Atlantic salmon are extremely surface oriented. A variety of reasons for this behavior can only be speculated. They have very keen vision capabilities like the brown trout due to their very similar genetic tree lineage (salmo salar/salmo trutta)- it is thought that salar evolved from brown trout roots and found passage to the oceans and evolved in the Atlantic/Baltic rim. The acuteness in vision is evident by the hundreds of different Atlantic salmon patterns that are complex in the variety of their color combinations – sometimes a tiny hint of yellow, red or blue as sparse as it may be can elicit a take when switching flies to stubborn Selective/Reflective fish.
Atlantic salmon youth/parr can spend up to three years in the river systems before smolting to the ocean/sea. Here they feed like stream brown trout and look very similar and have a great focus on aquatic insects. From the time they are born in the river and become small parr, their larger then stream trout pectoral fins are a vital part of their physiology. These powerful propellers will eventually allow them to jump incredibly high waterfalls and obstacles unimaginable by other Pacific salmonids. These fins as parr allow them to take up positions in very fast runs/riffles not many other resident trout could command. The love for faster, shallower oxygenated, boulder/pocket water lies when the fish are parr, not only keeps them out of larger deeper pools, dominated by fish that would eat them, but here is where a majority of the emerging aquatic insect life occurs. Hatching, egg laying and darting nymphs and larva are always present throughout their juvenile river environment. Very important in this salmon natal behavioral imprinting stage is the fact that the aquatic insects are easily propelled upwards to the surface in these shallow environments and can be trapped in the turbulent surface and be easy prey for the juveniles. Also surface vision is very good in shallow environments due to the amount of light infused into these skinny water habitats. Looking up for food and surface curiosity will be a major behavioral protocol for the rest of the Atlantic’s life because of this behavioral repertoire of its youth. Keep in mind also that both brown trout and Atlantics are very photophobic and shun light and prefer low light conditions in the A/A and S/R states. In low light situations the surface environment is easily discernible verses a very bright sunny day where glare and the direct powerful sun’s rays make surface feeding and items of prey/curiosity difficult to detect.
Since an Anadromous fish like the Atlantic salmon’s behavior is a dual interaction between natal river experiences and what occurs in the ocean/sea environments, another surface orientation behavioral takes place in the big water prey hunting grounds. Atlantics are pelagic (meaning upper water) baitfish hunters. They are in a constant search for schools of herring, capelin, smelt and all sardines. These schools are visible in the dark ocean/sea environments as the baitfish schools dark cluster is backlit by the light filtered surface waters. If the salmon cruised the surface to hunt and looked down into the dark abyss these schools of prey would not be detected as well. Thus coming up from the bottom with camouflaging steel gray/blue heads and torsos, which the prey cannot detect, has been an evolutionary successful hunting style. Once they corral the baitfish they smash their bodies upwards into the schools stunning the fish and eating them. So from the early youth through adulthood, the surface orientation is constant for the Atlantics. Also keep in mind that when the parr are in shallow water river environments they are extremely vulnerable to birds of prey like osprey, hawks, kingfishers and the like. Keeping one keen eye upward at all times is paramount to survival.
But thanks to the strong evolutionary significant life survival strategy, constantly referred to by John Malcolm Elliott, in his excellent work, QUANTITATIVE ECOLOGY AND THE BROWN TROUT” ,(Oxford Press), young of the year salmon can survive given this spatial difference from all its predators. In the lifecycle strategy, reproductive traits in populations such as brood size, size of young and distribution interact with reproductive traits like growth and survival. Thus looking up/surface orientation behavior is an innate behavior through the parr habitat preference/necessity which is thus genetically transmitted from generation to generation due to spatial life strategy spacial occupation. Given shallower water lies, predators from above like birds of prey: Osprey, kingfishers, hawks and eagles etc., the young salmon's eyes are tuned in to incoming predators, and also on the hunt for surface prey like hatching mayflies, stoneflies, caddis or smaller baitfish searching for the same."