Seasonal nursery ponds allow for time for the pond to rest between each stage of new fry, preventing unwanted algae and fish from getting a foothold, and helping remove unwanted toxins. Yearround designs cost more in energy costs, through growlights, heaters, generators, etc. Fry typically don't handle extreme cold or heat, meaning that nursery ponds may be left dormant in these months anyway.
Allowing the pond to fully dry out and be exposed to sunlight can be a powerful way to further disinfect and refresh your nursery pond. Sunlight can oxidize stubborn compounds like sulfides, and helps breakdown any leftover waste or organic matter.