Fish Keepers
- Thread starter Orleander
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John Connellan
Valued Senior Member
Not if those large fish have already reproduced and passed on their "large" genes to the next generation. What we could be doing is killing off smaller fish before they reproduce and the effect of this would be like an accelerated natural selection which would cause them to evolve quickly into large fish. We have seen this in many forms of farming of both plants and animals.
Yup. Large catch selection has already modified the average genetic value of at least one West Coast salmonid population. Estimated breeding values for size are declining. John, increases in size and yield in agriculture are the result of retention and selection on broodstock. You can't select for increasing individual size by selecting against large fish. You brood-test and retain the sires and dams with high EBV.
Keeping larger fish also selects in favor of reproduction at a smaller size - resources put into reproduction are not available for growth, and if the compensating advantage of being able to reproduce more heavily at the larger size is abrogated (by having been eaten by people, say) then the advantage would be to reproduce as quickly as possible, when still small.
Something similar is happening to trees, and not always by direct human agency: American elms have been observed seeding at shrub sizes in the northeast, where Dutch Elm has had its longest impact. These trees will be slower growing, and probably never attain the dimensions of their ancestors even if spared the fungus.
Something similar is happening to trees, and not always by direct human agency: American elms have been observed seeding at shrub sizes in the northeast, where Dutch Elm has had its longest impact. These trees will be slower growing, and probably never attain the dimensions of their ancestors even if spared the fungus.
Yes and that "abrogation" can be by being eaten by larger fish as was the case in a proof of evolution experiment done in Brazil. Read it here:
http://www.sciforums.com/threads/an...e-once-believers.136289/page-117#post-3122010
But here is part of that text (There is also proof that new species can evolve in only 10,000 years in that post, if conditions are very unusual):
"There are well controlled experiments. My favorite was done in Brazil. Grad students moved some tiny fish, which laid only a few eggs when less than a year old, (Because they were eaten by bigger fish who also live below a water fall). Moved to above the water fall. In less than 10 years, those "tiny fish" delayed egg laying a couple of years and laid many hundreds as above the falls they were not eaten.
I. e. The genes that said :"Lay four eggs quick" were not well represented in the next generation. - They lost out in the struggle for the limited food supply. The genes that said: "Wait a couple of years and lay 500 eggs." became the gene pool norm - just as evolution had predicted - and several other predictions were confirmed too.
That "ability" to use a theory to predict, do a testing experiment, and find the predictions confirmed is POWERFUL support for the theory much more so than thousand so observations that are consistent with the theory. Theory of evolution has done this more than a dozen times.- String Theory of physic, has not one confirmed prediction, AFAIK.
Evolution is quite possibly one of the best tested theory man has ever made;
John Connellan
Valued Senior Member
I assume you meant "selecting against small fish" here right? I never claimed an increase in size of the individuals. I meant the average size of the current stock would increase by killing off the smaller fish, and through this selection process, the average size of the fish stocks in future generations would also increase.
Also, you are wrong in your agreement without qualification with the OP. There will be no selection if those large fish have already reproduced and passed on their "large" genes to the next generation
I think we're discussing at cross purposes here. I wrote:
You can't select for increasing individual size by selecting against large fish.
Select against large fish, and you get small fish. You can definitely select for large fish size by selecting against small fish. Or was that a typo?
Are you talking total catch or something? Generally speaking, economic gains in harvesting efficiency increase with larger fish, so that rendering time per pound of meat is higher for larger catch. Your comment above though appears to conflict with your earlier statement on this point:
If selection (say, trawler selection) is against large fish, then across the board you're eliminating large fish. Some of those might be older, which might translate into poor growers per annum, but on average it's more likely you're going to get the ones that achieve large maximum size at equivalent ages to the small fish, which results in selection against large fish. This means declines in frequency for those alleles responsible for large size in this case. I don't think your model of 'killing off smaller fish before they reproduce' can thereby work. You wouldn't be killing off small fish before they reproduce, but rather selecting against larger fish, say according to trawler gear size. If you mean that you'd be killing off larger fish and therefore that smaller fish would have more opportunity to grow larger - say by having relatively more food resources - then this would be possible, but as it's contingent on lower population density, it wouldn't be stable and presumably not heritable either, unless you're invoking epigenetic modification. This would come with a whole new set of stable population density states and so I don't know that it's a good idea per se. Moreover, it would still presumably operate in favour of producing large fish at low density and being epigenetic, wouldn't be classically heritable.
Yes and no. It would work specifically in the case of harvesting of post-breeding single-event breeders. Salmonids (Teleostei: Salmonidae) are a good example. Salmonids come in two flavours: iteroparous, in which individuals have repeated breeding events yearly, and semelparous, in which individuals experience only a single breeding event. In the case of semelparous species - a lot of Pacific salmon in this group, several members of Oncorhynchus - they die immediately after breeding, hence the semelparous part. A few do survive and go back to sea, but not a substantial number against the huge number that die. You could do this, but why not just harvest all of them? They're not breeding again anyway. No need for a gradient of size selection at all. If you select before breeding, you hit the same classical problem in which selecting out larger animals effaces alleles resulting in large size.
Now, say instead we're talking about the iteroparous or repeated-breeder condition. We can select those after breeding, and no worries, since they've already bred. Unfortunately, however, that's the point of iteroparity: coming back to the same river system to breed. If you select against large size at this point, you're reducing the number of large individuals that come back in succeeding generations. This is a net selection against alleles causing large size - not absolute, but certainly present. The problem becomes worse with additional seasons of reproduction, since the chance of being caught as a large fish increase, and the relative advantages of being small increase proportionately, or probably nearly proportionately. The big guys are gone, and you're looking increasingly attractive to the mates remaining, like closing time at East Side Mario's. So for that first generation, sure, it's no biggie - but it gets worse each time you go back for another round of breeding. Looking at it like a binary trait, the odds are stacked against big fish are probably 3:1 for two-generation iteropars, and 4:1 or more for three generations. I saw a presentation a while back from a West Coast salmonid guy about declines in breeding value for size as a consequence of trawling selection.
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John Connellan
Valued Senior Member
No typo. I understood that the OP was asking if the fish were getting smaller (a bad thing) and I was saying not necessarily. To reassure him, I also said that we could actually select for large fish by killing off the smaller fish so that their genes do not get reproduced in future generations.
How is it in conflict? In both sentences I am talking about killing off smaller fish so that future generations are larger. I cannot see how this could be misinterpreted!
Write4U
Valued Senior Member
IMO, selecting the size of the environment plays a great part in the growth of fish.
A goldfish is a carp and when confined in a small aquarium, they remain small. Breed the same goldfish in a large pond will result in continued growth and the small goldfish becomes a 6 pound carp.
A goldfish is a carp and when confined in a small aquarium, they remain small. Breed the same goldfish in a large pond will result in continued growth and the small goldfish becomes a 6 pound carp.
https://en.wikipedia.org/wiki/Carp
This is possibly theoretically, but rarely practically. The OP question concerns fishing pressure and presumably trawler pressure:
Were you speaking of potentialities? If so, sure, this is possible. But in practice, selection in fishing resources is always directed against larger fish, since it's a bit difficult to design excluder gear that keeps out large fish and presumably because of the increased handling time for smaller individuals.