The Free Online Aquaculture Dictionary



see ultra violet


Everywhere at the same time


See Ulcerative dermal necrosis 


A lesion in the skin or mucous membrane. Disintegration and necrosis of the tissues. Often caused as a direct result of an infection or a lesion caused by physical damage that has failed to heal and become infected.

Ulcerative dermal necrosis

(U.D.N.) Disease affecting primarily salmonids at spawning time. Shows itself in the form of ulcers, especially on the head area. Ulcers become quickly infected by Saprolegnia, which often causes the diagnosis to be a fungal disease. The fungus is however merely opportunistic, and not the cause of the disease. Cause unknown. Not generally a problem in farmed fish, only wild fish as they enter freshwater to spawn.


High frequency sounds used by some marine mammals to communicate. Can also be used as a sterilisation method (used for some medical instruments).  


Type of light consisting of a specific band of wavelengths. In the same way that there are different types of visual light (i.e. different colours and shades of those colours), there are different types of Ultra violet light, it's just that the human eye is incapable of seeing it. The bulbs used in ultra violet systems are designed to produce a light of a specific wavelength (around 254 nanometers), to ensure maximum take up of the light, by the DNA in the living organisms. Ultra Violet (or UV) light has been used for disinfection of water since about 1910. The effectiveness of the light depends on a number of factors including bulb wattage, age, cleanliness, the distance between the bulb and the organism you are trying to kill, what the organism you are trying to kill is, and the duration and intensity of light that the organism is exposed to and the clarity of the water. The way UV works is similar to the things that happen when we sit out in the sun. The UV light in the suns rays, kills off the DNA in the outer layers of our skin, causing the cells to die off. As the cells are dead, the skin sheds them...hence we peel. If we were exposed to much greater amounts of UV, many more cells would die off….and in the would we! As a rough guide water borne algae and bacteria require a dose rate of 15,000 - 30,000 mW-sec/cm2, whereas protozoa require doses in the region of 45,000 mW-sec/cm2. The basic rule (with the exception of some viruses) is that the larger the pathogen, the higher the dose required to achieve a high kill rate. Remember, if you are using UV on water where turbidity may be a problem (as in some recirculation systems) it is better to err on the safe side and order a unit with a higher dose rate, than would be used in clean water. No table for disinfection rates is given as there is so much conflicting information. Care should be taken when reading papers on the effective dose rates, as the methods used in the laboratory are often very different from those in commercial aquaculture.


Single celled.

Unionised Ammonia


The toxic fraction of ammonia. Ammonia is present in two forms in water, the higher the pH and temperature, the higher the percentage of the toxic fraction (un-ionised). The toxicity increases logarithmically with pH so a shift in pH from 7.2 to 7.4 will result in a greater rise of toxicity than a shift from 7.0 - 7.2. High ammonia levels in the water mean that the fish cannot excrete the ammonia from it's body (most of which is done across the gills) and so the ammonia builds up in the blood, poisoning the fish. Fish can often be observed breathing much quicker or crowding round inlets (where the ammonia concentrations are less) in an effort to get rid of the ammonia from their system. The amount of unionised ammonia can be calculated in two ways, either by using the calculation or look it up in the table (both provided above). See also biological filtration and ion exchange for methods to remove ammonia from the water.


Unplasticised Polyvinyl Chloride. The most widely used of all plastics and commonly used for pressure pipes. Rigid, suitable for above and below ground applications. Good chemical resistance, odourless, tasteless. For use with liquids and gasses with temperatures 0-60oC (for higher temperatures see C-PVC) at a wide range of operating pressures. Some poorer quality PVC can leach chemicals into that water which can build up in recirculation systems, however most modern pipe is built to specific standards (e.g. BS3505/6, ASTM D 1785, ASTM D 2241, DIN 8061/2, KIWA 49, BS4346 PART 1, DIN 8063) and as long as the pipe is rated as such, there should be no problems. Usually joined using a push fit solvent cement joint, requiring no special tools. See pipe pressure ratings, plastic welding


A process where water flows from below to above. Used to describe water flow in egg trays and filters, as well as the phenomenon where nutrient rich deep waters rise to the surface (see stratification)


Chemical used in some solutions for fertilising eggs. Used for carp (Cyprinus carpio) eggs at 3g (+ 4 g NaCl) per litre for fertilisation solution and 20g (+ 4g NaCl) per litre for dissolving solution


Collective name for a group of resins that form polyurethane's, which are used in a variety of plastics, paints and foams.

Urogenital pore (papilla)

Common external outlet from the fish for urine and eggs/milt. 


A device for the oxygenation or aeration of water. A tube or pipe is sunk into the ground and water directed down one side and up through the other side. A bubble diffuser is situated at the entrance to the tube. The cross sectional area of the downward leg of the tube is smaller than the upward leg. The downward leg is sized for a water velocity of that greater than the buoyant velocity of the bubbles so that they are carried down with the water (usually about 1.3m/sec, although this velocity is dependant on the bubble size). The upward leg is ideally sized so that the water travels up at the same rate as the bubbles. This results in a maximum contact time between the oxygen and the water and therefore a maximum transfer from the gas into the water. The depth of the tube creates increased pressure at the bottom of the tube, which accelerates oxygen absorption into the water. Undissolved gas is sometimes captured and reintroduced into the U-tube to increase the efficiency. The efficiency of a U-Tube (12m deep) is in the region of 30-50%, but when off gas recycling is included, this rises to 60 - 90%. The benefits of such a systems are the low head loss (typically less than 1-2m) and the ability to use water which is carrying particulate matter, without breaking it up (as would occur during pumping). This is somewhat offset in some areas by the cost of installing such a system if the ground conditions are not favourable for drilling. If gas flow through the U-tube exceeds 25% of the water flow, the flow will be interrupted and irregular, due to the formation of slugs of gas and temporary air locking.