Volckaert et al. Genetics Selection Evolution 2012, 44:15 http://www.gsejournal.org/content/44/1/15 Page 3 of 5 Figure 1 Scatter plot of distribution of sea bass plasma cortisol in relation to sampling time. Tanks 1, 2 and 3 represent groups of fish analyzed from different tanks (n = 1687); please note that the Y-axis is on a log scale; the slopes of the three curves range between −0.15 and 0.11 (7.64 × 10-4 < r2 < 2.11 × 10-3). Oncorhynchus kisutch [19], 0.12-0.47 in brown trout, Salmo trutta [20], 0.09-0.44 in carp, Cyprinus carpio [21,22], 0.38- 0.79 in Nile tilapia, Oreochromis niloticus [23,24], 0.64 (± 0.12) in cod, Gadus morhua [25], 0.38 ± 0.07 in gilthead seabream, Sparus aurata [26]. To our knowledge, studies on the heritability of cortisol response to stress in fish have been limited to salmonids [27-29] and cyprinids [21], which limits generalizations. In addition, lines with high and low cortisol response to stress have been selected in rainbow trout [29]. However, the her- itability of cortisol response to stress appears to be variable even among related species: 0.27-0.50 in rainbow trout, Oncorhynchus mykiss [27,29,30] and 0.60 in carp [21], but only 0.05 in Atlantic salmon Salmo salar [27,31]. These dis- crepancies can be partly explained by the differences in spe- cies and methodologies used to determine the cortisol response. It should be noted that the time-dependence of cortisol response to stress is a key element and a potential source of error. However, the methodology used in our study seems reliable since no apparent drift in cortisol Table 2 Heritabilities (bold), genetic correlations (upper triangle) and phenotypic correlations (lower triangle) and standard deviations (in brackets) for different traits in European bass (n 922) BW CORT SL BW 0.54 (±0.21) −0.04 (±0.05) 0.94 (±0.04) CORT −0.60 (±0.44) 0.08 (±0.06) −0.05 (±0.06) SL 0.94 (±0.07) −0.55 (±0.44) 0.65 (±0.22) BW Body weight, CORT Cortisol response, SL Standard length. levels with time was observed after applying the confine- ment stress (Figure 1). A comparative study on stunning methods used in different fish farms for European sea bass reported mean levels of cortisol response similar to that obtained here, corresponding to a 5-fold increase in cortisol compared to resting values when using ice [32]. In a pilot study on gilthead seabream using the same methodology, a significant change in cortisol levels of a control group not subjected to stress was observed, but there was no signifi- cant additional effect of ice-water on a group subjected to confinement stress (A. Canario, unpublished observations). Thus, the methodology adopted here for European sea bass is appropriate and even 2 h after stunning, the levels of cor- tisol obtained are due to the response to confinement stress and should reflect individual variation. In conclu- sion, in European sea bass, the growth traits measured have a moderate to high heritability but the cortisol level, as an indicator of response to stress, has a low heritability. Whether this low heritability derives from an artefact or an unbalanced family structure or whether it has a true biological base needs further clarification. Additional files Additional file 1: Multiplex assignment to linkage group. Loci included in each multiplex set and assigned to D. labrax linkage groups, primer concentrations and comments on the 98 microsatellite markers used to scan the genome of European sea bass. Additional file 2: Parentage assignment statistics. Locus-specific PIC (Polymorphic Information Content) values, test of Hardy-Weinberg equilibrium (**: P < 0.05), alleles, cumulative parentage assignment (in percent) with one and no parent known at high (95%) and low (80%) stringency.