There are lots of reasons for food shortages. You thought of one, that their food might be moving. But if this is a regular thing, we would expect the whales to just go where the food goes.

Whales eat different things. Some filter plankton from the water. Others have teeth and eat fish, squid, and other foods. The supply of these foods can be low because their food supply is low.

In other words, the plankton that do photosynthesis (phytoplankton) may not have enough nutrients or the right temperature to grow and reproduce rapidly. This would mean fewer of the zooplankton that eat them.

The food species could have low populations because of diseases.

The animals that compete with whales for their food could be taking more than usual.

As you can see, lots of things can happen to change whale food supplies. How do you think changes in ocean currents might influence their food?