Aquacultural Engineering 49 (2012) 23–34 Contents lists available at SciVerse ScienceDirect Aquacultural Engineering journa l h omepa g e: www .elsevier.com/locate/aqua-online Ceramic clay reduces the load of organic matter and bacteria in marine fish larval culture tanks Kari J.K. Attramadal a,∗ , Bjørnar Tøndel a , Ingrid Salvesen b , Gunvor Øie b , Olav Vadstein c , 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 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 6 May 2011 Accepted 19 February 2012 Keywords: Microbial control Ceramic clay Marine larvae Green water Microalgae Gadus morhua a b s t r a c t Ceramic clay has been increasingly used to improve contrast and prey detection in tanks for rearing of fish larvae. In contrast to live microalgae or algae pastes, clay increases turbidity without contributing to the organic matter load. In addition, clay may aggregate and sediment organic matter and bacteria, facilitating its removal. Marine larvae are sensitive to infections by opportunistic bacteria. Fish, algae, and live feed increase the microbial carrying capacity of the rearing water which allow exponential growth of bacteria and favor fast-growing opportunists. Reducing substrate levels by replacing microalgae with clay may reduce bacteria proliferation and benefit larvae. We compared the effects of three rearing regimes including live Isochrysis galbana, Nannochloropsis oculata paste, and ceramic clay on the bacterial commu- nity, concentration of organic matter, and growth and survival of Atlantic cod larvae (Gadus morhua L.). The application of clay resulted in reduced substrate levels for bacteria in the rearing water compared to the addition of live algae or algae paste. To some extent, clay aggregated and transported organic matter to the bottom of the larval fish tanks, where it could be effectively removed. Fish tanks receiving clay showed a lower abundance of bacteria in the water than tanks added algae paste or live algae. Fish tanks with algae paste showed a higher abundance of bacteria and a higher share of cultivable bacteria and TCBS counts than the other two treatments. Tanks with live algae showed low relative abundances of opportunistic bacteria and TCBS counts in both water and rotifers. Cod larvae in tanks with clay or live algae initiated exponential growth earlier than larvae in tanks with algae paste. Larvae in tanks receiving clay had significantly higher dry weight than larvae in tanks receiving algae paste at day 5 and 20 post hatching. The survival of larvae in the tanks added clay was variable. Two of the three tanks with clay had significantly higher larval survival than the tanks with live algae or algae paste. However, one tank with clay underwent 100% mortality. It is not possible to conclude whether this was related to the use of clay or an incidental development of a harmful microbial community in this tank. The effects of clay addition on larval performance should be studied further. Clay addition appears to be an easy way to reduce bacterial load during early first feeding of marine larvae without compromising the beneficial effects of turbidity. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Production of juveniles is a bottleneck in marine aquaculture, characterized by variable performance of larvae, which has been linked to negative interactions with microbes (Vadstein et al., 1993, 2004). Newly hatched marine larvae rely on the general immune system and are vulnerable to infections by opportunistic bacteria. Non-selective reduction of bacteria is one of three key elements suggested in a strategy aiming for microbial control in the rearing of marine larvae (Vadstein et al., 1993). According to this strategy, ∗ Corresponding author. E-mail address: kari.attramadal@bio.ntnu.no (K.J.K. Attramadal). actions aimed at limiting the abundance of bacteria include meth- ods that focus on reducing the microbial carrying capacity (CC) of the system by reducing input and increasing removal of organic matter. Dissolved organic matter (DOM) supplied from decom- posing hatching remnants, fecal matter, and live feed is the main growth-limiting substrate for heterotrophic bacteria in rearing water. Different types of particles are commonly added to the rear- ing water during the first feeding of marine larvae. We hypothesize that the addition of ceramic clay reduces the load of organic matter on the rearing tanks, and hence the abundance of bacteria, com- pared to addition of algae paste or live microalgae. Addition of microalgae to the culture water has a beneficial influence on survival and growth of marine larvae in intensive rear- ing systems (Howell, 1979; Naas et al., 1992; Reitan et al., 1993; 0144-8609/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaeng.2012.02.003