Pacific oysters (Crassostrea gigas) have been introduced worldwide for aquaculture, which can benefit food sovereignty and economic development; however, established non‐native populations may present risks for local organisms and habitats. We reviewed the documented introductions of C. gigas by country to evaluate factors related to introduction, establishment and production, and to determine impacts of feral populations. To date, C. gigas has been introduced in 64 countries and 10 territories, has been established in 32 and contributes to cupped oyster production in 36 (additionally, three native range countries). Oysters were more likely to be introduced and to establish in higher‐latitude countries, but the economic factor of per capita Gross Domestic Product per capita (GDPpc) served as a stronger explanatory variable than latitude for introduction. Earlier introduction and the use for extended periods in aquaculture, not just in trials, further increased establishment. Commercial production increased with coastline length and when established populations were present, but in contrast to establishment, declined at higher latitude. Effects of feral populations have been studied in 11 countries, spanning a range from negative to facilitative as expected for an ecosystem engineer, while reaching levels of concern warranting removal from coastlines in the UK, Norway, Sweden Australia and South Africa. The use of GDPpc in analyses revealed a widespread inequity in terms of low investment in Pacific oyster aquaculture in poorer countries, while richer countries were able to invest not only in aquaculture but also in studies of environmental impacts of feral oysters.

In bivalve mollusk aquaculture, massive disease outbreaks with high mortality and large economic losses can occur, as in northwest Mexico in the 1990s. A range of pathogens can affect bivalves; one of great concern is ostreid herpesvirus 1 (OsHV-1), of which there are several strains. This virus has been detected in the Gulf of California in occasional or sporadic samplings, but to date, there have been few systematic studies. Monthly samples of Crassostrea gigas, water, and sediment were taken in the La Cruz coastal lagoon and analyzed by PCR. The native mollusk, Dosinia ponderosa, which lives outside the lagoon, was sampled as a control. The virus was found throughout the year only in C. gigas, with prevalence up to 60%. In total, 9 genotype variants were detected, and genetic analysis suggests that linear genotypic evolution has occurred from strain JF894308, present in La Cruz in 2011. There has been no evidence of the entry of new viral genotypes in the recent past, thus confinement of the virus within the lagoons of the Gulf of California could promote a native genotypic diversity in the short term.

ABSTRACT. Infectious diseases have been a major limiting factor for large scale production of oyster farming. Several factors have contributed to the emergence and spread of infectious diseases in all cultivation sites around the world. Therefore, the prevention and control of diseases has become a priority for the sustainability of global aquaculture. Crassostrea corteziensis is a native species that contributes to the Mexican Pacific oyster production and used in this study to assess seasonal simultaneous infective events between distant areas in the Gulf of California. The results of the molecular analysis showed a higher prevalence of Perkinsus marinus in the north area and Marteilia refringens in the south. OsHV-1 was only present in summer and autumn with low prevalence in the two areas. The histological analysis of the PCR-positive organisms presented alterations characteristic of infections. The presence of M. refringens in a new location on the Gulf of California suggests that this pathogen is already well established in the area and the dual presence of pathogens in C. corteziensis is reported for the first time.