Inbreeding is a major problem in small populations. Unfortunately small populations are all we have. Very few of us can maintain hundreds of a single fish species just for breeding purposes. It just isn't feasible. So in order to maintain a high degree of genetic integrity (fitness and diversity) the population size is increased by maintaining several lines that can be crossed periodically to boost genetic fitness and variability. This only works if distantly related lines are used for the cross. Crossing brother/sister lines is still inbreeding. Alleles will be lost, some will accumulate and the fitness of the strain will deteriorate. This is why a pedigree is so important. With a pedigree chart the most distantly related stock can always be chosen for the outcross.

Unless we keep accurate records and construct pedigrees like below we will eventually end up with a poor strain that is doomed to extinction. As some of the captive strains are already extinct in the wild or facing extinction it is very important that we implement pedigrees now before more critical alleles are lost.

How often should an outcross be done? My feeling is every third generation at the very least. If possible an outcross should be made every generation but this is an ideal most of us won't realize.

The above figure is a rather fancy rendition and not really what is expected. It does outline the principle: cross out every 3 generations to a distant relative.

The pedigree could just of easily of been written out like such:

A0  →→→  AxB
  9  A1  →→→  A1x(BC)
  9  A2  →→→  A2x(BC)

In essence each arrow represents a generation. A cross is designated by a "x" and the line it is being crossed out to is represented by the letter(s) e.g.: BC which is a cross between B and C as seen in the figure above.

Lets go three generations forward... It is now time for AB to be crossed out, it will bow be crossed out to BC so the pedigree will read:

A0  →→→  AxB  →→→  (AB)x(BC)
  9  A1  →→→  A1xB.C  →→→  A1.B.CxB.C.C.A
  9  A2  →→→  A2xB.C  →→→  A2.B.CxB.C.C.A

What is worth noting is that the outcross need not be to the main line but could be to a X1 or X2 strain. What is important is that the line being crossed to is as distantly related as possible.

Over time the pedigree will become complex to interpret especially as other lines are established and they are in turn crossed out to other lines. The above is a rather complex example but the real situation can get more daunting or simpler. A two line tree will only contain the designations A & B so all crosses will be AxB etc... A four line tree will include a 4th letter D so you will have AB, BC, CD and DA after the first cross. Needless to say good book keeping will be very important to the task of species maintenance.