Nitrite is a potential problem in aquatic environments. Freshwater fish actively take up nitrite across the gills, leading to high internal concentrations. Seawater fish are less susceptible but do take up nitrite across intestine and gills. Nitrite has multiple physiological effects. Its uptake is at the expense of chloride, leading to chloride depletion. Nitrite also activates efflux of potassium from skeletal muscle and erythrocytes, disturbing intracellular and extracellular K(+) levels. Nitrite transfer across the erythrocytic membrane leads to oxidation of haemoglobin to methaemoglobin (metHb), compromising blood O(2) transport. Other haem proteins are also oxidised. Hyperventilation is observed, and eventually tissue O(2) shortage becomes reflected in elevated lactate concentrations. Heart rate increases rapidly, before any significant elevations in metHb or extracellular potassium occur. This suggests nitrite-induced vasodilation (possibly via nitric oxide generated from nitrite) that is countered by increased cardiac pumping to re-establish blood pressure. Nitrite can form and/or mimic nitric oxide and thereby interfere with processes regulated by this local hormone. Steroid hormone synthesis may be inhibited, while changes in ammonia and urea levels and excretion rates reflect an influence of nitrite on nitrogen metabolism. Detoxification of nitrite occurs via endogenous oxidation to nitrate, and elimination of nitrite takes place both via gills and urine. The susceptibility to nitrite varies between species and in some cases also within species. Rainbow trout fall into two groups with regard to susceptibility and physiological response. These two groups are not related to sex but show significant different nitrite uptake rates.

Nitrite disrupts multiple physiological functions in aquatic animals.

The economics of most modern aquaculture operations require that animals be cultured at high densities. A consequence of high‐density aquaculture is the increased probability that the animals will be exposed to elevated concentrations of nitrogenous wastes, particularly ammonia and nitrite. Ammonia toxicity is dependent primarily on the concentration of ammonia and the pH of the environment High concentrations of ammonia will decrease survival, inhibit growth, and cause a variety of physiological dysfunctions. Nitrite toxicity is dependent on the concentration of nitrite and, in many species, the concentration of chloride in the water. Nitrite toxicosis is characterized by reduced survival and growth, methemoglobinemia, and other physiological dysfunctions. Both ammonia and nitrite act as stressors in that they stimulate the release of corticosteroid hormones into circulation. Elevated concentrations of circulating corticosteroids have been linked to impaired immune function and, consequently, de...

Toxicity of nitrogenous wastes to aquaculture animals

The toxicological and environmental significance of N-containing effluents discharged to seawater from fish farms is difficult to establish. Environmental quality standards for N compounds in seawater are hard to derive in the context of aquaculture because the toxicity of NH3 and NO2- to marine fish is poorly understood. Furthermore, details of aquacultural effluents are not routinely reported. Marine teleosts excrete N via the gills, skin and faeces, but do not have the metabolic capacity to cause breaches in discharge consent conditions. The most likely cause of discharge consent breaches will be poor farming practice. Nitrogen pollution will arise from food wastage, poor N absorption, and N retention. It is estimated that 52–95% of any N added to the culture system as food will ultimately pollute the environment.

Nitrogen pollution in mariculture: toxicity and excretion of nitrogenous compounds by marine fish

The anatomy of the gastrointestinal tract of fishes follows the same basic plan as in other vertebrates with a degree of variation reflected in phylogeny and ontogeny, diet, and environment. Morphological studies provide us with a context for understanding the spatial organization and relationship of physiological and biochemical data, and the molecular machinery that is rapidly being elucidated through molecular techniques directed at the genome, transcriptome, and proteome. Morphological data are also key to understanding fish nutrition in ecology and aquaculture, and during development as well as mechanisms for physiological adaptations to a changing environment. A number of the multifunctional roles of the fish gut that are discussed in the following chapters (e.g. respiration and ion regulation) of this volume incorporate distinctive morphological features that will be highlighted in this chapter. The stomach represents a significant vertebrate innovation and is also the most highly diversified region of the gut, yet it has undergone a number of independent secondary losses with stomachless fishes accounting for approximately 20% of species. The diversity of fishes makes the identification of the causes of stomach loss elusive. An updated survey is given.

Morphological diversity of the gastrointestinal tract in fishes

Aquaponics is the combination of aquaculture (fish) and hydroponic cultivation of plants. This review examines fish welfare in relation to rearing water quality, fish feed and fish waste and faeces to develop a sustainable aquaponic system where the co-cultured organisms, fish, bacteria in biofilters and plants, should be considered holistically in all aquaponics operations. Water quality parameters are the primary environmental consideration for optimizing aquaponic production and for directly impacting fish welfare/health issues and plant needs. In aquaponic systems, the uptake of nutrients should be maximised for the healthy production of the plant biomass but without neglecting the best welfare conditions for the fish in terms of water quality. Measures to reduce the risks of the introduction or spread of diseases or infection and to increase biosecurity in aquaponics are also important. In addition, the possible impacts of allelochemicals, i.e., chemicals released by the plants, should be taken into account. Moreover, the effect of diet digestibility, faeces particle size and settling ratio on water quality should be carefully considered. As available information is very limited, research should be undertaken to better elucidate the relationship between appropriate levels of minerals needed by plants, and fish metabolism, health and welfare. It remains to be investigated whether and to what extent the concentrations of suspended solids that can be found in aquaponic systems can compromise the health of fish. Water quality, which directly affects fish health and well-being, is the key factor to be considered in all aquaponic systems.

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Fish Welfare in Aquaponic Systems : Its Relation to Water Quality with an Emphasis on Feed and Faeces : A Review

1.

The relationship between rate of chloride uptake and external chloride concentration was investigated in Rainbow trout,Salmo gairdneri, and Perch,Perca fluviatilis. The relationship between nitrite uptake and external nitrite and the inhibition of chloride uptake by nitrite was also investigated. Nitrite tolerance tests were performed on a variety of freshwater animals, including Carp,Cyprinus carpio, Tench,Tinca tinca, Pike,Esox lucius, Eel,Anguilla anguilla, and tadpoles,Rana temporaria.




2.

The chloride uptake mechanism is saturable, with maximum uptake rates of 368 μMh−1kg−1 and 429 μMh−1kg−1 for the trout and perch, respectively. The half saturation value (Km, the affinity constant) is 159 μMl−1 for trout and 333 μMl−1 for perch.




3.

Net nitrite transport was determined in trout, net movement being into the fish against a concentration gradient, with a maximum uptake rate of 281 μMh−1kg−1; theKm is 198 μMl−1. This suggests that nitrite enters the fish via an active uptake process.




4.

The data suggest that nitrite is a simple competitive inhibitor of active chloride uptake in both trout and perch. Trout are less tolerant of nitrite than perch (24-h LC50 values are 0.7 mMl−1 for trout and 1.2 mM l−1 for perch) and also have a greater affinity for nitrite.




5.

The spectrum of nitrite sensitivity seen in freshwater animals is discussed in relation to chloride uptake kinetics. These data support the hypothesis that nitrite uptake is an active process and furthermore uptake is linked quantitatively with chloride uptake, suggesting that chloride and nitrite enter the fish via the same route.

E. M. Williams

Papers

Chloride uptake in freshwater teleosts and its relationship to nitrite uptake and toxicity