Date of Award


Publication Type

Doctoral Thesis

Degree Name



Biological Sciences


Expensive tissues; Hemisphere specialization; Laterality; Lateralization; Salmon; Tissue Trade-offs


Higgs, Dennis




Specific hypotheses have been put forth to help explain and guide further studies of patterns of brain or body growth, as well as lateralized outcomes in fishes. In terms of laterality, what I refer to as the genetic variation and laterality hypothesis has been proposed, stating that there is an inverse relationship between the genetic variation of an organism and measured laterality. The expensive tissue hypothesis, on the other hand, has been proposed as an explanation of differences in brain size, stating that for increased brain growth there must be a compensatory trade-off with other ‘expensive’ tissues. In the present dissertation I have used a salmonid species, Chinook salmon (Oncorhynchus tshawytscha), to explore both the differential investment into brain and body growth, and drivers behind morphological and behavioural laterality. In the first data chapter, as an examination of the genetic variation and laterality hypothesis, I investigate how four different ‘inbreeding levels’ affect morphological laterality of the hemispheres of two main brain regions, the optic tectum and cerebellum. As well, I examine how fish in the ‘inbreeding levels’ differ on a brain-to-body ratio measure as a test of how genetic background might affect expensive tissue investment. In the second data chapter, I use juvenile salmon of six different genetic backgrounds, three domestic and three outcrosses, as well as a manipulation of flow direction (clockwise or counter-clockwise) in the rearing barrels of the fish, to investigate the genetic background, environmental, and gene-by-environment (GxE) interaction effects on both behavioural (C-start; mirror inspection) and morphological (brain; whole eye) laterality. The third chapter examines the effect of population differentiation of seven Chinook populations on morphological brain laterality, again, as an examination of the genetic variation and laterality hypothesis. In this chapter I also looked at the differences between populations on the expensive tissue trade-offs of the brain and the body, the brain and the gut and the gut and the body. Finally, in the fourth data chapter I examine the brain-to-body trade-off on six populations of Chinook salmon over three years: 2014, 2015 and 2016. As a whole, the laterality results demonstrate that there is some genetic effect on morphological laterality of the brain hemispheres, but not following the pattern suggested by the genetic variation and laterality hypothesis. From behavioural examinations I note that the manipulation of flow direction and the GxE interaction show the most significant effects on laterality. Results of expensive tissue trade-offs show that there is differential investment into the brain versus the body in Chinook salmon, and this investment also shows differences between populations examined, indicating that there are drivers to expensive tissue trade-offs which require more exploration. Investigating these areas may hold important information for aquaculture facilities, especially with regards to differential tissue investment, where often times a larger body is the end goal. However, investment into the brain may be a reflection of cognitive ability which would be of greater importance for those hatcheries rearing fish for conservation purposes: higher cognitive ability may very likely equate to higher overall survival. In regard to lateralization, the further exploration of laterality, both morphological and behavioural, can help us to better understand how and why laterality developed, its advantages, and how, and perhaps why, it has been maintained throughout the evolution of vertebrates.