The Mysterious Fish at Han’s Reef

The Mysterious Fish at Han’s Reef

It started out as a typical Saturday for the Gili Shark Conservation Research Team. We spent the morning diving and then headed to our home base to prepare for the afternoon’s Community Day Event. While collecting materials for our upcycling workshop, we received an unexpected call from our partner dive shop, Oceans5 Dive Resort.

One of their captains had spotted a Whitetip reef shark, belly up, floating on the surface of the water, and they wanted our shark team to have a look. We felt saddened by the news of a dead shark but were eager to take this research opportunity and discover precisely why it had died.

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A very unusual case

Interestingly enough, the shark had been found at Han’s Reef, a dive site located on the Northeast side of Gili Air, Lombok, Indonesia. Han’s Reef is a macro lover’s paradise, best known for muck diving. You’re likely to find frogfish, nudibranchs, and seahorses at Han’s Reef,  but it’s not a typical place to spot a Whitetip reef shark.

We headed over to the Oceans5 boat where the captain had hauled the shark onboard for further examination. Unfortunately, it was in much worse condition than we had initially expected. The shark was a male, about 81 centimeters long, and weighed approximately 3.4 kilograms. Over half of the shark’s head was gone, and it was missing both eyes.

However, apart from its head, the rest of the body seemed to have no external damage. There were no signs of damage from a spear gun, fishing hook, or any other damage to the external organs. We quickly noticed that this shark had two spines in front of its dorsal fins, something which Whitetip reef sharks lack. Blacktip reef sharks are also commonly found around the Gilis, but we could rule that species out.

If this wasn’t a Whitetip reef shark or a Blacktip reef shark, then what was this?

Identifying this shark became the next challenge for the research team, and without most of its head, it was going to be difficult. Fast forward through a movie-worthy montage of phone calls to colleagues, books, and poring over the internet research and we had a confident conclusion. This shark was actually a Mandarin dogfish shark.

Mandarin dogfish are a bottom dwelling species of shark, usually found at depths of 149 – 650 meters. Commonly found in waters around and near Indonesia, Taiwan, and Southern Japan, they have little to no value at the fish market. So what was this fish doing at Han’s Reef? What was it that killed this animal?

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Looking for answers

The only way to find out was to take a look inside the dogfish. The first thing we noticed was just how large this fish’s liver was – it took up about 80% of its internal organs! Sharks are known to have large livers to begin with, but this size seemed abnormal. Instead of having a swim bladder to maintain buoyancy, a shark’s livers is filled with an oil that is lighter than water.

Due to the current state of our oceans and the sheer amount of pollution we see, we firmly believed that the killer of this dogfish would be a stomach full of plastic. Unfortunately, it’s an all too common that ingested plastic is a leading cause of death for marine life found floating on the surface. However, there was one problem – we could not see the stomach. Baffled, we searched and searched, knowing that this fish had to have a stomach somewhere inside. 

Eventually, the stomach was found, and it was virtually unrecognizable. The fish’s stomach was so small, it looked more like a gallbladder, especially when comparing it to the shark’s oversized liver. To our dismay, when we cut open the stomach to analyze its contents, it was empty. This dogfish hadn’t died due to ingesting an obscene amount of plastic or being attacked by a bigger fish. It had starved to death.

How was this possible that this fish died from starvation?

As our research team regularly conducts “roving survey” dives where we collect data on the abundance of indicator species, we knew that this dogfish should’ve had no problem finding its next meal.

We, quite literally, needed to dig deeper into the dogfish. We continued our dissection with an incision along the throat, where we found a fishing hook lodged in the shark’s esophagus. The hook in the shark’s throat was stuck in such a way that nothing the shark ate would’ve been able to pass through to the stomach.

The dogfish was likely caught by a fisherman and released, or got away somehow. The exact scenario is unknown, but when the dogfish swallowed the fishing hook, its fate was sealed. The bite out of its head was likely from a large grouper and was most likely made after the shark had already died.

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Our research wasn’t over yet…

When we finished recording the data and drawing conclusions, our research and use of the dogfish was not over. We are able to use the fish as bait for our Baited Remote Underwater Video Camera (BRUV). The BRUV uses bait to attract sharks to a video camera, which allows us to observe sharks in their natural habitat. It’s about as close as we can get without actually being in the water with them! We use the footage of the BRUV to collect abundance data which helps us answer many questions about the Gili Matra Marine Park, and whether or not the abundance of sharks and the size of the marine park are related.

We are especially looking forward to this upcoming footage featuring the Dogfish because we expect other sharks to really go after the oily meat of the dogfish, a delicious treat compared to our usual bait of mackerel!

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