John Hepburn has been one of Wembury Marine Centre’s “casual” volunteers since 2003 and regularly helps with our Rockpool Safaris, he is also the secretary of the Wembury Marine Conservation Area Advisory Group. A member of the Devon stranding network he is occasionally called on to record cetaceans and seals washed up on local beaches. As a STEM Ambassador and Marine Conservation Society Sea Champion, he works with schools in Plymouth to raise awareness of the importance of the marine environment and maritime activities. He also sails in the Bristol Channel Pilot Cutter, “Cornubia” taking disabled children day sailing in Plymouth Sound. As well as being first mate he delivers the marine science elements of the day, which includes looking at pontoon life and plankton through a microscope and seabed life with a baited lander.

In this guest blog he talks about a citizen science project he has been doing for the last two years studying Catshark or Dogfish eggcases at Wembury Point in partnership with The Shark Trust. You can contact him at and follow him on Twitter @johnmewstone

Here he is in action!

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At Wembury Point you will find dogfish (more properly known these days as catshark) eggcases if you know where to look. They are laid by Scyliorhinus stellaris, the Bull Huss or Nursehound, in the very low intertidal, making it possible to study them by snorkelling. I found my first one in December 2009, and started tagging them in April 2014, after asking the experts at the South West Marine Ecosystems Conference (SWMEC) if it would be useful, possible and ethical, to put tags on them to see how long they took to hatch.

20140428 - 08 - Wembury Pt slip - survey site (Medium) 20100823 - 02 - Scyliorhinus stellaris, Bull Huss or Nursehound egg case (Medium)
Here’s the survey site – Wembury Point (left) and an egg case in situ (right)
From the start, the Shark Trust supported the idea, and in the months to come Cat Gordon and Ali Hood were incredibly helpful. Because they understood the current state of knowledge they were vital in defining the research objectives, which became:
• Finding out how long they take to hatch
• What the success rate is
• When they are laid
• Whether biofouling has any effect.

Plus The Shark Trust’s Great Eggcase Hunt might get a bonus from the recovery of any stranded tagged cases indicating the distance eggcases might travel from laying sites.
They agreed that records of temperature and wind velocity (as a proxy for wave energy) would be useful to see if they affect hatching success. At first temperature was just down to me recording the temperature on my watch on each dive. Attempts to create a waterproof container for a data logger all failed. But the Shark Trust managed to spring a proper scientific logger from the university and we’re waiting for the first time series to pop out of the system. Weather records just involve looking at the last 24 hours on the Met Office website and recording peak wind velocity if the speed is over 20mph.

20150306 - 05 - Wembury Pt slip - Temp (Medium) 20160116 - 10 - Wembury Pt slip - Scyliorhinus stellaris, Bull Huss or Nursehound egg case temp sensor (Medium)

Temperature data – on John’s watch (left) and the scientific logger (right)

The tags were an interesting problem. On the first ones I wrote a number on with permanent marker. This turned out to be very temporary in salt water. After a short trial showed retagging with the same method was impractical I devised the “binary tag.” Complicated to explain! The only problem with this is doing the mental maths on the higher numbers at the end of a two hour cold snorkel. Fortunately I always take a photo of the tag so when I compile the record I can correct the field record if necessary.

20140804 - Binary tags (Medium) 20150205 - 14a - Wembury Pt slip - Scyliorhinus stellaris, Bull Huss or Nursehound egg case #126 (Medium)

Binary tags (left) and tag in situ (right)

At first I did the tagging solo, notwithstanding the BSAC recommending against solo snorkelling. But when Cat Gordon started joining me it became clear that another pair of hands and eyes made the process much more effective. And a second, or even third person looking means you find more eggcases. This advantage cancels out the advantage of quicker tagging, and the dives become even longer! Last year’s Marine Centre full-time volunteers Steph, Grace and Kieran joined Cat Gordon and me on a couple of trips as well as a few other friends and relations. Let me know if you’re up for a 2 hour snorkel. The water’s about 10oC at the moment, but it can get up to 19!


So, what have we found?
For a number of reasons, we don’t know how long they take to hatch. The early problems with illegible tags meant there wasn’t a clear audit trail from laying to hatching. Some eggcases weren’t spotted when freshly laid, either they were missed on an earlier dive, or were laid early in the interval between dives. It’s surprisingly difficult to tell when an eggcase has hatched. Heavy biofouling and tight binding to the weed they are laid on can prevent shining a torch through the eggcase to get an X-ray view. There are also many “false negatives” – dives when an eggcase is not seen, but is shown on a subsequent dive to have been present – which can extend the error range on the date of hatching. And some are just not seen again when they appeared to be on the point of hatching. The average looks like 196 days – shorter than expected.

20150814 - 18e - Wembury Pt slip - Scyliorhinus stellaris, Bull Huss or Nursehound egg case #162 (Medium) 20150603 - 23d - Wembury Pt slip - Scyliorhinus stellaris, Bull Huss or Nursehound egg case #117 (Medium)

Biofouled eggcase (left) and empty eggcase (right)
I have no real idea of the success rate. With less than 20 reasonably certain complete records out of nearly 100 taggings it’s tempting to say the rate is low. But the false negative rate makes it difficult to be certain. This is further complicated by the discovery that perhaps more than 20% of them have either no embryo, or one that doesn’t develop – known as a ‘wind egg’, or that the eggcase is poorly made. Steph had seen a “wind egg” rate of 40% in the eggcases being reared in Newquay aquarium while working there before her time at Wembury, and this came as a bit of a surprise. And for reasons mentioned above it’s not always possible to get any “X-ray” images, let alone good ones. We are keeping our eyes open for signs of predation, but so far we have seen none during the period of this project.

20150324 - 16b - Wembury Pt slip - Scyliorhinus stellaris, Bull Huss or Nursehound egg case #141 (Medium) 20150105 - 05f - Slip - Scyliorhinus stellaris, Bull Huss or Nursehound egg case, #131 (Medium)

Poorly made eggcase (left) and good x-ray view through an eggcase using a powerful torch (right)

With nearly two years of records it looks fairly clear that they are laid throughout the year. April has the lowest average (1), with July the highest (5.5).
Hardly surprisingly, given the difficulty in determining success rates, I have not been able to reach any conclusions on whether biofouling affects embryo development or hatching time, or whether embryo development affects biofouling. But it is possible to say that there is considerable variation in the rates and types of biofouling even on adjacent eggcases.

One thing is very clear, though, is that eggs laid on anything other than the seaweed Cystoceira tamariscafolia do not do well here. This seems well known to the catshark (or sharks) laying the eggs, as only two were found laid on anything else and they were only seen once.

20141222 - 19e - Slip - Scyliorhinus stellaris, Bull Huss or Nursehound egg case, #124a (Medium) 20141222 - 04b - Slip - Scyliorhinus stellaris, Bull Huss or Nursehound egg case, #132 (Medium)

Adjacent eggcases with different biofouling rates (left) and eggcases attached to Thongweed, Himanthalia elongata rather than Cystoceira tamariscafolia (right)


As a result of this project the Shark Trust’s Great Eggcase Hunt now has a record of one eggcase found on the strandline that they know exactly where it was laid, about 500 metres away.

20150420 - Scyliorhinus stellaris, Bull Huss or Nursehound eggcase #102 (found on beach 20150418 SX 504 484 - restaged) (Medium)

Eggcase washed up on Wembury beach with John’s tag still attached
So far neither wind nor temperature appears to have a significant effect on the hatchery. Indeed it seems surprisingly resilient to strong wave conditions. That it is well sheltered is obvious to us, and maybe we should not be surprised that its advantages are clear to catsharks, too.
Other similar sites are rumoured to exist in Cornwall, and we would be very happy to share the lessons learnt. There is also a similar project in the Bay of Naples. At 30 metres they can only reach them therewith SCUBA gear, but the visibility is always good (unlike at Wembury Point) and the water is always warm (also unlike at Wembury Point!).


Cat Gordon presented a poster for the European Elasmobranch Association and at the Fisheries Society of the British Isles, which you can see at


John will give a short presentation on the project to SWMEC in April 2016.

The project is very much work in progress, but I hope it shows that citizen science is capable of making a useful contribution to our knowledge of marine life.