Fisheries oceanography

1. DEPARTMENT OF AQUATIC ENVIRONMENT MANAGEMENT
SUBJECT- FISHERIES OCEANOGRAPHY (512)
TOPIC:- OCEAN TIDES
Presented by
Pragati inwati
mfk2112

2. OCEAN TIDE
INTRODUCTION
 Tides are produced by the attractions of the moon and sun on
the waters of the ocean.
 They occur in the ocean as progressive or standing
long waves.
 Among all the oceanic long waves, tides are of maximum
importance for people living near the sea shore and for those
travelling the sea.
 The rise and fall of the tide is a continues phenomenon and
varies from day to day and from place to place.

3. DEFINITION-
Tides are bulges of water produced by the gravitational attraction
of the Sun and Moon on the ocean and by the centrifugal force
resulting from rotation of the Earth –Sun and Earth-Moon
systems.

4.  Gravity is the force of attraction
between two bodies.
 The greater the mass and smaller
the distance, the stronger the
gravitational attraction.
G α M1M2/d2
 Where G is gravity M1 and
M2 represent the mass of body 1
and body 2 and d is distance
between the center of the two
masses.

5. Tide changes proceed via the following stages:
1. Sea level rises over several hours, covering the
intertidal zone, flood tide.
2. The water rises to its highest level, reaching high
tide.
3. Sea level falls over several hours, revealing the
intertidal zone, ebb tide.
4. The water stops falling, reaching low tide.

6. Cause of tides
 The gravitational pull of the Moon.
 Gravitation pull of Sun.
 Centrifugal force, the force that works as a counterbalance of
gravity.
 Gravitation pull and centrifugal force are responsible for the
creation of two major tides on earth.
On the moonward side of the earth, tidal bulge is caused by the
gravitational pull from the Moon, and the opposite side of the earth's
bulge caused by centrifugal force.

7.  Highest tides experienced on Bay of Fundy in Canada; it
is approx 15-16 m high.
The highest tide in India is recorded at Okha, Gujarat.

8. Types of Tides
 Tides vary in their frequency, direction and
movement from place to place and also from time
to time.
 Tides may be grouped into various types based
on their frequency of occurrence in one day or 24
hours or based on their height.
Tides based on Frequency
1. Semi-diurnal tide
2. Diurnal tide
3. Mixed tide

9. 1. Semi-diurnal tide
 A semi-diurnal tidal cycle is the one with two nearly equal
high tides and two low tides each day.
 The successive high or low tides are approximately of the
same height.
 The interval between the high and the low tides is of around
12 hours and 25 minutes.
 Semi-Diurnal Tides are most widespread in the Indian
Ocean.

10. 2. Diurnal tide
 There is only one high tide and one low tide during each
day. The successive high and low tides are approximately of
the same height.

11. 3. Mixed tide
Tides having variations in height are known as mixed
tides.
 These tides generally occur along the west coast of North
America and on many islands of the Pacific Ocean.

12. Tides based on the Sun, Moon and the
Earth Positions
 The height of rising water (high tide) varies
appreciably depending upon the position of sun and
moon with respect to the earth.
1. Spring tide
2. Neap tide

13. 1. Spring tides
The position of both the sun and the moon in relation to
the earth has a direct bearing on tide height.
When the sun, the moon, and the earth are in a straight
line, the height of the tide will be higher.
These are called spring tides and they occur twice a
month, one on the full moon period and another
during the new moon period.

15. 2. Neap tides
 When the Moon is at first quarter or third quarter, the Sun
and Moon are separated by 90° when viewed from the Earth.
 The Moon’s attraction, though more than twice as strong as
the sun’s, is diminished by the counteracting force of the sun’s
gravitational pull.
 Normally, there is a seven-day interval between the spring
tides and neap tides.
 Like spring tides, these tides also occur twice a month.

16. Magnitude of tides based on Perigee and Apogee of
moon
 Once in a month, when the moon’s orbit is closest to the
earth (perigee), unusually high and low tides occur.
During this time the tidal range is greater than normal.
 Two weeks later, when the moon is farthest from
earth (apogee), the moon’s gravitational force is limited.
The tidal ranges are less than their average heights.

17. Magnitude of tides based on Perigee and Apogee of earth
 When the earth is closest to the sun (perihelion), around 3rd
January each year, tidal ranges are also much greater, with
unusually high and unusually low tides.
 When the earth is farthest from the sun (aphelion),
around 4th July each year, tidal ranges are much less than
average.

18. Fig. Magnitude of tides based on Perigee and Apogee of
moon

19. Importance of Tides
1. Navigation
2. Fishing
3. Desilting
4. Tides are used to
generate electrical
power

20. The tidal bulges on wide continental shelves have
greater height.
In the open ocean tidal currents are relatively weak.
When tidal bulges hit the mid-oceanic islands they
become low.
The shape of bays and estuaries along a coastline can
also magnify the intensity of tides.

21.  Funnel-shaped bays greatly change tidal magnitudes.
Example: Bay of Fundy - Highest tidal range.
 The large continents on the planet, however, block the
westward passage of the tidal bulges as the Earth rotates.
 Tidal patterns differ greatly from ocean to ocean and
from location to location.

22. Impact of Tides
1. Tides raise the level of seawater and hence exposes a
large part of the ocean for erosion.
2. It is helpful for the tidal ports that have shallow water
which is a constraint for the big ships to enter.
3. Tidal currents are a very potential source of tidal
energy which is harnessed by many developed
countries on a very large scale and to some extent in
India as well.

23. 5. It can be devastating in cases where the tide gets too huge
and results in the flooding of the nearby coastal regions.
6. Tides are very helpful for ecosystems such as the
mangrove forests and coral reefs to grow and sustain.

24. Tidal Bore
 A tidal bore is a large wave or bore caused by the constriction of the
spring tide as it enters a long, narrow, shallow inlet.
 The in and out the movement of water into a gulf through channels
called a tidal current.

25. Impact of Tidal Bore
 Tides are stable and can be predicted. Tidal bores are less
predictable and hence can be dangerous.
 The tidal bores adversely affect the shipping and
navigation in the estuarine zone.
 Tidal bores of considerable magnitude can capsize boats
and ships of considerable size.

26.  Strong tidal bores disrupt fishing zones in estuaries and
gulfs.
 Tidal bores have an adverse impact on the ecology of
the river mouth.
 The tidal-bore affected estuaries are the rich feeding
zones and breeding grounds of several forms of wildlife.

27. Reference
• oceanservice.noaa.gov