1. Can One Dorsal Fin Show Which Dolphin It Is?
When you look at the sea from a boat, you see a dolphin’s dorsal fin cut through the water. Most of the body is hidden underwater, so only the fin and part of the back are visible. Yet, researchers can identify each dolphin as "that one" [2]. The clues are the nicks and cuts on the back edge of the fin. Marine Mammal Research Australia explains that individual identification is based on these naturally occurring nicks and cuts [2]. The fin is thin and easily damaged. Because nicks usually remain, they are used for identification over long periods [3]. The method of identifying bottlenose dolphins by their fin nick patterns began with research in the 1970s and continues in current studies [1]. So, where do these nicks come from?
For this method to work, the nicks must be different for each individual and last a long time. Next, we look at where and how these nicks form.
2. How Do Nicks Appear on Dolphin Fins That Are Smooth at Birth?
Dolphin nicks are not birthmarks. According to the Irish Whale and Dolphin Group, bottlenose dolphins are born with smooth, unblemished dorsal fins [3]. As they grow, nicks appear on the edges due to interactions with other dolphins, rocks, seaweed, and boat propellers. The position, size, and shape of these nicks are different for every dolphin [3]. A long-term research group in Sarasota Bay, Florida, writes that many nicks are natural, such as bite marks from sharks or other dolphins. Some are caused by human interactions, like entanglement in fishing lines or boat propellers [5].
Researchers use nicks, not scratches, for identification. Scratches heal and disappear, but nicks usually remain, making them useful for long-term identification [3]. Therefore, the nicks on a fin are a record of the time that dolphin has spent in the sea. How many of these "name tags" have been collected?
3. How Many Dolphins Have Researchers Identified from Fin Photos?
How many photos of these "name tag" nicks have been collected? The Sarasota Bay research program identifies each dolphin by its nick pattern, comparing it to human fingerprints. They state that about 95% of dolphins can be identified by eye. This 95% figure applies only to the dolphins in Sarasota Bay and cannot be directly applied to other seas. Their photo catalog includes more than 5,000 individuals from multiple areas on the west coast of Florida [5].
The U.S. National Oceanic and Atmospheric Administration (NOAA) laboratory also maintains a catalog of dorsal fin photos since 1995, which lists 2,423 individuals [6]. According to the same article, a January 2018 survey in North Carolina estuaries took 1,011 photos, identified 547 dolphins, and matched 228 with past catalog records. Photo matching also used a code called FinFindR [6].
As photos pile up in catalogs, the chances of saying "that is that dolphin" increase. But what happens if a nick changes?
The catalogs rely on the assumption that nicks last a long time.
4. What Do Researchers Do When a Dolphin's Nicks Change Over Time?
Nicks have weaknesses. According to the Irish group, nicks change over time. Several nicks may connect to form one long line, or new nicks may be added, changing the fin’s appearance [3]. Encounters with sharks or boats can also make identification difficult [4].
To help, researchers use body pigment patterns. A study used 30 years of photos of wild bottlenose dolphins. Both experts and undergraduates were asked if two photos showed the same individual. Even with only 10 seconds, both groups guessed correctly at a rate higher than chance [4]. This does not mean fin identification is unreliable. It is a strategy to use fin nicks as a base and supplement changing parts with other clues.
What do we learn when names and records are accumulated for a long time? In Japan, there is an island where this research has continued.
According to the conservation group, nicks merge into long lines or new nicks are added, changing the fin’s look [3]. So, if names and records are kept long-term despite changing nicks, what becomes visible?
5. How Are Dolphins Identified and Watched at Mikura Island?
Wild common bottlenose dolphins live around Mikura Island in the Izu Islands of Tokyo. The Mikura Island Tourism Association has conducted individual identification surveys for over 20 years, assigning names or ID numbers to each dolphin. About 120 dolphins are believed to live in the area (as reported in 2016) [7]. Names are said to be given based on features like body scars and fin nicks [8]. Names like "Ringo-chan" and "Hocchan" are easy to call.
Tracking each dolphin by name revealed new insights. Kindai University reported that "Hocchan," a young female (8 years old at the time), cared for the calf of "Ringo-chan" (the biological mother, 15 years old at the time), including nursing, for 102 days [7].
The Kindai University team has continued underwater video observations in Mikura since 1994. With many individuals’ actual ages and videos accumulated, they announced that belly spots begin to appear around the genital slit at about 6.5 years old and increase by age 27, differing between males and females [9]. This is research on belly spots, not fins. It is said to lead to a way of knowing age without touching the body.
Nicks provide clues for naming, names support continued records, and records generate new discoveries. At the end of these small nick name tags, the life of each individual dolphin becomes visible.
The flow so far is: nicks become name tags, weaknesses are supplemented by other clues and long records, and tracking by name leads to observing daily life.
6. How Can You Make a Paper Fin with Its Own Nick Pattern?
You need construction paper, scissors, and a pencil. Choose scissors with rounded tips and use them with an adult. Cut the construction paper into three triangular shapes resembling dorsal fins. Use the scissors to make 1 to 3 small nicks on the back edge of each. Sketch the position of the nicks on each fin separately. The next day, look only at the sketches and try to guess which paper fin belongs to which sketch.
Finally, add one more nick to one fin. Compare it with the sketch. How does it change the ease of identification? It is said that in real dolphin fins, nicks increase or connect, changing the appearance [3].
If you want to read more, the Science Portal by the Japan Science and Technology Agency has an article about dolphin parenting in Mikura. It is available in Japanese.
Use scissors with rounded tips and work with an adult. You can verify by hand the limit that nicks merge or increase, changing the appearance [3].