Limpets at Frenchman’s Steps, Pembrokeshire

Limpets by Lara Howe

Limpets by Lara Howe

John Archer-Thomson was studying limpets in Pembrokeshire before the Sea Empress grounded in 1996. Here he tells the story of his work on the coastal ecology before and since the disaster, and the wider impacts of man’s activities on the two seashore species.

I have been studying the population of limpets on a rocky shore called Frenchman’s Steps since I started work at Dale Fort Field Centre in 1982. Residential student groups collected the data as part of their biology field course and I kept the data sets, as it seemed like a good idea at the time! When the Sea Empress oil spill occurred in 1996 I had a good set of “normal” data to compare the effects of the oil to.

In essence the oil reduced the population by approximately 50%. Students collected data by measuring the longest diameter, in millimeters, of every limpet in their randomly placed quadrats at different heights up the shore from a base height, a known marker point 2.25m above Chart Datum. They then worked up the shore in 0.75m height jumps until they ran out of limpets. Because we had size data we could also say that the oil had warped the age structure of the population by killing proportionally more young limpets. We assumed, for this shore, that the smallest limpets were the youngest, it might not actually be this straightforward but it’s probably not too unfair an assumption in general.

Student data is all very well but I wished to assess the recovery of the population from the oil and then go on to look at the limpets each year to see what represented “normal” variation. For this I wanted the data to be as reliable as possible so I enlisted the help of some MSc students, from the University of Leuven, to collect the data as I taught them every year. To my surprise numbers of limpets had recovered to what might be considered “normal” within a year of the spill but the population age structure was still askew. Two years later numbers were on the high side of “normal” but, to my delight, the age structure had returned to “normal” as well.

I continued to monitor the population using Dale Fort teaching staff in the latter years as Leuven students found measuring limpets less fascinating than hitherto! All proceeded normally (numbers and population structure) until 2010. To my surprise the numbers of limpets were much greater than ever before (so much so I wondered if the data, collected by Dale Fort placement students, was reliable). In 2011 I collected the data myself, with Dale Fort teaching staff, to check its reliability and to my astonishment numbers were even higher again! In both years the structure of the population was normal.

The explanation for these off the chart limpet numbers is where life gets even more interesting, rather speculative and a little more complex.

Dogwhelk by Lara Howe

Dogwhelk by Lara Howe

Another rocky shore denizen is the dogwhelk, a carnivorous snail that on the shores around Dale Fort eats barnacles (and young limpets at a push). When I arrived at Dale Fort in 1982 you had to search really hard to find any dogwhelks on the shores from the Fort to Dale village, they were incredibly rare. The reason for their rarity was a spectacularly toxic anti-fouling paint called Tri-butyl Tin (TBT). This was applied to the hulls of yachts, tankers etc. to prevent fouling organisms settling on the hulls and therefore slowing the boats passage through the water by friction. This was the most toxic substance ever deliberately introduced into the marine environment by humans; a teaspoon-full in an Olympic-sized swimming pool is enough to adversely affect marine organisms. Since dogwhelks are carnivores they received a bigger “hit” from the TBT as their suspension-feeding prey concentrated it. Female dogwhelks grew a non-functional male reproductive organ, which blocked their oviduct and prevented successful reproduction. Numbers plummeted in estuaries around the UK as a consequence of TBT poisoning. TBT was banned from small vessels in 1987 (and a complete ban has been in place on all vessels since 2008) and as a result dogwhelk numbers have increased spectacularly on the shores around Dale Fort. My suggestion is that the dogwhelk population spike has impacted their main prey, barnacles, freeing up space on the rocks for young limpets to settle on and allowing a population increase above “normal” limits.

After 2011 limpet numbers have decreased again and my suggestion for the decrease is as follows. Dogwhelks may have impacted the barnacle population to such an extent as to need to switch prey species and I think they then targeted young limpets (old limpets’ shells are too thick to be worth the effort). This has led to a decrease in the limpet population to “normal” again possibly by 2013. I will monitor the population again in April 2016 to continue the story!

All this shows the value of long-term data sets, ideally without missing years. The above explanation for the patterns observed is reasonable but not definite, other variables may well have contributed but it does make an interesting story and allows students to see how data they have collected fits into a broader picture and also how their efforts are valuable in long-term monitoring.