When would male be expected to improve his fitness by attempting to mate at sea rather than establishing a harem?

The correct answer for the above question is;
A small male will do better mating at sea when suitable beaches are rare and successfully defended by large males.
The condition that would make the female on the other hand to improve her fitness by mating with a small male at sea rather than with a large male defending harem on shore is If the female offspring's survival is improved more by her continuing to feed at sea than the superior genes of the beachmaster. 

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