Managing the Microbial Community of Marine Fish Larvae: A Holistic Perspective for Larviculture

Abstract

The availability of high-quality juveniles is a bottleneck in the farming of many marine fish species. Detrimental larvae-microbe interactions are a main reason for poor viability and quality in larval rearing. In this review, we explore the microbial community of fish larvae from an ecological and eco-physiological perspective, with the aim to develop the knowledge basis for microbial management. The larvae are exposed to a huge number of microbes from external and internal sources in intensive aquaculture, but their relative importance depend on the rearing technology used (especially flow-through vs. recirculating systems) and the retention time of the water in the fish tanks. Generally, focus has been on microbes entering the system, but microbes from growth within the system is normally a substantial part of the microbes encountered by larvae. Culture independent methods have revealed an unexpected high richness of bacterial species associated with larvae, with 100–250 operational taxonomic units associated with one individual. The microbiota of larvae changes rapidly until metamorphosis, most likely due to changes in the selection pressure in the digestive tract caused by changes in host-microbe and microbe-microbe interactions. Even though the microbiota of larvae is distinctly different from the microbiota of the water and the live food, the microbiota of the water strongly affects the microbiota of the larvae. We are in the early phase of understanding larvae-microbe interactions in vivo, but some studies with other animals than fish emphasize that we so far have underestimated the complexity of these interactions. We present examples demonstrating the diversity of these interactions. A large variety of microbial management methods exist, focusing on non-selective reduction of microbes, selective enhancement of microbes, and on improvement of the resistance of larvae against microbes. However, relatively few methods have been studied extensively. We believe that there is a lot to gain by increasing the diversity of approaches for microbial management. As many microbial management methods are perturbations of the microbial community, we argue that ecological theory is needed to foresee and test for longer term consequences in microbe-microbe and microbe-larvae interactions. We finally make some recommendations for future research and development.

Live Feeds Used in the Larval Culture of Red Cusk Eel, Genypterus chilensis, Carry High Levels of Antimicrobial-Resistant Bacteria and Antibiotic-Resistance Genes (ARGs)

“…To our knowledge, this is the first report on the occurrence of antimicrobial-resistant bacteria in the live feeds that are most frequently used in marine fish larvae rearing, as most of the previous studies have attempted to reduce bacterial load in these cultures [19,[25][26][27][28][29] or characterize the bacterial species associated with the live feeds [7][8][9][10][11]13,15,16,18,19,43]. Our findings have demonstrated that live-feed cultures used in a Chilean commercial hatchery of red cusk eel have high levels of resistant bacteria in both untreated and treated live feeds, suggesting that the usage of florfenicol is not a necessary causal condition for the development and maintenance of elevated frequencies of florfenicol resistance.…”

K-Selection as Microbial Community Management Strategy: A Method for Improved Viability of Larvae in Aquaculture

“…In addition to interactions ongoing within the rearing tank, various inputs and losses from the tank also affect this microbe dominated food web. Figure from Vadstein et al (2018) .…”

“…The species composition of the bacterial community of the system is determined by (1) the composition of the bacteria in the external sources mentioned above, and (2) the growth potential and the selective forces acting on the microbial community within the system. The growth within the system (header tanks, tubes, and rearing tanks) can be substantial, and may contribute with 10 8 bacteria L -1 h -1 ( Attramadal et al, 2014 ; Vadstein et al, 2018 ). Depending on the rearing conditions, this is 1.2–14 times higher than the bacteria brought in with the incoming water.…”

“…This has resulted in new knowledge on the functional roles of such microbes, including healthy development of the and a wide range of diseases not considered to be infection related, like diabetes and autism (e.g., Kanther and Rawls, 2010 ; Sekirov et al, 2010 ; McFall-Ngai et al, 2013 ; Lerner et al, 2017 ). The importance of microbe-fish interactions in host development and health are less studied, but this area of research is also rapidly progressing (see Kanther and Rawls, 2010 ; Vadstein et al, 2018 ; Vestrum et al, 2018b ). This rapid progress is largely due to a revolution in molecular and imaging methods ( Vestrum et al, 2018a ).…”

Impacts of the Marine Hatchery Built Environment on Mucosal Microbiome Colonization Across Ontogeny in Yellowtail Kingfish, Seriola Lalandi

“…Ocean net pen fish generally had lower microbial diversity than indoor reared fish for all fish body sites, but was most pronounced in the gill. This further suggests that the mucosal, even in adult fish that are least susceptible to BE impacts, has a high capacity to change which is critical when considering time scales for probiotic effects [27,79,80]. Probiotic treatments in fish are common but little is known about dosage for a given treatment along with frequency of administration for having a lasting effect.…”