You need to do the biobay tour when there is NO moon.
COmpanies are not alloowed to fo the Vieques biobay tour 1 day before and 1 day after full moon day, meaning at least 3 days of each month , you cannot do the biobay tour.
Bioluminescent Bay Moon Phase Calendar
One of the most popular activities for visitors here in Puerto Rico is to visit the bioluminescent bay (bio bay). Puerto Rico is fortunate to have not one but three bioluminescent bays; Mosquito Bay in Vieques, Laguna Grande in Fajardo and La Parguera in Lajas. Best one is by far in Vieques.
The bio bay at Mosquito Bay is the largest and brightest (healthiest) and was recently named in Coastal Living’s Top Ten Coastal Wildlife Hot Spots.
The bio-bays glow is caused by micro-organisms (dinoflagellates) which glow whenever the water is disturbed, leaving a trail of neon blue. The best time to view the bioluminescent bay is when the night sky is dark due to little or no moonlight.
You can contact any of the bio-bay tour operators for information about the expected conditions for the night that you would like to take the tour.
The calendar below is based on the the various phases of the moon for the upcoming months. As you can see there are certain days during each month, when a tour of the bio-bays are not recommended. The best days to go are either side of a NEW moon and you should try to avoid days either side of a full moon.