Description: The effects of a floating fish farm in Kiel Fjord, Western Baltic, have been studied in the summer 1991 by underwater video, sediment and benthos samples. Significant alterations of the benthos and sediment geochemistry as compared to control stations were documented. The sediment under the farm is anoxic, organically enriched (1.5 to 3.5 fold), covered by sulfur bacteria, and almost free of benthic macro fauna. Rates of decay of organic carbon and oxygen uptake (derived from porewater profiles) are high and account for 100–150 mmol m‐2 d‐1 in summer.

Description: Effects of moderate hypoxia and oscillating oxygen conditions on growth of European sea bass (Dicentrarchus labrax L.) were investigated. Groups of four to six sea bass (initial weights 40–90 g) were exposed to one of three oxygen regimes (40% air saturation; oscillations between 40–86% with a period of 770 min; 86% as a control) at 22°C and a salinity of 37 for 1 month. All fish survived and gained weight, but relative to the controls, the sea bass exposed to hypoxic conditions consumed significantly less food, exhibited a reduced growth, and had a lower condition factor. Oscillating groups were intermediate, and not statistically distinguishable from either normoxic or hypoxic treatments. Feed conversion efficiency and variation in body size were not significantly affected by oxygen conditions. Growth was correlated with feed intake, suggesting that reduced growth under moderate hypoxic or oscillating oxygen conditions is primarily due to reduced appetite and not a consequence of a decrease in feed conversion efficiency.

Description: The early life-history of Chinese rock carp Procypris rabaudi was investigated during a 56-day rearing period: 318 artificially propagated P. rabaudi larvae were reared throughout metamorphosis in a small-scale recirculation system (345 L water volume, 10 × 18 L rearing tanks, 150 L storage and filter compartment with bioballs, 20–30 larvae L−1) at the Institute of Hydrobiology, Wuhan, Hubei Province, China. The newly hatched larvae had an initial total length of 8.93 ± 0.35 mm SD (n = 10) at 3 days post-hatch and reached an average total length of 33.29 mm (±1.88 mm SD, n = 10) 56 days after hatching. Length increment averaged 0.45 mm day−1, resulting in a mean growth of 24.4 mm within the 56-day period. High mortality rates of up to 92% derived from an introduced fungus infection and subsequent treatment stress with malachite green. Our results indicate that Chinese rock carp can be raised successfully from artificially fertilized eggs. We therefore assume this species to be a candidate for commercial aquaculture.

Description: The monthly variation in reproductive cycle, condition factor (fatness), gonad index (GI), ovarian egg diameter and biochemical composition [RNA, DNA content and their ratio RNA/DNA (R/D)] were compared to determine the spawning season of the purplish Washington clam, Saxidomus purpuratus, collected from the west sea of Taean, Korea. All these factors were correlated to the spawning season from May to October. Monthly variation and concentration of the R/D ratio especially represented indicative information about the processes of gonad development and spawning patterns. The increases in R/D ratio in the ovaries corresponded with increases in ovarian egg diameters and the GI during the spawning season. The peak of R/D and RNA content in females was a good indicator of sexual maturation and the DNA content in males. During winter between November and January, the value of fatness, GI, egg size, RNA and DNA content were lower than in other seasons, indicating depletion of energy reserves and lower metabolic demands because of colder temperatures.

Description: The aim of this study was to evaluate the potential of a compound diet as a live prey substitute for feeding European sea bass larvae (Dicentrarchus labrax L.). The effect of a commercial diet (Nippai ML feed) and live prey (Artemia nauplii) on tryptic enzyme activity, protein content, growth (standard length) and survival rates of sea bass larvae were tested during a 27-day rearing experiment. Sea bass larvae were divided into two groups. The live food group (control group) was fed exclusively on newly hatched Artemia nauplii (Inve AF grade), the test group was fed exclusively with the compound diet from day 15 onwards. As trypsin has been demonstrated to be a useful indicator for evaluating digestibility of food and the nutritional condition of fish larvae, individual tryptic enzyme activity was determined in both feeding groups. Larvae older than 14 days after hatching and fed on live food showed a significantly higher tryptic enzyme activity than larvae fed the compound diet. A similar relationship between tryptic activity and standard length in both test groups was detected only in small larvae (standard length 〈 7 mm). The usefulness of proteolytic enzyme activity measurements in larval fish research, as well as its use in aquaculture nutrition research, was confirmed. Protein content, increase in length and survival rates of the sea bass larvae were additionally determined in order to evaluate an influence on the diet. The protein content of larvae fed the Artemia nauplii was higher and the growth of larvae fed the compound diet was reduced. Larval mortality was not affected by the diet given.

Description: The respiration of coho salmon, Oncorhynchus kisutch, weighing between 20 and 45 g was measured at gradually declining oxygen levels and at temperatures ranging between 14 and 17 °C. The maximum and minimum oxygen concentrations tested were 235 and 41 μmol/L, respectively. Respiration rates were measured for 24 h at 235 μmol/L before the oxygen concentration was lowered stepwise to 157 and 81 μmol/L. In one single trial, the oxygen level was lowered to 66, 53, and 41 μmol/L. Respiration was highly variable in time. Peak activities always occurred during the night. The standard metabolic rate at normoxic conditions was estimated to be around 4 μmol oxygen/g/h. The highest rates reached values close to 15 μmol oxygen/g/h. At reduced oxygen levels the standard oxygen demand slightly increased to 4.5 μmol oxygen/g/h, indicating a higher demand for vital metabolic functions. Due to the decrease of swimming activity, the maximum oxygen uptake rates dropped to 〈 8 μmol oxygen/g/h below 81 μmol/L oxygen concentration. Under long-term conditions, physiological and behavioural adaptations play an important role for survival and need to be considered for the design and operation of fish farm facilities.

Description: Aquaculture in Germany is a small industry, practiced only in a few specifically suited areas. Marine finfish farming is almost nonexistent but mussel farming is a steady activity with a notable level of production. However, this industry is presently very restricted with no hope for expansion as regulations are stringent and new licences in both fresh and marine waters are extremely difficult to obtain. In addition to State-specific conditions within the Federation, the complexity of the regulatory framework is exemplified by focusing on some key aspects for finfish farming and by specific aspects of mussel farming activities in the Schleswig-Holstein Wadden Sea where environmental protection measures dictate the level of control, mainly as a result of the recent establishment of the National Wadden Sea Park.

Description: This study determined the effects of two test diets — a dry-phytoplankton and a trout-fry feed — and a control diet (Artemia nauplii) on tryptic activity, growth and survival rates during early life stages of Nile tilapia (Oreochromis niloticus) larvae. In addition, during a second experimental series, the interaction between trypsin and CCK (cholecystokinin) secretion was monitored in larvae fed with a PHA (phytohemagglutinin, a protein extract from the red kidney bean) enriched commercial trout-diet and compared with the data resulting from the use of the same but non-enriched feed. Subgroups were taken from the experimental units and kept under starvation. Oreochromis niloticus was chosen as a model species, since the larvae are able to intake artificial diets by the time of first feeding, and thus featuring the experiments with a manipulated micro-diet. The results demonstrated that larval mortality and growth are affected by the diet given and this was mostly observed in the group fed on dry-phytoplankton (12.2% mortality, 1.45 mg/d; control group: 2.9% mortality, 3.19 mg/d). The same larval group showed also a higher tryptic activity compared with all the other groups, which in combination with the bad survival and poor growth performance gives evidence for inadequate nutritional quality of the dry-phytoplankton feed for larvae aged more than two weeks after hatching. Every other feeding group showed good growth rates (trout-fry feed: 3.04–3.19 mg/d, with PHA enriched trout-fry feed: 2.85 mg/d), similar to the larvae fed with live prey (3.19–3.35 mg/d). A reduction of tryptic activity characterised the starvation process. These results confirm the usefulness of monitoring the individual tryptic activity as an indicator for evaluating the quality of a diet and the nutritional condition of fish larvae, but also the necessity of combining data of tryptic activity with growth and survival data for a correct interpretation. An interaction between trypsin and CCK secretion was also confirmed with this experimental approach, since induction and reduction of tryptic activity followed a reverse pattern compared with the concentration of CCK.

Description: Water and sediment quality conditions in and around a cage culture unit located in Wismar Bay (Western Baltic Sea, Germany) were investigated during summer 1994 using bacterial group indicators (Enterobacteriaceae, saprophytic bacteria and Vibrio, determined on selective media). Fish were fed once daily with extruded commercial pellets. Over a period of 8 days samples were taken before and after feeding at different sampling points close to or at the cages. The results showed that a drastic (up to 10 times) increase in different bacterial groups occurred inside the cages 10 min after feeding as well as occasionally in the surrounding water. Additional electron-microscopic observations of the bacterial biofilm on the cage nets revealed that a decrease (in comparison with the original number) in total bacteria on the net materials of 9.5 up to 42.8% occurred after feeding. It was concluded that water mixing caused by frenzied swimming just before and during feeding caused a release of bacteria which were growing vigorously on the surfaces of the cage system. Since the pathogenity and hydrophobicity of bacteria are closely related, this process of redistribution of bacteria into the water column could constitute an important stress factor for fish, which could probably also have a negative effect on fish production.