected to continuous light.

We used customized fish tanks as model fish ponds to observe grazing, swimming, and conspecific social behavior of common carp (Cyprinus carpio) under variable food-resource conditions to assess alterations in feeding niche. Different food and feeding situations were created by using only pond water or pond water plus pond bottom sediment or pond water plus pond bottom sediment and artificial feeding. All tanks were fertilized twice, prior to stocking and 2 weeks later after starting the experiment to stimulate natural food production. Common carp preferred artificial feed over benthic macroinvertebrates, followed by zooplankton. Common carp did not prefer any group of phytoplankton in any treatment. Common carp was mainly benthic in habitat choice, feeding on benthic macroinvertebrates when only plankton and benthic macroinvertebrates were available in the system. In the absence of benthic macroinvertebrates, their feeding niche shifted from near the bottom of the tanks to the water column where they spent 85% of the total time and fed principally on zooplankton. Common carp readily switched to artificial feed when available, which led to better growth. Common carp preferred to graze individually. Behavioral observations of common carp in tanks yielded new information that assists our understanding of their ecological niche. This knowledge could be potentially used to further the development of common carp aquaculture.

This study was undertaken to quantify the rate of nitrogen excretion by the cultured Japanese flounder as a function of body weight and water temperature. Measurements of the d i d rate of nitrogen excretion for the cultured flounder were made at mean water temperature of about 16,19, and 23 C. The excretion rate of nilmgen was estimated by adding the nitrogen excreted in the forms of ammonia and urea nitrogens. The diurnal pattern in the excretion rate of nitrogen was governed mainly by the feeding time. The proportions of excreted ammonia to total nitrogen were 77.9% at 16 C, 84.5% at 19 C, and 83.2% at 23 C. The average was 81.9%. The relationship between the body weight and excretion rate of nitrogen (EN, mg N / h h per h) is expressed as EN = a,.W', where a, = nitrogen excretion rate at temperature 8. The parameter a, is dependent on water temperature, such that a, = Q~. Q~, ( -~) , where am = nitrogen excretion rate at 20 C, and Qo, = temperature coefficient. The values of the kinetics parameters for the cultured flounder were found to be 7 = 0.57, am = 0.060, and Q, = 1.12.The water supply for a culture pond of the Japanese flounder Paralichthys oliva-

We compared growth efficiency, feeding consumption, metabolism, excretion, and energy allocation in populations of juvenile Atlantic halibut from Norway, Iceland, and Canada reared at low (8 C), medium (12 C, 15 C) and high (18 C) experimental temperatures. Our findings show that protein utilization and energy allocation in juvenile Atlantic halibut varies among different populations, as the Norwegian population displayed the highest protein efficiency ratio, protein production value, and energy conversion efficiency compared to the Canadian population, while the Icelandic population showed overall intermediate growth performance. The results do not conform to a simple thermal adaptation model but might represent an example of: 1) local adaptation of fish populations; or 2) countergradient variation where shorter growing season at higher latitudes is compensated with higher physiological efficiency. These findings have implications for halibut culture, particularly in selection work focusing on growth performance.

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