Pebble to the Metal: A Boulder Approach to Enrichment for Danio rerio

Abstract

Zebrafish are an established and widely used animal model, yet there is limited understanding of their welfare needs. Despite an increasing number of studies on zebrafish enrichment, in-tank environmental enrichment remains unpopular among researchers. This is due to perceived concerns over health/hygiene when it comes to introducing enrichment into the tank, although actual evidence for this is sparse. To accommodate this belief, regardless of veracity, we tested the potential benefits of enrichments presented outside the tank. Thus, we investigated the preferences and physiological stress of zebrafish with pictures of pebbles placed underneath the tank. We hypothesized that zebrafish would show a preference for enriched environments and have lower stress levels than barren housed zebrafish. In our first experiment, we housed zebrafish in a standard rack system and recorded their preference for visual access to a pebble picture, with two positive controls: visual access to conspecifics, and group housing. Using a crossover repeated-measures factorial design, we tested if the preference for visual access to pebbles was as strong as the preference for social contact. Zebrafish showed a strong preference for visual access to pebbles, equivalent to that for conspecifics. Then, in a second experiment, tank water cortisol was measured to assess chronic stress levels of zebrafish housed with or without a pebble picture under their tank, with group housing as a positive control. Cortisol levels were significantly reduced in zebrafish housed with pebble pictures, as were cortisol levels in group housed zebrafish. In fact, single housed zebrafish with pebble pictures showed the same cortisol levels as group housed zebrafish without pebble pictures. Thus, the use of an under-tank pebble picture was as beneficial as being group housed, effectively compensating for the stress of single housing. Pebble picture enrichment had an additive effect with group housing, where group housed zebrafish with pebble pictures had the lowest cortisol levels of any treatment group.

Fig 1. Zebrafish preference for enrichment depended on enrichment position and social housing.

a) Singly housed zebrafish preferred enrichment regardless of position (T_1,43 = 4.51; P<0.0001); group housed zebrafish only preferred enrichment in the back of the tank (F_1,43 = 17.98, P = 0.0001). b) Preference did not differ between visual enrichment type (pebble picture versus visual access to conspecifics) (F_1,43 = 0.7686; P = 0.3855). Data depict singly and group-housed fish and are plotted as LSM ± SE. * indicates LSMs with a significant preference versus the null hypothesis of no preference (dashed line).

Fig 2. Chronic stress, measured by tank water cortisol, is reduced by social housing and pebble picture enrichment.

Both enrichment with pebble pictures (F_1,10 = 7.171; P = 0.0232) and group housing (F_1,10 = 9.8339; P = 0.0106) significantly reduced tank water cortisol. These effects were additive and equivalent, such that singly housed enriched tanks did not differ significantly from the barren, group housed tanks (F_1,10 = 0.1049; P = 0.7527), and group housed zebrafish with pebble pictures had the lowest cortisol levels of all treatment groups. Data are plotted as LSM ± SE. * indicates significant differences between Barren versus Pebble enriched tanks.