Salt water fish have an ability to move all over and rarely stay in one place. This is an advantage during normal bad times, but during a long term problem like what happened at the end of the KT divide everywhere is equally dismal.
Fresh water fish must be able to survive bad times locally, for most can NOT swim out into the ocean. Fresh water fish have a membrane that keeps salt inside, so the fish has access to it when it is needed. Salt water fish membrane is made to keep salt OUT, if the fish needs salt it just takes in some salt from the ocean. Thus in fresh water fish it is important to keep salt inside, while a salt water fish keeps it outside. Some marine animals can do both, but those are exceptions AND more a temporary ability then anything permanent (for example the Salmon which spawns in Fresh water, but the rest of the Salmon's life is spent in salt water).
Anyway, the theory is that fresh water fish MUST accept what ever is dished to them on the river they are in. Fish water fish thus developed long term solutions for bad conditions, for example surviving low water conditions (Dig in mud OR just spawn before hand and the eggs settle in the mud as it drys, and when the water returns grow into adult fish then).
Notice the survival mechanism of Salt water fish is to swim away from the problem, in the case the mass extinctions, there is no place to swim. Fish water fish lay eggs during the bad times, and the eggs only develop during good times, so during the mass Extinctions, the adult fish water fish all died off, but their eggs survived so when good times came back the species came back.
Amphibians also survived the KT mass extinction without any noticeable disappearance of Species, probably for the same reason as fresh water fish, the adults may have died off, but the eggs would have survived.
For more see the following paper:
http://www.ugcs.caltech.edu/~presto/cenozoic.pdfThis site says sharks and rays were hard hit but not other salt water fish:
http://palaeo.gly.bris.ac.uk/Communication/Goddard/page1.htmlAnother article makes this comment ("Stream Communities include fresh water fish):
In stream communities few groups of animals became extinct; because stream communities rely less directly on food from living plants and more on detritus that washes in from land, buffering them from extinction. Similar, but more complex patterns have been found in the oceans. Extinction was more severe among animals living in the water column than among animals living on or in the sea floor. Animals in the water column are almost entirely dependent on primary production from living phytoplankton, while animals living on or in the ocean floor feed on detritus or can switch to detritus feeding.http://www.absoluteastronomy.com/topics/Cretaceous-Tertiary_extinction_eventIn simple terms if the species was dependent on Algae for food, it was hard hit by what ever happened in the K-T period, but if it was NOT, either if it was deep water OR Fresh water, it survived (This is a better theory then I have read in the past, which depended on the ability to move around as explained above). It also so I was half-right (and also half-wrong). I was right that fresh water fish were NOT affected, but wrong when I said salt water fish were affected. Deep ocean fish were NOT affected, but fish dependent on Algae, directly or indirectly, were affected by the die off of the Algae caused by whatever happened in the K-T period.
A little research helps but my memory was not that bad given what I found on the net about the K-T period of fish.
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