The term nursery is a concept used in scientific literature to describe areas where species come in large numbers to breed or spawn and reside there as they grow towards maturity.
Nurseries have been previously defined as specific areas that contribute more to the population of a species than would otherwise occur elsewhere (Beck et al., 2001).
Due to a mandate that required all Fishery Management Plans to document and include the safeguarding of Essential Fish Habitat (NOAA 1996 as read in Heupel et al., 2007), and a decline in population numbers, researching areas of possible shark nurseries increased.
Scientists have been aware of elasmobranch species using nurseries since the beginning of the 1900s. As most shark species are found in coastal waters; in the early years, there was often some confusion as to what exactly constitutes a specific area as a nursery.
Correct classification and identification are critical for effective conservation efforts. Wrongly identifying areas which are too large or not even a nursery at all dilutes credibility and diverts resources away from areas that need it most.
In previous years, any areas where young or juvenile sharks were found were mistakenly referred to as nurseries. Recognising this was an issue, clearer guidelines were established to prevent the misidentification of shark nurseries; the area in question must have a higher number of sharks present than other areas of the coastline, sharks must remain here for some time or else return here for long periods and the area is used multiple times over years (Heupel et al., 2007).
Nurseries are commonly accepted to be areas of high productivity, such as estuaries and mangrove ecosystems which are full of biodiversity and ensure enough food for the juvenile (Castro,1993); and offer a lower risk of predation than open waters (Springer 1967 as read in Heupel et al., 2007).
Some species may use shallow tropical bays, while other species may use more open areas with a higher risk of predation. In general, it is species who are slower to reach maturity and therefore spend more time being vulnerable and susceptible to predation, that use the sheltered bays (Branstetter, 1990).
As adult sharks can grow to large sizes and become difficult to track and manage, when numbers began to decline; conservation efforts focused on the newborns as they were easier to manage.
When studying the value of shark nurseries Kinny & Simpfendorfer (2009) found that ensuring adult and subadult members of the population were protected is a vital step in shark conservation.
Protecting those who were close to reaching or had already reached sexual maturity allowed them to procreate and ensure there was a constant supply of pups. Prioritising neonates (newborns) and failing to protect those who can reproduce will lead to a species decline (Kinny & Simpfendorfer 2009).
Within these nurseries, it has long been suggested that different areas are used in different stages of development. Long-term passive monitoring of blacktip sharks (Carcharhinus limbatus) off the Florida coast showed juvenile blacktips using specific areas of the nursery as they matured (Heupel et al., 2004).
First using a small, sheltered section of the bay, before steadily increasing its area as the juvenile increased in mass and age. As they mature, more energy is required and this is what led them to utilize larger areas and leaving behind sheltered warm waters, for colder waters richer in food.
With this movement comes an increased risk of predation. Juveniles faced a trade-off between avoiding predation and securing enough food and energy (Feldheim et al., 2002). The same study proved this species showed a strong site fidelity, proving that shark species can be philopatric, returning to the same nursery for multiple years.
Knowing this information will help future conservation decisions within the bay, ensuring the needs for each stage of development are met; such as maintaining sheltered habitats in the area of the bay neonates utilise.
Philopatry has also been strongly documented while studying a lemon shark nursery in the Bahamas. The females were found to return to the nursery to give birth every couple of years (Feldheim et al., 2002).
Philopatry may explain why some shark nurseries are not always in areas of high food production. This also puts populations at risk who may return to the same nurseries only to find they have been negatively impacted by anthropogenic factors. Areas once abundant with food and good water quality may no longer be suitable.
Lastly, It should also be noted that nurseries are not species-specific. Multiple shark species have been found using different areas of the bay at the same time.
Lemon sharks and blacktip sharks studied off the coast of the Virgin Islands confirmed that spatial and temporal habitat positioning can occur.
Lemon sharks were found in the shallower waters, occupying a seagrass and mangrove habitat while blacktip sharks were found in the deeper waters (DeAngelis et al., 2008).
In summary, nurseries play a crucial role in shark conservation. Nurseries provide essential shelter, protection and food for neonate and juvenile sharks.
The examples pointed out showcase strong site fidelity to these nurseries, therefore, we must ensure they remain viable and as minimally impacted by anthropogenic factors as possible such as removal of mangrove ecosystems.
As the studies mentioned in this article have shown, focusing conservation efforts entirely on neonates and juveniles, while failing to protect members of the population that can reproduce, will ultimately lead to population decline.
Conservation strategies must include measures that also protect older individuals. This, however, will likely vary with the species and will require more localised strategies when dealing with a specific species.
Thank you Anne Macy Hall (@anmhal) for letting me share this awesome video of a juvenile lemon shark: https://www.instagram.com/p/Bt7DtBGFlhf/
Written by Dion Mc Daid
BIO:
Dion is a postgraduate marine biology student at University College Cork. He is also editor of ClimateClues.com; a website aimed at providing short, bite-sized and easily digestible environmental information for people in a hurry.
Website: climateclues.com
Instagram: @climateclues
References
Beck, M., Heck, K., Able, K., Childers, D., Eggleston, D., Gillanders, B., Halpern, B., Hays, C., Hoshino, K., Minello, T., Orth, R., Sheridan, P. and Weinstein, M., 2001. The Identification, Conservation, and Management of Estuarine and Marine Nurseries for Fish and Invertebrates. BioScience, 51(8), p.633.
Branstetter, S., 1990. Early Life-History Implications of Selected Carcharhinoid and Lamnoid Sharks of the Northwest Atlantic. Elasmobranchs as Living Resources: Advances in the Biology, Ecology, Systematics and the Status of Fisheries, pp.17-28.
Castro, J., 1993. The shark nursery of Bulls Bay, South Carolina, with a review of the shark nurseries of the southeastern coast of the United States. Environmental Biology of Fishes, 38(1-3), pp.37-48.
DeAngelis, B., McCandless, C., Kohler, N., Recksiek, C. and Skomal, G., 2008. First characterization of shark nursery habitat in the United States Virgin Islands: evidence of habitat partitioning by two shark species. Marine Ecology Progress Series, 358, pp.257-271.
Feldheim, K., Gruber, S. and Ashley, M., 2002. The breeding biology of lemon sharks at a tropical nursery lagoon. Proceedings of the Royal Society of London. Series B: Biological Sciences, 269(1501), pp.1655-1661.
Heupel, M., Carlson, J. and Simpfendorfer, C., 2007. Shark nursery areas: concepts, definition, characterization and assumptions. Marine Ecology Progress Series, 337, pp.287-297.
Heupel, M., Simpfendorfer, C. and Hueter, R., 2004. Estimation of Shark Home Ranges using Passive Monitoring Techniques. Environmental Biology of Fishes, 71(2), pp.135-142.
Kinney, M. and Simpfendorfer, C., 2009. Reassessing the value of nursery areas to shark conservation and management. Conservation Letters, 2(2), pp.53-60.
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