Enrichment Increases Aggression in Zebrafish

Abstract

Environmental enrichment, or the enhancement of an animal’s surroundings when in captivity to maximise its wellbeing, has been increasingly applied to fish species, particularly those used commercially. Laboratory species could also benefit from enrichment, but it is not always clear what constitutes an enriched environment. The zebrafish, Danio rerio, is used widely in research and is one of the most commonly-used laboratory animals. We investigated whether changing the structural complexity of housing tanks altered the behaviour of one strain of zebrafish. Fish were kept in three treatments: (1) very enhanced (VE); (2) mildly enhanced (ME); and (3) control (CT). Level of aggression, fertilisation success, and growth were measured at regular intervals in a subset of fish in each treatment group. The VE fish were more aggressive over time than either ME or CT fish, both in the number of attacks they made against a mirror image and in their tendency to stay close to their reflection rather than avoid it. Furthermore, VE fish were shorter than CT fish by the end of the experiment, though mass was not significantly affected. There was no significant effect of treatment on fertilisation success. These findings suggest that the way in which fish are housed in the laboratory can significantly affect their behaviour, and potentially, their growth. The zebrafish is a shoaling species with a dominance hierarchy, and so may become territorial over objects placed in the tank. The enrichment of laboratory tanks should consider aspects of the species’ behaviour.

Zebrafish models of impulsivity and impulse control disorders

“…In rodents, environmental modulation, such as environmental enrichment (EE) or regular exercise, correct various behavioral deficits (Bailoo et al., 2018; Bayne, 2018; Toth, Kregel, Leon, & Musch, 2011), including obsessive‐compulsive disorders (OCD)‐related aggression (Haemisch & Gärtner, 1997; Haemisch, Voss, & Gärtner, 1994) and impulsivity (Binder, Droste, Ohl, & Reul, 2004). Various EE models are currently available in zebrafish (Giacomini et al., 2016; Marcon et al., 2018; Volgin et al., 2018), showing high sensitivity to EE modulation of aggression (Woodward, Winder, & Watt, 2019), sociality (Schroeder, Jones, Young, & Sneddon, 2014) and anxiety‐like behavior (Marcon et al., 2018). Collectively, this suggests that EE‐based aquatic models may also be relevant to ICDs, fostering the search for their biobehavioral therapies.…”

Enrichment for Laboratory Zebrafish—A Review of the Evidence and the Challenges

“…This viewpoint is understandable, given that the word ‘enrichment’ implies an improvement, and is sometimes argued for on the basis that it might help avoid modifications being made to animal environments without proper validation. However, ‘enrichment’ is still frequently used for modifications that have not been shown to improve animal welfare [ 34 , 35 ], or is applied to modifications which could be considered to provide basic necessities for animals rather than an additional welfare benefit (see discussion of social enrichment for zebrafish, below). This review uses the broad definition given above to select papers in order to ensure good coverage of the relevant literature.…”

“…A concern that is often raised when considering adding physical structures to zebrafish tanks is that structures may lead to an increase in aggressive behaviour, perhaps because zebrafish may try to monopolise high-value resources. Evidence here is mixed: some have found increased aggression in zebrafish housed with structures [ 35 , 105 ] compared to those in barren tanks, and another found that the initial higher levels of aggression seen in newly set up tanks took longer to settle when structures were present [ 34 ]. However, Hamilton and Dill [ 90 ] found that enrichment did not result in higher levels of aggression, and others have found lower levels of aggression, injury, and mortality in the presence of structures [ 94 , 106 , 107 ].…”

Welfare of Fish—No Longer the Elephant in the Room

“…The number of cultured individuals arguably rivals any other commercially farmed species, yet even when the subject of welfare in aquaculture started to be addressed, zebrafish were apparently left behind. Say no more because two studies by Woodward et al [8] and Deakin et al [9] focus on two important topics that impact zebrafish welfare: the first shows that environmental enrichment in zebrafish housing promotes aggression and risk-taking behaviours in zebrafish [8], and the authors explain this with the…”

“…The number of cultured individuals arguably rivals any other commercially farmed species, yet even when the subject of welfare in aquaculture started to be addressed, zebrafish were apparently left behind. Say no more because two studies by Woodward et al [8] and Deakin et al [9] focus on two important topics that impact zebrafish welfare: the first shows that environmental enrichment in zebrafish housing promotes aggression and risk-taking behaviours in zebrafish [8], and the authors explain this with the social and territorial behaviour of the species, in which the enrichment structures provide resources to monopolise; the second suggests a novel method to analyse pain responses to standard experimental procedures in this species. Using fractal analysis of behaviour, the authors create (and validate) a pain intensity scale for zebrafish, and propose that variations in complexity of movement should be a good indicator of welfare in this species [9].…”