>I wanted one more chance to do better and became obsessed with finding another cookiecutter shark. Looking for ways to do blackwater dives from my home island of Oahu, I found weather windows and made friends with people who had boats. A sea anchor allowed us to go out weekly, and every week we found a completely different assortment of squids, jellies and fishes.
>It didn't take long before my fascination with cookiecutter sharks expanded to include the other unusual creatures that greeted us every Saturday. I started to keep track of the times we would see pelagic seahorses, box jellyfish, exotic larval fishes and pyrosomes. I eventually moved to Kona, where I picked up a part-time gig guiding blackwater dives.
>I still had a lot of unanswered questions lingering from my time on Oahu. The unknowns that plague blackwater divers are often simple, such as "When is the best time and/or place to go?" I developed a more robust and comprehensive study from the notes I took on Oahu, so I could compare my observations against environmental variables such as moon phase, seasonality, bathymetric depth and water temperature. I also looked at more esoteric factors such as the El Niño–Southern Oscillation (ENSO), the North Pacific Gyre Oscillation and the development of cold-core eddies.
>Through an analysis called a generalized additive model, I was able to see how these multiple variables influenced the biodiversity in my data. The most common creatures we encountered were all soft-bodied animals such as salps, siphonophores and ctenophores. Moreover, I found that bathymetric depth, water temperature and the ENSO index were some of the best predictors for oceanic diversity.
>ENSO is a periodic set of ocean conditions resulting in what we know as the El Niño and La Niña events. It is difficult to predict, but it contributes to productivity. The peak depth range of the ocean bottom was around 5,000 feet (1,500 meters), a zone where the island's slope starts to plateau. The best water temperature was around 78°F and correlated with late spring and autumn temperatures, which is also when we tend to see more larval fishes. It seems that the fishes know exactly when to spawn so their young can develop in food-rich water. Using only water temperature, depth and ENSO, we were able to explain 43 percent of the variability in diversity over the study period, which is a good start.
>"What is that thing?" is another basic question that is common to blackwater veterans and novices alike. There aren't yet any field guides to blackwater diving, so we put together identifications piecemeal from all kinds of sources. The more sources we have, the more accurate the identifications tend to be. The reality is that the ocean will constantly offer surprises, and we will occasionally have to revise identifications. While guiding a dive one night, I looked down and saw a small species of shark approaching one of my divers. I assumed it was a cookiecutter shark going in for a scoop, so I swam down to lend assistance. The animal didn't look right for a cookiecutter though. It was darker, bluish and lacked the collar that cookiecutters sport, so I snapped a photo and sent it to an expert, who told me that it was a pygmy shark.
>Recently while doing a safety stop, I was about to leave the water when Sarah flashed her light across my mask and made the sign for "shark." I immediately recognized the small, unusual fish swimming past us and easily caught up with it. This time I was prepared and spent a few minutes swimming alongside the shark, enjoying the encounter. It was a little more than a foot long and had tan coloring and a dark collar at its gills. Finally, 10 years after I originally missed what I thought was a once-in-a-lifetime opportunity, I was able to take 14 crisp photos of a live-swimming cookiecutter shark.
>The 10 years of searching enriched my life in ways I could have never predicted. By asking some basic questions, I made my time in the black far more important than just some simple photos of an unusual shark.
>Learn more about the cookiecutter shark in this video.
>© Alert Diver — Q1 2020