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Healthy gills showing the clearly defined primary and secondary lamella.
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Serious hyperplasia is evident with some sections of the gill showing fusion of primary lamella. No secondary lamella as such can be seen.
Although the main function of the gills is that of gas exchange, this multipurpose organ is also the dominant site for osmoregulation, ammonia excretion and acid-base regulation.
Its normal function is vital in the fishes well being and it has been said that the gill is the ‘mirror of fish health.’ For such an important organ the gill is extremely exposed to environmental challenges as, unlike our own lungs, it is situated externally within the buccal cavity and protected only by the operculum (Figure 1).
There are four gill arches on either side of the head and each gill arch contains two rows of filaments or primary lamella (Figure 2). The surface area of each primary lamella is further increased by semilunar folds on both the ventral and dorsal surfaces giving a surface area exceeding the total surface area of the body of the fish itself. These folds; the secondary lamella, are staggered so that they are positioned at the spaces between the secondary lamella of the adjacent filaments and together present a matrix through which the water must flow. Gas exchange takes place across the surface of the secondary lamella (Figure 3) and is sufficiently efficacious so as to extract between 60-80% of the oxygen in the water passing over and through the gill structure.
Monitoring gill health
Monitoring gill health in the hatchery is both one of the easiest, and at the same time, most important routines available to the hatchery technician, especially during the early life stages. There are many irritants that can affect the gills, ranging from the fishes own metabolites such as ammonia, through to uneaten feed and dissolved metals.
The gills generally respond to insult in the same manner, irrespective of the cause – by increasing mucus production and by cell proliferation. As this is not immediately apparent, it is wise to monitor gill health regularly using a light microscope in order to detect problems before there is clinical expression.
As the fish normally have sufficient respiratory reserve, even with some proliferation or hyperplasia, the farmer may not immediately detect that there is a problem before it is too late. Early detection allows changes in fish husbandry in order to deal with the underlying cause. Non-detection can either lead to secondary problems such as bacterial or fungal invasion of the gill surfaces and/or colonisation with protozoa such as costia and chilodonella.
A good microscope
Good microscopes nowadays are relatively inexpensive and often come with digital capture incorporated into the microscope head like the instrument shown in Figure 4. These microscopes can capture either still pictures or video directly to a computer and display the live view on a monitor making them ideal for both teaching other staff and enabling second opinion from co-workers or health services via email. The purchase of a light microscope and its use in regular gill checks will invariably pay for itself in a very short period of time.
A typical deterioration in gill quality through proliferative changes resulting in severe hyperplasia can be seen in Figure 5. Early changes to the gill structure are normally expressed as general proliferation and mucus secretion leading to thickening of the secondary lamella as can be seen in images 3 and 4 in Figure 5., or classic ‘clubbing’ of the lamella tips as shown in Figure 6. These changes will not be evident to the farmer, neither through increasing mortality or behaviour, and without regular microscope checks, the gill damage can quickly progress to that seen in images 5-10 in Figure 5.
Once hyperplasia is present the situation in gill health can deteriorate very quickly as excess mucous production by the fish is an ideal substrate for colonisation by secondary invaders such as the protozoans or bacteria. All of these can also be seen with the use of a good light microscope and can influence greatly the decision whether or not to treat with products such as formalin or chloramine-T.
Care must be taken at this stage as topical treatments such as those mentioned above can have a further negative effect on the gills causing even more proliferation and mucous production and the whole situation becomes a bit like a revolving door where the treatment gives some temporary respite, normally through mucous removal, but very quickly becomes worse due to the gills reaction to the treatment itself.
The use of the microscope in deciding whether or not to treat is therefore of vital importance and can only be achieved through regular monitoring. It is advisable to check gills as frequently as twice a week at the early stages although this can be reduced to a weekly or monthly check once the fish are bigger.
In the next column we will look at Water quality and Environmental disease before going on to cover the transmissible aspects of disease in later editions.
– Alan Dykes
Alan Dykes is Fish Health Services Manager at FishGuard Scotland.