1. Introduction
Intensive rearing of marine fish larvae suffers from heavy mortalities, which may be Ž
attributed to bacteria introduced in the rearing system with live food Nicolas et al., .
1989; Keskin et al., 1994 . The high organic load associated with intensive production of Ž
. live food cultures selectively induces an increased proportion of opportunistic r-selected
Ž .
bacteria, which may be pathogenic to the fish larvae Skjermo and Vadstein, 1999 . Disinfection, although beneficial, may not prevent a re-colonization of the live food
Ž .
within a short time period Munro et al., 1999 . Replacement of the opportunistic bacteria with other less-aggressive bacteria may provide a solution.
The rotifer Brachionus plicatilis and the brine shrimp Artemia franciscana are common live food organisms used for the rearing of marine fish larvae. These have been
considered as possible vectors for the delivery of different substances, such as nutrients Ž
. Ž
. Watanabe et al., 1983 , antimicrobial agents Mohney et al., 1990; Dixon et al., 1995 ,
Ž .
Ž .
vaccines Campbell et al., 1993 , and probiotics Gatesoupe, 1994 . These are live bacterial additives that may have a positive effect on the host organism by improving the
Ž .
properties of the indigenous microflora Havennar et al., 1992 . This positive effect of Ž
probiotics may be attributed to their ability to outcompete other bacteria Austin et al., .
Ž 1995 , or to produce micronutrients important for the development of fish larvae Sugita
. et al., 1991; Ringø et al., 1992 .
The quantitative and qualitative properties of the bacterial microflora of live food Ž
must be adjusted to avoid the negative effects of an overload of bacteria Benavente and .
Gatesoupe, 1988; Nicolas et al., 1989; Skjermo and Vadstein, 1993; Keskin et al., 1994 , Ž
and at the same time accomplish a successful colonization of the larval gut Munro et .
al., 1999 . A short-term incubation of live food organisms in a bacterial suspension consisting of one or several probiotic strains is a possible approach to replace oppor-
tunists with other less-aggressive bacteria. Live food organisms may reside in the rearing Ž
. tanks for several hours before ingested by the larvae Reitan et al., 1993 . Once the
bacteria have been bioencapsulated in the live food, it is important to determine the rate of loss of the bioencapsulated bacteria, and whether the changed bacterial composition
persists, as live food organisms may be depleted of the specific bacteria before they are ingested by fish larvae. The detection of specific bacteria in the live food or the larvae,
Ž . Ž
has been accomplished by use of enzyme-linked immunosorbent assay ELISA Ringø
. Ž
. et al., 1996 , measuring the amount of lipopolysaccharide Kawai et al., 1989 , or by
Ž . Ž
. counting the number of colony-forming units CFU
Gomez-Gil et al., 1998 . Ž .
The aims of this study were: i to control the composition of the bacterial flora of live food by adding specific bacteria to the culture water and letting them feed for a
Ž . short time period, ii to evaluate if the changed bacterial composition was maintained
when the live food organisms were transferred to conditions similar to those in a first Ž .
feeding tank, and iii to compare two types of immunoassays as methods for detecting specific bacteria in the live food. For comparative purposes, the live food organisms B.
plicatilis and A. franciscana were used, and two bacterial strains were tested for each of the live food organisms.
2. Materials and methods
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