Roa Island Shore Walk July 2021

July 29th, 2021
Discarded fishing line.
Discarded fishing line – sadly our first find…

Our first Roa Island shore walk this year took place on Saturday 24th July.  Understandably, given the late decision to hold this, turnout was lower than normal but 5 of us had an interesting time scouring the shore for life. Sadly the first find of any interest was 30-40m of discarded fishing line tangled in the wrack under the walkway to lifeboat station. This took about 20 minutes of patient work to disentangle it so that it could be taken away for disposal. Thankfully the rest of the beach was relatively clear of litter!

Pacific oysters (Crassostrea Gigas). Photo Lewis Bambury.
Pacific oysters (Crassostrea Gigas)

We saw quite a large number of Pacific oysters Crassostrea Gigas (sometimes known as Portuguese Oyster).  These are a non-native commercial species; it’s possible that they have originated from oyster farms in the Menai Straits, although there is an oyster hatchery on Walney Island that puts some immature stock in the Bay to grow on before selling them on to other farms to mature and this may be the source too.

Dislodged sponge, photo Lewis Bambury.
Dislodged sponge

It was noticeable that there were quite a few large pieces of dislodged sponge and sea squirts, probably from deeper water, at the low water line.  It wasn’t clear how these had been dislodged, whether it was manmade disturbance – we didn’t see a dredger while we were there for instance – or a natural process, but it did bring some species to view that we may not have seen otherwise.  For instance these Oaten Pipe hydroids (Tubularia indivisa) living on one of the detached sponges.

Mysid shrimp. Photo Lewis Bambury.
Mysid shrimp captured in a shallow water trawl. Photo Lewis Bambury.

Barry tried some plankton trawls in the shallows where large numbers of mysid shrimp and small (juvenile) fish were swimming against the incomming tide, hoping for it to bring them supper! Microscopic examination revealed a phytoplankton community dominated by pennate diatoms (Proboscia alatum and Rhizosolenia sp.), though we did see one centric diatom Odontella mobiliensis. The trawl also contained some lanceolate Naviculacea (Pleurosigma sp. – probably angulatum), and one example of Bacillaria paxillifera; these are typically benthic/surface dwelling diatoms, but very commony found in shallow water trawls. There were also a number of periwinkle (Littorina littorea) eggs and newly hatched ‘velligers’. Juvenile periwinkles (the ‘velligers’) are planktonic, and use cillia covered extensions of their ‘foot’ (called a ‘vellum’) to swim. This mode of propulsion is very effective in the smaller juveniles, allowing them to make respectable swimming speeds (Olympic qualifiers – for ther size!) as they actively hunt for food, which is usually smaller zooplankton.

Periwinkle egg about to hatch. Micrograph by Barry Kaye.
Periwinkle egg containing two velliger embryos – very close to hatching! Micrograph by Barry Kaye.

Lockdown has meant that the best tides for this have passed us by this year, but this was the first of two dates that we picked as having the chances of interesting finds.  The timing of Spring tides around Morecambe Bay means that the very lowest tides – when we have the best chance of finding some of the creatures that are normally hidden – happen around 6 or 7 am, of 6 or 7 pm, so daylight times are a factor in choosing dates too. 

Our next walk is on the calendar for Wednesday 8th September at 19:00 (Low water 0.95m at 19:42, just before sunset).

Report by Lewis Bambury with additions on microscopy by Barry Kaye.

Posted in Events, Science, Shore walks