A deeper look; Does Heincke´s law apply to Atlantic cod (Gadus morhua) in the Barents Sea and what is the effect of eye size on prey encounter rate?
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- Master theses 
The Atlantic cod (Gadus morhua) is arguably one of the most valuable fish we have, and the Barents Sea stock, Northeast Arctic cod (NEAC), is the largest stock in the world. Despite the long history of fishing and the substantial amount of research conducted on this stock, there are still age-related aspects to the NEAC that we do not fully comprehend and no one has looked at how depth distribution changes with increasing age for this stock, a phenomenon known as ontogenetic deepening or Heincke`s law. The aims of this study are: 1) Investigate whether Heincke´s law applies to the NEAC. 2) Parameterize a model to investigate how visual range and prey encounter rate changes with cod size and depth. 3) Determine whether increased prey encounter rates as a consequence of increased visual range in larger cod can compensate for the increased energy needs associated with larger sizes. 13 years of data (2003-2016) on depth-of-capture from the joint Norwegian-Russian ecosystem surveys (BESS) was analyzed to establish average depth for different NEAC length groups. Measurements of lens size were taken from 21 cod heads acquired from a local fish shop and correlated to body size. The effect lens size would have on visual range, encounter rates and net energy balance was predicted using a visual range model. The NEAC showed a significant deepening between length groups, indicating that Heincke´s law applies to this stock. Increased lens size was predicted to increase visual range ceteris paribus. The positive effect increased lens size had on visual range was not sufficient to compensate for loss of light at the greater depths experienced by the larger individuals however. Additional hypotheses trying to explain Heincke´s law in other areas of the Atlantic or different species have been discussed. This thesis´ inability to explain what drives ontogenetic deepening in the NEAC goes to show the importance of understanding the intricate relationships between physical and biological factors and how they together can shape distributions.