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GONHS Marine Bioblitz at Sandy Bay

23 September 2020

The prime objective of our first marine bioblitz was to engage the public in a citizen science initiative which also served to raise understanding of the dynamic coastline we have in Gibraltar.

This was the first official GONHS outing since the COVID lockdown and turnout on the day was high, with around forty people attending, including some friends and experts from Spain.

Sandy bay is dominated by a sandy substrate and its perimeter demarcated with rocky groins. The sampling area was restricted to this beach to protect participants from the wavy seas induced by strong easterlies over the previous days. This varied benthos allows for species that live above the sediments on hard surface and for others that live within it; diversity was expected to be high.  It was expected that both rocky and sandy regions would be dominated by the molluscs as this phylum contains the largest amount of species. The water column was expected to be dominated by teleosts (bony fish) as these are the specialists for this zone.

Results from the day did not support the expectations for several reasons. The main reasons are that the data were not collected systematically and some participants only produced presence and absence for individual species. Consequently, this has had a demonstrable effect on the data collected with an obvious skew; the highest counts appear to be for species which most people easily recognised. This is further supported by the fact that no single participant encountered barnacles on the rocky zones, inspite of the fact that this zone is covered in them. This leads to the conclusion that the data has a high skew towards larger bodied organisms.

Figure 1: Presence and absence data for various invertebrate and vertebrate groups found at Sandy bay


Bony fish clearly dominate the waters of Sandy Bay with 25 different species observed on the day. There were no abundance data collected for the fish so it was impossible to determine which species were dominant within the sample site.

Molluscs were the next group, with 10 species encountered. Of all the molluscs, it was various species of limpets that were most observed, followed by the marine snails. Of particular interest was the presence of the Mediterranean Ribbed Limpet (Patella ferruginea), which is a protected species and appears to be represented on all parts of our rocky coastline.

The Echinoderms were represented primarily by the sea cucumbers, of which 3 species were observed. This was followed by a single species of sea urchin. Interestingly, it was the Black Sea Urchin which showed the highest abundance (297) of all the species in Sandy Bay.

The Cnidarians were only represented by the anemones, both snakelocks and beadlet, though abundance was low. This is possibly testament to the fact that Sandy Bay is particularly exposed to wave action despite the protection offered by the groins.

The presence of the invasive algae Rugulopteryx okamurae was noted, although no abundance or coverage data were collected. Further, the shoreline was covered in the washed-up algae which emitted a pungent aroma as it dried in the sun.

Finally, the other notable observation was the presence of the crab Xanthoporesa. This was the first record of this species ever at Sandy Bay and the only decapod to be observed on the day.

The event was deemed to be a great success with much positive feedback from participants. We were able to determine some good presence/absence data and add to the existing knowledge base of species at the site. The Marine section wishes to thank everyone whom was involved and looks forward to repeating the event next year!