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Scottish Marine and Freshwater Science Reports

Formal report series, containing results of research and monitoring carried out by Marine Scotland Science


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The Thermal Environment of an Inter-tidal Pacific Oyster Farm

Scottish Marine and Freshwater Science Vol 11 No 15

This report describes the opportunistic deployment of recording temperature loggers at the seaward and landward edges of a Scottish inter-tidal shellfish farm between October 2016 and May 2018. The farm, situated in Loch Creran, Argyll, cultures the Pacific oyster Crassostrea gigas (Thunberg, 1793). Additionally, the presence of the non-native invasive sea squirt Didemnum vexillum (Kott, 2002) has been recorded on the farm since 2015. Results presented here confirm that inter-tidal habitats periodically exposed to the air on the Scottish west coast experience greater temperature extremes than sub-tidal habitats which are only exposed to sea temperatures.

Two research priorities were identified during this study:

1) In order to be able to manage the risk of naturalisation of the Pacific oyster in Scottish waters, a priority research area is understanding the thermal conditions required for successful reproduction and recruitment using stock already exposed to Scottish conditions.

2) In order to understand the reproductive potential of Didemnum vexillum in Loch Creran, research is needed into the potential development of larvae under local conditions, as well as the effect of exposure to high temperatures in air inter-tidally on the sexual development of Didemnum vexillum.

Turrell, W.R., Dalgarno, E., Hermann, G., Graham, J., Lacaze, J-P. and Rabe, B. (2020). The Thermal Environment of an Inter-tidal Pacific Oyster Farm. Scottish Marine and Freshwater Science Vol 11 No 15, 51pp. DOI: 10.7489/12332-2
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UK Open Government Licence (OGL)
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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.

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