Depth Perception in our Oceans

Reaching the bottom of the ocean has fascinated man and challenged engineers for millennia. The bottom, at approximately 35,800 feet below the surface of the Pacific Ocean was identified and named Challenger Deep for the British research vessel Challenger II in 1949. The recent victory by James Cameron noted film director, screenwriter and deep-sea explorer, in arriving at Challenger Deep is a noteworthy example of man’s ingenuity and persistence. It was not the first time – the first time was 52 years ago and it was accomplished by David Walsh and Jacques Piccard – but this time as one would hope, the equipment was upgraded. Apparatus including grab claws, “slurp guns”, sediment samplers, water quality gauges and who knows how many cameras were included in the mission. The crowning achievement was the design of a vertical vehicle that could withstand the tremendous pressure and yet not stir up the bottom sediments as it descended. However consider this, because of characteristics of water that change with depth, the “bottom” can be different depending on perspective, and the problems and results for dealing with them can be equally diverse.

Not surprisingly, the ocean does not have uniform characteristics. Light and temperature as well as pressure change dramatically with descent. For instance it is generally agreed that fish cannot detect surface light below about 3200 ft., although the visual acuity of other animals may reach even deeper. With the loss of light, the character of the mesopelagic or middle-ocean community changes and it is even more dramatic than the diurnal/nocturnal changes on land. The dark ocean offers no shelter so you might agree that the saying “eat or be eaten” is a biological prescription for biochemically produced light (bioluminescence) to either lure in food, hide behind its camouflaging effects or perpetuate species by allowing for the attraction of mates. Many engineering solutions to study this phenomenon, and the revelation of the fact that bioluminescence is a normal modus operandi at these depths can be credited in part to Dr. Edie Widder, resident and esteemed researcher of south Florida. Interestingly enough the most abundant fish in our oceans are the bioluminescent bristlemouths living and dying by this law of light at the bottom of light.

Fish respond to temperature as well as light. A thermocline is a layer in the ocean marked by a significant temperature change at a distinct depth and down through to the ocean bottom there can be more than one. An easily understood example is the thermocline that forms in calm summer weather when surface water increasingly warmed by the summer sun becomes significantly less dense than the cold water beneath it and so it floats at the surface. This warming, in the absence of high winds, causes an abrupt thermocline both intensifying and extending deeper as the warm season progresses. Thermoclines are characterized by not only temperature change but by the concurrent density change and both can act as a barrier to forage fish, the reason why game fish will descend to the interface. Nearly forty years ago Charlie White and his manufacturing friend Blayney Scott from the Pacific Northwest revolutionized fishing by engineering the downrigger that made it possible to get trolled bait down to the depths where fish are feeding in response to these thermoclines. Fish finders sensitive to density identify these layers so both solutions have made it possible to expand recreational fishing to these depths.

The month of June is Oceans Month. Take the time to celebrate the depths of the oceans and what they mean to so many people. The perception of ocean depth is relative and whether you are a filmmaker, a scientist, an angler or even a beachgoer that never goes deeper than your waist, consider the challenges that have been met from so many perspectives.

Ellie Van Os is Director of Education and Exhibits for the Florida Oceanographic Society. She can be contacted at and by phone at (772) 225-0505 ext. 113.