Fertile Waters

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June 27, 2012

Fertile Waters

Outdoors Dave Sartwell The Gloucester Daily Times Wed Jun 27, 2012, 09:44 PM EDT

The waters off Gloucester are some of the most fertile fishing grounds in the world. We are the beneficiaries of a set of geological circumstances that sets us apart from the rest. A look at the charts and an understanding of global currents tells the story of why. First we have to understand how currents in the ocean work. In general, surface currents move warm water from the equator toward the cooler poles. In our case the the most obvious of these is the Gulf Stream. The currents also move cold water from the poles toward the equator. We will see in a minute that this means for us the Labrador Current. Deep water currents are somewhat effected by this as well. However, their movement is mostly driven by density that is changed by temperature and salinity. To add to this is a movement called the Coriolis force which has to do with the rotation of the earth. Oddly enough this force was first written about in 1651 by an Italian named Riccioli detailing the effect of the firing of a cannon ball. If it was fired north it would deflect to the east. But for this article it is only important that you understand that the northward movement of warm water and the rotation of the earth causes a large gyre or huge ocean whirl pool that rotates clockwise throughout the Atlantic. As the Gulf Stream flows northward along the Eastern seaboard, it runs up along the shallow Georges Bank. Here the bank acts like a bumper, protecting the Gulf of Maine. As the Gulf Stream heads out toward Iceland, the much colder Labrador Current flows south along the coast of Nova Scotia over the Scotian Reef and Browns Bank. Lying between these two features and Georges Bank is a huge hole called Georges Basin that drops down some 1200 feet. All along this collision of streams are some very significant thermoclines, where there can be significant difference in temperature in a matter of feet. These huge temperature changes are important. For example, a study by Boudreau in 1992, suggests that lobster postlarvae would be unlikely to cross through that cold water. Leading out of the basin is a deep gully called the Northeast Channel that funnels the Labrador current and a bit of back eddy from the Gulf Stream into the Bay of Fundy. As that water swings up into the bay it creates the famous tidal bore, the highest tidal variation in the world, that comes roaring in over the flats. As the water leaves the bay it slowly makes its way southward along the coast of Maine and New Hampshire, picking up the fresh water runs of some sixty rivers. This fresh water flows out over the salt water, resulting in great spawning and growing medium for all sorts of life forms. It also starts a counterclockwise movement that is called the Gulf of Maine Gyre. Think of it as a huge back eddy to the Gulf Stream. This whole flow comes slamming into a big rock called Gloucester/Rockport, turns east, and flows out the Great South Channel south of Georges Bank. There the waters collide with the northern current and head toward Iceland. It takes about three months for the surface water to make this whole rotation. The first scientist to study this phenomenon in detail was a fellow named Bigelow back in 1927. He discovered, using a large number of surface drifters, how the summer flow was effected by persistent wind, tidal rectification, seasonal heating and cooling, the deep water flows and the intrusion of the fresh water runs of the rivers. Scientists today remark on how incredibly accurate were his early observations. Now we have replaced vessel observation with satellite tracking. Using electronically charged sensors, observers can get results from the whole area every few days. This allows them to follow the current movements even in the roughest weather. Recent observations reveal that the salinity levels on Georges Bank are effected by all of the fresh cooler water that flows into the Gulf of Maine. The northern end of the Bank has less salinity than the southern during the winter. All of this mixing of the Gulf of Maine gyre and the Gulf Stream gyre out over the Grand Banks make it one of the most productive ecosystems in the world. The total annual production on the bank is about two-to-three times higher than the mean value of the annual production over other continental shelves. Fish and other life forms in the ocean, like plankton, follow these currents and are changed by them. Some just "ride the current", following it along in piggyback style, dispersing at will. Others use the currents to propel them northward. In either case, if the temperatures of the different currents waters tend to become the same, if the degree of difference in temperature moves toward a steady state, how will that effect the life forms that move with them? Using this information and consulting your charts should give you a better understanding of where to fish and when. It is a big ocean out there, but fish, like land animals, follow a path. They have habits that are influenced by bottom structure and prevailing currents, tides and wind. Figuring it out can increase your chances of success.