ABSTRACT Though many fish larvae exhibit phototaxis, its adaptive value is underexplored. Larval fish rely on cues to guide them to suitable nursery habitats and phototaxis enables larvae to react to environmental cues. In the Anthropocene, innate phototactic behaviour might also be disrupted via climate change and habitat alteration. It is known that larval coregonines (Salmoniformes) in Lake Constance (which borders Germany, Austria and Switzerland) inhabit the shallow littoral as larvae then migrate into deeper water as juveniles. Hence, we aimed to establish whether the ontogenetic development of phototaxis during the early‐life development of coregonines acts as a mechanism that induces migrations to and from shallow nursery sites in Lake Constance. We conducted Y‐maze choice trials of light and light‐colour preference throughout the stages of larval ontogeny in Coregonus wartmanni , which served as a model pelagic lacustrine fish. We raised fish and conducted experiments under two temperature regimes to examine the influence of temperature on early‐life phototactic behaviour. Initially fish showed strong positive phototaxis and preference for mid‐spectrum wavelength light, supporting the premise that phototaxis guides larvae to well‐lit shallow nurseries enriched in mid‐spectrum wavelength light. Larvae subsequently showed a reduced attraction to full and mid‐spectrum light, constituting a behavioural change that induces larvae to leave their shallow nursery habitats. Higher rearing water temperature accelerated larval growth and disrupted the natural phenology of phototactic behaviour by precociously reducing attraction to mid‐spectrum light. Results suggest that progressive changes in phototactic behaviour during larval ontogeny can act as a mechanism that promotes larval ingress into shallow littoral nurseries and prompts the shift to a deeper, pelagic juvenile habitat. Events such as heatwaves also have the potential to alter early‐life migration phenology, which may be detrimental to population recruitment. Additionally, the strong phototactic response that larvae exhibit could mean that artificial light attracts them into anthropised areas that are unsuitable as nurseries. This study shows the progression of early‐life phototactic behaviour in a pelagic lacustrine fish. These innate responses are likely to influence commonly observed patterns of early‐life distribution and phenology of many fish species. Furthermore, anthropogenic influences, such as heatwaves and artificial light may disrupt natural behavioural patterns, with potential negative effects on recruitment success and population persistence.
Roberts et al. (Wed,) studied this question.