Aquacultural Engineering 46 (2012) 27–39 Contents lists available at SciVerse ScienceDirect Aquacultural Engineering journa l h omepa g e: www .elsevier.com/locate/aqua-online Recirculation as a possible microbial control strategy in the production of marine larvae Kari J.K. Attramadal a,∗ , Ingrid Salvesen b , Renyu Xue c , Gunvor Øie b , Trond R. Størseth b , Olav Vadstein d , Yngvar Olsen a a Department of Biology, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway b SINTEF Fisheries and Aquaculture, Department of Marine Resources Technology, 7465 Trondheim, Norway c Pre-clinical Medical and Biological Science College of Soochow University, Suzhou 215123, PR China d Department of Biotechnology, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway a r t i c l e i n f o Article history: Received 9 May 2011 Accepted 26 October 2011 Keywords: Recirculating aquaculture system RAS Microbial control Marine fish Cod larvae Water treatment Bacteria Gadus morhua Juvenile production a b s t r a c t Marine hatcheries represent high value, low waste systems for larvae that are sensitive to general infec- tions by opportunistic bacteria. In intensive cultivation several procedures destabilise the microbial community of the rearing water and favour growth of potentially harmful microbes. Recirculation aqua- culture systems (RASs) have properties that may contribute to microbial stabilisation, including long water retention time and a large surface area of biofilters for bacterial growth. Moreover, the microbial community in RAS biofilters has the possibility to stabilise at a similar carrying capacity as the larvae tanks, which could potentially limit the chances of proliferation of opportunistic microbes in the rear- ing water. The development of the microbial community in a RAS with moderate ozonation (to 350 mV) was compared to that of a conventional flow-through system (FTS) for the same group of Atlantic cod, Gadus morhua. The feed and intake water were the same for the two groups. The RAS developed and maintained a more diverse and stable microbial community composition compared to the FTS. Water treatment regime explained most of the variation in microbial composition during the live feed period, and in addition it accounted for ten times higher variation in the composition of bacteria in the FTS than in the RAS. Less variability in bacterial composition was found between replicate fish tanks of the RAS than between tanks of the FTS. The RAS had a more even microbial community structure with higher species diversity and periodically a lower fraction of opportunists. The fish in the RAS performed better than their siblings in the FTS, despite being exposed to an apparent inferior physiochemical water quality. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Marine fry production of cold water species is steadily increasing, but challenges like suboptimal performance and poor reproducibility between replicate tanks are still typical in culti- vation of early stages of cod, halibut, turbot, lobster and scallop (Y. Olsen et al., 1999; Verner-Jeffreys et al., 2003; Jensen et al., 2004; Magnesen et al., 2006). The poor reproducibility has been attributed to detrimental larvae–microbe interactions based on improvements obtained in studies using antibiotics (Munro et al., 1994; Skjermo et al., 1997; Verner-Jeffreys et al., 2004) or micro- bial maturation of water (Skjermo et al., 1997; Salvesen et al., 1999; Skjermo and Vadstein, 1999). Newly hatched marine fish larvae lack a fully functional specific immune system (Vadstein, 1997; Schrøder et al., 1998), and are vulnerable to general infections by ∗ Corresponding author. E-mail address: kari.attramadal@bio.ntnu.no (K.J.K. Attramadal). opportunistic bacteria (Munro et al., 1994; Skjermo et al., 1997; Hansen and Olafsen, 1999; Skjermo and Vadstein, 1999; Verner- Jeffreys et al., 2003; Sandlund et al., 2010). Clearly, there is a need for reliable and cost efficient microbial control strategies. Vadstein et al. (1993) used a number of criteria based on the ecological theory of r/K-selection (MacArthur and Wilson, 1967) to describe the state of microbial maturity of rearing water. Gener- ally, a low maturity situation with a high share of fast-growing r-strategic bacteria is believed to be detrimental for fish larvae (Vadstein et al., 1993). The carrying capacity (CC) is the number of bacteria that can be sustained in the system over time. The supply of organic matter is typically the growth limiting factor defining CC for heterotrophic bacteria, the group interacting directly with fish larvae. Microbially matured water contains a more diverse and resilient microbial community of K-selected specialists in abun- dance close to the carrying capacity (CC). The microbial state of the rearing water depends on the supply of bacteria and organic matter together with selective forces in the tank and in the water sources. The main contributors of bacteria to 0144-8609/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaeng.2011.10.003