Effects of nocturnal illumination on fitness-related traits of the New Zealand common triplefin (Forsterygion lapillum)

Identifying determinants of variation in fitness for organisms with complex life histories has been a longstanding challenge for ecologists. Night-time conditions encompass half the lives of many organisms (Gaston et al., 2022), yet the impacts of varied nocturnal illumination on fitness-related traits across ontogeny are rarely considered. Many organisms exhibit differing patterns of development, growth, and reproduction in conjunction with natural seasonal variations in the timing, strength, and duration of light:dark periods. Interruptions to these cycles can be particularly disruptive to organisms that rely on environmental light to entrain and synchronise development or reproduction. Elucidating the unique impacts of nocturnal illumination during different stages of life history can be particularly difficult. In this thesis, I assessed the effects of nocturnal illumination on fitness-related traits of Forsterygion lapillum (the New Zealand common triplefin) in early (embryonic) and adult life history phases.

In Chapter 2, I conducted a laboratory experiment to appraise the impacts of the strength and timing of nocturnal light (including lunar patterns), on fitness-related traits for adults. I addressed three questions: 1) Does nocturnal illumination alter adult body condition? 2) Does growth vary in different conditions of nocturnal light? 3) How does nocturnal illumination affect reproductive behaviour? I exposed adult triplefin to four different nocturnal light treatments (regular lunar cycle, dimmed lunar cycle, 24-hr artificial light, and dark at night) over the course of three months. I then evaluated the impact of nocturnal light on the relative change in body condition for each individual during the experiment. Additionally, I extracted triplefin otoliths to reconstruct a portion of life history during the experiment and conducted growth analyses assessing if somatic growth varied between treatments. Lastly, I progressively photographed egg clutches and quantified the influence of nocturnal illumination on the timing and frequency of reproduction. Body condition was not influenced by light treatment but differed with sex and pre-experimental body condition. Female fish experienced greater reduction in body condition than males, and body condition degraded to a greater degree over the course of the experiment as pre-experimental body condition increased. While light treatment was not a determinant of body condition, the interaction of light treatment, sex, and standard length caused significant variation in growth increment width. This interaction was particularly pronounced in the 24-hr light treatment, where male growth rates increased as body size increased, but the inverse relationship was seen for females. The interaction of sex and body size varied in the other three treatments. There were no apparent lunar patterns in growth. Reproduction was also impacted by nocturnal illumination. Fish in the 24-hr light and dark at night treatments were more likely to reproduce than those in the lunar treatments. Furthermore, fish in the 24-hr light treatment tended to lay more eggs than those in the dark at night treatment. The number of eggs laid in lunar treatments also followed semi-lunar patterns. Fish in the regular lunar treatment exhibited greater numbers of eggs laid at the first and third quarter moons, while fish in the dimmed lunar treatment had asymmetrical peaks during just after the full moon and just before the new moon. Water temperature and the number of females in each tank did not influence the likelihood of reproduction nor the number of eggs laid during reproductive events. These results suggest that nocturnal illumination has distinct and significant impacts on fitness-related traits for adult F. lapillum that interact with other life history traits.

In Chapter 3, I estimated the length, structure, and success of embryonic development for eggs laid during the laboratory experiment to address two questions: 1) How does nocturnal illumination influence the duration and structure of embryonic development? 2) Does exposure to nocturnal light impact hatching success of embryos? Using photographs of egg clutches taken progressively throughout the experiment, I tracked the fates of each clutch and estimated their dates of laying, eye development, hatching, and their hatching success. I used these estimates to assess the influence of nocturnal illumination on fitness-related traits for offspring. Light treatment did not impact the length or structure of development, however, eye development and overall development length followed lunar patterns. Clutches laid during the new moon had longer development periods than those laid during the full moon, and eye development was longer when it coincided with the first and third quarter moons. Warmer water temperatures at laying resulted in shorter periods of eye and total development and increased the rate of eye development relative to total development time. Conversely, clutches that had faster relative eye development also took less time to hatch. Hatching success was likewise not impacted by light treatment but followed lunar patterns. Clutches that hatched during the full moon tended to have lower hatching success than those hatched during the new moon. Larger clutches experienced much greater hatching success at higher temperatures, and water temperature did not influence hatching success for small clutches. These results emphasise the complicated interactions of environmental cues on fitness-related traits during early life history phases for common triplefin (F. lapillum) and highlight the need for further research into this subject.