1. There are some seas
where fish swarm in millions...
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2. and plankton blossoms
in vast clouds.
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3. The sheer quantity of life here is
unmatched anywhere in the oceans.
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4. These are the most productive seas
on Earth.
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5. They are the Seasonal Seas.
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6. The Seasonal Seas border
the temperate zones,
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7. where conditions change
through the year.
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8. The seasons also affect
the underwater world.
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9. The power of the sun
is constantly changing.
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10. In the far north,
during the summer,
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11. there are long hours of sunlight,
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12. but in winter, that dwindles,
and there can be weeks of darkness.
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13. The summers are warm and gentle,
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14. the winters
racked by savage storms.
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15. In conditions like this,
life of any sort
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16. has to struggle to survive.
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17. January on Sable Island
off the coast of Nova Scotia.
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18. Gray seals
have managed to get ashore
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19. through the crashing breakers.
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20. Gales here can blow
for days on end.
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21. Sable Island has the world's
largest colony of gray seals.
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22. 100,000 come here
to breed each year,
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23. just when the weather
is at its worst.
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24. To add insult to injury,
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25. the pups, having suckled for
only 18 days, are then abandoned.
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26. Their mothers must return to
the sea to find food for themselves.
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27. As yet unable to dive,
the pups are marooned,
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28. sustained only
by their fatty blubber.
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29. It will be five weeks before
they are strong enough to swim,
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30. but then it will be spring and
the ocean will be teeming with food.
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31. By May, spring has reached
the coasts of Scotland.
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32. Underwater, it arrived
rather earlier.
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33. In March,
the seas had warmed enough
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34. to trigger
a magical transformation.
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35. These are phytoplankton -
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36. tiny floating algae,
each much smaller than a pinhead.
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37. They multiply with amazing speed
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38. to produce more annual growth
than all the plants on land together -
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39. six billion tons of it.
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40. This immense bloom spreads
across the face of the ocean
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41. and, within a couple of months,
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42. turns vast areas of it
a dense green.
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43. Animal life
reacts to the blooming sea.
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44. These look like small anemones,
but they're about to change.
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45. They separate, revealing themselves
to be tiny common jellyfish.
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46. They are less
than three millimeters across.
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47. Within a few months, they will
have assembled into vast swarms.
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48. Minute copepods
are part of their staple diet.
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49. These appear every spring
in vast numbers
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50. and graze
on the phytoplankton bloom.
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51. Their beating legs create currents
that sweep the little algae
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52. into the filters
round their mouths.
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53. On this microscopic scale,
water is so viscous
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54. that the phytoplankton
can't swim against the current.
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55. Whilst feeding,
sensors on the copepod's antennae
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56. give warning of dangers ahead.
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57. Lighting by lasers
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58. reveals that feeding copepods
leave wakes behind them,
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59. like jet trails in the sky.
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60. Slightly larger floating predators
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61. are able to use these trails
to find their prey.
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62. A close call, but some predators
are simply too large to avoid.
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63. Jellyfish.
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64. They may appear to be delicate
as well as beautiful,
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65. but they are deadly hunters.
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66. With every pulse
of the delicate bells,
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67. plankton-rich water is drawn
into their lacy throats
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68. and pushed out again, leaving
copepods stuck to the membranes.
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69. A single sea-nettle jellyfish,
only a few centimeters across,
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70. can collect thousands
of copepods in a day.
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71. So, the killing power of giants
like these is hard to estimate.
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72. Each of these jellyfish
weighs up to 30 kilos
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73. and has tentacles
stretching over eight meters.
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74. But there are even greater dangers
awaiting the copepods.
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75. By late spring, the baby
common jellyfish are fully grown.
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76. They gather in millions,
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77. forming swarms
hundreds of meters long,
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78. which filter out all the small
planktonic animals in their path.
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79. Nevertheless, there are such
astronomic numbers of copepods
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80. that enough will survive
to form swarms of their own.
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81. A basking shark.
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82. This huge eight-ton fish
feeds on plankton and nothing else.
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83. Every hour it filters 1,000 tons
of seawater through its gills.
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84. Numbers of these shark appear
when the bloom is at its peak
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85. and travel along currents
where the plankton is thickest.
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86. Even so, the supply of plankton
is so abundant
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87. that another group of animals
can feast-
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88. Every centimeter of rock is covered
with invertebrates of some kind -
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89. anemones, sea squirts, gorgonians -
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90. all trapping plankton
from the passing water.
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91. Not all life in the Seasonal Seas
depends upon plankton.
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92. It is early spring
in British Columbia.
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93. Seaweed has started to grow slowly
in the cold water.
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94. As the hours of sunlight increase
and the water warms,
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95. these small plants turn into
great beds of bull kelp.
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96. The immense 30-meter long strands
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97. have small gas-filled floats
which keep them high in the water,
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98. within reach
of the energy-giving sunlight.
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99. Further south,
the sunshine is more powerful,
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100. so here,
on the coast of California,
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101. the biggest kelp of all can grow.
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102. This is giant kelp,
and by midsummer,
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103. each plant is growing in length
by nearly a meter a day.
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104. The main stem is fixed
to the bottom 35 meters below
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105. by a small
but immensely strong holdfast.
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106. Fully grown,
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107. these gigantic plants
are 100 meters long
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108. and they form vast marine forests.
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109. Great schools of fish
find sanctuary here.
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110. These are blacksmith fish.
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111. They are plankton feeders
and, whenever it seems safe,
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112. they head into open water to feed
where the plankton is thickest.
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113. But at the first hint of danger...
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114. they rush back to the safety
of the forest.
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115. Fish are not the only creatures
that shelter here.
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116. Californian sea otters
gather in the kelp forest
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117. to rest and snooze in safety.
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118. To prevent themselves
being carried away
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119. into dangerous open water
where the big predators cruise,
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120. they anchor themselves by winding
straps of kelp around their body.
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121. Sooner or later,
they have to find food,
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122. and that lies on the seabed
a long way below them.
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123. Sea otters, however, can stay
underwater for up to ten minutes,
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124. ample time to find
the shellfish they need.
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125. Many of the smaller creatures
that live in these forests,
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126. such as urchins, graze on the kelp
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127. and can seriously damage it
if their numbers are unchecked.
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128. The sea otters
feed on some of these grazers
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129. and prevent them
getting too numerous.
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130. So, in effect, the sea otters
are the guardians of the forest.
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131. Garibaldi fish do not,
in fact, damage the kelp.
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132. If anything, they help it
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133. by picking off tiny animals
that encrust the leaves.
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134. They graze on bryozoans,
tiny colonial animals
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135. which build their colonies
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136. like a patchwork of white skins
on the leaves.
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137. When night falls, there are
fewer predatory fish around,
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138. and the bryozoans emerge
from their white shelters.
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139. Now, just like coral polyps,
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140. they start filtering out the
plankton under cover of darkness.
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141. They're not the only animals
to venture out at this time.
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142. This is an amphipod,
just two centimeters long,
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143. and it does eat kelp.
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144. In turn,
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145. it is excellent food
for many predators.
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146. To protect itself, it produces
silk, just like a spider,
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147. and uses it to sew together
two sides of a kelp frond
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148. and so form a shelter.
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149. This one is in particular need
of a secure home.
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150. She's a mother.
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151. There are over 50 youngsters
clustered on her abdomen,
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152. so her home is becoming
somewhat cramped.
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153. They will soon
be old enough to leave,
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154. and, whenever she can,
she kicks them out
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155. to get a taste of the outside world.
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156. Dawn.
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157. Beds of eel grass
grow in the shallows
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158. between the kelp forest
and the shore.
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159. A harbor seal
has found sanctuary here,
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160. But not for long.
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161. A male seal gives a wake-up call.
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162. More seals are attracted
from all directions.
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163. It's June, the time
when young male harbor seals
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164. start their strange
mating displays.
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165. One listens attentively to the
grunting noises made by the other.
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166. These calls are almost certainly
a way of establishing
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167. which of the two will be dominant.
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168. But if the animals are closely
matched in size and experience,
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169. grunts won't settle the issue.
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170. The rivals
will have to come to blows.
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171. And seals can be
surprisingly violent.
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172. In a month's time,
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173. the breeding season will start
and then fights will be in earnest,
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174. but now, in midsummer,
these exchanges are harmless.
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175. They're just playing.
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176. Many of the creatures
that shelter in the kelp
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177. have to venture out
in order to feed.
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178. The bat ray for one.
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179. Unlikely though it seems,
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180. the sandy floor of the open sea
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181. is, for the bat ray,
a rich feeding ground.
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182. There is food
hidden within the sand,
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183. and the bat ray has
a special technique for finding it.
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184. It uses jets of water
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185. to blow the sand aside
and expose small invertebrates.
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186. A kelp bass hangs about alongside
waiting for scraps.
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187. Other hunters
are also on the prowl.
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188. The fan-tailed sole.
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189. There are mantid shrimps here,
living in tunnels.
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190. But, once again, hunger compels
them to venture into the open.
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191. That, of course, is a gamble.
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192. They will either eat...
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193. or be eaten.
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194. A sea slug called Janolus.
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195. Its colors suggest
that it is poisonous,
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196. and so it is, to everything
except another kind of sea slug...
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197. the predatory Navanax.
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198. Navanax pulls itself
along the trail of slime
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199. that Janolus leaves behind it.
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200. Once caught,
Janolus rolls into a ball.
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201. All Navanax gets
is a few yellow tentacles.
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202. And Janolus is swept to safety
by the current.
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203. It's now midsummer, and the sun is
shining at full strength.
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204. The increasing warmth is the cue
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205. for an Atlantic lobster
to start on a long journey.
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206. She's spent the winter 250 meters down,
far beyond the reach of the storms.
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207. But it was cold down there and now
she needs to find warmer water,
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208. so she's marching
towards the shallows.
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209. They, however,
are 150 kilometers away.
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210. After a month of walking, she arrives
at her favored sand bank.
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211. But she is not the first here.
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212. Dozens of other lobsters
have already dug homes in the sand
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213. and they don't intend
to surrender them to newcomers.
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214. Size counts for everything
in these battles.
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215. The new arrival
is in need of a pit,
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216. and since she weighs a hefty seven
kilos, she'll probably get one.
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217. She has won.
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218. These battles continue
for the next two months,
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219. and they are crucial,
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220. for the females must have shelter
and warm water to raise young.
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221. For the last seven months,
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222. each female has been carrying
about 20,000 fertilized eggs,
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223. but their task
is approaching its end.
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224. The warmth of the shallows
is speeding the eggs' development.
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225. Two more months
and the eggs are ready to hatch.
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226. At first, they're not very good
at swimming.
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227. But within a few minutes,
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228. the babies are able to set off
in a purposeful way.
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229. At this time of the year,
the sea is full of larval animals.
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230. This one is a day old lobster.
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231. And this - a three-week-old crab -
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232. just ready to start its life
on the sea floor.
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233. Its feet touch the bottom
for the first time.
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234. At this stage, it's a vegetarian
with a taste for sea lettuce.
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235. As it grows,
it will repeatedly molt
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236. and expand into a bigger,
thicker skin.
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237. The chances are it will be eaten,
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238. but if it survives for five years,
it will become an armored giant.
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239. Now it eats meat,
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240. and special adaptations
enable it to hunt in the dark.
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241. Its jointed feet
are covered in sensors
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242. which detect the slightest chemical
change in its surroundings.
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243. As it walks through the darkness,
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244. its feet can, literally,
taste the sand.
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245. As soon as it finds suitable food,
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246. it passes it forward
to its powerful crushing claws,
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247. which make light work
of the soft flesh.
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248. And those claws are also
very useful for defense.
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249. A one and a half meter long
common octopus glides by.
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250. The jet-propelled giant
is both powerful and very clever.
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251. With octopus about, it's risky
for even a crab to be in the open.
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252. Even in the dark,
the octopus's eyes are sensitive
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253. to the slightest movement.
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254. Against a hunter like this,
the crab's claws are useless.
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255. Late summer in South East Alaska.
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256. The water is still warming,
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257. and mysid shrimp
swarm near the surface.
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258. It's a final feast
for Pacific salmon
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259. returning to the coast
from the open Pacific.
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260. They are heading inshore to breed
and they arrive in huge numbers.
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261. They have to swim
far up the rivers to spawn,
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262. but this river's level
is still too low.
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263. They'll have to wait
till rain causes it to rise.
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264. For now, they are trapped
in the sea, close to the shore,
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265. the worst place to be.
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266. A three meter long salmon shark,
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267. a close relation
of the great white.
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268. This one has sensed
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269. minute electrical signals
from the salmon nearby.
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270. Shark can maintain
their blood temperature
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271. at a higher level
than the surrounding water.
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272. And that means they have the energy
to be quick.
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273. Quicker than salmon.
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274. Early autumn in Vancouver Island
in Canada, 600 miles to the south.
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275. The ocean temperature
is slowly dropping.
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276. 40 meters below the surface,
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277. this year's baby herring feed
on the last of the summer plankton.
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278. Their movements attract attention
from the skies above.
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279. Gulls can't dive,
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280. so for now,
the fish are still safe.
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281. But there are birds which CAN dive.
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282. Auklets and murres swim with ease
down beneath the school.
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283. The panicked herring are forced
towards the surface.
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284. They gather into a giant
defensive ball of swirling fish.
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285. The commotion attracts
yellowtail rockfish.
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286. They, too, are hunters.
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287. The marauding fish
scatter the herring.
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288. Repeated attacks split the ball
into numerous smaller groups.
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289. Now, it's easier for the divers
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290. to keep the confused fish
penned at the surface.
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291. Even the gulls can get at them.
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292. Attacked from all sides, the little
fish have virtually no chance.
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293. More and more divers
are attracted to the scene.
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294. They harry the shrinking numbers
of herring
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295. right down
to the very last individual.
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296. Far bigger predators
cruise here, too.
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297. Pacific white-sided dolphin.
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298. But the dolphin
are mainly nocturnal hunters.
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299. During the day,
they concentrate on socializing.
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300. They display by releasing streams
of bubbles
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301. and they play games.
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302. Games like pass the seaweed,
for example.
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303. Exactly eight months ago,
in the winter off Western Scotland,
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304. an egg was laid and securely fixed
to a strand of kelp.
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305. Inside, a tiny embryo
started to develop.
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306. Protected by the tough egg case,
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307. it endured
the worst of the winter storms.
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308. By summer, it was half-grown.
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309. Now, at last, in late autumn,
it's nearly ready to hatch.
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310. A fully-formed miniature shark
swims free.
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311. This year's plankton will soon die,
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312. but the dogfish is so well developed
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313. that it can hunt immediately
for larger prey.
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314. The days are getting shorter now.
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315. In British Columbia,
the water begins to chill.
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316. This bizarre-looking creature
is searching the kelp for food.
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317. This is Melibe -
the hooded sea slug.
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318. It catches plankton
with its net-like head.
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319. As winter approaches,
plankton is becoming scarce,
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320. but Melibe
is an assiduous searcher.
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321. It can swim.
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322. It flaps away to look
for a better feeding spot,
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323. but its search is becoming
increasingly difficult.
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324. Within the next two weeks, most of
the plankton will have disappeared.
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325. As the sunlight
becomes ever more feeble,
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326. the kelp starts to die,
gradually rotting away to nothing.
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327. Soon, it will be winter.
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328. But 9,000 miles to the south,
the sun is rising on a new spring day.
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329. The southern hemisphere, too,
has temperate regions.
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330. And now the plankton is beginning
to bloom around Tasmania.
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331. Just as in the north,
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332. the Southern Seasonal Seas
have areas of rich green water...
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333. with their own kelp forests...
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334. and their own swarms of plankton.
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335. Some of the inhabitants
of these seas
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336. live only
in the southern hemisphere.
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337. This is one of them -
the handfish -
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338. that strolls around
on modified fins.
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339. But, when needs must,
it can resort to tail power.
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340. Every summer, visitors come
to the shallows around Tasmania.
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341. These are Australian squid,
about half a meter long.
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342. They are here to breed.
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343. The larger males compete
for the attentions of a female,
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344. displaying towards her
and to one another,
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345. by putting on a ballet,
continually changing costume color.
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346. Eventually,
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347. they form pairs.
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348. A male passes a packet of sperm
across to the female.
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349. After the eggs
have been fertilized,
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350. they are deposited
in tough, rubbery egg cases
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351. that other creatures find poisonous.
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352. Within three weeks,
the babies are ready to hatch out.
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353. They are already
able to change color,
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354. but aren't so good at swimming.
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355. There is another animal here
that is a rather more devoted parent.
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356. This is a male leafy sea dragon,
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357. an exquisitely-decorated relative
of the sea horse.
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358. He is carrying his partner's eggs
around with him.
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359. They would be a nutritious snack
for any predator that found them.
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360. But that's not easy
because the eggs are attached
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361. to their father's
perfectly-camouflaged body.
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362. They could scarcely be
in a safer place.
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363. By November,
10,000 miles to the north,
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364. winter has arrived.
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365. Norway now gets less than
five hours of daylight in every 24.
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366. The temperature is falling rapidly.
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367. But, despite the cold,
the sea is far from deserted.
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368. Every winter, 500 million tons
of adult herring
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369. seek shelter in these deep waters.
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370. They will stay here
for four months,
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371. living entirely
on the fat they accumulated
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372. during the summer feasts
of plankton.
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373. But they are not alone.
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374. Orca.
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375. This pod is part of a population
of some 500 killer whales
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376. that specialize in hunting
North Atlantic herring.
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377. Using their echo location,
they have detected a shoal of herring
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378. 50 meters below them.
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379. With enough air
for a ten-minute dive,
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380. they swim deep below the herring
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381. and drive the fish upwards.
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382. Bubbles stream from the rising fish
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383. as gas in their swim bladders
expands and escapes.
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384. Even an orca finds it difficult
to catch a healthy herring.
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385. But they have a devastating weapon
all their own.
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386. They use their tails to club the fish
with waves of water pressure.
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387. Then it's simply a matter
of collecting the stunned casualties.
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388. The herrings have no chance.
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389. Both orca and gulls
will eat as much as they can
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390. every day for the next four months.
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391. But there are so many fish
wintering here,
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392. over five billion individuals,
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393. that the losses
are almost unnoticeable.
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394. Violent as this winter weather
may be,
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395. it's essential for the renewal
of the riches of the Seasonal Seas.
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396. Out in the open oceans,
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397. the surging waters stir up
nutrients from the depths.
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398. By the end of winter,
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399. the Seasonal Seas will be
full of minerals once more,
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400. ready for the return of the sun
and the next great plankton bloom.
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