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Coho Salmon (Oncorhynchus kisutch) Prefer and Are Less Aggressive in Darker Environments.

Gaffney LP, Franks B, Weary DM, von Keyserlingk MA - PLoS ONE (2016)

Bottom Line: Fish are capable of excellent vision and can be profoundly influenced by the visual properties of their environment.Moreover, having darker backgrounds in the environment resulted in less aggressive behaviour throughout the tank (p < 0.0001).These results provide the first evidence that darker tanks are preferred by and decrease aggression in salmonids, which points to the welfare benefits of housing farmed salmon in enclosures containing dark backgrounds.

View Article: PubMed Central - PubMed

Affiliation: Animal Welfare Program, Faculty of Land and Food Systems, University of British Columbia, 2357 Main Mall, Vancouver, BC, V6T 1Z6, Canada.

ABSTRACT
Fish are capable of excellent vision and can be profoundly influenced by the visual properties of their environment. Ambient colours have been found to affect growth, survival, aggression and reproduction, but the effect of background darkness (i.e., the darkness vs. lightness of the background) on preference and aggression has not been evaluated systematically. One-hundred Coho salmon (Oncorhynchus kisutch), a species that is increasing in popularity in aquaculture, were randomly assigned to 10 tanks. Using a Latin-square design, every tank was bisected to allow fish in each tank to choose between all the following colour choices (8 choices in total): black vs. white, light grey, dark grey, and a mixed dark grey/black pattern, as well as industry-standard blue vs. white, light grey, dark grey, and black. Fish showed a strong preference for black backgrounds over all other options (p < 0.01). Across tests, preference strength increased with background darkness (p < 0.0001). Moreover, having darker backgrounds in the environment resulted in less aggressive behaviour throughout the tank (p < 0.0001). These results provide the first evidence that darker tanks are preferred by and decrease aggression in salmonids, which points to the welfare benefits of housing farmed salmon in enclosures containing dark backgrounds.

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Related in: MedlinePlus

Dark refuges decrease aggression throughout the tank.Overall aggression (total number of aggressive acts occurring per 10-min trial) throughout the tank decreased with average tank darkness (p < 0.01), but especially when the tank contained an area available with darkness of .55 or greater (p < 0.0001). Dots represent aggression counts in a trial for each tank. Background darkness ranges from 0 (white) to 1 (black). Filled-in circle (◯) colour corresponds to the average tank darkness and the triangle (▿) indicates the average tank darkness of the pattern vs. black trial. The bi-colour panel at the bottom of the plot shows the two colours tested in the trial.
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pone.0151325.g003: Dark refuges decrease aggression throughout the tank.Overall aggression (total number of aggressive acts occurring per 10-min trial) throughout the tank decreased with average tank darkness (p < 0.01), but especially when the tank contained an area available with darkness of .55 or greater (p < 0.0001). Dots represent aggression counts in a trial for each tank. Background darkness ranges from 0 (white) to 1 (black). Filled-in circle (◯) colour corresponds to the average tank darkness and the triangle (▿) indicates the average tank darkness of the pattern vs. black trial. The bi-colour panel at the bottom of the plot shows the two colours tested in the trial.

Mentions: Background darkness dramatically decreased aggression (df = 157, t = 11.06, p < 0.0001) Fig 2): aggression was nearly four times greater on blue backgrounds (the industry standard) than on black backgrounds. The effect of blue and patterned background on aggression was entirely explained by their darkness value or, in other words, aggression on the blue side was no different than aggression on the light grey side (which had the same darkness value) and aggression on the patterned side was consistent with what would have been expected given the patterned background’s average darkness value. There was virtually no variation in aggression per tank—the ICC (intraclass correlation coefficient) was 0. Examining aggression at the tank level, we found that increasing the average darkness of the environment decreased the number of overall aggressive acts (z = 2.79, p < 0.01; Fig 3). Interestingly, however, the decrease in aggression with background darkness was not linear even on a log scale. When the tank contained a dark ‘refuge’—a side of the tank with a darkness value of 0.55 or higher—overall aggression was reduced beyond what would be expected by the effect of mean tank darkness alone. Fish engaged in only one third as many aggressive acts in trials with a dark ‘refuge’ compared to trials without a dark ‘refuge’ (Fig 3; z = 9.41, p < 0.0001).


Coho Salmon (Oncorhynchus kisutch) Prefer and Are Less Aggressive in Darker Environments.

Gaffney LP, Franks B, Weary DM, von Keyserlingk MA - PLoS ONE (2016)

Dark refuges decrease aggression throughout the tank.Overall aggression (total number of aggressive acts occurring per 10-min trial) throughout the tank decreased with average tank darkness (p < 0.01), but especially when the tank contained an area available with darkness of .55 or greater (p < 0.0001). Dots represent aggression counts in a trial for each tank. Background darkness ranges from 0 (white) to 1 (black). Filled-in circle (◯) colour corresponds to the average tank darkness and the triangle (▿) indicates the average tank darkness of the pattern vs. black trial. The bi-colour panel at the bottom of the plot shows the two colours tested in the trial.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4814047&req=5

pone.0151325.g003: Dark refuges decrease aggression throughout the tank.Overall aggression (total number of aggressive acts occurring per 10-min trial) throughout the tank decreased with average tank darkness (p < 0.01), but especially when the tank contained an area available with darkness of .55 or greater (p < 0.0001). Dots represent aggression counts in a trial for each tank. Background darkness ranges from 0 (white) to 1 (black). Filled-in circle (◯) colour corresponds to the average tank darkness and the triangle (▿) indicates the average tank darkness of the pattern vs. black trial. The bi-colour panel at the bottom of the plot shows the two colours tested in the trial.
Mentions: Background darkness dramatically decreased aggression (df = 157, t = 11.06, p < 0.0001) Fig 2): aggression was nearly four times greater on blue backgrounds (the industry standard) than on black backgrounds. The effect of blue and patterned background on aggression was entirely explained by their darkness value or, in other words, aggression on the blue side was no different than aggression on the light grey side (which had the same darkness value) and aggression on the patterned side was consistent with what would have been expected given the patterned background’s average darkness value. There was virtually no variation in aggression per tank—the ICC (intraclass correlation coefficient) was 0. Examining aggression at the tank level, we found that increasing the average darkness of the environment decreased the number of overall aggressive acts (z = 2.79, p < 0.01; Fig 3). Interestingly, however, the decrease in aggression with background darkness was not linear even on a log scale. When the tank contained a dark ‘refuge’—a side of the tank with a darkness value of 0.55 or higher—overall aggression was reduced beyond what would be expected by the effect of mean tank darkness alone. Fish engaged in only one third as many aggressive acts in trials with a dark ‘refuge’ compared to trials without a dark ‘refuge’ (Fig 3; z = 9.41, p < 0.0001).

Bottom Line: Fish are capable of excellent vision and can be profoundly influenced by the visual properties of their environment.Moreover, having darker backgrounds in the environment resulted in less aggressive behaviour throughout the tank (p < 0.0001).These results provide the first evidence that darker tanks are preferred by and decrease aggression in salmonids, which points to the welfare benefits of housing farmed salmon in enclosures containing dark backgrounds.

View Article: PubMed Central - PubMed

Affiliation: Animal Welfare Program, Faculty of Land and Food Systems, University of British Columbia, 2357 Main Mall, Vancouver, BC, V6T 1Z6, Canada.

ABSTRACT
Fish are capable of excellent vision and can be profoundly influenced by the visual properties of their environment. Ambient colours have been found to affect growth, survival, aggression and reproduction, but the effect of background darkness (i.e., the darkness vs. lightness of the background) on preference and aggression has not been evaluated systematically. One-hundred Coho salmon (Oncorhynchus kisutch), a species that is increasing in popularity in aquaculture, were randomly assigned to 10 tanks. Using a Latin-square design, every tank was bisected to allow fish in each tank to choose between all the following colour choices (8 choices in total): black vs. white, light grey, dark grey, and a mixed dark grey/black pattern, as well as industry-standard blue vs. white, light grey, dark grey, and black. Fish showed a strong preference for black backgrounds over all other options (p < 0.01). Across tests, preference strength increased with background darkness (p < 0.0001). Moreover, having darker backgrounds in the environment resulted in less aggressive behaviour throughout the tank (p < 0.0001). These results provide the first evidence that darker tanks are preferred by and decrease aggression in salmonids, which points to the welfare benefits of housing farmed salmon in enclosures containing dark backgrounds.

Show MeSH
Related in: MedlinePlus