Lipids and their constituent fatty acids are, along with proteins, the major organic constituents of fish, and they play major roles as sources of metabolic energy for growth including reproduction and movement, including migration. Furthermore, the fatty acids of fish lipids are rich in ω3 long chain, highly unsaturated fatty acids (n-3 HUFA) that have particularly important roles in animal nutrition, including fish and human nutrition, reflecting their roles in critical physiological processes. Indeed, fish are the most important food source of these vital nutrients for man. Thus, the longstanding interest in fish lipids stems from their abundance and their uniqueness. This review attempts to summarize our present state of knowledge of various aspects of the basic biochemistry, metabolism, and functions of fatty acids, and the lipids they constitute part of, in fish, seeking where possible to relate that understanding as much to fish in their natural environment as to farmed fish. In doing so, it highli...

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Metabolism and Functions of Lipids and Fatty Acids in Teleost Fish

Oogenesis in teleosts: How fish eggs are formed

In this review I show that the '3/4-power scaling law' of metabolic rate is not universal, either within or among animal species. Significant variation in the scaling of metabolic rate with body mass is described mainly for animals, but also for unicells and plants. Much of this variation, which can be related to taxonomic, physiological, and/or environmental differences, is not adequately explained by existing theoretical models, which are also reviewed. As a result, synthetic explanatory schemes based on multiple boundary constraints and on the scaling of multiple energy-using processes are advocated. It is also stressed that a complete understanding of metabolic scaling will require the identification of both proximate (functional) and ultimate (evolutionary) causes. Four major types of intraspecific metabolic scaling with body mass are recognized [based on the power function R=aMb, where R is respiration (metabolic) rate, a is a constant, M is body mass, and b is the scaling exponent]: Type I: linear, negatively allometric (b 1) to one or two later phases of negative allometry (b<1). Ontogenetic changes in the metabolic intensity of four component processes (i.e. growth, reproduction, locomotion, and heat production) appear to be important in these different patterns of metabolic scaling. These changes may, in turn, be shaped by age (size)-specific patterns of mortality. In addition, major differences in interspecific metabolic scaling are described, especially with respect to mode of temperature regulation, body-size range, and activity level. A 'metabolic-level boundaries hypothesis' focusing on two major constraints (surface-area limits on resource/waste exchange processes and mass/volume limits on power production) can explain much, but not all of this variation. My analysis indicates that further empirical and theoretical work is needed to understand fully the physiological and ecological bases for the considerable variation in metabolic scaling that is observed both within and among species. Recommended approaches for doing this are discussed. I conclude that the scaling of metabolism is not the simple result of a physical law, but rather appears to be the more complex result of diverse adaptations evolved in the context of both physico-chemical and ecological constraints.

Beyond the '3/4-power law': variation in the intra- and interspecific scaling of metabolic rate in animals.

Factors affecting egg quality are determined by the intrinsic properties of the egg itself and the environment in which the egg is fertilized and subsequently incubated. Egg quality in fish is very variable. Some of the factors affecting egg quality in fish are known, but many (probably most) are unknown. Components that do affect egg quality include the endocrine status of the female during the growth of the oocyte in the ovary, the diet of the broodfish, the complement of nutrients deposited into the oocyte, and the physiochemical conditions of the water in which the eggs are subsequently incubated. In captive broodfish, the husbandry practices to which fish are subjected are probably a major contributory factor affecting egg quality. Our knowledge of the genetic influences on egg quality is very limited indeed. We know that parental genes strongly influence both fecundity and egg quality, but almost nothing is known about gene expression and/or mRNA translation in fish oocytes/embryos. This is surprising because the products synthesized in ovoand the mechanisms controlling their expression are likely to play a central role in determining egg quality. The genetic mechanisms underpinning oocyte and embryo growth and development are a priority for research

Egg quality in fish: what makes a good egg?

In the context of future scenarios of progressive accumulation of anthropogenic CO2 in marine surface waters, the present study addresses the effects of long-term hypercapnia on a Mediterranean bivalve, Mytilus galloprovincialis. Sea-water pH was lowered to a value of 7.3 by equilibration with elevated CO2 levels. This is close to the maximum pH drop expected in marine sur- face waters during atmospheric CO2 accumulation. Intra- and extracellular acid-base parameters as well as changes in metabolic rate and growth were studied under both normocapnia and hypercap- nia. Long-term hypercapnia caused a permanent reduction in haemolymph pH. To limit the degree of acidosis, mussels increased haemolymph bicarbonate levels, which are derived mainly from the dissolution of shell CaCO3. Intracellular pH in various tissues was at least partly compensated; no deviation from control values occurred during long-term measurements in whole soft-body tissues. The rate of oxygen consumption fell significantly, indicating a lower metabolic rate. In line with pre- vious reports, a close correlation became evident between the reduction in extracellular pH and the reduction in metabolic rate of mussels during hypercapnia. Analysis of frequency histograms of growth rate revealed that hypercapnia caused a slowing of growth, possibly related to the reduction in metabolic rate and the dissolution of shell CaCO3 as a result of extracellular acidosis. In addition, increased nitrogen excretion by hypercapnic mussels indicates the net degradation of protein, thereby contributing to growth reduction. The results obtained in the present study strongly indicate that a reduction in sea-water pH to 7.3 may be fatal for the mussels. They also confirm previous obser- vations that a reduction in sea-water pH below 7.5 is harmful for shelled molluscs.

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Effects of long-term moderate hypercapnia on acid-base balance and growth rate in marine mussels (Mytilus galloprovincialis)

Biochemical composition of copepods for evaluation of feed quality in production of juvenile marine fish

First feeding of marine fish larvae: Are free amino acids the source of energy?

The simultaneous effect of temperature (5, 7, 10 and 13°C) and light on the rates of oxy- gen consumption and ammonia excretion of larval and early juvenile Atlantic cod Gadus morhua was examined Larvae increased their mean dry body mass by 2000 times within 48 d Instantaneous growth rate exceeded 30% d -1 towards the end of the study period, and proportionality of growth fol- lowed a triphasic pattern, during which body water content significantly declined but no inflection could be detected in the metabolic exponents Data were rigorously tested via Model-I (least squares) and Model-II (geometric mean) regression techniques, and the aerobic metabolic rate was found to scale allometrically with both dry and wet body mass The metabolic exponent was not affected by increasing temperature, but was significantly decreased by the presence of light (b = 088 to 089 for light-adapted larvae; b = 090 to 091 for dark-adapted larvae) The effect of light on small larvae (4 to 7 mm standard length, SL) caused a 30 to 40% increase in metabolic rate, while no effect was observed in larger juveniles (40 to 60 mm SL) Acute temperature acclimation of Atlantic cod of 4 to 60 mm SL (004 to 350 mg dry mass) demonstrated normal thermal sensitivity with Q10 values of 24 for dark-adapted larvae and 26 for light-adapted larvae Rates of ammonia excretion also scaled allo- metrically with wet and dry body mass and showed greater variability in dark-adapted compared to light-adapted larvae Comparison of the molar rates of ammonia excretion and oxygen consumption revealed that Atlantic cod larvae have a high reliance on amino acids as fuel for energy dissipation With lipids as the assumed co-substrate, amino acids were estimated to account for 70 to 95% of total substrate oxidation for larvae up to 7 mm SL (first 3 to 4 wk of post-hatch development) Beyond 7 mm SL, the reliance on amino acids as fuel began to decline, but even in juveniles of 40 to 60 mm SL, amino acids still represented the dominant source of fuel For juveniles of between 10 and 20 mm SL, both the rates of oxygen consumption and ammonia excretion remained unaffected by the presence of food in the gut For short-term fasted juveniles (35 to 60 mm SL), however, a substantial decline in the rate of ammonia excretion was observed This indicates that during short-term fasting (8 to 12 h) early juvenile Atlantic cod conserve amino acids, rather than funneling them into the tricarboxylic acid cycle

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Fuel and metabolic scaling during the early life stages of Atlantic cod Gadus morhua

Investigations during the last decade have shown that free amino acids (FAA) abound in marine pelagic fish eggs The concentration in the egg (about 150 mM) is well above what is typically found in adult teleostean tissues FAA in fish eggs were initially studied in order to evaluate their role as osmolytes More recently, however, FAA have been implicated as a fuel in the energy metabolism of developing marine fish eggs and larvae Further aggregation of data verifies that pelagic eggs of marine teleosts generally have a high content of FAA The large pool of FAA is almost exclusively contained within the yolk‐sac compartment The relative composition of the FAA pool shows little interspecific variation The FAA pool is depleted during development and reaches low levels at first feeding The observed decline in the FAA content during development is not due to a loss of FAA to the ambient water, but rather results from endogenous metabolic turnover FAA seem to be utilized to varying extents for 

Metabolic aspects of free amino acids in developing marine fish eggs and larvae

Developing eggs and larvae of laboratory-reared gilthead sea bream (Sparus aurata) maintained in filtered seawater (40 ppt) at 18°C, were measured for oxygen uptake, ammonia excretion, contents of free amino acids (FAA), protein, fatty acids (FA) accumulated ammonia, and volumes of yolk-sac and oil globule. Absorption of the yolk coincided with the consumption of FAA and was complete ca. 100 h post-fertilisation. Amino acids from protein were mobilised for energy in the last part of the yolk-sac stage. Absorption of the oil globule occurred primarily after hatching following yolk absorption, and correlated with catabolism of the FA neutral lipids. Overall, FAA appear to be a significant energy substrate during the egg stage (60 to 70%) while FA from neutral lipids derived from the oil globule are the main metabolic fuel after hatching (80 to 90%).

Hans Jørgen Fyhn

Papers

Biochemical composition of copepods for evaluation of feed quality in production of juvenile marine fish

First feeding of marine fish larvae: Are free amino acids the source of energy?

Fuel and metabolic scaling during the early life stages of Atlantic cod Gadus morhua

Metabolic aspects of free amino acids in developing marine fish eggs and larvae

Energy metabolism during development of eggs and larvae of gilthead sea bream (Sparus aurata)