Neffect of rearing density on the growth and welf are indices of juvenile spotted wolffish, anarhichas minor (olafsen)

Stress

Le stress se définit comme la condition d’ un organisme dont l’équilibre dynamique, ou homéostasie, est menacé ou perturbé par l’action de stimuli intrinsèques ou extrinsèques (Chrousos & Gold, 1992, cité par Wendelaar Bonga, 1997). Il agit à trois échelles différentes. La première implique, chez les poissons, la stimulation de l’axe hypothalamopituitairo- interrénal (HPI) [équivalent de l’axe hypothalamo-pituitairo-surrénal (HP A) chez les mammifères (Metz et al., 2006)] qui relâche l’hormone adrénocorticotropique (ACTH). Celle-ci stimule les tissus interrénaux à sécréter les corticostéroïdes, dont le cortisol, fréquemment employé pour mesurer la réponse au stress (Donaldson, 1981; Schreck, 1981 ; Wedemeyer et al. , 1990; Barton & Iwama, 1991; Wendelaar Bonga, 1997; Barton, 2002). La réponse secondaire est induite par la réponse primaire et s’exprime par des changements au niveau des paramètres métaboliques, sanguins, structuraux et hydrominéraux (Barton & Iwama, 1991). Ainsi, une élévation du glucose [très faible (Lays et al. , 2009), voire non détectable chez le loup (S. Lamarre, données non publiées)] et du lactate [non détectable chez le loup (S. Lamarre, données non publiées)] , des variations de l’hématocrite, de l’ indice hépato-somatique et de la quantité totale de protéines de même que des changements de taille au niveau des cellules interrénales sont quelques uns des indices de stress secondaire pouvant être évalués chez les poissons (Wedemeyer et al., 1990; Barton & Iwama, 1991; Barton, 2002).

Une augmentation de la perméabilité des branchies occasionnant des échanges d’eau et d’ions Na+ et cr plus importants (Mazeaud et al., 1977; Wendelaar Bonga, 1997) est également possible. Cela résulte, chez les espèces marines comme le loup, en une augmentation de l’osmolalité et des concentrations d’ions puisque plus d’ ions entrent et qu’une plus grande quantité d’ eau sort, alors que le contraire est observé chez les poissons dulcicoles. La réponse tertiaire au stress considère tous les impacts à long terme comme les effets sur la croissance, le taux métabolique, la résistance aux maladies, la reproduction et la capacité à tolérer des stress additionnels (Wedemeyer et al., 1990; Barton & Iwama, 1991; Wendelaar Bonga, 1997). L’ activité des lysozymes plasmatiques, des composants humoraux du système de défense non-spécifique (Magnadottir, 2006), constitue un indicateur potentiel de l’effet du stress sur l’ immunité. Un stress de courte durée résulte généralement en une augmentation de l’ activité des lysozymes plasmatiques alors qu’ un stress plus important provoque plutôt une baisse de ce paramètre (Mock & Peters, 1990; Demers & Bayne, 1997; Maricchiolo et al., 2008). La réponse au stress est propre à chaque espèce et peut même varier en fonction de l’âge et du stade de vie (Wendelaar Bonga, 1997; Ramsay et al. , 2006). Même si plusieurs sources de stress sont inévitables en aquaculture, connaître les particularités de chaque espèce cultivée est important afin d’adapter les pratiques et d’assurer un minimum de stress aux animaux (Ashley, 2007).

Experiment 1 (increasing densities): 50-100 g Juvenile wolffish (n = 199) were individually tagged with electronic microchips (12 mm Musicc Fecava, Avid Canada) and acclimatized to the experimental tanks for 1 month at a density of la kg·m-2 . They were then randomly distributed among six tanks (between 32 and 34 fish per tank) at initial densities of ~ la, 20 and 40 kg·m-2 [tank volume (L x W x D), respectively ~ 44.5 x 36.5 x 14 cm, 24.5 x 36.5 x 14 cm and Il x 36.5 x 27 cm, n = 6]. Densities were allowed to increase as the fish grew to co ver a wider range of densities. An adjustable plastic screen allowing water exchange was placed inside each tank to adjust the length and ob tain the desired density. The triallasted for 120 days. Fish were measured (wet weights and totallengths) at the start of the experiment (51.6 ± 21.9 g, 18.6 ± 2.5 cm) and on days 30, 59 and 120. Fish were not fed for 2 days before sampling was conducted, a reasonable fasting period since the average intestinal transit time for the close relative Atlantic wolffish, Anarhichas lupus L., is approximately 15 ho urs (Le François et al. , 2004). To facilitate handling, fish were anesthetised directly in the tanks by adding benzocaine (ethyl 4-aminobenzoate, Sigma E1501, St. Louis, Missouri, USA) to the water at a concentration of 50 mg·L-I .

Condition factors (CF) and SGR were calculated for each individual fish. Blood samples were also taken of three fish per tank on days 0, 15 and 30 from the caudal vein with a sodium heparinised hypodermic syringe. Blood was kept on ice prior to centrifugation at 1100 g for 10 min at 4 oC. Plasma was stored at -80 oC. Plasma concentrations of Na+ and K+ were determined using an automatic ion chromatograph (model ICS-3000, Dionex, Oakville, ON, Canada). Condition factors were calculated as CF = 100 x (WILl), where W is the weight of the fish (g) and LT is its total length (cm). Specific growth rates were calculated as SGR = 100 x (ln W 2 – ln W 1) 1 (h – tl), where W 1 and W 2 are the weights of individual fish at times tl and t2. Fish were kept under a 12-h light:12-h dark artificial photoperiod. They were fed in excess three times a week, previously determined in our facilities to be sufficient for 19 wolffish of this size, with formulated feed (3 or 4 mm, Skretting Bayside, NB, Canada) containing 55% protein and 15% fat. Temperature, dissolved oxygen saturation and salinity were monitored daily (YSI Dissolved Oxygen Meter, with multiprobe 85110 FT, Yellow Springs, Ohio, USA). Means (± SD) were 8.3 ± 0.8 oC, 84 ± 6% and 28.7 ± 1.1 g’L-1 , respectively. Nitrite and total ammonia nitrogen (TAN) concentrations were measured weekly using commercial kits (HA CH, Loveland, CO, USA) and remained low (N02: 0.022 ± 0.016 mg·L-1; TAN: 0.14 ± 0.06 mg-L-1 ) throughout the experiment.

Experiment II (fixed densities): 100-160 g A total of 189 untagged fish were divided in nine groups of 21 individuals and stocked at 20 kg·m-2 for a I-month acclimatization period in the experimental tanks (n = 9). At the beginning of the experiment, fish were measured (95 .5 ± 41.3 g, 21.2 ± 2.7 cm) and randomly distributed among nine raceways, one per group, to yield densities of ~ 20, 30 and 40 kg’m-2 in triplicate (tank volume, respectively ~ 26.5 x 38 x 16 cm, 18 x 38 x 16 cm and 13 x 38 x 16 cm). As for experiment I, adjustable plastic screens were used to obtain adequate lengths and fixed densities. Individual wet weights and total lengths were measured on days 29, 59 and 90 of the experiment and densities were then readjusted to initial values by moving the adjustable screens. Specifie growth rates could not be calculated because fish were not individually tagged and so mean GR were calculated. Fish were not fed for two days before measurement and anesthetised with a 50 mg’L-1 benzocaine solution. Tanks were supplied with flow-through water (6 L’min-1 ) at a temperature of 7.9 ± 0.8 oC, a dissolved oxygen of 95 ± 4% and a salinity of 26.6 ± 1.4 g·L-I . Fish were kept under a 12-h light:12-h dark artificial photoperiod and fed in excess three times a week with fonnulated feed (4 mm, Skretting) containing 55% protein and 15% fat. Nitrite and TAN concentrations remained low (0.013 ± 0.009 mg’L-1 and 0.09 ± 0.02 mg’L-1 , respectively) throughout the experiment. The growth part of Experiment II was terminated at day 90 when fish weighed on average 160 g because at day 117, failure of the water cooling system caused the death of 87 fish in total (temperature increased from 8.5 to 18 OC).

The surviving fish showed normal growth and feeding behaviour within 2 weeks. After 6 months at the three different rearing densities, stress and immune response were evaluated. On day 186, four fish per tank were individually nette d, avoiding any disturbance to the remaining fish, held in the air for 60 seconds (Acerete et al. , 2004) to mimic aquaculture handling and placed in a different tank. After 60 min, these individuals were anesthetised with a 50 mg’L-1 benzocaine solution. The remaining wolffish were anesthetised directly in the tank on day 187. Four fish from each tank were immediately sampled to establish baseline values for unstressed fish at the three densities. Blood samples were taken from the caudal vein with a sodium heparinised hypodermic syringe. Haematocrit was measured using a Hct centrifuge (3 min at 13 858 g, Jouan industries S.A.S ., Château-Gontier, France) and blood was centrifuged to obtain plasma, which was stored at – 80 oC until assessment of total proteins (TP) and lysozyme activity. Fish were killed by a blow to the head. Livers were removed and weighted to calculate the hepatosomatic index (HSI = 100 x (LW / W), where LW is the wet liver weight and W is the total wet weight of the fish) . A piece of muscle and liver were also collected and dried in a oyen (ModeI287, Despatch oyen CO., Minneapolis, MN, USA) at 60 oC for 72 hours to evaluate the water content.