Environmental enrichment influence on physiology and behaviour
Structural environmental enrichment has been used in captive fish as an attenuator of maladaptive or aberrant traits (Näslund & Johnsson 2016; Roberts et al. 2014; Roberts et al. 2011). Shelter-like structures (e. g. perforated logs, pipes) have generally beneficial effects, such as decreased metabolic rates (Fischer 2000; Millidine et al. 2006) and reduced plasma cortisol levels (Näslund et al. 2013), particularly in aggressive species (Näslund et al. 2013). Our results indicate that parental fish reared on enriched environments have lower basal metabolic rates and waterborne cortisol levels. While metabolic rate did not appear related to the rearing environment of the parents, the tight correlation between waterborne cortisol levels and basal metabolic rates in parental fish, suggests that parental individuals reared in enriched environments were less stressed and spent less energy to maintain basal metabolic rate than individuals reared in barren environments.
In fish, structural environmental enrichment tends to decrease activity, mainly due to increased sheltering (Moberg et al. 2011; Roberts et al. 2011; von Krogh et al. 2010), and exploratory activity and boldness tend to be positively correlated (Champneys et al. 2018; Mazué et al. 2015). Here, parents reared on enriched environments were slightly more active, but also more neophobic, than individuals reared in poor environments, suggesting no clear boldness-exploratory relationship in response to environmental enrichment in K. mamoratus . In this sense, plastic behavioural responses during the ontogeny of this species have been previously suggested, based on its variable responses to conspecific presence and simulated predation risk (2013).
Although behavioural effects of environmental enrichment on fish are well known (Jonsson & Jonsson 2014; Näslund & Johnsson 2016), the understanding of their potential inter- or transgenerational effects is limited, as most studies have focused on one generation (Näslund & Johnsson 2016). Our results indicate that offspring activity and neophobia were influenced by the parental rearing environment. In general, offspring from parents reared in enriched environments had higher activity levels, regardless of their own environment, suggesting a sustained parental effect on activity levels. Previous studies in this species suggested that life-history traits (offspring size), but not behaviour (exploration, boldness and aggression), were affected by the parental environment (2013). However, in mammals there is ample evidence of parental effects caused by environmental enrichment, where the offspring from enriched environments tend to be more exploratory (Dell & Rose 1987; Mychasiuk et al. 2012), have increased learning capacity and memory formation (Bygren 2013), than those reared in non-enriched environments. While in fish increased cognitive capacity due to environmental enrichment was known (Roberts et al. 2011; Salvanes et al. 2013), this is the first evidence of behavioural intergenerational (parental) effects.