Water current is normally the dominant environmental force on a sea cage farm. Several factors may create a current in the water, including:
· Local water flows, such as rivers
· Large global oceanic currents or coastal streams such as the Gulf Stream.
Currents create both horizontal and vertical movements in the water. In fish farming the focus is normally on the horizontal currents.
There are large variations in the current from site to site, and the overall current picture comprises all the different single currents. A proper description of the current conditions on site must therefore be included in the site measurements.
Current is created in the water when the wind blows over the surface, because there will be a drag from the wind on the water surface. The velocity of the created water current depends on the strength of the wind. Because the drag is on the water surface, wind created current will be highest near the surface and decrease with depth. The following equation may be used to calculate the current created by wind in open water:12
U sv( z)= 0.02U10( 50 – z/ 50)
U10 = wind velocity measured 10 m above the water surface
z =distance downwards from the surface to the depth at which the velocity of the wind generated current is to be found.
Find the current generated by wind at depths of 5 mand 10 m. There is open water and the wind is neargale force.
First the near gale force wind velocity is found from the Beaufort wind scale to be between 13.9 andx17.1 m/s. Therefore use a velocity of 15 m/s.The current velocity at 5 m depth is then
At 10 m depth the wind creates a current of:
= 0.24 m/s
These calculations also show that the velocity is reduced by the depth.
As can be seen from the first equation, in this subsection, the surface current velocity is 2% of the wind velocity (U10) calculated for open sea. In shallow water the wind generated current velocity will normally be somewhat higher, partly due to stratification and reduced thickness of the water layers being dragged by the wind, and under practical conditions may be up to 5% of the wind velocity.
Tidal current is created by tidal range. The tidal range varies from site to site around the world. In Canada, a tidal range of up to 14 m occurs. In narrow fjords and narrow necks the tidal current is especially high, because the tide forces the water in and out. In North Norway a tidal current of up to 16 kn (8.23 m/s) has been measured in Saltstraumen in the inlet to Skjerstadfjorden, which is the world’s strongest tidal race.
In open waters the current caused by the tide is not as high, and depends on the size of the tidal range in the area. If Norway is taken as an example, the tidal current varies from 0.2 to 0.8 m/s along the coast.14 Tidal current is approximately equal through the total water column and does not vary with depth. The strongest tidal current occurs in the middle between high and low tide.
Solar heating, variation in water density, wind, gravity and the Coriolis force have influences over the large ocean currents. The Gulf Stream is one of the strongest currents known. It starts in the Gulf of Mexico, passes along the east coast of the USA and crosses the Atlantic Ocean past Ireland and Great Britain and continues up past the west coast of Norway. Where its velocity is 0.4–0.5 m/s. Coastal currents present the largest velocity close to land and decrease with depth.
When evaluating a site, the water current must be measured. Specially designed instruments are used to measure the direction and the velocity of the current. They are an integral part of a floating buoy, and also include a recorder to store the results. Some also have a transmitter for downloading the monitored results. It is recommended that the measuring buoy stays out for quite a long period, preferably for a whole year or at least in the periods when the strongest and weakest currents occur. The results of the measurements can be shown in a current rose in the same way that the wind data can be presented.
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