Above: Juvenile Blacktip reef shark (Carcharhinus melanopterus). IUCN red list: Near Threatened.
The growing concern over the decline of elasmobranch (sharks, rays and skates) populations has led to increasing effort to conserve nursery areas, which are thought to be critical for population sustainability. Science is helping us to discover nursery regions, but how effective is protecting them for conservation?
A nursery area can be defined as an area in which the juveniles of a marine species undergo growth and development; in these regions young receive increased protection from predators, plentiful food, and grow at faster rates, all of which increases recruitment into the adult population (Beck et al. 2001). Once they reach a certain size and health, individuals will usually migrate out of nursery regions to breed themselves.
Nursery areas have received significant attention in recent years; just this year a rare manta ray and a hammerhead shark nursery were as discovered off the coast of Texas and the Galapagos, respectively. In 2016 the first ever great white shark nursery was identified, leading to changes in management policies around the North Atlantic Coast.
Above: Great white shark (Carcharodon carcharias). IUCN red list: Vulnerable.
A strong emphasis has been placed on protecting these juvenile- rich regions, primarily due to the difficulties involved in managing adult elasmobranchs, which often migrate over large distances and across geopolitical boundaries. However, due to past confusion over the definition of a nursery habitat large areas of coastline have been identified as nurseries, making conservation impractical. Although this problem has somewhat been clarified as more and more nursery areas are mapped and defined, less attention has been paid to the true value of nurseries for the recovery of exploited shark populations.
A prime example of this issue is the protection of the liver-oil shark (often called the school or tope shark, Galeorhinus galeus) in southern Australia. As early as the 1920s this shark was fished for its liver-oil, a principal component of many traditional medicines and dietary supplements. By the 1950s concerns were already being raised over the collapse of liver-oil shark populations in the area, and in the 1960s numerous nursery habitats around Tasmania were protected and access was heavily restricted (Kinney & Simpfendorfer, 2009).
Unfortunately, by the 1990’s it was clear that juvenile numbers within the nurseries had still continued to plummet; in a 1992 study no sharks were caught in 23 hours of fishing and the catch rate for liver-oil shark declined by over 50% from 1983 to 1997. Despite 30-years of nursery‐focused protection, populations had declined because fishing of adults outside nursery regions had continued unsustainably. Infact, the lack of recruitment to the managed nursery areas meant they contained so few juveniles that their classification as nurseries became unwarranted (Kinney & Simpfendorfer, 2009).
Above: Liver-oil shark (Galeorhinus galeus). IUCN red list: Vulnerable.
More and more evidence, primarily from demographic (population structure) modeling, points to the fact that life stages outside of nurseries may be more important in this regard. Modeling of populations of the dusky shark (Carcharhinus obscurus), for example, have shown that the most important age group in terms of maintaining positive rates of population increase are large juveniles reaching maturity. For the dusky shark, this means 17-23 years olds, which are much larger than those protected in designated nurseries (Simpfendorfer et al. 2002).
Above: Juvenile Dusky shark (Carcharhinus obscurus). IUCN red list: Vulnerable.
Such findings have strong implications for conservation of shark populations. As mature individuals are caught outside of nurseries, in turn recruitment to nurseries becomes low. This low-recruitment effect is even more pronounced in elasmobranch populations because of their life history characteristics: they are slower‐growing, longer‐lived, and reach maturity at a later age. This means they do not breed very often (low fecundity) and mature individuals can be caught before they have a chance to breed. Hence, sharks, skates and rays often have low rates of population increase, which hinders the recovery of depleted populations.
But this isn’t to say that nursery areas should be disregarded entirely; depletion of nursery stocks and/or destruction of nursery habitats could result in a situation where the availability of nurseries becomes a limiting factor to population increase. Alone they may not be sufficient to protect elasmobranch species, instead they should form an integral part of a conservation strategy that encompasses several life-stages, including older individuals.
This kind of site protection alone may be sufficient for species that show strong site-fidelity (return the same area frequently), but in overfished populations marine protected areas (MPAs) must be coupled with reduced catch in order to ensure fishing doesn’t just move elsewhere. In cases where species are highly migratory, particularly those that only gather to breed, protecting breeding individuals or individuals of a certain size could prove to be more effective (Kinney & Simpfendorfer, 2009).
It is these kinds of species-specific solutions that will ensure the effective management of elasmobranch populations, and it is essential that we implement them for the most vulnerable species.
Beck et al., (2001) The identification, conservation, and management of estuarine and marine nurseries for fish and invertebrates. BioScience 51:633–641
Kinney & Simpfendorfer (2009). Reassessing the value of nursery areas to shark conservation and management. Conservation Letters. 2(2); 53-60.
Simpfendorfer et al., (2002) Validated age and growth of the dusky shark, Carcharhinus obscurus, from Western Australian waters. Mar Freshw Res 53, 567–573.