And it didn't stop with fish:

Hunting,  too,  was  for  many  ages  considered  as forbidden  during  Lent :  the  spirit  of  the  holy  season was  too  sacred  to  admit  such  exciting  and  noisy  sport. Pope  St.  Nicholas  L,  in  the  ninth  century,  forbade it  the  Bulgarians,1  who  had  been  recently  converted to  the  Christian  faith.  Even  so  late  as  the  thirteenth century,  we  find  St.  Raymund  of  Pegnafort  teaching that  those  who,  during  Lent,  take  part  in  the  chase, if  it  be  accompanied  by  certain  circuмstances  which he  specifies,  cannot  be  excused  from  sin.  This prohibition  has  long  since  been  a  dead  letter;  but St.  Charles  Borromeo,  in  one  of  his  Synods,  re- established it  in  his  province  of  Milan.

But  we  cannot  be  surprised  that  hunting  should be  forbidden  during  Lent,  when  we  remember  that, in  those  Christian  times,  war  itself,  which  is  some- times so  necessary  for  the  welfare  of  a  nation,  was suspended  during  this  holy  season.  In  the  fourth century,  we  have  the  emperor  Constantine  the  Great enacting  that  no  military  exercises  should  be  allowed on  Sundays  and  Fridays,  out  of  respect  to  our  Lord Jesus  Christ,  who  suffered  and  rose  again  on  these two  days,  as  also  in  order  not  to  disturb  the  peace and  repose  needed  for  the  due  celebration  of  such sublime  mysteries.  The  discipline  of  the  Latin Church,  in  the  ninth  century,  enforced  everywhere  the suspension  of  war  during  the  whole  of  Lent,  except in  cases  of  necessity

Could the shortened lifespan of man have anything to do with no longer having a water canopy over the earth, thereby allowing the sun's rays to do damage to living creatures sooner and with greater effect?

Absolutely.  Also, i've read from saints who described that one of the punishments/changes that God enacted post-Flood was that nature/animal kingdom was hostile towards mankind, whereas before it was not.  For example, pre-Flood, there was no rainfall, as we know it.  The earth was watered by geysers/mists of water that sprang up at certain times.  There were no storms or anything like this because the firmament was still in place.  The earth was like a tropical paradise, not too hot or not too cold, all year round, which not much variation in temperature/weather, etc.


Also, there had never been earthquakes, flooding, tornados, hurricanes, etc.  The first time mankind saw rainfall was when God sent the Great Flood.  Then, at the peak of this chastisement, you had all of nature revolt/attack mankind as a punishment.  When the "waters of the deep" sprang forth and the firmament's water rained down, this was mixed with earthquakes, tornados, etc.

And, post-flood, nature is still "in turmoil" against mankind, as a reminder of God's punishment of the Flood.

Then with the animal kingdom, Noah was able to gather "2 animals of each kind" fairly easily because animals were much more docile.  Genesis says that God gave mankind to eat the "plants with seed-bearing fruits" and everything else was for the animals.  So, being that mankind didn't eat meat pre-Flood, it makes sense that animals didn't kill each other either.  Animals were also "vegan".  And animals were also friendly with humans...much more so than they are today.  So when Noah gathered the animals into the Ark, they would've obeyed him as if he were a friend.

Then you have the insect world.  Some saints have said that ANOTHER punishment for the Flood was that the insect world also attacked humans (i.e. flies, bees, mosquitoes, etc).  Whereas before, they didn't.  When you think about how certain insects eat farmlands and destroy crops or gardens...this probably never happened pre-Flood.  Insects only ate certain plants, animals ate certain plants, and humans ate seed-bearing plants.  Everything in nature was in order, was organized, was calm and peaceful (probably not as organized/peaceful as in the Garden of Eden, but MUCH more peaceful than now).

It seems that after Adam's sin in the Garden, God punished humans with chaos of human nature (i.e. concupiscence, death) but nature/animals were still orderly.  But after the times of Noah, when mankind had corrupted earth/animals (i.e. some say with genetic experiments and unholy satanic rites), then God punished humans with chaos of weather/animals, in addition to shortening man's life and making our health more fragile.

Pre and post Flood is a stark difference in humanity and the world.  It's fascinating to think about "what was" vs "now".  Even when God sends us punishments (i.e. tornados, mass snowfalls, rainstorms, etc) these are still beautiful events, which artists capture pictures of, all over the world.  And even the most deadly of animals is still beautiful and awe-inspiring in their strength and abilities.  And, of course, beer/wine are one of God's most perfect creations. 

There are many reasons to have a flock of sheep, even if you don't eat them -- clothing, fertilization of crops, cultivation of land, etc