Hot Spot

Every year in the calm, glistening waters off the coast of California, leopard sharks arrive in their droves, to pass the summer months in dense aggregations, hundreds strong. Coming right into the surf, these peaceful sharks can be interacted with, at ease, whether you are snorkeling or just paddling in the shallows. It is truly an incredible site and an experience that is likely on every shark lover's bucket list. But why do so many leopard sharks come to these waters? What is the appeal of this exact spot? And what causes them come and go, like clockwork, year after year?

Hundreds of leopard sharks aggregate in California during the summer months (Image Credit: Reuters / Youtube)

Leopards Don't Change Their Spots

Leopard sharks (Triakis semifasciata) are relatively small sharks (maxing out around 1.5 metres in total length), that are only found along the west coast of the North American continent; from Oregon, USA, to Mazatlán, Mexico. Yet despite only having a relatively small distribution, these sharks are disproportionally well known thanks to their exhibition in many aquariums around the world and their instant recognisability, thanks to the iconic spots that earned them their name (Compagno, 1987).

In some place zebra sharks are known as leopard sharks (Image Credit: Lewis Burnett / Shutterstock)

Many people ask why these little sharks are known by this common name when the zebra shark (Stegostoma tigrinum) actually looks more like a leopard in comparison. In fact, in some parts of the world, this species is refered to as the leopard shark. However, zebra sharks got their name from the fetching black and white striped they sport at birth, where leopard sharks earned their name because they are spotted throughout their whole lives (Compagno, 1987).

The beautiful leopard shark is named for it's iconic spotted skin (Image Credit: Mr. Meijer / Shutterstock)

On the Move

Whilst on rare occasions individual leopard sharks have been known to make very long distance migrations (one is known to have traveled as far as 900km!), they generally have quite small home ranges and only migrate over relatively short distances along the coastline (Ebert et al. 2013; Nosal et al, 2014).

In the summer, leopard sharks aggregate in high numbers in La Jolla, California to make the most of the warm waters (Image Credit: Nosal et al, 2013)

Along the Pacific coast leopard sharks travel seasonally to form huge aggregations in shallow water. These emigrations are as regular as clockwork. For example, every summer thousands of leopard sharks gather in the waters off La Jolla, California (Ebert et al, 2013; Nosal et al, 2013a; Nosal et al, 2014).

La Jolla boasts bountiful foraging grounds, within kelp forests and submarine canyons, and the location sits within a lee, so is very calm and consistently warm. Perfect conditions for the leopard sharks (Nosal et al, 2014).             

There's No Place Like Home

After equipping leopard sharks with tags, scientists have found that they return again and again to almost the exact same spot in La Jolla year after year. This is known as "site fidelity". In one study 60% of female leopard sharks came back to the study area at some point during the three year study period. In another project, in Santa Monica Bay, a leopard shark was resighted 10 years later (Nosal et al, 2014).

Tracking studies, like this one following great white sharks, have shown that many species of sharks return to the same locations year after year (Image: Jorgensen et al, 2010)

Scientists think that the sharks are actually returning to the site of their own birth to give birth to their own young. This is known as "natal philopatry" and ensures that females can provide a safe habitat for their pups (Nosal et al, 2014).

It's thought that they may use geomagnetic cues, low - frequency sound cues or their sense of smell to navigate to their old "nursery habitats", but it is remarkable because sharks do not receive any parental care, and therefore, are never shown the way by an experienced individual. They make these journeys based on only their instincts (Nosal et al, 2014). To learn more, head over to Homeward Bound.

Waters off the coast of California are perfect for leopard sharks, as there is ample food and suitable environmental conditions for them to thrive (Image Credit: Barbara Ash / Shutterstock)

California Girls

Scientists have found that the temperature of the water and the length of the day are what trigger leopard sharks to make their move. For instance, in Tomales Bay, leopard sharks leave when the water drops below 10–12 °C. They move south to find warmer regions during the winter and move polewards again as the water starts to warm up (Hight & Lowe, 2007; Nosal et al, 2013a; Nosal et al, 2014).

Leopard sharks are most densely aggregated in California waters during the summer months, but males and females occupy different areas (Image Credit: Nosal et al, 2014)

However, the different sexes don't respond to temperature in exactly the same way. Females tend to prefer warmer waters compared to the males. This is known as "sexual segregation". For instance, the aggregation in La Jolla is made up predominantly of females, many of them pregnant. Wheras the males tend to hang out in nearby Del Mar. Experts suspect warmer waters may allow females a metabolic boost during pregnancy, so their young can develop more quickly (Hight & Lowe, 2007; Nosal et al, 2013a; Nosal et al, 2013b; Nosal et al, 2014).

Female and male leopard sharks segregate by sex whilst in their aggregations in California (Image Credit: Nosal et al, 2013)

Other scientists disagree and suspect that female leopard sharks use La Jolla to escape sexual harassment from males. This is known as "refuging". Mating very commonly causes injuries to female sharks, as males bite down on their fins and flanks as they get it on. For some female sharks, mating again is worth the risk because they can birth litters with many different fathers, maximising the return on their investment (to learn more, see Who's Your Daddy?), but for some species, it's not worth the hassle and so they will not mate again once they are already pregnant. This may explain why female leopard sharks may choose to spend time in La Jolla, far away from any pesky, amorous males (Nosal et al, 2013b; Nosal et al, 2014).

I think the ladies out there will probably understand that!

References

Compagno LJV (1987). Sharks of the World: An Annotated Catalogue of Shark Species Known to Date. FAO Species Catalogue. Access online.

Hight BV & Lowe CG (2007). Elevated body temperatures of adult female leopard sharks, Triakis semifasciata, while aggregating in shallow nearshore embayments: evidence for behavioral thermoregulation? Journal of Experimental Marine Biology and Ecology, 352. Access online.

Jorgensen SJ, Reeb CA, Chapple TK, Anderson S, Perle C, Van Sommeran SR, Fritz-Cope C, ACBrown, Klimley AP & Block, B. A. (2010). Philopatry and migration of Pacific white sharks. Proceedings of the Royal Society B: Biological Sciences, 277:1682. Access online.

Nosal AP, Cartamil DC, Long JW, Lührmann M, Wegner NC & Graham JB (2013a). Demography and movement patterns of leopard sharks (Triakis semifasciata) aggregating near the head of a submarine canyon along the open coast of southern California, USA. Environmental Biology of Fishes, 96. Access online.

Nosal AP Lewallen EA & Burton, RS (2013b). Multiple paternity in leopard shark (Triakis semifasciata) litters sampled from a predominantly female aggregation in La Jolla, California, USA. University of San Diego Previously published Works. Access online.

Nosal AP, Caillat A, Kisfaludy EK, Royer MA & Wegner NC (2014). Aggregation behavior and seasonal philopatry in male and female leopard sharks Triakis semifasciata along the open coast of southern California, USA. Marine Ecology Progress Series, 499, 157–175. Access online.

Smith SE (2005). Leopard shark mating observed off La Jolla, California. California Fish and Game, 91:1, 128-135. Access online.

By Sophie A Maycock for SharkSpeak