7/26/2013: Upwelling

Last night the ship mapped some areas closer to the edge of the continental shelf. They found a long ridge of rocks that looked promising for holding fish. During the day today we set traps at these locations. The videos taken from both ends of the traps had lots of rocks and areas which looked like they would have a lot of fish living in them. However, putting the CTD over the side, we found that the water temperature went from 26°C at the top of the water column to 16°C at the bottom in 55 meters of water. This steep decrease in water temperature is called a thermocline, and can easily be seen on a graph of water temperature.

Fish who don't want to eat

More fish not eating

This cold water on the bottom of the ocean has two main effects on the fish we are hoping to video tape and catch in the traps. First, fish are exothermic (cold blooded), so their metabolism slows down when the water around them gets cold. This means that they don’t need to eat as much and are not attracted to the bait. They will be more inclined to sit outside the traps, not entering them if the water is cold.   Second, many of the species we are interested in capturing are not particularly cold-tolerant. They prefer to remain at temperatures above 20°C or so. They will often move out of a cold water area in search of warmer waters. When the bottom temperature gets too low, the whole neighborhood just empties out. We set our traps in areas which had both of these situations. Some of the video cameras observed fish that may often go into traps, but not were caught. 

Beautiful neighborhood. No one home.

Other cameras saw beautiful habitat, where we would expect to at least see fish, but none were present at all in the area.

Another abandoned subdivision

We think that the thermocline we are observing on the continental shelf in this area is due to upwelling.

Upwelling is a pretty common phenomenon along coasts where the prevailing winds are from land to ocean. The wind will blow the top layer of water away from the coast. A colder, more dense layer of water moves in to replace the water that was blown away.  The west coast f the U.S. has LOTS of upwelling. We don’t normally think of the U.S. east coast as having much upwelling. However, it has been known to occur in localized areas.