That is somewhat perturbing. I just got back from Bonaire and the reefs were healthy with lots of fish everywhere. I'll be on Brac in a few weeks. I hope that the contrast will not be as severe as it sounds.
Go to the "Cayman Picture of the Day" thread at the link below and then search "This Thread Only" for "Brac" to see very many underwater photos taken by Scubaboard members while diving Cayman Brac. I don't think you should worry about finding healthy reefs and lots of fish around Cayman Brac.
Cayman - Picture of the Day
I love Bonaire and the Caymans, they are each wonderful in similar/different ways. I can tell you that you are likely to see more turtles, rays, and groupers on Brac then on Bonaire and you have a better chance of seeing sharks.
I was just looking at some articles published by reef research organizations. Bonaire, Little Cayman, and Grand Cayman have been studied extensively and have documented their determination to protect their reefs and natural beauties. I saw references to Bonaire as the "biodiversity hotspot of the Caribbean" and to Little Cayman as one of the best places in the Caribbean to "try for a Century Dive" to spot 100 species on a single dive.
Brac hasn't been studied as much as these other sites - mostly just to confirm that they have a large, healthy, breeding, grouper population. But Brac is only 5 miles from Little Cayman and they are governed by the same environmental protection regulations so you are likely to find healthy reefs and fish IMO.
The water in Cayman is colder during January than in Bonaire, I don't know if there is a seasonal variation in fish numbers in the Caribbean due to water temperature? Maybe somebody else has the answer to this question?
But if the weather cooperates - which is big factor for any dive vacation - I think that you will have a lovely dive trip to Cayman Brac; and it is such a small, funky, friendly little island - with natural beauties both above and below the water - that you may come to love it, too.
I will be interested to read your trip report!
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