I am on a steep slope, at 25 meters depth, in warm water and with good visibility of about 15 meters, a gentle current carrying me along—10 minutes into the dive, I realize that this is a special site, and that we are diving it at the right time. A large school of a few hundred spotted, dark fish, each about 40 cm long, cruises below me. They are female squaretail coral trout Plectropomus areolatus, and I know what they are up to: They are getting ready to visit their male counterparts who must be hanging out above me, on the reef flat, in spawning colors while fighting with other suitors for the best spot. My colleague and friend Purwanto is following them to get some video. Purwanto has been monitoring reef fish aggregations for decades, but seeing this large school of squaretail females is special, even for him. Later in the dive we visit the area where the males have their territories—they are aggressive to each other, and many have bite marks. Never thought I would get to see a spawning aggregation site again, but here I am: The place is Meaterialam Atoll, Sermata Islands, and it is 5 November 2020, a couple of days after full moon.
The animal kingdom has many examples of individuals from one species getting together for the purpose of rest and reproduction. Many species of sea birds do so, for example the boobies and frigate birds of Manuk Island, and some mammals, like sea lions, from spectacular colonies on isolated islands. Bats gather in caves, or, like Indonesia’s flying foxes, on mangrove islands a few tens of kilometers away from feeding grounds on the main land. Komodo National Park has two such mangrove islands, eerily similar in size, shape, and distance from the mainland, and seeing the flying foxes depart en masse for their feeding grounds just after sunset is a highlight of a naturalist’s field trip. Less well known, but equally spectacular, are the spawning aggregations of reef fish such as groupers.
Let me first describe how a typical grouper aggregation looks like to a SCUBA diver. Most dives start deep, so the first species a diver would encounter are tiger grouper Epinephelus fuscoguttatus, between 25 and 40 meter. These large, bulky fish are not rare, but they like to hang out in crevices and between corals, so they normally do not draw the diver’s immediate attention. At a spawning site during spawning season, however, they are hard to miss. The diver would first see a few, then a few more, until she realizes that the place is crawling with them. Moreover, they have become more conspicuous then they normally are: Their coloration, normally a fairly boring brown with black blotches, changes into a flashing white with black patches on the cheeks. That’s a male saying “Here I am, and you are only welcome if you are female with a belly full of eggs”. There is a fair bit of fighting, and some of the fish have bite marks: Sex and aggression often go together.
Some of the sites that have a tiger grouper aggregation also feature an aggregation of a species that looks similar, but that is a bit smaller: The camouflage grouper Epinephelus polyphekadion. They are usually a bit shallower than the tiger groupers, typically between 25 and 15 meters deep. Then, towards the end of the dive, the diver may run into an aggregation of squaretail coral trout, Plectropomus areolatus. This is a pretty red coral trout with vertical bars and large spots—hence the alternative name polkadot coral trout. There may be hundreds of them just above the reef slope, on the reef flats, between 15 meter and 5 meter deep. Most of these are males, guarding a small territory of a couple of square meters, preferably on a sandy patch between corals. The females of the squaretails are a bit smaller than the larger territorial males, and they form schools numbering in the low hundreds on the slope just below the reef flats, their bellies distended from their payload of eggs. They look a bit darker than the males, and the pattern of bars that is quite pronounced on the males is less clear on the females. There are variations on this theme, and often the grouper aggregation sites feature spawning aggregations of other fish as well, as we shall see. By the way: The actual spawning happens at night, and witnessing that event is still on my bucket list.
Fisheries scientists have been aware of the existence of reef fish spawning aggregations for some decades, and Indonesia’s fishers know about them much longer than that. For the community of underwater naturalists, however, this phenomenon is still new, and few have seen one in its full glory. There are various reasons why so few underwater naturalists have seen an active spawning aggregation site.
Firstly, reef fish prefer to spawn at a few current-prone sites, often at the extreme tip of a reef. These are not “easy” dive sites! Dive them at the wrong tide or enter the water at the wrong point, and the group will drift away from the main attraction before they know what’s going on. It’s not necessary to be a champion swimmer to enjoy these sites, and at the right tide these sites can be easy diving even, but the failure rate is a bit higher. So the rarity of spawning aggregation sites, combined with somewhat challenging conditions for diving, means that few underwater naturalists have had the opportunity to witness this phenomenon.
Second, as any male or female of most species will agree, timing is of the essence if one plans to mate. Spawning aggregation sites are only “happening” a few months per year—here in Indonesia, October, and also April appear to be the favored months for reef fish to set up the next generation for their pursuit of happiness. Within these months, aggregations “peak” at either full moon or new moon. So an eager underwater naturalist only has a few days, during a few months, to get up and close with the private life of reef fish.
Finally, and it’s sad to end on this note, there are not that many aggregation sites left! As I mentioned above, fishers have been aware of these spawning aggregations sites for centuries or longer. Before globalisation of the reef fish trade, fishing on an aggregation site was just a quick and easy way to get one’s daily fix of animal protein. Over the last five decades, however, reef fish, and especially groupers, became the darlings of Southeast Asia’s seafood markets. Especially as a live product, groupers fetch high prices in Hong Kong, Singapore, and other urban wealth centers. The fishers of Indonesia realized that there was money to be made, and it did not take long before supply lines connected fishers living at Indonesia’s remote islands to the up-market restaurants of Hong Kong. Well, nature can handle modest fishing on an aggregation site, but which fisher goes home after catching a few fish when the fishing is still good? Consequently, many spawning aggregation sites have all but disappeared before underwater naturalists ever laid their begoggled eyes on them: Fished out, nothing left, game over.
So, diving an active, “happening” aggregation site is a rare treat, and for my part I never thought I would see one again in Indonesia. Back in the nineties, we monitored the aggregation sites of Komodo National Park, but only later we realized that we were looking at the few fish that the reef fish trade forgot to clear out. I know of a couple of sites in Indonesia that still feature reef fish aggregations, but they are few and far between, and none of them are pristine. I had to travel to Manus Island in Papua New Guinea to appreciate how a pristine spawning site looks like: Not tens, but hundreds or perhaps thousands of fish. So, let’s get back to the “forgotten” spawning aggregation sites of the Sermata, and have a closer look at them!
First, there is Meaterialam Atoll, a large reef encircling a lagoon at 128°10′ East and 8°10′ South, stretching about ten nautical miles from North to South and about five nautical miles from East to West. We first dived the western tip of the central part of this massive atoll. There were two fishing canoes hanging out there, which usually indicates that the site is fishy. They were from a nearby village, and after the dive we had a chat with them—they had a caught a couple of squaretail coral trout, which they kept alive for transfer to the holding pens in the lagoon of the atoll. We got to see a couple of tiger groupers at the beginning for the dive, but I did not see any camouflage groupers. There were many squaretail coral trout on the shallower part of the dive, numbering into the low hundreds.
The most numerous fish on the reef flats were humpback snappers Lutjanus gibbus—massive schools of them, clearly getting ready to spawn. The humpback snappers feature in the opening shots of the video posted on this page. Another species that was very common on this site was yellowlip emperor Lethrinus xanthocheilus—that’s the species caught by the fisher in the video—and it would not surprise me if these emperors were at this site for the same purpose as the squaretails.
Later in the day we dive the southern tip of the atoll, which had an even larger aggregation of squaretail coral trout. This was the site where we spotted the schooling females. The major surprise of that dive was a small aggregation, probably tens of individuals, of the largest of the coral trouts, the Chinese footballer Plectropomus laevis. I had not seen Chinese footballers for a couple of years, as they have become very rare. So finding oneself in a group of them is exceptional indeed. Chinese footballers have a striking color pattern as juvenile and as females: Black bars on a white body, with yellow fins. You can see one of them in the shot of the fish in the holding pen, towards the end of the video. Like many other groupers, females turn into males once they grow to a certain size, and as large males they lose there spectacular coloration, turning dark with black saddles.
Later in the afternoon, we visited the holding pens within the lagoon, which had squaretail coral trout, some Chinese footballers, and a variety of other groupers and snappers. These fish are valuable, hence the presence of army personnel… A couple of canoes were fishing in the channel to the east of the atoll, also catching squaretails (see the aerial shots in the accompanying video). That channel, marked with a fish symbol in the accompanying map, may actually be the major spawning site of that atoll. We had a chat with the manager at the holding pens, who told us that Meaterialam atoll is fished by one nearby community.
In eastern Indonesia it is common for villages to claim use rights of nearby fishing grounds, and this usually provides some level of protection. After all, if a village can be sure that any fish left in the sea will be available for them in the next fishing season, then they are less likely to engage in a “race to the bottom”: Catching what they can before others do. The ownership that the villagers take over the atoll opens opportunities for working with the villagers to ensure sustainability, possibly in combination with development of responsible dive tourism. The biodiverse and pristine atoll is among the last of its kind, and it deserves careful stewardship.
The third site we dived in Sermata, Tanjung Pomelu on Pulau Kelapa was a complete surprise. We were promised batfish (which we got) and maybe hammerhead sharks (which were probably just out of sight, making fun of us). This site, too, had tiger grouper at 25 meter or so, and squaretail coral trout a bit shallower. We decided to dive the same site, but a bit more to the northern part of the Tanjung (cape). That turned out to be a mind-blowing experience. I had seen sailfin snappers Symphorichtys spilurus before, but never by the thousands! They are the prettiest snappers you can imagine: bright yellow with blue stripes, with a steep head and a very high dorsal fin. I now wonder whether that school is a happy jest of Mother Nature, or if this is actually how things are supposed to be in the absence of mankind… Freak occurrence or last chance to see how things used to be, that school was a phenomenal sight. Nanda got some of it on video, and I enjoy watching it, but the actual in-the-water experience was of a different level.
Well, this report covers just two days out of a 25-day conservation trip with Seven Seas to Komodo, the Forgotten Islands, Banda, and Seram. I got something out of every day we spent at sea, whether it was spotting for whales and Fish Aggregating Devices, diving with sea snakes, chatting with fishers, or talking about conservation and sustainable fisheries with my fellow travelers. The groupers and snappers of Sermata, however, were special.
Source: The Seven Seas