Stop mobs from despawning

Heyyww!

Im spawning some entities at set locations!

Code:

    for (int i = 0; i < 2; i++) {
       Silverfish silverfish = Bukkit.getWorld("DepartedWorld").spawn(murtlapSpawnup1, Silverfish.class);
       silverfish.setCustomName(ChatColor.YELLOW + "Murtlap");
       silverfish.setCustomNameVisible(true);

     }

     for (int i = 0; i < 11; i++) {

       Silverfish silverfish2 = Bukkit.getWorld("DepartedWorld").spawn(murtlapSpawnup2, Silverfish.class);
       silverfish2.setCustomName(ChatColor.YELLOW + "Murtlap");
       silverfish2.setCustomNameVisible(true);

     }

So, they spawn (all fine) but the current problem is, they despawn upon leaving the area, when the chunck despawns they also do. Any1 has a solution? :3
(Or a better idea how I can load the mobs on every onEnable() so you can encounter them always?) :3

 

Get chunk on ChunkUnloadEvent, get entities in chunk, check if they're a Murtlap silverfish, save them in config with the chunk as key, respawn them on ChunkLoadEvent.