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Nassau Grouper Spawning Aggregations

February 7 2018

In the Cayman Islands, there used to be five Nassau grouper spawning sites and all but one is still active. This expedition is to take a look to see if some of those dormant sites have been revived post regulation. The expedition is made possible by and is under the direction of the CI Dept. of Environment.

February 7 2018


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Debriefing Stage

The spawn is over this year. The folks over at the DOE did another few dives in the last few weeks on the "lesser moons" and it turned out there was activity as well but not the height of SPAG that can be seen on the major moons.

The Trident has been sent back to California for a maintenance cycle and then back out to some adventure.

Before closing this, there are a few ways you can help/get involved. The first is head over to the website and make sure you are spreading the word about that organization and the great work they do. The second is to make sure you and all your dive buddies are supportive of conservation efforts around the great Nassau grouper.

Until next year, Zack out.

Mission Underway

The window for me had closed and I needed to get back to California. I left my Trident behind as it sounded like the weather was going to clear up on Monday.

The team, now well trained and equipped, seemed to have some great success finding grouper on East End. I will let the video do the talking.

Even though we didn't get to hit the SPAG during a spawning event to do a full population survey we were able to more or less confirm that these "dead" sites are still so. This makes the work to protect the active sites even more important for the folks at REEF and the DOE. The Trident seemed like it could really help in this mission again. The active sites, even through we didn't catch it mid-spawn, was definitely showing lots of grouper activity which is a good indicator that we were in the right place but just at the wrong time.

I will definitely get down there next year.

I'm going to upload the second half of the amazing dive highlights in just a bit along with some ways you can help out too, for now it's time to sign off.

Massive thank you to REEF for making this possible and to the DOE for their tireless work and hospitality.



As the weather would prevent us from working out on East End, it was decided that I would leave the Trident behind with the DOE team to try later in the week if the weather permitted. Today, we could use to get them trained up as pilots and practice our freeboating skills.

We set out in the mid-morning from the south-end docks and motored to West End. Everyone got a chance to fly, manage the tether, and drive the boat. All required a unique set of timing and micromanagement skills. We really started to click after a few deployments.

A few lessons learned: *When freeboating, reversing the boat towards the direction of the tether seems counter-intuitive but was actually quite effective. It allowed the boat to steer more like a car and kept the tether visible to the person running the tether. *How much tether is in the water is directly proportional to the amount of steering authority the vehicle has. Paying out too much can have a big effect. The ideal amount of tether in the water seems to be roughly twice your depth. Keeping the boat over the Trident is necessary for this to work. *When the pilot needs some emergency steering authority to chase something or get a look at something the tether op can throw about 5m into the water. Keeping 10-15m on hand for emergencies is very important. *Even if there appears to be a good deal of slack in the line, pulling the tether affects the steering of the ROV even across a big area. It's best to notify the pilot when you're pulling some in so they can tell the difference between current and the tether being pulled in. The exception to this is if you can back the boat up just by the tether to create actual slack at which point the tether op should take this opportunity to pull in as much as they can to increase the steering authority of the ROV. *The tether op and skipper need to keep in verbal contact to avoid pulling the tether into the prop. The best practice is to back up with the tether down current slightly as if you were parking beside the tether. *Having a daylight readable screen was very important. Finding a good, low-cost one is high on my to-do list now.

After some practice runs we went up to the Northeast end of the Island where we managed to find quite a surprising number of Grouper. No Nassau's but there were some spectacular Tiger Groupers in the area. Check out the video!

I was really sad to be leaving before getting some video of the Grouper in full-colors at a SPAG...but as Bradley said, the bad weather is probably the best thing for them. Nobody fishing or bothering them so they can do their thing.



The wind and seas were still looking quite bad for a deployment out to "East End" but it was going to get worse so the decision was made to go. The windward side of the island is not only subject to stronger on-shore winds and groundswell but also has some notorious currents where the deep ocean hits the "wall" of the shelf surrounding the island. Perhaps because of the regularly rough conditions this is also where our Nassau grouper spawning aggregations are still active. The tricky part is discovering exactly where.

The importance of these aggregation spawns is something that seems to be of growing importance the more I hear about them. Of particular interest in finding them with Trident is so that we can get some good survey videos of the area for a population estimate. If this is one of the last SPAGs in the area, then it stands to reason that all the individuals in the area will be in one place and easy to count.

The challenge of open-ocean deployment in heavy seas and nasty 3-dimensional bottom currents was apparent from the first dive. The boat had a very hard time reversing into the wind with person-high rollers crashing over the back of the 29-foot boat. Meanwhile, being the pilot, I had to wrap my arm around the upright of the welded metal Bimini top that was providing meager shade for the tablet to keep me and the tablet in the boat.

The most effective thing we found was to get slightly ahead of our target area and use the 1-2 minutes it took to be blown over the area to dive the Trident straight down at full speed. Once near the bottom, we could then take a quick look and get hauled back in. The practice from yesterday was really crucial to getting this right as it required 4 people to coordinate. Paul would manage the tether, which required quick and deft spool out to get as much tether in the water to keep up with the straight dive while not throwing any kinks or twists over. Bradley would drive the boat backwards to slow down the drift while turning into the rogue windswell set waves that would approach at 90 degrees and rattle the DOE "Pegasus" maliciously. Claire was watching an external monitor we hooked up to my tablet to keep an eye out for Grouper which was a good idea since I had to try to keep the Trident on course, stay in the boat, and not loose my lunch...which I wasn't very good at it turned out.

In the end we did find a few Nassau groupers which was very encouraging but they always seemed to move just out of reach or just before running out of tether as the current pushed the Trident off the shelf. There were some amazing dives out there though, the best are captured in the video.


Bradley explained that the area we'd be surveying is an old spawning aggregation site (SPAG) that had been "fished out." He expected to find other fish like snapper and perhaps some other species of grouper but that today would be more or less a training day to test the equipment and get used to the chaotic orchestra that is freeboating while running transects in the open-ocean

He went on to explain that the Nassaus are picky about where they show up. They come from all over the island and previous population surveys indicate that there's a large population in and around Grand Cayman. The juvenile fish join in the SPAG as well and importantly this is how they learn where the sites are and more or less how to get down to it once they are there. These sites are often too deep to monitor and this is why there is still so much unknown about them. Because sites can become "fished out" it indicates that fish who spawn at one site don't know about the other sites and further indicates that protection is needed for the few remaining SPAG sites.

It wasn't until recently that it was known that something needed to be done and just how easy it was for a few number of fishermen to hit a SPAG site and really deplete the population. Things were starting to get really critical before the DOE was able to get regulations in place to protect the remaining sites.

Currently there's reason to believe that some of these sites might be returning but that it may also be the case that these sites are gone for good.

We launched the boat from the put in and motored up to a shallow mooring to do a systems test. Mostly went well but getting a hang of putting in and pulling out the tether was challenging. After a few dives we got our confidence up and motored to the area around the SW point that we wanted to survey.

As you can see by the video there was a lot going on down there but no grouper to be seen. Lot's of black durgon, snapper, and a few surprise visitors (see video).

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DAY 1 - Morning Breifing

Morning Briefing. 9AM at the DOE building.

Bradley, Research Officer at the Department of Environment was leading the mission. Paul and Claire would be on hand to provide support. The plan was to check out the Southwest Point. The weather was quite bad and heavy seas and strong winds would prevent us from getting out to the other sites on the perimeter of the island.

These areas would be between 25 and 50 meters depth. Most of the sites would be at the edges of walls or steep drop offs. The bottom would be mix sand and rock with some structures up to 2-3 meters proud of the bottom to avoid. Any rock would be covered with sensitive corals, fans, and sponges so it would be crucial to have good tether management and keep well off the bottom and we'd either have to use the existing moorings or maneuver the boat above the ROV using the motor to compensate for the current/winds. Called, "freeboating" this can be quite tricky.

In terms of what to film we primarily wanted to see if there were any Nassau grouper in the area. At this time of day, close to a spawn, if we were in the right spot we would see them sort of sparsely hanging out near the bottom. The idea would be get Trident onto the edge of the wall and either drive over it and survey down, nape of the earth style flying, and record any grouper sightings. Often time other grouper besides Nassaus can be a giveaway too that you're in the right area.

I gave a brief introduction on Trident and went over some of the basics of operating Trident. We loaded up the boat. Made a run for sandwiches and launched the boat.

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Preparation Stage

Adventures often start with a phone call. This one was with Brice Semmens at Brice wanted to see if I could get a Trident and a pilot down to Grand Cayman for the weekend to see if I couldn’t use it to help find evidence of an annual grouper spawning aggregation. I had plans to go check out the super blue blood moon eclipse which was supposed to be a once-in-a-lifetime sort of thing here in San Francisco--but so might a chance to use Trident to see something like that in person. I learned that there’s a group of folks from REEF that volunteer their time in the Cayman Islands to help the authorities assess the population and general health of the Nassau population during these spawns. More specifically, on Grand Cayman, there’s the need to look evidence of any rebound of the Nassau Gropuer spawning aggregation sites that are supposed to be dormant or possibly rebounding. The challenge is that the sites are often quite deep so divers can’t stay down long and can’t visit that many sites. They’d be interested to explore the possibility of using the Trident to see if it can help. It seems that the super blue moon wasn’t just interesting to us...but for the Nassaus as they attend these spawning aggregations on the first few days after a full moon in January.

“You’ll love working with the DOE guys,” he said. “Sounds good. Can’t wait.”

Packing list: *Two Trident beta prototype units, “Argon” and “Potassium” (all are named after elements) *1 25m Tether *1 100m Tether Topside wifi modules, cables, and tablets *Snorkel/Mask should the need arise *GoPro *Dramamine. It was supposed to be quite rough.

Expedition Background

The Nassau grouper is one of the Caribbean's most iconic fish. They are friendly, curious, and huge draw for divers from all over the world. Each year during a winter full moon, the usually solitary grouper travel to specific sites to spawn. These spawning aggregations, (SPAG) have been their downfall as fishermen lucky enough to find them can remove huge amounts of fish. In the Cayman Islands, there used to be five Nassau grouper spawning sites and all but one is dormant and depleted.

Tireless work by conservationists, environmentalists, and volunteers have led to regulations that have been in place for some time and this expedition is to take a look to see if some of those dormant sites have been revived. The expedition is made possible by the wonderful people at who I have had the pleasure of working on a previous expedition.

This expedition is under the direction of the Cayman Islands Department of Environment.

There's a great documentary about this by Changing Seas:

For further reading:

(photo credit: