Determining restoration potential by transplanting mussels of different size classes over a range of aerial exposures

Abstract

Restoration of mussels typically focuses on either subtidal or intertidal habitats, although it is important to consider the full historical range of a species. However, it remains unclear how environmental changes can impact the ability of mussels to survive in tidal heights where they occurred historically. Additionally, there is limited research on the viability of reducing mussel stock size for restoration purposes. In this study, green-lipped mussels Perna canaliculus of 2 size classes (80 and 60 mm) were assessed when transplanted as a single size class or as mixed cohorts in 9 m2 plots at 3 shore heights (i.e. neap low tide, spring low tide, and subtidal). The mussels were sampled over a 1 yr period to understand the effect that shore height and size class had on mussel metrics, such as survival, growth, and condition. The results revealed that shore height had a greater effect than size class on mussel survival, with a total loss of mussels transplanted into areas that were exposed at neap tides in contrast to 39% mussel survival transplanted into areas that were only exposed on spring low tides. Further, mussels transplanted in the adjacent subtidal had higher overall survival (74%). This suggests that aerial exposure time determines the upper vertical limit for restoration by transplantation of mussels sourced from aquaculture, despite their historical distribution. The results of this study also support the use of smaller mussels (~60 mm) for transplantation for mussel reef restoration, as a 25% reduction in size resulted in 50% more mussels being deployed.

Aerial exposure and critical temperatures limit the survival of restored intertidal mussels

“…A recent global review on coastal ecosystem restoration determined that one of the main reasons for unsuccessful restoration was site selection, a process further constricted by a poor understanding of how restoration protocols will function under future climate scenarios (Fraschetti et al 2021). Positive results from the first set of experiments to test intertidal restoration of green-lipped mussels show restoration in the intertidal zone is viable (Benjamin et al 2023a;Toone et al 2023b), but this study emphasizes that mussels will need to be deployed where aerial exposure is very limited to protect against both high temperatures (>25 C) and repeated exposures to those temperatures. The summer of 2021-2022 was the fifth hottest summer on record in New Zealand (NIWA 2022) and July 2023 had the highest ever recorded global average temperature which was 1.12 C warmer than the twentieth century average of 15.8 C (source: https://www.weather.gov/ncep/).…”

“…To assess the effects of ambient temperatures among the experimental plots, one HOBO temperature logger (Onset Computer Corp., Bourne, MA, U.S.A.) was placed in the middle of each of the 12 intertidal plots at 3 months post-deployment and recorded hourly temperature until the final sampling. Temperature data from a full 29-day tidal cycle was analyzed to quantify aerial exposure time on each intertidal plot and any hourly data point that was one standard deviation colder or hotter than the mean temperature was quantified as exposed (Benjamin et al 2023a;Toone et al 2023b). Temperature metrics were determined for each plot over a 90-day period from 19 October, 2021 to 17 January, 2022 (i.e.…”

“…This study assessed the feasibility of mussel restoration in the intertidal zone by examining the in situ influence of the current summer temperature regime on mussel survival. This experiment was part of a series of three experiments conducted to initiate intertidal restoration using green-lipped mussels sourced from subtidal aquaculture farms in New Zealand (Benjamin et al 2023a;Toone et al 2023b). The primary objective of this specific study was to investigate the impact of environmental stress on the success of intertidal mussel restoration, with particular focus on mussel survival.…”