Re: Why don't anadromous fish suffer inbreeding depression?

TO: Alan Hargus

Yes, other than a bit of straying, salmon typically return to their 
neonatal stream to spawn.  Depending on the size of the areas available 
for spewing  (a certain amount of area is required for each nest, or redd) 
the population of that stream may be concentrated or stretched out over a 
great distance.  I am told by my salmon specializing counterparts that 
typically those prime areas upstream are utilized first, with a 
progression back downstream as the best sites are taken.  So even within a 
population native to a particular stream, since the fish arrive at the 
site at different times, often over the course of several weeks after 
returning from the ocean, the chances of an adult reaching the exact spot 
it was spawned on are very slim.  The chances of another (sibling) salmon 
reaching exactly the same place at the same time are even smaller.  Oh, 
yes, reduce this chance by another 50% to take into account the 
male/female ratio required for spawning. (And to complicate things 
further, not all the eggs hatch at the same time from the same spawning.) 
But for argument sake, let’s say this all fell into place and happened.  
In reality, in a very small number of cases it does.

A common but artificial situation that makes a good example is fish 
populations in farm ponds.  Some of these ponds have been genetically 
isolated for many, many years, and no outward sign of abnormalities are 
evident there.  The stunting typically seen in such ponds is invariably 
due to overpopulation, rather than genetic miss-selection.  In aquaria, 
fresh water “tropical” fish can be cross and back bred for several 
generations without an excessive number of outwardly harmful physical (or 
other) traits being expressed.  This crossing is frequently done on 
purpose to emphasize a particular color, fin length, or what not in order 
to make the fish somehow more popular to the aquarium trade.  However, if 
things go too far and the recessive genes (and associated traits) are 
expressed, two things usually happen.  

The first is that the fish develops some physiological trait that reduces 
that fishes ability to successfully: feed, assimilate its food, escape 
predators, resist disease, attract a mate, or breed.  These can perhaps be 
best thought of as typical “birth defects”- missing fins, dain bramage, no 
gill covers, bad coloration, miss-shaped mouth, etc.  In the case of the 
salmon, anything major would most likely prevent the fish from making the 
stream-ocean-stream anadromous round trip successfully.  Somewhere along 
the line, it would be taken out and assimilated into the food web (lunch).

Should the fish make it past all the obstacles and initiation rites to 
become a real dues-paying adult salmon, and actually make it to the point 
of breeding, it would encounter the (typically) second MAJOR result of 
“inbreeding”; the inability to produce fertile eggs and viable offspring.  
Typically, one of two things happens if the “inbred” parent proves 
actually fertile.  The dominant genes of the non-inbred (hopefully) parent 
completely repress the recessive genes/traits back to the point of being 
repressive (at least not expressed) and the result is an outwardly normal 
offspring.  Quite often the eggs do not develop, or the eggs which hatch 
do not survive beyond embryo stage; and the parent dies (typically after 
spawning), and that is the end of that genetic line.  (If not the details 
of paragraph 3 start all over again.)  This process is accelerated if both 
spewing parents are in-bred.

Some of the genetic changes which occur under this inbreeding occasionally 
may lead to the expression of traits which may enhance the abilities of 
individual fish to survive and spawn successfully, and these traits would 
then be passed along.  Remember, Darwin was right.

A small number of salmon from distinctly different chromosomal profiles 
could still (and do) interbreed.  It is known that a certain percentage of 
fish stray, get lost (small percentage), or are unable to reach their own 
natal stream (big percentage with hydroelectric dams).  This would allow 
the fish to breed with others from another isolated population.  In this 
case, this would intermingle the genetics beyond what would have normally 
been found within each individual population.  Evolutionarily speaking, 
the balance of inbreeding, and recruitment of “stray” breeding adults into 
the population is balanced out by the environmental conditions the fish 
must survive under.  I believe this would only be a concern in the wild if 
an extremely small population of salmon spawned in a very inaccessible 
location.  Then this particular population would indeed die out.  Since 
nature abhors a vacuum, I imagine some other salmon would eventually stray 
into that area, and start the cycle all over again.

A good web site with more details and specifics is:

 http:/www.bluefish.org/nmfs_sum.htm

Hope this helped!

Justin Strynchuk
BS Aquaculture Technology/BS Environmental Technology
Florida Institute of Technology