This manual will cover alternative approaches to dissection observed, applied, or developed over years of practice with different species. The associated images to each step are intended for illustration for the particular structure-organ being described and may come from different species. However, the full set of images of the procedure for the species representing each group covered by this manual can be found in the fish carousel. Additional information applicable for that group will be found in the captions. Users are encourage to read the general procedure and then select the group of interest for specific details. Irrespective of the fish species and the necropsy objectives however, the chosen approach should aim for a streamlined access to structures, organs or tissues, in a systematic manner, minimising distortions and contamination, and facilitating a quick approach to avoid the influence of post-mortem changes.
Opercula and gills
Integument and fins
When examining integument and fins, pay special attention to the colour of the specimen to determine whether it is normal for the species. Notice also integrity of the skin, condition and tensile strength of scales, if relevant (species with scales), amount of mucus, presence of furuncles, ulcers, nodules, cottony masses or ectoparasites (see FPA). If you need to know the age of the specimen, now is the time to take samples of scales for reading and estimating age . In some farmed fish, fins are often be absent, very eroded or only present as small appendages or stumps , which should be recorded. When the aim of the necropsy is a parasitological examination, prepare slides for microscopic inspection of the integumentary mucus and certain areas of skin, fins and orifices. For example, it is important to check the internal face of the pectoral fin insertion, which is the preferred site for some ectoparasites. To do so, scrape samples of skin for observation under optical microscope or subsequent analysis. Place a coverslip, slide, scalpel or spatula at an angle of about 45º to the base of the fin and gently scrape the surface in cranio-caudal direction. If lesions such as furuncles or ulcers are found, samples may be needed for microbiological and histological analysis.
Examination of nostrils, mouth and oropharyngeal cavity
Examination of the anus and genital and urinary openings
The internal examination includes the assessment (and frequently the sampling) of the organs and tissues of the abdominal, pericardial and cranial cavities. There is no a single “correct” way to do a dissection or a unique order to follow, rather we will say there is the best procedure adapted to suit the requirements and objectives for which the necropsy is performed. The sequence described in this manual is influenced by the approach usually applied when samples for diagnostic purposes are required, reflecting therefore an order that respects the preservation of tissues, particularly for histopathological examination. Fish are normally placed with the head to the left, which means on their right side for fusiform fish, and left or right for flat fish which may have their eyes migrated to one or other flank, depending on species (sinistral or dextral). This position follows a generalized convention, facilitates the dissection for right handed people and is particularly useful for immediate visualization and direct access to organs that in a majority of species, are placed to the left of the body cavity, like the liver and frequently the spleen, without the need to displace other structures. A good practice before starting and especially if microbiological samples are required, is to disinfect the external surface of the fish before proceeding. This is normally achieved by spraying with a solution of 70% alcohol.
Opening of the abdominal and pericardial cavities
Examination of the pericardial cavity and heart
Examination of the digestive tract
Examination of the liver- gall bladder and the spleen
Examination of the gonads, air bladder and kidney
The gonads are usually pairs bodies, located on a ventral position to the air bladder. Their appearance, size and colour vary, depending on the fish species, the sex and the state of development. Size and colour are usually recorded to assess their degree of development. To remove the gonads pull lightly to locate the connection cranially, sometimes simple pulling detaches them but it is preferable to cut rather than tear. Very mature male gonads (testicles) may easily release sperm which should be avoided. Loose sperm in the histological samples of other tissues can lead to misinterpretation. After removal, they are placed in a suitable container and reserved usually for reproductive or parasitological studies. The air bladder is located dorsally to the gut and easily to observe if it has not been deflated. The colour, shape and wall consistency varies widely among species. It can be almost transparent and delicate as in salmonids, or solid pearl white in colour and strong wall consistency on cyprinids. Its integrity must be assessed and the possible presence of parasites and exudates of different characteristics, haemorrhage, petechiae or hyperaemia, among others. To remove separate from the digestive tract and carefully using a non sharp tool like a spatula, detach the edges from the body wall leaving the kidney behind, with the scope of dissecting the bladder without loss of gas. The bladder would remain together or not the digestive tract, depending on whether the fish is physostmous or physoclist, respectively. In the first case the pneumatic duct needs to be severed. The bladder is placed in a container and kept moist for subsequent observation, especially for the purpose of parasitological examination. Once the gonads and air bladder are removed, the kidney becomes accessible. This organ is located dorsally to the abdominal cavity, intimately attached to the spine. The peritoneum sheet must be cut to have real access to the organ. Check for observable macroscopic changes, which frequently affect the posterior (excretory) part, including increase in size, swelling, thickening, loss of brightness, vesicles and pustules. Nodular or granulomatous changes are also possible observations. The presence of the the “corpuscles of Stannius”, located in the ventral wall of the organ approximately in the middle zone, marks a transition between the excretory and the haemopoietic portions of this organ and should not be confused with granular lesions.If required, proceed to the sampling for histopathology or microbiological examinations. The kidney is an important organ in the detection of bacterial and viral agents as well as target by fungal agents and sensitive to water quality issues. Sometimes fresh tissue samples are checked as imprints or smears stained and analysed in situ.
Examination of cranial cavity and brain
For accessing the cranial cavity and brain the skull needs to be opened. Before attempting to cut open the hard tissues of the head, secure the fish with a cloth or paper towel to prevent sliding, as it can be dangerous when using sharp tools. Depending of the fish size, a scalpel may suffice, or a sharp knife or a saw might be required in larger specimens. If using scalpels or knifes, hold firmly the head of the fish by introducing a tooth forceps on the mouth cavity, and proceed to make a decisive sharp cut on a horizontal plane in a line just above the eye balls, continuing slightly into the dorsal musculature. The section has to aim to go through from side to side of the head in the same cut, to open clean the roof of the skull.Once exposed, observe in situ the cavity and the brain, its general appearance, the colour and consistency, excessive fluids, haemorrhages, as well as the eventual presence of macroscopic parasites. The killing method applied and the time elapsed since death can cause alterations which must be taken into consideration. When histology or microbiological samples are required, these must be taken prior to any further manipulation. To extract the whole brain, depending on the correctness of the dissecting plane, the tissue will have remained on one piece either on the skull cap (having severed most of the neural connections) or would be still in the cranial cavity, but now clearly visible from top. Both options allow a clean and easy collection; if on the top with immediate access as no connection to nerves or spinal cord remains; if in the cavity, after sectioning the olfactory and optical nerves that will hold to the front and the spinal cord, to the back. For the detection of protozoa, fungi and certain parasites, fresh tissue preparations are checked in situ. The open skull will also provide access to locate and remove the otoliths, which are used in the age determination, mainly in fish without scales.