Table 2.

Key results of studies relating to physical enrichment for laboratory zebrafish. Studies have been included where they have either addressed a preference (such as might be used to identify conditions which might promote welfare), or tested the effects of a condition intended to provide enrichment. Unless otherwise specified, all results here relate to the presence of the enrichment condition noted in the table compared with an absence of that condition.

Study	Enrichment	Indicator(s)	Effect(s)
[82]	Gravel vs. barren	Preference (occupancy in enriched compartment)	Preference for gravel
	Sand vs. barren		Preference for sand
	Gravel vs. sand		Preference for gravel
	Submerged plant vs. barren		Preference for submerged plant
	Floating plant vs. barren		Preference for floating plant
	Floating plant vs. submerged plant		Preference for floating plant
	Gravel & floating plant vs. sand and submerged plant		Preference for gravel & floating plant
	Gravel and submerged plant vs. sand and floating plant		Preference for gravel & submerged plant
	Gravel image vs. barren		Preference for gravel image
	Sand image vs. barren		Preference for sand image
	Air stone vs. barren		Preference for barren
[83]	Real plants (Ceratopteris thalictroides) and clay pots	Preference (occupancy in enriched compartment)	Preference
		Behavioural diversity	No effect
[84]	Sandy substrate and variety of plastic plants	Preference (occupancy in enriched compartment)	Preference; greater preference when combined with water flow
[85]	Black tank walls vs. barren	Preference (occupancy in enriched compartments)	No preference
	Underwater image on walls vs. barren		No preference
	Sloped gravel vs. barren		Preference for gravel
	Flat gravel vs. barren		Preference for gravel
	Gravel vs. plastic plants		Preference for gravel
	Gravel & plastic plants vs. gravel or plastic plants		Preference for gravel & plastic plants over gravel or plastic plants alone
	Number of plastic plants		Preference for greater number of plants
	Visual contact with neighbouring tanks		No preference
[86]	Plastic plants and PVC pipes	Preference (occupancy in enriched compartment)	Preference
[89]	Gravel, real plants (vallis, Vallisneria spp. Including V. spiralis, V. elongata and V. tortifolia, and water trumpet, Cryptocoryne wendtii)	Preference (occupancy in enriched compartment)	No preference
		Anxiety	↓
		Survival at 30 dpf	↑
		Body size at 60 dpf	↓ (but no effect at 120 dpf)
[81]	Shade vs. barren	Preference (occupancy in enriched compartment)	Preference for barren
	Artificial plants vs. barren		No effect
	Shade vs. artificial plants		No effect
[90]	Cover	Preference (occupancy in enriched area)	Preference
	Artificial plants		No preference
[60]	Sand and gravel, caps for refuge and natural plants (two branches of Cabombaceae and Pontederiaceae)	Cortisol (after chasing with a net)	↓
[91]	Gravel, plastic ‘ruin’, three submerged plastic plants (two 10 cm tall and one 20 cm tall)	Anxiety (unstressed fish)	↑
		Anxiety (after exposure to unpredictable chronic stress)	↓
		Cortisol (unstressed fish)	No effect
		Cortisol (after exposure to unpredictable chronic stress)	↓
		Levels of reactive oxygen species (unstressed fish)	No effect
		Levels of reactive oxygen species (after exposure to unpredictable chronic stress)	↓
[92]	Gravel and two 20 cm tall Acorus spp. plastic imitations	Activity	↓
		Cortisol	↑ (but not as high as in fish exposed to a stressor)
		Proliferating cell nuclear antigen-expressing cells in the telencephalon	↑
[94]	Floating plastic plant	Aggression-induced morbidity and mortality	↓
		Cortisol	↑ (after 5 days); ↓ (after 10 days)
[95]	Sand, plants, artificial rock formation	Anxiety	↓
		Exploratory behaviour	↑
		Inhibitory avoidance	↓
		Telencephalic expression of genes related to stress response	↓
[47]	One artificial plant	Anxiety	↓ (when combined with presence of other fish)
[96]	Two plastic plants, one plastic shelter, gravel substrate and a novel object (white PVC pipe, rock, different coloured plants or a plastic bottle—changed weekly).	Anxiety	↓
		Learning	↑
		Brain size	↑
[99]	50 haphazardly placed 50 mm lengths of artificial Elodea canadensis	Body length	↓
		Rate of learning	↑
[100]	Artificial plants	Initial time to solve maze task	↓
		Rate of learning	↑
		Memory retention	↑
[103]	Plastic grass or plastic leaves	Number of eggs	↑ with plastic grass; no effect with plastic leaves
		Number of fry (6 dpf)	↑ with grass when parents were 110 or 160 dpf; ↑ with leaves when parents were 173 or 180 dpf
		Survivability of fry (6 dpf)	No effect
[104]	Gravel, plastic ‘ruin’, three submerged plastic plants (two 10 cm tall and one 20 cm tall)	Levels of reactive oxygen species in response to unpredictable chronic stress	↓
[105]	Four or five submerged plastic plants	Aggression	↑
		Latency to feed	↓ (one wild strain only)
		Shoaling distances	No effect
[35]	One artificial plant, one upturned flower pot and aquarium backing with blue seascape design on rear tank wall	Aggression	↑
		Body length	↓
		Fertilisation success	No effect
		Number of eggs	No effect
[34]	Three groups of 12 opaque black glass rods, 50 mm, 100 mm and 180 mm in height	Time for aggression levels to settle.	↑
		Activity	No effect
		Cortisol	No effect
		Shoaling density	No effect
		Space use	No effect
[106]	12 strips of plastic bag in a 3 × 4 arrangement to simulate vegetation	Aggression	↓
		Food monopolisation	↓
[107]	Three artificial plants (15.24 cm tall, moneywort imitations) and aquarium gravel	Aggression	↓
		Fecundity	No effect
[108]	Refuge created by partial wall	Aggression induced by exposure to lead	↓

↑ = increase; ↓ = decrease.