Growth and metabolic scaling of fish: Unravelling how variation in growth affects metabolic scaling

Growth and energy use (metabolic rate) constitute two essential parameters for both individuals and populations as a whole. These are, however, not fixed and will vary depending on both environmental conditions, but also notably on the size of the organism. The relationship for both metabolic rate and growth rate with body size is, however, not linear. Instead, they both follow an allometric relationship where growth rate and metabolic rate increase disproportionately less compared to mass, as mass increases. This is known as scaling since a power function can describe the relationship, 𝑀𝑅 = 𝑎 ∙ 𝐵𝑀𝑏, where MR is metabolic rate, BM is the body mass, a is the scaling coefficient, and b is the scaling exponent. Here, the scaling exponent (b) determines how much metabolic rate or growth rate will increase for a given increase in mass. This thesis will explore how both metabolism and growth increase within fish as they grow in size during ontogeny, and importantly, look at how their scaling exponent correlates. This is to get a better understanding of why growth seems to be a driver of variation in metabolic scaling and explore the possibility of a trade-off between them.

We first examined how food availability in a laboratory environment affected both how fast individuals grew and if it affected their scaling exponent of metabolic rate. This was done both to increase our general knowledge in the phenomenon, but also to see if the correlation is affected by food availability and perhaps only occurs in a more food-restricted natural environment. Combining a previous study on damselfish with an experiment I did on zebrafish, we found that across treatments, there was a positive correlation between the scaling exponent of standard (maintenance) metabolic rate (SMR) and the scaling exponent of growth rate. Thus, fish that had a higher increase in growth rate with increasing body mass also had a higher increase in SMR. The same was not found for maximum metabolic rate (MMR), which means that both aerobic scope (AS) and factorial aerobic scope (FAS) increased at a lower rate as the increase in growth rate is higher. Both AS and FAS are estimates of how much energy an organism has to perform work beyond just maintenance.

We then tested if changes in FAS determine when fish start investing into reproduction to see if this happens at a threshold value in guppies. We found that while fish with a steeper scaling exponent of FAS start sexually maturing at a bigger mass, there is no indication of sexual maturation starting at a threshold value of FAS. Unlike for damselfish and zebrafish in the previous study, we found no correlation between the scaling exponent of SMR and growth rate, but we found that both MMR and FAS had a steeper scaling exponent for fish growing faster.

Combining the data for the zebrafish, damselfish, guppies and several previous studies, we examined across-species patterns for the correlation between the scaling of metabolism and growth rate. Across individuals for the seven species we examined, we found an overall pattern of the scaling of SMR being steeper in individuals with steeper scaling of growth rate. For AS and FAS, the opposite correlation was found, with individuals having a steeper scaling exponent of growth having a lower scaling exponent of FAS and AS. Additionally, it was found that there was substantially more variation within the scaling exponent of growth rate compared to within the scaling of metabolic traits.

Lastly, we examined how an anthropogenic stress in the form of artificial light at night (ALAN) influenced the scaling of growth and metabolism in a natural environment. To do this, I did repeated measurements on clownfish and dascyllus that were very site-attached to sea anemones. This was done on Mo'orea, French Polynesia, using bungalows as sources of light pollution. We found that overall relative growth (SGR) was higher for fish exposed to ALAN, and that there was a positive correlation between SGR and the scaling exponent of SMR. However, we also found the opposite response, with the scaling exponent of growth rate being negatively correlated with the scaling exponent of SMR. Interestingly, we did find that when accounting for the effect of the scaling exponent of growth rate, fish under ALAN had a steeper scaling exponent of SMR, something not visible in the scaling exponent of SMR, which was compared across control and ALAN.

Combining the results, we find evidence for a potential trade-off where fish growing faster have a higher maintenance metabolism and lower aerobic capacity to perform tasks such as feeding, growth, and reproduction. This indicates that both metabolism and growth rate must be considered when assessing the impact of a stressor on a population.