Or, still fitting:

Scientists Find Eastern Baltic Cod Shrank due to Overfishing Affecting Genepool

Actually directly altering the genes would have to involve mutation or direct engineering which is a bit more involved.

I think a good way to think about this is with human dwarfism. Many humans with achondroplasia get it through de novo mutation, but some get it by a combination of having two recessive loss of function genes that get transmitted by both parents (often of normal height)

Now imagine a laser beam that went and killed every human above a certain height. Young people would be spared along with adults with dwarfism. Over many generations, previously rare genes for dwarfism would increase in frequency shifting the average height of the population lower and lower.

It is the change in frequency that matters here more than the underlying explanation of what changed the genes.

In what sense? Is being bigger Platonically better than being smaller?

as a data point, a recent change in regulations regarding eastern Baltic cod had no statistical effect on reported catch https://link.springer.com/chapter/10.1007/978-3-030-03308-8_...

>Data quality for stock assessments has deteriorated, discarding of cod has not decreased despite a reduced minimum size and there are no indications of increased gear selectivity in the fishery

I read about this being tested in large fish tanks using either cod or trout some 20+ years ago, where they removed fish either randomly, or let the small ones go. They came to the same conclusion: letting small fish go results in reduction of average size of mature fish after a few generations.

The authors of the submitted paper references this[1] article, which points out the following:

Despite a theoretically strong conceptual basis, evidence of genetic change unequivocally attributable to wild-capture fisheries has been elusive. Among the top five threats to biodiversity, evidence for genetic trait change is strongest for studies of pollution and weakest for studies of overexploitation (and habitat change). Determining whether phenotypic change in declining populations is the result of evolution, as opposed to other influences on growth, survival, and fitness, or gene flow from adjacent populations, has proven challenging.

So this paper seems to provide evidence that the lab results holds up in the wild.

[1]: https://www.pnas.org/doi/full/10.1073/pnas.2105319118

> Size obviously matters when it comes to mating for salmon. However, being a small male can also succeed when it comes to scoring a female. The so-called "jacks" that are found in chinook and coho salmon are male individuals that return to their natal streams a couple of years earlier than expected.

> Although they are much smaller than a fully grown male, they are also sexually matured when they reach the spawning ground. What advantages do these smaller fish have? It is obvious that they will not win when confronted by a fully grown male. Behavioural biologists believe that these jacks are "sneaker males". Their duty is to simply stand by when larger males are fighting for territory, and sneak in while unnoticed to mate with the females that are also waiting for the fights to end. As you can see, being big does not always have all the advantages, sometimes being small can be very beneficial too.

IIRC the female already laid the eggs, and the big males start to fight. During the fight the small one sneaks a fertilice them. "Waiting" and "Mate" are misleading.

This makes sense if you think deeply about it; evolution will only happen when the 'normal' genetic expression stops surviving, otherwise the random variations will even out.