Fundamentals of Aquatic Veterinary Medicine. Группа авторов

      Acknowledgement

      Special thanks to Dr. Craig Harms for the input on exploratory surgery in fish in chapter 6.

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1 Aquatic Environment and Life Support Systems

       Sherry Kasper, Olanike K. Adeyemo, Trista Becker, David Scarfe, and Julius Tepper

      Learning Objectives

      Students should demonstrate knowledge of the elements of life support systems and water found within these systems.

1.1 Introduction

      For veterinarians to adequately evaluate the health or disease problems of aquatic animals, it is imperative that they understand the impact of, and assess the conditions in which the animal lives. Like terrestrial animals, the health of aquatic species is impacted by their environment. Suboptimal conditions can result in non‐infectious diseases or create situations favorable for opportunistic pathogens, which can lead to decreased growth, increased morbidity and clinical disease. It is therefore imperative to include an assessment of water quality and a history of any changes in the environment as part of a clinical evaluation of any health problem.

      Fish health management is a term used in aquaculture to describe management practices designed to prevent fish disease. Once fish get sick it can be difficult to salvage them. Successful fish health management begins with prevention of disease rather than treatment, accomplished through, among other considerations, good water quality management. Without this foundation it is impossible to prevent outbreaks of opportunistic diseases. Fish are constantly bathed in potential pathogens, including bacteria, fungi, and parasites. Suboptimal water quality generally associated with stressful conditions allows potential pathogens to cause disease.

      The biotic integrity of an ecological system is often reflected by the health of organisms that reside in that system. Proper management of the aquatic environment therefore offers optimum environmental conditions for the growth and better health of the cultivated fish. It also strengthens the defense mechanism of the body to fight against invading disease‐producing organisms. Eradicating any predatory or invasive fish species, disinfecting the pond, selecting quality and healthy seed for stocking, maintaining proper species ratios and stocking density, regulating water quality, as well as feeding and handling fish properly, are the various steps of these management measures.

With a very large number of cultured aquatic species, and hence different water systems in which they may live, the task of evaluating optimal conditions for every species may seem daunting. However, if considered on an ecosystem level for those that thrive naturally in freshwater, brackish or marine aquatic ecosystems, and evaluating these as life‐support systems for different taxa, the task is simplified. Furthermore, many biotic and abiotic physicochemical features of fresh, brackish or marine water bodies are similar, even for very different taxa.

      This chapter provides general information about basic features of aquatic environments and life‐support systems important to the health of finfish, amphibians, mollusks, crustaceans and invertebrates that veterinarians might encounter. As aquatic mammals, birds and other homeothermic species are generally covered in veterinary curricula, they are not addressed here.

1.2 The Life‐Support System in Aquaculture and Ornamental Fish Care

      Aquaculture is the farming of aquatic organisms including fish, mollusks, crustaceans and aquatic plants, with some sort of intervention in the rearing process to enhance production, such as regular stocking, feeding, and protection from predators. Farming also implies individual or corporate ownership of the stock being cultivated. Aquatic ecosystems provide the basis for aquatic production. The worldwide practice of aquaculture runs the gamut from low‐technology extensive methods to highly intensive systems. Although used for housing and maintaining ornamental fish, similar aquasystems are found in large public aquaria and private homes and are discussed in this chapter. The term ‘life‐support system’ is understood as a complete system that satisfies the physiological needs of an aquatic organism in question. Diagrams and photographs of examples of life‐support systems can be found in Chapter 3.

      Managed ecosystems form the basis for aquaculture production, which has been widely offered as a means of complementing and ultimately supplementing traditional fisheries. Here, unlike fisheries, inputs, production processes and quality of output can be at least partially controlled, and ownership, care and environmental responsibility might be more easily established. By removing natural constraints to survival and productivity, and by husbandry and management, production need be limited only by availability of simple inputs such as land, water, seed (hatchling, spawn, fry or fingerling), fertilizers and feeds. The design of a life‐support system is dependent on the type of aquatic system in question, being either a flow‐through system or a closed system. In a flow‐through system, the main elements include a spigot or waterfall that is adjustable for the amount of water flow necessary, a drain that is usually positioned at the opposite end of the system, potential for increased aeration, and, possibly, temperature controls.

      A closed system may be a pond, tank or aquarium. In a pond, the most important elements are circulation and aeration, and it may also include a filtration system. Tanks and aquaria usually require a filtration system, aeration, and often temperature control. The source of the water is important when deciding on the type of aquatic system and the type of life‐support system. A large well, river or lake is necessary for a flow‐through system because of the high volume of water that is needed to maintain proper water quality. If city water is being used in a system, it is important to have a reservoir to dechlorinate any new water entering the system. In recirculating systems, water circulation and purification is accomplished with the use of powered pumps and filtration systems. The type of equipment used will depend on the type of fish, number of fish and the expected growth rate of those fish.

In aquaculture, high‐intensity growth of fish leads to higher ammonia levels and waste production. The output of the pump should allow for a flow of the full volume of water in the system to pass through the filter every two to three hours. This is referred to as the “turnover time”. It is calculated by dividing the water volume in the system by the volume per time rating of the circulation, measured at the return (the volume/time is variously described in gallons/minute or hour as GPM, GPH or metric m³/hour). In tanks and ponds, flow rates are very variable, from less than five minutes in the smallest aquaria to more than 10 hours in larger ponds. Submersible pumps with low maintenance sealed impellors may be used in small ponds and aquaria, but larger aquasystems typically use external pumps. The filtration system should contain biological media with an appropriate surface area to allow for detoxification of ammonia based on the number of fish. On average, 1 m² of active surface area can process 1 gram of