Vaccination is considered crucial for disease prevention and fish health in the global salmon farming industry. Nevertheless, some aspects, such as the efficacy of vaccines, can be largely circumvented during natural coinfections. Sea lice are ectoparasitic copepods that can occur with a high prevalence in the field, are frequently found in co-infection with other pathogens, and are highly detrimental to fish health. The aim of this case-control study was to evaluate the interaction between the detrimental effects of coinfection and the protective effects of vaccination in fish. We used the interaction between the sea louse Caligus rogercresseyi, the bacterial pathogen Piscirickettsia salmonis, and their host, the Atlantic salmon Salmo salar, as a study model. Our results showed that coinfection decreased the accumulated survival (AS) and specific growth rate (SGR) of vaccinated fish (AS = 5.2 ± 0.6%; SGR = −0.05 ± 0.39%) compared to a single infection of P. salmonis (AS = 42.7 ± 1.3%; SGR = 0.21 ± 0.22%). Concomitantly, the bacterial load and clinical signs of disease were significantly increased in coinfected fish. Coinfection may explain the reduced efficacy of vaccines in sea cages and highlights the need to test fish vaccines in more diverse conditions rather than with a single infection.

Harmful Algal Blooms (HABs) have been classified depending on the causative organism and its impacts: non-toxic HAB (microalgae capable of affecting tourism and causing oxygen deficiency, which generates mortality of marine organisms), toxic HAB (microalgae capable of transferring toxins to the food chain), and ichthyotoxic HAB (microalgae capable of generating mechanical damage in fish). HABs represent a worldwide problem and have apparently increased in frequency, intensity, and geographic distribution at different latitudes. This review details the occurrence of HAB events in the Southeast Pacific, Chile, over a 65-year period, analysing two of the three types of HAB described: toxic and ichthyotoxic HABs. For this, we conducted a review from many different scientific sources and from the written press and social media, that have mentioned HAB events in the country. In Chile, the microalgae involved in HAB events are dinoflagellate (52%), diatoms (33%) and silicoflagellate (10%), with a total of 41 species and/or genera described in the literature. A total of 501 HAB events were recorded in Chile between 1956 and 2021, where 240 (47.9%), 238 (47.5%), 14 (2.7%), 8 (1.5%) and 1 (0.2%) event were caused by diatoms, dinoflagellate, silicoflagellate, raphidophycean and haptophyte, respectively. An apparent increase in the frequency of HAB events is observed since the first record in 1956, with a maximum of 46 events during the years 2017 and 2019. The highest incidence in fish is caused by the group of silicoflagellate, raphidophycean and haptophyte (23 events), where 10 events caused mortalities in salmon with an incidence rate of 43.4%. Unlike what is observed with diatoms and dinoflagellate, the events associated with these groups are less frequent, but hold a much higher salmon mortality rate. During the last 65 years, HAB’s geographic extent shows an apparent trend to increase south-to-north. However, the identification of events is closely linked to the areas where much of the country’s aquaculture is located and, therefore, it could be biased. In turn, it is observed that the apparent increase in HAB events could be associated with a greater monitoring effort after major events (e.g., after the 2016 HAB event). On the other hand, it is also recognized a lack of knowledge about harmful algae throughout the Chilean Humboldt Current system, particularly in the northern regions, such as Atacama and Coquimbo. Therefore, the total number of blooms that have occurred in fjords and channels, particularly those that have caused minor economic impacts for artisanal fishermen and the salmon and mussel farming sector, might be underestimated.

Vaccination is a widely used control strategy to prevent Piscirickettsia salmonis causing disease in salmon farming. However, it is not known why all the currently available commercial vaccines generally fail to protect against this pathogenic bacteria. Here, we report, from two different populations, that between-family variation is a strong intrinsic factor that determines vaccine protection for this disease. While in some full-sib families, the protection added by vaccination increased the survival time in 13 days in comparison with their unvaccinated siblings; in other families, there was no added protection by vaccination or even it was slightly negative. Resistance to P. salmonis, measured as days to death, was higher in vaccinated than unvaccinated fish, but only a moderate positive genetic correlation was obtained between these traits. This disputes a previous hypothesis, that stated that both traits were fully controlled by the same genes, and challenges the use of unvaccinated fish as gold standard for evaluating and selecting fish resistant to P. salmonis, particularly if the offspring will be vaccinated. More studies are necessary to evaluate if variation in the host immune response to vaccination could explain the between-family differences in resistance observed in vaccinated fish.