Why do Arctic predators accumulate vitamin A?
The phytoplankton at the bottom of the food chain have high levels of vitamin A and precursors like beta-carotene.  At each step up the chain, vitamin A bioaccumulates.  Top predators in the Arctic deal with this by storing the vitamin A in the liver (bears, cetaceans) or blubber (pinnipeds).  

This is the same mechanism that causes accumulation of pesticides like DDT in animals.  https://en.wikipedia.org/wiki/Bioaccumulation  

Carotenoids are part of the photosynthetic system, and also protect the organism from sunlight.  Phytoplankton are unicellular, so each cell needs protection.  Therefore land plants contain much less carotenoids, since most of the cells are interior and protected from the sun.  For example, grains, roots, and wood have very low levels of carotenoids.  

This occurs in polar regions due to up-welling nutrient-rich waters supporting high productivity.  Temperate and tropical deep ocean regions are typically deserts due to low nutrient levels, particularly iron and fixed nitrogen.