Extrinsic Factors Affecting Fish Health

The extrinsic or environmental factors which are known to compromise the health status of fish individually or collectively can be grouped into the following categories according to their location within the system: (1) water-associated; (2) pond-associated; (3) nutrition-associated; (4) management-associated. To some, this classification scheme may seem somewhat simplistic and/or conceptual. Nonetheless, the process of investigating the nature of a disease episode has been enhanced by identifying and quantifying the associated factors. The quantitative evaluation of the prime factors has permitted the raising of fish with a minimum of problems.

(1) Water-associated factors: Among water-related factors identified as affecting the productivity of aquaculture systems, water temperature and dissolved oxygen content have the most significant insidious effects on fish health. They are inherent in all water supplies and are subject to fluctuations to which the fish in the system must adapt. The physiological effects of the fluctuations are very broad, ranging from a change in metabolic rate to altering the susceptibility to pathogens. In this regard, the documentation of environmental changes and the occurrences of infectious and noninfectious disease processes could be an invaluable aid in predicting the likelihood of a subsequent disease episode. Examples of this are legion but not sufficient to be a widespread practice.

(2) Pond-associated factors: The primary effect here is requiring a particular fish to live in a pond configuration which does not meet its behavioral requirements. 

(3) Nutrition-associated factors: One would think that in this day and age of high quality commercial diets, nutritional problems would not occur, but they do, all too frequently.  Our studies over the past 4-5 years on production forecasting have shown the health of fish can be compromised when the fish are fed at a rate permitting less than 80% of the Allowable Growth Rate. We have found growth rate to be a very reliable indicator of the health of the population. Deviations of as little as 1% from the expected growth rate can be measured quite accurately and evaluated with a high degree of statistical validity. 

Overfeeding a population is another health-compromising situation. In this case there are frequently abnormal amounts of abdominal fat and hepatic glycogen deposits. The effects are not often seen immediately, but can be implicated in the milieu of casual factors of an unhealthy state.

(4) Management-associated factors: The primary health-threatening factors in this category emanate from exceeding one or more of the pond carrying capacities, from inadequate housekeeping practices, from inadequate record-keeping practices, and from undue physical stressors.

Intrinsic Factors Affecting Fish Health

The intrinsic or somatic factors originate within the fish itself. They are largely governed by the genetic make-up which by and large dictates the physiological and psychological responses of the fish to the extrinsic factors. 

Perhaps the major intrinsic factor which fish health managers have some control over is the generation of endogenous ammonia (NH ). We all know that free ammonia (NH ) is deleterious to the health of fish when specified limits are exceeded. The main impact, as we now understand it, is on the gill lamellae in the form of epithelial hypertrophy and hyperplasia which reduce the oxygen uptake by the fish, thus impacting the physiological well-being of the fish. The control of this process is to reduce the dietary protein, or to decrease the retention time of ammonia in the pond by increasing the water flow, or to reduce the population of fish in the pond, or to reduce the water temperature, or to decrease the pH of the system. Any one of the foregoing will serve to preserve the health of fish, within limits. A second intrinsic factor over which we can exercise some control is the healthy or chronic asymptomatic carrier of infectious agents. It is apparent from our studies with Renibacterium salmoninarum and Aeromonas salmonicida carriers that the presence of these bacteria within the fish negatively impact their growth potential and represent a measurable threat to the uninfected portion of the population.

Summary and Conclusions

Up to this point, this presentation has been rather gloomy, in my opinion. But, we must face reality - we could do better with preserving the health of our fish if we only would. To that end, I would offer the following as food for thought, and, hopefully, subsequent action by both the fish farming community and the fish health management profession, collectively. 

1. Practice preventive medicine through detection and elimination of the carrier states of bacterial and viral pathogens, through mass immunization of fish against pathogens, through implementation and maintenance of health-preserving management practices, and through implementation and enforcement of live fish transportation regulations.

2. Apply the principles of epidemiology to investigations of disease occurrences.  

3. Maintain open lines of communication among all facets of the aquaculture community.

Gill Diseases of Fish
By:  George W. Klontz, D.V.M.

Introduction

The gill diseases of fish under confined farm ponds or raceways, and free-living conditions are very complex and, in many cases, not well understood from epidemiological and etiological standpoints. This group of diseases, in the opinion of many, is one of the major production-limiting factors in farmed fishes. Subclinical episodes are often quite difficult to detect due to their insidious onset. Clinical episodes are frequently dramatic in terms of the mortality involved, which has a rapid onset and often an exponential daily increase.

One of the major cost-incurring factors in this group of diseases is the chemotherapeutic regimens, which could best be described as "categorical." That is, fish with the clinical signs of rapid, shallow respiratory movements, grossly enlarged gill tissues, and incomplete opercular closure are "treated" with one of the many chemicals added to the pond water. The results have ranged from "rewarding" to "well, we guessed wrong." the latter cases could have been prevented, perhaps, by elucidating the nature of the problem prior to treatment.

Nature of Gill Diseases of Fish

The known causal factors in gill diseases of confined and free-living fishes are myriad. These factors can be grouped into those which are (1) infectious or are (2) noninfectious, with each group being further subdivided into those factors which are primary and those which are secondary.

The primary causal factors are those which have a direct effect on the respiratory tissues and/or processes. Included in this group are the nonsystemic myxobacteria, eubacteria and fungi, the external protozoans and monogenetic trematodes, the water-borne chemical and physical agents, and the genetic and nutritional factors.

The secondary causal factors are those which affect the respiratory tissues and processes indirectly. Included in this group are the systemic bacterial and viral pathogens, the systemic protozoans and digenetic trematodes, the environmental chemical and physical stressors and certain metastatic neoplastic processes. These factors, in most cases, exert their pathogenic capabilities throughout the body with the gill involvement being just a part of the somatic process.

The major factors predisposing fish to subclinical or clinical episodes of gill diseases are (1) the stress response; (2) age; (3) environmental conditions favoring the proliferation of infectious agents; i.e., myxobacteria, eubacteria, protozoa and fungi. The three factors are virtually always involved in clinical episodes.

The stress response, particularly to chronic stressors such as high population densities, has been shown to create pathological changes in the gill tissues which are conducive to the involvement of secondary infectious and/or noninfectious factors.

The susceptibility to noninfectious and infectious respiratory disease-inducing factors decreases with age. The highest incidence of respiratory diseases occurs in the sac fry stage during which the most common malady is sestonosis with subsequent myxobacterial and protozoal involvements. This condition is typified by the accumulation of particulate materials and fungal elements on the buccal aspect of the gill rakers. Death is usually by suffocation. Because of its nature, sestonosis is virtually untreatable.

Strict cleanliness is the only sure method of prophylaxis. The bacterial gill diseases are quite common in populations of fishes from the fingerling stage through the mid-juvenile stage. This group of maladies is most frequently a sequel to a noninfectious process, environmental gill disease (EGD). The EGD syndrome is considered to be, first, stress-mediated and, second, environmentally-mediated. By itself; i.e., uncomplicated by pathogens or noninfectious factors, it is more a debilitating process than it is lethal. This aspect is, perhaps, what makes it such an economically significant disease process. There is no specific recommended treatment regimen largely because the causal factors are often quite obscure, if evident at all.

Pathological Changes in Respiratory Diseases of Fish

A. Noninfectious causal factors: The gross and microscopic pathological changes in gill tissues, especially the lamellae, are dose response dependent. The inflammatory process ensuring from the initial exposure begins with lamellar epithelial hypertrophy in which there is focal-to- generalized involvement of the squamous epithelial cells. If the irritant exposure is short-lived, the hypertrophic condition subsides within a few days, usually without notice.

B. Infectious causal factors: The pathological changes associated with direct and indirect infectious causal factors are often quite confusing for the diagnostician. In the directly associated causal factors the pathogen usually occupied an opportunistic role. That is, it "sets up housekeeping" on gill tissues already affected by some moderately long-standing noninfectious process.

Prevention and Control of Respiratory Diseases of Fish

There are several approaches to preventing and controlling respiratory disease episodes in confined fish populations. The first is avoidance of the conditions which are conducive to the occurrence of subclinical and clinical episodes. This is best accomplished by (1) maintaining the fish within the environmental "no-effect" limits with respect to settleable and suspended solids, ammonia-nitrogen, dissolved oxygen and population density and by (2) routine examination of gill wet mounts, production data and feed conversion ratios.

The "no-effect" limits of environmental factors have been established for some species of salmonids under certain, albeit limited, conditions. The best suggestion is for the individual aquaculturist to establish the unique limits for his/her facility. To accomplish this one must establish a protocol for measuring the environmental parameters and their effects on fish growth and gill tissues. This process should begin with sac fry and continue throughout the production cycle. One caveat is that the process is time-consuming and often frustrating, but always rewarding in the long term.

During the process of establishing the specific "no-effect" limits of environmental parameters, wet mounts of gill tissues must be examined on a regular basis. Fish should be taken from the "healthy" and the "unhealthy" or "sickly" portions of the populations. The fish are prepared for examination by killing them with a sharp blow to the dorsum of the head and exanguination by severing either the causal peduncle or the spinal cord immediately posterior to the base of the skill. An entire gill arch is removed and placed into 10% neutral buffered formalin for no longer than 1-2 minutes. It is either examined in toto (small fish only) or a few of the filaments are removed with scissors, mounted in pH 7.2 phosphate-buffered normal saline and examined using the 10X and 100X objectives.

If and when a clinical episode of respiratory disease occurs, then an accurate diagnosis must be made prior to initiating any therapeutic regimen. The sequence of changes occurring in the gill tissues is the best indicator of the nature of the causal factors involved. Lesions such as hypertrophy, ECS, and occlusive hyperplasia all suggest basic physiological upsets which may be reflected by alterations in other systems. The presence of bacteria and the so-called gill parasites often is a reflection of an underlying environmental problem, most common of which is "poor housekeeping."

Once the problem is defined; i.e., the major causal factors identified, the next step is to "balance" the system. This is best accomplished by, first, withholding feed for 3-4 days, if the fish are of sufficient size to permit this. This will (1) reduce the oxygen demand of the fish; (2) reduce the ammonia-nitrogen generation by the fish and (3) reduce the fecal and uneaten solids in the system. Second, administer sufficient salt (as granulated NaCl) to the system to obtain a 1-2% solution. This will (1) reduce the blood ammonia-nitrogen levels; (2) stimulate mucus secretion; and (3) have an astringent effect on the gill tissues. Third, reduce the population density to approximately one-half the oxygen-related carrying capacity of the system. This should be accomplished without unduly stressing the fish.

If, in addition to the environmental factors being corrected, there are infectious agents involved, the following treatment regimen has been effective. First, select a candidate chemotherapeutant which can be administered by water exposure. Second, conduct a bioassay. The fish to be tested for efficacy and tolerance to the chemotherapeutant should be obtained from the clinically affected and "healthy" portions of the affected population. At the end of the bioassay period - usually an hour for pond conditions, the treated fish should be examined. If the target organisms were killed or removed from the gills, the treatment was effective - and safe, if the test fish survived. These fish should remain under separate conditions for 12-18 hours following the bioassay to detect any delayed effect.

Expandable Fiberglass Rearing Tanks, Gemini Fiberglass Products, 303-278-0033

Gemini fabricates their tanks with hand-laid mat and woven fiberglass which is approximately 10% stronger than a chopped fiberglass tank. Gemini tanks are finished with an outside surface of gelcoat, which cosmetically makes for an improved final product, but more importantly, it eliminates the painful glass slivers that are often present with chopped fiberglass tanks. For straight-walled tanks requiring less than c" flex and for the extreme stress on transport tanks, Gemini uses a stiffening method which puts a heavy coat of gelcoat against the glass-smooth mold, than multiple layers of resin-soaked mat and woven fiberglass cloth which sandwiches a light-weight layer of balsawood between the surface layers and the back-up layers of mat and woven fiberglass sealed with a final sprayed layer of gelcoat.

Raceways

These fiberg1ass ba1sawood-Laminated tanks can stand alone without additional supports. There s no need for external bracing, therefore it s easy to walk and work between rows. Where space requirements are less demanding and working areas are more open, fiberglass tanks with bracing offer exceptional rigidity and strength. With gel-coating on all surfaces, tanks require no painting and very require low maintenance. Additional options include slots for screens, location of inlets, drains, standpipes, fish evacuation openings and tank-to-tank connecting channels with gate valves.

Specialty tanks

Gemini also has specialty tanks of many kinds that will meet your design and specifications. One-piece or sectional, rectangular, circular or cone-shaped, sloping or flat bottomed, with or without skirting are all possible.

Panel tanks

With the Gemini panel design, you can have large diameter tanks in varying widths and depths. These tanks have two bottom pieces and several flanged side sections and allow for easy access to buildings with limited door openings. An ideal tank for intensive heavy production, the paneled sides and bottoms nest together to allow for economical shipping. Nuts, bolts and washers are furnished with each tank. Also included is either a closed-cell, self adhesive pvc gasket or the fiberglass materials to seal the seams.

Culture tanks

The round fish culture tanks come in various sizes in sectional or one-piece design and are available with or without skirting. These tanks have a reinforced top lip and are gelcoated on inside and outside surfaces. Walls are slightly tapered to allow nesting together for economy in shipping. Like all Gemini tanks, a variety of pvc drain fittings, stand pipe assemblies, fish evacuation portals with gate valves and any other customized needs are easily built into these tanks.

Fish Disease Treatments
By: FishDoc

The treatment of fish disease is a subject fraught
with difficulties and misunderstandings. 
This is in part due to the need to oversimplify of what is a complex subject.

In all probability there are as many fish killed, directly or indirectly, by medications as there are cured. I am sure that we would all agree that this would be totally unacceptable with any other companion animal. Just imagine the uproar if cats and dogs routinely died as a result of treatments for fairly minor ailments.  Clearly, the main reason that other animals fare better is that they are usually treated by trained professionals, i.e. veterinarians, whereas fish, by and large, are treated by their owners. There are broadly three reasons why treatments often fail:

1. An inaccurate diagnosis (or guess) is made as to the nature of the problem, which in turn results in the wrong treatment being used. This is often the situation where dartboard medication is used, whereby, a guess is made as to the nature of the disease (usually based solely on clinical signs rather than an examination)  and a succession of different treatments is applied in the hope that one of them will work. This can often exacerbate an existing condition or mean that the condition simply gets worse. This really is a kill or cure approach - sadly too often a kill result. An accurate diagnosis is essential if health problems are to be treated effectively.

2. An accurate diagnosis is made but the selected treatment is not successful: Treatments may be unsuccessful for a variety of reasons. In some cases the germs and bugs (pathogens) are resistant to a particular treatment. It is not unusual for a treatment that works in one pond to be less successful against the same parasite in another pond. In such a situation another appropriate treatment should be used.

Another common problem that affects some disease treatments are variations in water chemistry. In these situations, water parameters such as hardness, pH or organic load can interfere with the action of the treatment, rendering it ineffective. There are also situations, particularly with severe parasite infestations, where, because of the hosts reactions to their presence, the parasites are protected by excess mucus.

3. A straightforward treatment overdose: This can arise from a simple miscalculation of dosages, or by the toxicity of the treatment being affected by water conditions such as pH and hardness.

A fourth reason stems, I believe, from a misunderstanding of the nature of many treatments used to combat disease problems. We can divide typical treatments into two types. First are the proper medications that have been formulated specifically to fight known pathogens. These would include antibiotics and other types of veterinary drugs and medicines. These tend to target the disease-causing organism, and while there may be some side effects, are unlikely to directly harmful to the patient at the correct doses.

The other class of treatments, and those most likely to be used by the hobbyist, are not medications in the accepted sense, but chemicals taken directly from the chemistry lab.  Their action is usually far less discriminatory. They are usually toxic to all life-forms at relatively low doses. The effective dose is one that is high enough to kill the smaller organisms such as parasites, but not high enough to kill larger animals such as fish. The safety margin is usually very fine. It is important for hobbyists to appreciate that the majority, including most proprietary treatments, work in this fashion and that such treatments should not be used in a cavalier fashion.

A further consideration are variations in water chemistry. Most chemicals, with the exception of salt, are affected to some degree by variances in water hardness, pH and temperature. Many chemicals also react with dissolved and particulate organic matter such as fish waste, algae and detritus - thus affecting or reducing their efficacy.

The three most important steps are:

 An accurate diagnosis.
An accurate dosage calculation.
A follow-up examination to monitor the effectiveness of the treatment.

When applying any treatments

• Make sure that the water is well oxygenated as many treatments can remove oxygen from the water. This is particularly important with any Formalin-based treatments.

• Turn off any UV lights as the ultraviolet radiation may degrade or affect the chemical.

• Consider whether it is necessary to by pass the filter system to avoid adversely affecting the filter biomass. This would in turn result in a loss of water quality, which would simply make the situation worse.

• Keep a close eye on fish under treatment as even at the correct doses some may react badly. Always be prepared to terminate a treatment under such circumstances. Consideration should be made as to how a treatment could be rapidly diluted if needed.

• Many treatments are degraded in strong sunlight, particularly potassium permanganate.

• Consider netting the pond as fish may well jump out when treatments are applied.

 Useful conversions are:

ppm = mg/litre (i.e.  5 ppm = 5 mg / litre)

mg / litre  x 3.785 = mg / gall (US)   (i.e  5 mg / litre = 18.9 mg / gall)

To convert imperial gallons to US gallons multiply by 1.2

Other useful figures: 

1 ounce = 28.35 grams

1% solution = 

10 ml per litre

10 gram per litre

38 gram per gal