One of the main causes of non-infectious losses in sea water is associated to runt fish.

The external characterizations of the runts are low condition, loss of scales and internally there is a lack of gut fat.

The runts are due to an alteration of the osmoregulatory capacity after they have been transferred to sea water (Slinks).

Introduction

Smoltification is a very complex process where there are physiological, morphological and biochemical changes that are being produced which allow the fish to become a parr-smolt. One of the main factors that are related to the fish being able to tolerate sea water is the increase of the gill Na+K+ATPase activity. Some of the main factors that regulate this process are the photoperiod, temperature and size.

After the smolts have been transferred to sea water, the ion levels in the plasma increase. This is associated to a smaller percentage of water in the tissues. This osmotic disturbance is followed by a stabilizing period of 4 to 8 days (Handelan et al, 2000).

There are many factors during the osmoregulatory development in fresh water, transfer from fresh water to sea water and the period just after they have been transferred which can cause the osmoregulatory process to fail. This is then seen by mortality in the water after transfer and/or runts present.

Some of the most common causes associated to the failure of the osmoregulatory after the fish have been transferred are the premature transfer (before the beginning of the transfer window), late transfer (after the transfer window has closed), a dispersion of the osmoregulatory capacity in the population, amongst others. Health factors and handling can be a negative influence to this process. On the other hand it must be kept in mind that even though the smoltification process was successful, eventually the stress, skin and gill damage during transport and evnironmental conditions when they are received can be a negative influence for the fish to adapt to sea water.

Clinical signs

The runts can be seen swimming at the edge of the cages. They have a low conditioning factor, dark colored body and various grades of scale loss. Internally there is scarce to no gut fat or prephyloric fat, the gall bladder is full of contents and normally there is no food in the digestive tract.

Histologically there is an increased storage of melanin and lipofuscin in the interstitial of the spleen and kidney tissue. In the kidney tissue you can see catabolism (Speare, 2003).

Figure 1: Atlantic salmon (Salmo salar) affected with runts. You can see many fish gathering at the border of the cages.

Figure 2: Atlantic salmon (Salmo salar) affected with runts. You can see many fish gathering at the border of the cages. The low condition of the fish can be seen in the details of this picture.

Figure 3. Atlantic salmon (Salmo salar) affected with runts. You can see many fish gathering at the border of the cages. The low condition of the fish is seen in the details of this picture.

Figure 4: Atlantic salmon (Salmo salar) affected with runts. In the picture the top three fish show low condition and scale loss in comparison with the last fish that has normal characteristics.

Figure 5: An Atlantic salmon (Salmo salar) that is a runt. The lack of gut fat can be seen in the picture.

Differential diagnosis

When diagnosing runt fish some of the conditions that must be kept in mind are the Infectious Pancreatic Necrosis (IPN) and Pancreas Disease(PD), an exotic disease found in our country.

Prevention and Control

Applying the Quality Assurance Program for the Smolt which includes transport and handling while receiving the fish are key elements for minimizing the impact to these kind of events.

Once it has been diagnosed, then it is necessary to decide on the percentage of the population that has been affected. This allows an earlier knowledge of the impact that this will be.

Traps set in the cage are one way to eliminate the runts since they are a risk factor to the population due to their vulnerability to infections.

Una de las principales causas de pérdidas no infecciosas en agua de mar se asocian a peces desadaptados.

Los desadaptados se caracterizan externamente por baja condición, descamación e internamente por escasa a ausencia de grasa visceral.

Los desadaptados son consecuencia de una alteración en la capacidad osmoregulatoria post tranferencia al agua de mar, lo cual se trae consigo o se asocia a una  falla en la alimentación post transferencia (Slinks). 

Introduction

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