Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 93

Salinity is the concentration of dissolved ions in water. The main ions in seawater are sodium and chloride, with calcium, magnesium, sulfur, potassium, and many other inorganic elements. The total salinity and the concentration of each ion affect behavior, physiology, disease dynamics, drug pharmacokinetics, and sensitivity to toxins. Since ions increase the density of water and make it easier to conduct an electric current, specific gravity and electrical conductivity can be used to indirectly measure salinity.

Marine and brackish water aquariums make use of natural or artificial seawater. Artificial seawater is typically made from dechlorinated municipal freshwater with salt mixtures added. These mixtures may be commercial (e.g. Instant Ocean®, Blacksburg, VA) or customized (Bidwell and Spotte 1985). For commercial mixes, sodium chloride alone and products with anticaking agents should be avoided. For proprietary mixes, the salts should be food‐grade or reagent‐ and analytical‐grade to reduce the risk of contaminants. Phosphates are rarely required in salt mixes for established aquariums as levels accumulate from the food fed. Bromides are less common in salt mixes now, as they do not appear to be required at the levels seen in natural seawater and can form harmful residual oxidants with ozone disinfection. Salts should be mixed in freshwater or brine in a dedicated, off‐exhibit tank and then tested, at least for salinity; salts should not be added directly to an aquarium system. Salt mixtures are often a significant part of the cost of marine aquariums.

Freshwater aquariums may use municipal, surface, or ground water. Some freshwater fish do best in low‐ion water; in these cases, ions may be removed from freshwater by deionization, reverse osmosis, or distilling.

Frequency of testing: Salinity should be assayed regularly; this often includes testing incoming water routinely or with each production batch, and testing each system every two to four weeks if stable. A full cation analysis (e.g. concentrations of Na⁺, Ca²⁺, Mg²⁺, and other ions) should be considered periodically, both as quality control for artificial salt water and in long‐established closed systems to check the ion balance and allow correction if needed. Salinity of transport water should be measured prior to acclimation to identify differences from the destination water; this may affect acclimation needs. For high‐risk fish transports, a full cation comparison is recommended prior to transport.

Sampling: Standard sampling is described in Box A2.1. Samples can be stored for a few days at room temperature or weeks when refrigerated or frozen.

Testing: The simplest test uses a refractometer or hydrometer to measure specific gravity; this measurement of density reflects total dissolved solids rather than true salinity, but gives a quick approximation of salinity. Refractometers need to be calibrated prior to each use and some require temperature correction. Handheld conductivity meters provide more accurate indirect measurements of salinity in salt water and total dissolved solids in freshwater. Ion‐exchange chromatography is used for cation analysis.

Units: Salinity in aquariums is usually measured in grams per liter (g/L), equivalent to parts per thousand (ppt). Other units include psu (practical salinity units), which are roughly equivalent to g/L values. If specific gravity is used, the results are between 1.005 and 1.030; conversion tables are available for temperature correction, if needed. In freshwater systems, electrical conductivity may be reported, typically in μSiemens per centimeter (μS/cm) or an older unit (μmho/cm); these units are equivalent.

Target values: The salinity of natural seawater is often 32–35 g/L (with a specific gravity of ~1.021–1.024 depending on temperature); many aquariums use a target range of 28–35 g/L. Freshwater salinity is typically <0.5 g/L or a conductivity of 60–2000 μS/cm (Table A2.2).

Practical considerations:

 Inappropriate salinity or rapid changes in salinity are both problematic. Effects are more severe in stenohaline species (e.g. reef species, most pelagic sharks).Box A2.1 How to Collect a Water Sample for Later AnalysisUse a clean polyethylene or polypropylene bottle with a screw cap for most sampling (borosilicate glass can be used for pesticides).Rinse the bottle in the system with the water to be tested.Select an area of concern or an area with sufficient mixing to be representative of the system.Dunk the bottle underwater and fill completely.Cap the bottle while still under water.Dry the outside of the bottle and label with system, time, and date.Refrigerate or keep the sample on ice.Most analyses should be run within a few hours; some analyses are still reliable after freezing and thawing.

 Low salinity in salt water may be seen due to freshwater contamination (e.g. rainfall, contamination from freshwater lines), insufficient salt additions, or crystallization of salts on the lids or life support system components (“salt creep”).

 High salinity in freshwater or salt water may be seen due to evaporation or contamination by salts or salt water.

 Rapid changes are more likely to cause problems than slow. Rapid changes are most common when fish are moved between water systems without acclimation or during freshwater or saltwater dips for ectoparasite diagnosis and treatment.