The Influence of Nocturnal Illumination on the Development of Early Life History Traits in Inanga, Galaxias maculatus
Amphidromous fish employ a bipartite life cycle with a freshwater adult stage and a marine larval phase. Galaxias maculatus, commonly known as inanga, is a widespread amphidromous fish indigenous to New Zealand. Inanga are a culturally and economically important species in New Zealand, partially due to harvest of the late-larval stage in the whitebait fishery. The species is classified as “At Risk-Declining” under the New Zealand Threat Classification System due to concern over population decline. However, limited knowledge on the larval phase and the processes that occur during the marine life stage have hindered management capability. The pelagic phase is a critical life stage for fish species, because many of the traits developed during this period have significant consequences for future fitness, survival, and recruitment. Larval fish can exhibit high plasticity in early life history traits in response to variation in environmental conditions. Light is an important determinant in larval development because of its influence on foraging success, behaviour, and physiological processes. Recent research suggests that variation in nocturnal light, both naturally over the lunar cycle and through anthropogenic processes, can influence larval development.
Here, I assess the influence of nocturnal illumination on the development of early life history traits for larval inanga. I reared larvae in captivity under four nocturnal light regimes. Two treatments were designed to assess the influence of development under different lunar cycles artificially maintained in a laboratory setting. The other two were designed to assess the consequences of anthropogenic manipulation of nocturnal light environments. The first set of treatment levels simulated natural lunar rhythms in moonlight, where the simulated rhythm aligned hatch-timing with either a full moon or a new moon. The second set of treatment levels simulated environments where anthropogenic processes altered the nocturnal light environment; one with a 24h photoperiod to simulate an extreme scenario of artificial light at night (ALAN treatment), and the other with a 10hL:14hD photoperiod to simulate an environment where moonlight is masked by cloud cover or increased sedimentation (dark night treatment). In addition, I explored patterns in growth in relation to the lunar cycle by analyzing otolith growth data from the laboratory reared larvae supplemented with data from wild larvae, kindly provided by Dr. Mark Kaemingk (University of North Dakota).
In chapter 2, I evaluated variation in survival rates and patterns of mortality over the first 50 days of development in the laboratory reared larvae. I conducted daily survival counts to construct a detailed history of mortality over development. Survival rates did not differ significantly between nocturnal light treatments. Notably, in the ALAN treatment (i.e., 24h photoperiod) survival rates varied between tanks from 75% survival to total mortality. The observed patterns indicate that if no exogenous stressors were encountered, the larvae exhibited high survival rates. However, the total mortality of some tanks indicated that these cohorts were susceptible to negative survival stressors such as infection. If this were to occur in the wild, then it would imply that these populations may be at risk. However, significantly more work would need to be done to demonstrate that link.
In chapter 3, I again used the laboratory reared larvae to evaluate variation in a range of fitness-related traits at 51-52 days of age. This included swimming performance, development of flexion, phenotype, and body allometry. The different nocturnal light environments during rearing did not significantly influence swimming performance, flexion stage or body size. However, otolith development was significantly influenced by exposure to artificial light at night (ALAN treatment). Using mixed effects linear models to evaluate variation in the relationship between phenotypic traits and body size between light treatments, I found that larvae reared in artificial light at night environments developed larger otoliths at a given body size than individuals reared in the simulated new moon hatch and dark night treatments. This observed uncoupling between otolith and somatic growth should be taken into consideration when considering the often-used practice of using otoliths as a proxy for body size. These results are consistent with the theory that otolith development may reflect the underlying physiological and metabolic status of the individual.
In chapter 4, I used otolith-based growth history reconstruction to evaluate daily growth rates across the different light treatments in the laboratory-reared larvae. I found that larvae reared in artificial light at night exhibited accelerated otolith growth rates, which was consistent with the larger otolith size identified in Chapter 3. Larvae with hatch-timing occurring at a simulated full moon exhibited significantly, albeit subtly, higher daily otolith growth rates than larvae with hatch-timing around a simulated new moon. This suggests that hatch-timing in wild inanga can have subtle developmental consequences as exhibited in otolith development. In addition, I statistically evaluated lunar periodicity in otolith growth using growth data from laboratory reared and wild larvae. I found that larval inanga exhibited significant lunar periodicity in otolith growth. I present evidence that larvae reared in captivity under simulated lunar cycles exhibited periodicity in otolith growth that matched the periodicity of the natural lunar cycle, even in the absence of natural lunar cues (i.e., tides or natural moonlight). I also observed significant periodicity in otolith growth with the lunar cycle in wild inanga larvae. These results suggest that nocturnal processes may be much more important for larval development than previously believed.
The combined findings of this thesis further our understanding of the early life development of the taonga species, Galaxias maculatus, and highlight the importance of considering nocturnal processes in fish ecology research and conservation management.