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Introduction

Does an Atlantic spotted dolphin have spots? Of course! Well… most of the time. Let me explain. These roughly 200lb, 5-7ft dolphins have a beautiful spotted coloration that changes with age, and actually starts off with no spots! That’s right, a calf is born with no spots at all, and can be mistaken for a baby bottlenose dolphin (though spotted dolphins are much more slender). This age class is known as two-tone and last from about 0-3 years of age. Then they begin to gain spots on their white bellies between 4-8 years, and this age class is knows as speckled (juvenile). Next they get white spots on their darker top between 9-15 years, and this age class is known as mottled (young adult). Finally, the spots begin to fuse together forming more of a coloration pattern vs. individual spots with age 15 years plus, an age class known as fused (adult). These spots allow two things: 1. the ability to identify individuals (as long as you get pictures regularly to keep track of the changing spots!) and 2. to know the age of the individual without knowing when it was born, which you cannot do for most species. This species has more spots than their pantropical spotted dolphin cousins, and interestingly are genetically more closely related to bottlenose dolphins than they are other spotted dolphins! So the taxonomy is still not fully understood.

Adult and calf Atlantic spotted dolphins. Note the difference in spotting patterns between the adult and the calf! Photo: Cultura RM/George Karbus Photography.

Distribution

Atlantic spotted dolphins are found in the warm temperate and tropical waters of the Atlantic Ocean. Their range includes U.S. East Coast waters (Gulf of Mexico to Massachusetts), the Bahamas, Brazil, the Azores and Canary Islands, and Gabon. It is thought that warm currents such as the Gulf Stream may impact this distribution.

Atlantic spotted dolphins typically prefer to be in waters along the continental shelf, and usually live in coast or continental shelf waters ranging from 65-820ft deep. However they have been found in deeper water in the more northerly parts of their range. Two population stocks are recognized here in the U.S.: Northern Gulf of Mexico stock, and the Western North Atlantic stock.

Diet

Atlantic spotted dolphins feed on a variety of fish and squid. Sometimes they will hunt cooperatively forming bait balls of small fish in the water column, taking turns darting through the ball to catch fish. But these dolphins also love bottom fish, and in coastal/shallow areas will often feed on hidden fish in the sand like flounder and razorfish. They don’t have to dig too far, as these fish are usually just below the sand surface. In the Bahamas they will also feed on squid and flying fish in the deep water at night in over 1000ft depth. They cannot digest the squid beaks and pens and will regurgitate them. Researchers can collect these specimens and often determine the species of squid they ate! Since they are mostly in shallow water their dives are usually under 30ft and 2-6 min in length, but they have been recorded diving 200ft and holding their breath for up to 10 min.

Behavior

Groups are usually 5-15 individuals, but can be up to 200 individuals. The following behavioral information is from the population of Atlantic spotted dolphins studied by The Wild Dolphin Project in the Bahamas; less is known about other populations. These dolphins have a fission/fusion social structure, meaning they live in groups that constantly change, but have some long-lasting relationships – very similar to human society! Males will have alliances with 1-2 other males, and these can last for life (talk about a best friend!). Sometimes these alliances will team up together, creating a second order alliance that may be used to fight off another alliance, or enemy such as a shark or bottlenose dolphin (sometimes the bigger bottlenose dolphins are aggressive with the smaller spotted dolphins). Female relationships center around reproductive status, so for example, females with calves will hang out with other females with calves – they have similar needs and let’s face it, it is always nice to have help raising the kids! Mothers and calves have the strongest bond for the first roughly 2-3 years, as the calf is fully dependent on its mother. Gestation lasts for about 11-12 months and mothers will nurse their calves for 3-4 years (usually until the next calf is born), this is when the calf learns a lot about the rules of the society!

The Wild Dolphin Project has tracked families of Atlantic spotted dolphins for decades. This is Bueno, the first fourth generation calf of the Wild Dolphin Project. Photo: Liah McPherson.

Two-tone and speckled individuals have a lot to learn about what they can and cannot do in the complex social structure, they have to learn the rules and how to interact with other individuals. They learn much of this through play. They will use fish for target practice to hone their foraging skills, play seaweed tag (or keep away), play fight (often under the watchful eye of mottled and/or fused individuals) to help develop the skills they need to navigate adulthood in their society. This type of play is an important part of many social species. They are also very and acrobatic swimmers and often flashy at the surface. They love to bow and wake ride (and will often approach vessels to do so, sometimes from a fair distance!) and breach (leap into the air). They will use bubbles and body posturing along with vocalizations (whistles, clicks and burst pulses) to communicate with each other. They have a complex repertoire of vocalizations and behaviors which is fitting for their complex social structure! If you want to know more about these amazing dolphins, check out www.wilddolphinproject.org (Research Director Dr. Cindy Elliser worked with The Wild Dolphin Project on their social structure, you can check our those publications on the PacMam website, www.pacmam.org).

Status

Total population estimates for the Atlantic spotted dolphin are hard to find, as so many of their populations are not well studied. However estimates for the U.S. stocks are approximately 25,000 – 31,000 animals in the Northern Gulf of Mexico stock, and 36,000 – 51,000 in the Western North Atlantic stock.

Threats

  • Entanglement: Unfortunately Atlantic spotted dolphins can become entangled or accidentally caught in commercial fishing gear. They are particularly susceptible to methods such as gillnets and purse seines. Injury or death can result from either entanglement or capture in nets.
  • Ocean Noise: Like other cetaceans, these dolphins depend on acoustic sounds to hunt, socialize, and communicate with one another. Thus sources of underwater noise pollution disrupt their echolocation and vocalizations. Most of this noise comes from ships, as well as industrial or military activities. These sounds can often disrupt important behaviors such as foraging, and can even result in hearing loss if loud enough.
  • Illegal Feeding & Harassment: Atlantic spotted dolphins often inhabit the same waters as fishermen and some have learned to follow boats in the chance of getting some discarded fish. Because of their proximity to the boats, some people begin actively feeding the dolphins and soon those dolphins become what are called “beggars”, following boats looking for food. This can be very detrimental to the dolphin both socially (often ignores or loses track of its comrades), and behaviorally (not practicing active foraging techniques). These dolphins are at a much higher risk of boat strike due to their behavior. Atlantic spotted dolphins are also occasionally hunted and killed in the Caribbean, South America, West Africa, and other offshore islands for food and bait.

Fun Facts

  • The Latin name Stenella frontalis literally translates to “narrow forehead”!
  • They can live up to 22-25 years in the wild, although some known individuals in the Bahamas have lived well into their 40s and 50s.
  • Dolphins will protect each other against shark attack.
  • They communicate through clicks, whistles, cries, and other high-pitched calls. Each of these has its own meaning: threat warnings, playful chatter or calling individuals (signature whistles). They also communicate through body language.

cindy.elliser@pacmam.org

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