This is the seventeenth entry in an ongoing monthly (or almost monthly!) series profiling the amazing identifiers of iNaturalist. Mark is also a member of the LGBTQIA+ community and we’re posting this for Pride Month! He “currently lives in Cambridge, Massachusetts with his husband, three ferrets, and a couple anemonefish.”
Mark Rosenstein joined iNaturalist nearly eleven years ago, and during that time he’s provided close to 500,000 identifications for other users. That includes over 235,000 ray-finned fish identifications, making him the top identifier of that taxon.
As a child growing up in San Antonio, Texas, Mark’s first connection to nature was as a butterfly collector, although he also liked watching Jacques Cousteau documentaries. “That's part of what prompted me to get a saltwater aquarium after college,” he explains, “[and] that led to me learning a lot about tropical fish, and eventually setting up a living reef tank.”
I was working in computers at MIT in 1992 when I heard Tim Berners-Lee give a talk about his recent invention, the world-wide web. I built my first website, FINS, about the aquarium hobby, in 1993. As part of this website, I created a listing of fish species available in the aquarium hobby, taking photos of fish in aquariums to illustrate it. This led to me learning a little about taxonomy for the first time. My interest in saltwater aquariums led to me getting scuba certified so that I could see them in their natural habitat, not just in a glass box on life support.
His first few diving destinations were to the Caribbean, but his eyes were opened on a later trip to Fiji with ichthyologist Gerry Allen. “This was the start of a more serious study of fish taxonomy on my part (as a hobby; by now I was running a web company),” Mark says.
He continued to visit spots in the Indo-Pacific region, upgraded his camera gear, and started taking lots of photos.
My regular dive buddy commented that I had probably photographed every fish in Fiji, and that led to the suggestion to produce a field guide, as we were disappointed with mistakes and missing info in the guides we were using at that point. I naively started that project, with no idea how big an undertaking it was. I spent about four years putting that together, and self-published Fiji Reef Fish shortly before the pandemic made everyone quit traveling or diving for a few years
Mark also started getting into birds due to seagoing avians he found while on the water, and it was at a bird club meeting in 2012 where he first heard about iNaturalist.
After entering some random recent photos that were mostly birds, I decided to try to enter my photos that weren't birds or fish, to use iNat to build a life list of other animals. After just playing with iNat for a few months, I was hooked, and started entering everything I photographed…
iNat then began to influence what I looked for and photographed in the field. I started trying to photograph one individual of each species of bird I saw each time I went out. Yes, that meant that every time I went out I would photograph another Robin, another House Sparrow. This gave me photography practice, and I have become much better at getting successful shots by doing this. And it gives me another way to see how common some species are, since I end up with many photos of them. I eventually started doing the same thing on dive trips. On a typical ten day drive trip to Fiji, I see about 500 species, and manage to photograph over 400 of them. And then once I get home, post them all to iNat.
Not a morning person, Mark was happy to find that insects, unlike birds, are active later in the day. “I spent a while working on odonates (dragonflies & damselflies) and Greg Lasley (@greglasley) who I met through iNat mentored me on those…I [also] resurrected my childhood interest in butterflies. I joined a local butterfly club, and also learned a lot from Liam O'Brien (@robberfly here on iNat). These days when I'm not traveling for scuba diving, I spend much of my time in the field searching for butterflies.”
When it comes to identifying, Mark explains that “for several years, I actually skimmed pretty much every observation posted, until that became too many to keep up with. These days, I do two kinds of identification: fish for anywhere, and lepidoptera for North America.” If he’s busy he’ll only add IDs to the obvious ones, but when he has time he’ll open up his field guides and get down to identifying species he’s not as familiar with. He’ll also sometimes go through Research Grade observations to look for any that need correcting.
I have written my own cheat sheets that list groups of confusingly similar species and what to look for to identify them. I've created public web pages with a few of these at https://fijireeffish.com/tips/ I also have amassed a collection of several hundred scientific papers from the primary literature and will refer to these for some identification challenges. As I learn new ways to identify fish, I also update the descriptions in my book. Someday I will publish a second edition with these updates.
One reason why he enjoys identifying on iNat is that, as someone who has experience cleaning up databases, he finds a “certain satisfaction” when he does that on iNat. “But the better reason,” he tells me,
is that I learn a lot while doing it…Identifying species I know well reinforces [my] knowledge so that a year from now when I next dive in that part of the world and see the species, I am more likely to remember it. And when I see an observation that I don't know, sometimes I will research it. I really learn a new species by then looking at all of the photos of that species on iNat, to get a sense of the range of variation within the species.
My primary area of study (reef fish) is something I can only do in person a couple of times a year. But I can look at pictures of reef fish every day and by identifying them for others, keep my ID skills sharp and the names fresh in my mind.
Some of Mark’s tips for taking identifiable fish photos:
- try to get a side view. Many people who are snorkeling or in a glass-bottom boat will take a photo looking down on a fish, but that often does not show necessary field marks.
If you're going to stand on a wharf and shoot fish in the water, the non-flat surface will distort the view. Take many many photos this way, and look through them to find one that has a pretty clear view of the fish. Or attach several of them to the observation.
- photos where the subject fish is tiny in the frame (such as frame grabs from GoPro video) are very difficult. Get as close as you can to the subject. If you nonchalantly drift towards a fish, you can often get quite close. Just don't expect the fish to let you charge at it full speed.
- those fish that are always moving, seemingly away from you, like wrasses--often they are swimming the borders of their territory. Just wait and it will complete the circle and come back towards you. If you do chase a fish into a hole, decide how badly you want the shot. If you back off ten feet and wait 3-5 minutes, it will probably reappear. But you may not want to wait that long.
- (this goes for any wildlife photography) Take multiple shots of each subject. Wild animals usually do not pose for you. A single shot may have an awkward pose. They might be pointing their pectoral fin directly toward the camera, so you can't see the pattern on it. With several shots, you can choose the best one to post.
- if you know that you are shooting a fish that is an identification challenge, try to get it with the fins spread and really sharp focus. Often the best way to separate similar fish is to count the little bones in the fins. But if you know this during the dive, and have the skill to get that kind of shot, you probably aren't looking for advice from me.
- those using scuba and a real camera rig probably already know this, but there are a few guidelines for good underwater photography. Always get as close as you can to your subject; water absorbs and scatters light a thousand times more than the same distance in air. Ideally get a little below your subject and shoot up at it; this will give you a blue background. If you are going to shoot deeper than 15 or 20 feet, you need artificial light--either a strobe or movie light--on your camera. Sea water absorbs red light, so as you go deeper you need to compensate, either with colored filters, adjusting the white balance, or using enough artificial light to counter the effect. Underwater photography is an excuse to throw money into the ocean.
Two of Mark’s go-to references for fish are Reef Fish Identification: Tropical Pacific by Allen, Steene, Humann & Deloach, and Reef Fishes of the East Indies by Allen & Erdmann. Both are available in physical and electronic form, and Mark says he’s written hundreds of notes in his copies.
