Resisting Global Change: Oyster Aquaculture and Disease in an Era of Marine Heatwaves

Climate change is expected to increase heatwaves (MHWs) and disease transmission, potentially incurring losses of aquaculture stocks. Ostreid herpesvirus (OsHV-1) is a temperature-associated pathogen that predominantly affects the commercially important Pacific Oyster (Magallana gigas) and other species around the world, with some µVars causing > 95% mortality of oyster stocks. Over the course of three studies, we explored the utility of stress hardening (SH) in ameliorating the effects of MHWs (Chapter 1) and disease outbreaks (Chapter 2) by exposing multiple oyster species to a stressor before the onset of an event as well as a comprehensive overview of OsHV-1 detections in different locations and hosts (Chapter 3). In Chapter 1, we exposed juvenile Olympia oysters (Ostrea lurida), Kumamoto oysters (Crassostrea sikamea), and M. gigas to a two-week-long SH phase that involved exposure to a combination of temperature (15˚C v. 21˚C) and tide (immersion v. 6-hour tidal cycle) prior to a 72-hour simulated MHW at one of four temperatures (15˚C, 18˚C, 21˚C, 24˚C). Once the MHWs ended, a portion of O. lurida seed were grouped by their SH treatments, outplanted in Tomales Bay, CA, USA, and then assessed for mortality after nine months. Outplanted O. lurida that experienced tidal SH had perished by the time they were retrieved 279 days later. In contrast, 53.4% of O. lurida fully immersed during SH at 21˚C survived outplanting, while only 13.3% of those in the 15˚C SH treatment remained. Less than 10% of oysters across all species perished during the SH and MHW phases of the experiment, with C. sikamea experiencing the greatest losses. For Chapter 2, we investigated the effects of laboratory-based stress hardening (SH) via temperature (15˚C/16˚C v. 21˚C) and tide (tidal cycle vs. full submersion) on M. gigas and C. sikamea with the expectation that short-term exposure to warmer conditions and a simulated tidal regime would improve the performance of outplanted oysters. In 2021 and 2022, we exposed oysters to SH mechanisms before outplanting oysters at three sites spanning a thermal gradient in Tomales Bay, where they were exposed to OsHV-1 outbreaks in the field. We compared these outcomes to a baseline study completed in 2020, where M. gigas that had not been conditioned were exposed to OsHV-1 at two sites. M. gigas submerged in the warmer SH treatment had lower mortality in 2021, and those that experienced tidal fluctuations during SH had higher growth rates in 2022. In 2021, we saw no effect of SH on C. sikamea growth or mortality, but oysters were infected with OsHV-1. To our knowledge, this is the first study to provide OsHV-1 copy numbers (mg tissue-1) for C. sikamea. In Chapter 3, we used peer-reviewed literature, government reports, and unpublished data to describe the timeline of OsHV-1 variant detections, countries where M. gigas has tested positive, and other known hosts. We also summarized mean prevalence and peak infection intensity when that information was provided in the studies we assessed. Overall, SH has the potential to improve the resilience of aquaculture impacted by MHWs and disease, but it, along with the database of OsHV-1 detections we collated here, will likely be best used as part of an array of solutions addressing the multifaceted effects of climate change