Updated: Apr 26
It can be hard to imagine that such awesome, fearsome predators as sharks start life as small and defenceless youngsters (just like all the rest of us). Juvenile sharks have it especially tough because neither their father nor mother looks after them as they grow up. After they are born, juvenile sharks develop to adulthood in "nursery habitats", without any intervention from their parents. These habitats are absolutely crucial to the survival of shark species and therefore, it is vital that we identify as many of them as possible.
Shark nurseries offer juvenile sharks excellent conditions to thrive and grow in, as they offer the youngsters a consistent supply of food and some protection from predators. For instance, some shark nurseries are in mangrove forests, where the small sharks can hide within the tree roots if a predator passes by (Heupel et al, 2007).
The nursery is in a very specific location, which is instinctively known to the female sharks. Pregnant (known as "gravid") females will travel long distances to ensure their young are born in that particular nursery habitat, and the area is used year after year, after year for the same purpose (Heupel et al, 2007).
For example, scientists have recently found that an region in La Salina, in the southwest of Cuba, is an important nursery habitat for juvenile lemon sharks (Negaprion brevirostris) (Ruiz-Abierno et al, 2020).
After surveying an estuary for over three years, scientists found that the majority of lemon sharks they saw in the area were very small; weighing less than 4.5 kg and measuring only around 75 cm in pre-caudal length (aka PCL - measured as the length from the tip of the snout to the base of the tail fin). They also reported many sharks they saw were "neonates", meaning they had only been born very recently. Based on the sizes of the sharks, the speculated that the sharks were probably born around January. The researchers did not find any lemon sharks which were longer than 110 cm PCL or heavier than 10 kg, meaning that all of them were juveniles; not yet sexually mature (Ruiz-Abierno et al, 2020).
Over the years of sampling, when they caught the same individuals repeatedly, the researchers noted that the juvenile lemon sharks grew an average of 10.47 cm per year. This shows that the nursery habitat offers them an ample food supply to be able to grow fairly rapidly. It is likely that the sharks were targeting shrimp, crabs and small fish for food (Ruiz-Abierno et al, 2020).
The La Salina nursery area is characterised as a tropical tidal estuary, with extensive sandbars and rocky / sandy bottoms. Over 186 square kilometers, the river empties into five different mouths to the Gulf of Cazones and the Caribbean Sea. Shallow lagoons (average depth of 49 cm) are only accessible to small animals. Algal blooms and seagrasses predominate throughout the water, and mangrove trees submerge their roots at the river banks. The habitat supports many species of fish, crustaceans, birds and mammals (Ruiz-Abierno et al, 2020).
I feel like I want to book a vacation immediately!
The site is within a wildlife refuge called the Zapata Swamp National Park. Therefore, the area is a part of a designated Biosphere Reserve, which is protected as an area of critical importance, according to the International Union for the Conservation of Nature (IUCN). The Cuban government strongly enforces protection of the natural environment within the park. For example, fishing is prohibited inside the refuge. Many areas of the park are inaccessible and isolated form large population centres, so activities like boating or water sports are incredibly uncommon (Ruiz-Abierno et al, 2020).
Conservationists have long-debated and experimented with the best methods to protect sharks. One tactic we have learnt, is to protect specific areas which are of critical importance to the sharks: areas where they feed or breed. Nursery habitats are certainly crucial because they are vital to ensure "recruitment" to the adult population. If more young sharks survive to maturity, then we will have more sharks breeding, and hopefully, depleted shark populations will start to recover (or at least stabilise). The fortuitous location of the La Salina shark nursery, means that these sharks are enjoying a relatively undamaged habitat, with minimal human interference. Now that scientists know of the importance of this particular area, they will be able to continue to protect this critical habitat, and contribute towards the international effort to conserve sharks (Ruiz-Abierno et al, 2020).
Heupel MR, Carlson JK & Simpfendorfer CA (2007). Shark nursery areas: concepts, definition, characterisation and assumptions. Marine Ecology Press Series, 337, 287-297.
Ruiz-Abierno A, Márquez-Farías JF, Hueter RE, Macías-Romero L, Barros-García JM, García-Córdova L, Hurtado A & Miller V (2020). Distribution and length composition of lemon sharks (Negaprion brevirostris) in a nursery ground in southern Cuba. Environmental Biology of Fishes,103, 1583–1594. Access online.