Sharks are a very important component of marine ecosystems. Many are top predators regulating the numbers of other species. Sharks tend to feed off old, sick and slow prey keeping prey populations healthy and population numbers optimum preventing ecosystem damage by, for example, over grazing.
There are a variety of sharks (approximately 25% of all sharks) that are friendly, easy to handle and reproduce by shedding eggs. The Whale Coast Conservation project focusses on these. This reproductive type is called ovipary. Eggs have different shapes, sizes and colours just like one finds amongst different bird species.
Many of the shark eggs have tendrils or hooks with which they attach to sea plants, rocks or other protrusions under water. Inside the egg there is an embryonic shark attached by a feeding tube to a yolk sac. The embryo is nourished by this yolk for up to nine months while it develops but this can vary depending on water temperature. When ready, the little shark hatches from the egg leaving the empty case behind. Two such hatchings are shown below.
The empty egg cases usually wash out of the sea and end up on the beach. If these ‘mermaids’ purses’ are found at a particular beach it indicates that the adult sharks or ‘parents’ are in the water nearby.
Through the Youth Environment Programme that Whale Coast Conservation implements, school learners became interested in these shark egg cases and wanted to match the egg cases found on our beaches with the adults laying them. By collaborating with the South African Shark Conservancy and the Two Oceans aquarium, a number of egg cases was identified. There are still some that are unknown. Regular shark egg case collections by volunteer groups along the Whale Coast shoreline, provides information on the egg cases found at different beaches. This gives an indication of the sharks occurring off shore at those locations. One such group is the Kleinmond Nature Conservation Society: shells and marine life interest group who have been monitoring the western portion of the coast. School groups, community members and Working for the Coast staff also collect regularly.
A previous study done by Carrie Pretorius (2013), noted that the two oviparous sharks she researched, had a preference for a particular plant or substrate that they entwine their egg cases to. Poroderma pantherinum, the leopard catshark, preferentially attaches its eggs singly onto sea fans and Haploblepharus pictus, the dark shy shark, attaches its egg cases predominantly in pairs, to the seaweed Bifurcariopis capensis. (Sea fans look like a sturdy plant as can be seen in Figure 2). We also want to know what the substrate preferences of the other sharks are. This is an observation task for divers in the inshore area. Paired with this is the task of setting up a sea plant herbarium for the Overstrand area. Material is collected at the shore, pressed, identified and kept at the Fernkloof Nature Reserve Herbarium. This project has been started by Whale Coast Conservation with the help of a Hermanus High School learner.
Details about the project are shown on information boards that have been erected at the three Overstrand Blue Flag beaches, in the media and at numerous presentations including at schools. Many citizen scientists send in results of their shark egg case collecting.
Scientists working on some of these oviparous sharks have observed that eggs are shed continuously throughout the year. However, in the course of the project, we found that there seemed to be many St Joseph shark (Callorhinchus capensis) egg cases at Grotto beach during autumn but far less during other months. This observation prompted the regular collection of shark egg cases at the three Blue Flag beaches during 2012.
Initially we wanted to answer the question, is there a pattern in the number of egg cases found at different times of the year which could indicate a definite breeding season? The pilot phase of the project has shown some surprising results. In the first seven months of collecting egg cases, the answer to this question is yes and no. The number of egg cases washing out on the beach remains constant throughout the year, with one exception. For the St Joseph shark the number of egg cases collected were very high during April, May and June, tapering off to a few only in the months September to February. To establish whether the same pattern occurs every year, the collections will continue for a number of years. During 2013 we established seven collection sites. Initial findings in Walker Bay are intriguing. Walker Bay is approximately 18km long, yet egg cases are deposited in a very small area of 500m to 1000m long. It would seem that in-shore currents pick up empty egg cases and carry them along the same pathway to be deposited in the same place on the beach. We are currently attempting to link this phenomenon with local oceanographic dynamics particularly atmospheric pressure, wind direction and speed.
We hope to enlist the help of divers to mark St Joseph egg cases underwater to give a clearer idea of the ocean pathways and to calculate what proportion of egg cases are actually deposited on beaches thereby determining a predictive model for population size.
About the author: Sheraine van Wyk is the Eco-learning manager at Whale Coast Conservation, an environmental services NGO in the Overstrand. Her focus is community orientated which includes formal and informal learning spaces. Sheraine facilitates citizen science projects in the Overstrand of which this is one. She can be contacted at email@example.com or 028 316 2527.