if we take into account laguna islands* as a whole respawn, yes it have the highest ammount tortoise, just because is a huge respawn of 3 whole islands filled with turtles, now if we talk about strcitly where you could kill the most ammount of turtles per hour, only turtles, i would say fenrock is better if u dont overkill it. if somehow you overkill it then laguna islands* are way better too.
Now the reasoning is simple, Meriana islands isn't 1 respawn, is composed by 3 HUGE islands filled with 3 floors of turtles, 1 single* island is hugest than fenrock respawn, and 1 single island of those 3 have more turtles than port hope (which is a huge respawn but poorly populated, most of the floors are filled with cocodriles and have few turtles scattered), now, the only trouble with laguna* is that simply is too huge, so talking strictly about turtles they are almost as scattered port hope ones (beside having likely twice the ammount), so in fenrock these are better distrirbuted but with only 1 medium sized floor of them, while laguna have 3 big sized floor filled with them, no point of comparation is literally impossible to run out of turtles in laguna islands being alone