The story behind the discovery of world’s first flapper skate nursery

On the blog today, one of our marine experts, Jane Dodd, explores the fascinating story of how the first report of an egg nursery for flapper skate was discovered and researched, leading to special protections for this critically endangered species and their habitat in the Inner Sound of Skye.

Flapper skate take over 10 years to reach sexual maturity and produce low numbers of offspring and therefore the population will be slow to recover from any impacts. The eggs are large (width 10-14 cm, length 13-23 cm excluding horns) and have a tough outer coat which protects the embryo during development. They lay their eggs on a seabed of boulder and cobble with relatively shallow waters is favoured; eggs remain on the seabed developing for over a year before hatching. The eggs can be sensitive to activities and disturbance while they are on the seabed – for example by abrasion from ropes or chains (such as scouring by mooring chains) or by towed fishing gear being smothered by silt, and incidental capture in fishing gear.

In October 2019, NatureScot received a report from commercial scallop divers operating in the Inner Sound of Skye that flapper skate eggs were “widely spread”. Subsequent reports, photos and video from recreational divers confirmed the reports and in March 2020, NatureScot visited a dive site which had become known as “Red Rocks” to collect tissue samples from the egg cases for genetic testing. We hope this work will provide more detail on flapper skate reproduction. If it shows that several egg cases collected from the area were laid by the same female, we will know if females return to the same site on multiple occasions to lay eggs.

As a result of the reports of egg cases from scallop and recreational divers, NatureScot’s visit to the site, and some historic records of flapper skate eggs in the area spotted in drop camera surveys looking for other priority marine features, the Red Rocks and Longay urgent MPA was designated in March 2021, with the site receiving permanent designation on 9 February 2023. The protection of the Red Rocks and Longay MPA has great potential to contribute to the restoration of the critically endangered flapper skate by ensuring that the vulnerable eggs within the site can develop to successful hatching. 

1 DDV survey location (latitude 57.322537, longitude -5.898492) where 8 flapper skate egg cases were observed in images recorded on 19/07/2018, 2 DDV survey location (latitude 57.312872, longitude -5.872370) where 4 egg cases were observed in images recorded on 20/03/2019, 3 Location (latitude 57.323533, longitude -5.921833) from where “widely spread” flapper skate egg cases were reported by scallop divers on 28/10/2019, 4 Shot line location (latitude 57.33325, longitude -5.927183) for dive site known as “Red Rocks” where flapper skate egg cases were collected on 04/03/2020 and flapper skate egg cases were collected and photogrammetry dive was carried out on 05/03/2020.

During the summer of 2021, NatureScot staff visited the Inner Sound of Skye several more times carrying out drop camera (where a camera is lowered to the seabed from a boat and allowed to drift in the current) and remote-operated vehicle (ROV, a mini-submarine with an on-board camera) surveys for skate eggs.

Over a thousand eggs were observed in total within the urgent MPA boundary and to the north, which resulted in the boundary of the MPA being extended to include these records.

The survey showed that Red Rocks and Longay met the criteria to be described as a flapper skate egg nursery, the first to be identified for the species in the world. The criteria were proposed by Dr Gerry Hoff who over a number of years has described egg nurseries for Alaskan skate, Aleutian Skate and Bering Skate in the eastern Bering Sea leading to their designation as habitat areas of particular concern, the first formal recognition of a skate nursery in the world. The criteria require that egg cases are at high density and in contact with the seabed, that the site is used over multiple years and that juveniles of the species associate with habitat which is different to that of the egg nursery.

The surveys targeted cobble and boulder habitat, but often the drop camera or ROV encountered other habitats such as sand maerl. Flapper skate eggs were only observed on boulder and cobble habitat in the Red Rocks and Longay site. The cobbles and boulders are very “clean”, with no kelp or silt which suggests it is quite tide swept. The eggs fitted nicely in the gaps between the boulders and we think the female flapper skate might be choosing this habitat to lay their eggs so that they are not moved away from the perfect conditions (temperature and water movement) for egg development by tides or currents.

The highest densities of eggs were observed at the edge of a deeper channel on the tops of geological features on the seabed known a belt moraines. These are piles of boulders and cobbles left behind by glaciers and we think the eggs might be densest on the tops of these to take advantage of increased water movement here, since water needs to be flushed through the egg cases for successful embryo development. Skate egg cases open (via the horns) at a certain point in their development and the embryo actively beats its tail to encourage water exchange, but increased water movement at the top of the belt moraines would help this process. Even in these areas, the distribution of eggs is clumped, with some areas containing no eggs and some containing very high densities even though the habitat is the same. This has also lead us to wonder if the females choose to lay their eggs where others are present for safety in numbers.

We learned in September 2020 that flapper skate eggs take 18 months to hatch after an egg we had collected from the Sound of Jura in April 2019 finally hatched in the aquarium at the Scottish Association for Marine Science.

Eighteen months is a long time to remain on the seabed, leaving the eggs vulnerable, particularly to anthropogenic activities which may disturb the sea bed.

The flapper skate egg nursery at Red Rocks and Longay is close to much deeper (100m+) water, a channel between Longay and the Crowlin Islands. Adult skate are known to spend a high proportion of their time in deep water, particularly in the summer months, however we don’t know for sure/have any evidence to date to indicate if adults are using the adjacent deep water, or if they are coming from further afield to lay eggs.

Large numbers of empty flapper skate egg cases wash ashore on the Orkney Isles (Orkney Skate Trust, Shark Trust, Great Egg Case Hunt) every year and egg cases have been reported from the seabed in Orkney, Shetland and Loch Melfort, Argyll, but not in the same numbers as at Red Rocks. It is possible that in these other areas the ‘perfect’ habitat for flapper egg case development doesn’t exist and therefore the drivers for female flapper skate to lay all their eggs in the same place are not as strong and eggs are laid at lower densities in ‘sub optimal’ habitats.

It is possible that other egg nurseries on the scale of Red Rocks do exist, but we were very lucky to receive a report and help to locate this one. Significant survey effort was required to describe it and define the boundary of the MPA. The description of the habitat will help us to look for flapper skate egg nurseries elsewhere in Scotland by searching for similar habitat and conditions, but we are likely to continue to rely on citizen science going forward. Commercial divers, recreational divers and interested community groups with access to drop cameras and ROVs are likely to be instrumental in identifying flapper skate egg nurseries in the future. We are also likely to have missed eggs at Red Rocks as there are still unexplored areas: the conditions are similar around the Crowlin Islands, where we looked for eggs but didn’t find any during our surveys, but they could be there.

Identifying the first flapper skate egg nursery in Scotland provides us with a fabulous opportunity to learn more about the reproduction of the species. Analysis of the genetic samples collected will hopefully provide some insight into the egg laying behaviour of the females, helping us to work out if they return to the site to lay eggs on multiple occasions and therefore how many females are using the site.

There are many other gaps in our knowledge about flapper skate. We know very little about their juvenile life stage: we think eggs, juveniles and adults occupy different habitats and that juveniles prefer mud habitats. Flapper skate are no longer targeted by commercial fisheries, and it is illegal to land those caught as bycatch in Scotland; they should be returned unharmed as soon as possible. Loch Sunart in the Sound of Jura MPA and Red Rocks and Longay MPA are both designated for the protection of flapper skate, with a focus on adult skate in Loch Sunart and a focus on skate eggs in Red Rocks and Longay. To help this amazing species recover, each of the life stages needs protection; therefore, our priority going forward should be to identify juvenile nurseries.

For more on the discovery of the skate egg nursery, see the full scientific paper.

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