Have heard this mentioned a few times in journals etc... but this is the first time I have personally seen an article in a popular magazine about it. Very interesting concept that the reduction in size of individuals in a population where the largest are removed is due to both the loss of genes for "largeness" and the increased fitness of genes for "smallness". It has been suggested fish like the atlantic salmon are evolving to become smaller as we kill the biggest.

Unnatural selection: Fish growing up fast - environment - 08 May 2011 - New Scientist

Dan

__________________
http://eclecticangler.blogspot.com/

Prof Andy Ferguson mentioned that there may be some evidence that this is happening on Corrib (and other western Irish loughs with big increases in angler visits and extensive catch and kill).

Another interesting hypothesis is whether a reduction in available marine forage fish reduces the competetive advantage of being a "multi sea winter" salmon - i.e. why not return earlier and breed since it is more difficult to get fat at sea?

This is something I've been warning loch fisherman about for decades. Selective culling can have good and bad effects on population dynamics.

Trout fishermen have a tendency to select big fish for killing, returning smaller fish. On wild waters with a small population of large fish this can be beneficial because all the trout have a tendency to grow big, so the removal of a big fish simply allows a territory to be vacated which will allow the new tenant the opportunity to match its former occupant in size and growth rate.

However, on waters where the general tendency is for small fish, the removal of a big fish can reduce the overall growth genes in the whole population unbalancing the size diversity.

I have always advised that small fish, in waters where the general size is big, should be nurtured and protected because each small fish is denied the chance to grow only by lack of opportunity. In waters where the population is great and composed of generally small fish, big fish should be returned to enhance the genetic tendency for growth, and small fish should be removed to provide greater feeding potential for fast growing fish.

Many waters thrive under a policy of controlled 'kill'. Blanket catch & release on waters where the trout population is large can encourage the development of smaller & smaller fish. Loch Craggie, near Lairg, is a prime example. When I first visited it the average rod catch was low but the average size of fish was approx 2 lb; nowadays the average rod catch is high but the average fish size has dropped by about 50%. This is soley due to an introduced, alien and misplaced anathema to killing fish. More fish is not always a good thing.

__________________
Stan Headley

"I started this life with nothing......and I've still got most of it left!"

This is a very common occurence with largemouth bass and panfish in the ponds back home. In the ponds on public land where there is a significant amount of public fishing and catch and keep fishing, you tend to have lots of fish just below the legal size limit (12" on most of my home waters) and many fewer above it. Even some of the larger lakes have the same problem if there is a significant amount of C&K angling. To combat this, the fish agencies have come up with some interesting set of requirements. Sometimes they set the size limit bigger (with the obvious shift in population to just below the new size limit). Some fisheries use a slot limit where any bass between 14" and 22" are required to be released. This is especially useful if the species has a specific age/size that is optimal for breeding. Some places have a slot limit where you have to release anything OUTside of the slot sizes. They often have a provision for keeping one fish over the slot size limit.

For a wild trout fishery here, I could see how the same sets of rules could be helpful in some situations.

Thanks
Rick

I expect that sustained heavy harvesting at sea will also select for non-migratory salmon (ie. parr-like fish which develop to sexual maturity at a small size; never develop into smolts and adopt a life history indistiguishable from resident brownies).

unnatural selection?-the effect of man

Eh, sorry?

__________________
Stan Headley

"I started this life with nothing......and I've still got most of it left!"

I've been told that the taking of large elephants (for their ivory) certainly affected the average size of African elephants - same principle? This article suggests some no longer grow tusks!

Super-Predators: Humans Force Rapid Evolution of Animals | LiveScience

Exactly the same principle, has been recorded in nearly every case studied where the largest are taken, red deer, elephants, salmon etc...

With regards to the non-smolting salmon my guess would be this is very unlikely. That would require a major genetic change (unlikely to occur without serious problems and loss of fitness) whereas getting generally smaller can easily happen with small mutations of certain genes linked to growth.

As far as I am aware un-natural selection is merely a play on words, as you say to highlight the fact it is our actions causing it.

Dan

__________________
http://eclecticangler.blogspot.com/

There is already a precedent for losing the anadramous habit in salmon to become river residents. For example, Prof Verspoor referred to an observed population in his talk to the WTT annual get together in 2009. The video is still up on the WTT website and here is the slide from his presentation showing the "precocious parr" (early maturing males) as well as the resident female and male populations that never run to sea (Newfoundland).