Temperatures regulate metabolism of marine ectotherms and thereby influence development, reproduction, and, as a consequence, dispersal. Despite the importance of water temperatures in the epidemiology of marine diseases, for the parasitic copepod Lepeophtheirus salmonis, the effect of high and low temperatures has not been methodically investigated. Here, we examined the effects of a wide temperature range (3–20 °C) on L. salmonis larval development, adult body size, reproductive outputs, and infestation success. Further, we tested if dispersal of salmon lice differed with two temperature-dependent development times to the infective stage (30 and 60 degree-days) using an individual-based dispersal model. Development times followed universal models of temperature dependence described for other marine ectotherms. Water temperatures had a negative relationship with development times, adult body size, and reproductive outputs, except at 3 °C, where larvae failed to reach the infective stage and all parameters were decreased, indicating low temperatures are more detrimental than high temperatures. The predictable effect of temperatures on lice development and reproduction will have important applications, such as predicting dispersal and population connectivity, to assist in controlling lice epidemics.

Disease and parasitism cause major welfare, environmental and economic concerns for global aquaculture. In this review, we examine the status and potential of technologies that exploit genetic variation in host resistance to tackle this problem. We argue that there is an urgent need to improve understanding of the genetic mechanisms involved, leading to the development of tools that can be applied to boost host resistance and reduce the disease burden. We draw on two pressing global disease problems as case studies—sea lice infestations in salmonids and white spot syndrome in shrimp. We review how the latest genetic technologies can be capitalised upon to determine the mechanisms underlying inter‐ and intra‐species variation in pathogen/parasite resistance, and how the derived knowledge could be applied to boost disease resistance using selective breeding, gene editing and/or with targeted feed treatments and vaccines. Gene editing brings novel opportunities, but also implementation and dissemination challenges, and necessitates new protocols to integrate the technology into aquaculture breeding programmes. There is also an ongoing need to minimise risks of disease agents evolving to overcome genetic improvements to host resistance, and insights from epidemiological and evolutionary models of pathogen infestation in wild and cultured host populations are explored. Ethical issues around the different approaches for achieving genetic resistance are discussed. Application of genetic technologies and approaches has potential to improve fundamental knowledge of mechanisms affecting genetic resistance and provide effective pathways for implementation that could lead to more resistant aquaculture stocks, transforming global aquaculture.

The aquaculture sector is currently experiencing a global disease crisis. Emerging bacterial diseases—often opportunistic or syndromic—have collapsed production in nations across the world. Losses in the shrimp industry associated with opportunistic Vibrio spp. exceed 40% of global capacity. This paper reviews potential drivers of syndromic diseases involving opportunistic bacterial pathogens affecting global aquaculture. We provide key examples from major industries where such conditions have prompted greater antibiotic use and have resulted in significant mortality. We challenge the conventional definitions of opportunistic pathogens and propose a fluid categorisation that acknowledges the continuum of host adaptation and the complexity of microbial ecology. We discuss the implications of environmental and dietary stressors such as climate change, coastal eutrophication and pollution, and the transition to plant‐based feeds, which have been linked to impaired epithelial barrier function, gut health disorders and increased disease susceptibility. We critique the ‘one‐pathogen one‐disease’ paradigm, suggesting that Rothman's causal pie model is more useful for understanding opportunistic infections as it emphasises the multicausal nature of disease. We provide examples of bacterial and viral interactions in aquatic disease and occurrence of bacterial diseases resulting from host damage from eukaryotic parasites or increasing frequency and severity of interventions to control such parasites. We recognise the need for corroborative evidence to validate the rise of opportunistic bacterial pathogens as a global trend, and we advocate for the application of nuanced disease causation models to reduce the incidence of opportunistic infections and improve the sustainability of the aquaculture industry.