Oysters, and colonies of the invasive sea squirt, experienced in situ temperatures of between -3.63° C and 34.02° C. The largest change of temperature over any 12 hour period was 19.3° C at the seaward edge and 29.1° C at the landward edge of the farm. During the observational period, in situ solar heating raised inter-tidal trestle temperatures on the shellfish farm by up to 20° C above air temperature. No widespread mortality was reported in either species during the observed period.

Using tidal data, and some assumptions about the upper and lower trestles, it would appear that the bulk of the shellfish farm lies (in the vertical) between 1.0 m (lower seaward edge) and 3.0 m (upper landward edge) above chart datum. From the tidal records, this would indicate that trestles on the farm are submerged in seawater between 96% (lower seaward edge) and 45% (upper landward edge) of the time over a year (2017). It would appear that both Pacific oysters and Didemnum vexillum can tolerate these exposure levels but growth of both species appears enhanced at the shorter exposure times (seaward edge of farm) compared to the longer exposure times (landward edge of farm).

The maximum number of degree-days (with Tref = 10.55° C) in Loch Creran during the observational period was 461 and 510 for the lower and upper trestles respectively, and 449 days for sub-tidal conditions with no air exposure. These are less than the 592 degree-days (Tref = 10.55° C) needed to achieve gametogenesis in Pacific oysters (Eno, 1994). Hence, for the observational period October 2016 to May 2018, thermal conditions in Loch Creran were never suitable for Pacific oyster reproduction.

If the biological reference temperature Tref is reduced to 9° C, then conditions in Loch Creran during the observational period could be categorised as having recruitment potential following the definition used by Syvret et al. (2008). It is not known if thermal adaptation can result in such reduction in the biological reference temperature but the evidence for successful reproduction of Pacific oysters in Scottish waters (e.g. Shelmerdine et al., 2017) suggests that some thermal adaptation may already have occurred.

An alternative to adaptation in reaching the required thermal exposure for recruitment potential is if in situ temperatures increased by 1.5° C. The general consensus is that our waters will have warmed by between 1° C and 4° C by the year 2100, but much depends on how society curbs its greenhouse gas emissions (Tinker et al., 2020).

Results presented here suggest that estimating thermal exposure using climatological sea temperatures can underestimate the recruitment potential of Pacific oysters in a region. In Loch Creran in 2017, using monthly averaged sea temperatures underestimated thermal exposure by 18% compared to using measured temperatures as experienced on the farm.

At the Loch Creran shellfish farm in 2017, sea temperatures were suitable all year round for asexual reproduction of Didemnum vexillum (ie. >8° C). Sea temperatures were suitable for the production of Didemnum vexillum larvae (i.e. >14° C) only in August and September. Didemnum vexillum on the upper trestles was additionally exposed to temperatures >14° C while in the air inter-tidally from March to September.