1
00:00:34,000 --> 00:00:38,597
There are some seas
where fish swarm in millions...

2
00:00:40,280 --> 00:00:45,071
..and plankton blossoms
in vast clouds.

3
00:00:46,800 --> 00:00:52,194
The sheer quantity of life here is
unmatched anywhere in the oceans.

4
00:00:59,360 --> 00:01:03,036
These are the most productive seas
on Earth.

5
00:01:07,000 --> 00:01:11,074
They are the Seasonal Seas.

6
00:01:14,240 --> 00:01:17,517
The Seasonal Seas border
the temperate zones,

7
00:01:17,680 --> 00:01:20,878
where conditions change
through the year.

8
00:01:28,680 --> 00:01:32,640
The seasons also affect
the underwater world.

9
00:01:33,640 --> 00:01:38,476
The power of the sun
is constantly changing.

10
00:01:41,600 --> 00:01:44,195
In the far north,
during the summer,

11
00:01:44,360 --> 00:01:46,795
there are long hours of sunlight,

12
00:01:46,960 --> 00:01:52,240
but in winter, that dwindles,
and there can be weeks of darkness.

13
00:01:53,560 --> 00:01:56,234
The summers are warm and gentle,

14
00:01:56,400 --> 00:02:00,440
the winters
racked by savage storms.

15
00:02:53,920 --> 00:02:57,277
In conditions like this,
life of any sort

16
00:02:57,440 --> 00:03:00,433
has to struggle to survive.

17
00:03:05,080 --> 00:03:09,472
January on Sable Island
off the coast of Nova Scotia.

18
00:03:09,640 --> 00:03:12,314
Grey seals
have managed to get ashore

19
00:03:12,480 --> 00:03:14,915
through the crashing breakers.

20
00:03:22,800 --> 00:03:27,352
Gales here can blow
for days on end.

21
00:03:45,600 --> 00:03:50,311
Sable Island has the world's
largest colony of grey seals.

22
00:03:50,480 --> 00:03:53,552
100,000 come here
to breed each year,

23
00:03:53,720 --> 00:03:56,872
just when the weather
is at its worst.

24
00:04:02,160 --> 00:04:04,880
To add insult to injury,

25
00:04:05,040 --> 00:04:10,160
the pups, having suckled for
only 18 days, are then abandoned.

26
00:04:10,320 --> 00:04:14,872
Their mothers must return to
the sea to find food for themselves.

27
00:04:18,520 --> 00:04:21,957
As yet unable to dive,
the pups are marooned,

28
00:04:22,120 --> 00:04:25,033
sustained only
by their fatty blubber.

29
00:04:27,160 --> 00:04:31,552
It will be five weeks before
they are strong enough to swim,

30
00:04:31,720 --> 00:04:37,910
but then it will be spring and
the ocean will be teeming with food.

31
00:04:51,680 --> 00:04:56,516
By May, spring has reached
the coasts of Scotland.

32
00:04:58,840 --> 00:05:02,151
Underwater, it arrived
rather earlier.

33
00:05:02,320 --> 00:05:04,994
In March,
the seas had warmed enough

34
00:05:05,160 --> 00:05:08,278
to trigger
a magical transformation.

35
00:05:20,400 --> 00:05:22,392
These are phytoplankton —

36
00:05:22,560 --> 00:05:26,839
tiny floating algae,
each much smaller than a pinhead.

37
00:05:27,000 --> 00:05:29,469
They multiply with amazing speed

38
00:05:29,640 --> 00:05:34,795
to produce more annual growth
than all the plants on land together —

39
00:05:34,960 --> 00:05:37,236
six billion tonnes of it.

40
00:05:39,000 --> 00:05:43,870
This immense bloom spreads
across the face of the ocean

41
00:05:44,040 --> 00:05:46,316
and, within a couple of months,

42
00:05:46,480 --> 00:05:50,315
turns vast areas of it
a dense green.

43
00:05:53,400 --> 00:05:56,677
Animal life
reacts to the blooming sea.

44
00:05:56,840 --> 00:06:01,790
These look like small anemones,
but they're about to change.

45
00:06:01,960 --> 00:06:07,672
They separate, revealing themselves
to be tiny common jellyfish.

46
00:06:10,560 --> 00:06:14,759
They are less
than three millimetres across.

47
00:06:17,960 --> 00:06:23,718
Within a few months, they will
have assembled into vast swarms.

48
00:06:27,840 --> 00:06:32,312
Minute copepods
are part of their staple diet.

49
00:06:32,480 --> 00:06:35,279
These appear every spring
in vast numbers

50
00:06:35,440 --> 00:06:39,320
and graze
on the phytoplankton bloom.

51
00:06:41,280 --> 00:06:45,832
Their beating legs create currents
that sweep the little algae

52
00:06:46,000 --> 00:06:48,720
into the filters
round their mouths.

53
00:06:53,080 --> 00:06:57,074
On this microscopic scale,
water is so viscous

54
00:06:57,240 --> 00:07:01,439
that the phytoplankton
can't swim against the current.

55
00:07:02,600 --> 00:07:06,514
Whilst feeding,
sensors on the copepod's antennae

56
00:07:06,680 --> 00:07:09,070
give warning of dangers ahead.

57
00:07:13,320 --> 00:07:15,118
Lighting by lasers

58
00:07:15,280 --> 00:07:19,274
reveals that feeding copepods
leave wakes behind them,

59
00:07:19,440 --> 00:07:21,432
like jet trails in the sky.

60
00:07:39,920 --> 00:07:42,276
Slightly larger floating predators

61
00:07:42,440 --> 00:07:47,071
are able to use these trails
to find their prey.

62
00:07:59,880 --> 00:08:05,672
A close call, but some predators
are simply too large to avoid.

63
00:08:13,600 --> 00:08:15,000
Jellyfish.

64
00:08:18,200 --> 00:08:22,877
They may appear to be delicate
as well as beautiful,

65
00:08:23,040 --> 00:08:25,396
but they are deadly hunters.

66
00:09:02,040 --> 00:09:04,760
With every pulse
of the delicate bells,

67
00:09:04,920 --> 00:09:08,880
plankton—rich water is drawn
into their lacy throats

68
00:09:09,040 --> 00:09:14,832
and pushed out again, leaving
copepods stuck to the membranes.

69
00:09:22,640 --> 00:09:27,760
A single sea—nettle jellyfish,
only a few centimetres across,

70
00:09:27,920 --> 00:09:31,709
can collect thousands
of copepods in a day.

71
00:09:42,760 --> 00:09:47,880
So, the killing power of giants
like these is hard to estimate.

72
00:09:52,680 --> 00:09:56,230
Each of these jellyfish
weighs up to 30 kilos

73
00:09:56,400 --> 00:10:02,112
and has tentacles
stretching over eight metres.

74
00:10:03,120 --> 00:10:07,956
But there are even greater dangers
awaiting the copepods.

75
00:10:08,960 --> 00:10:13,637
By late spring, the baby
common jellyfish are fully grown.

76
00:10:15,920 --> 00:10:17,991
They gather in millions,

77
00:10:18,160 --> 00:10:21,790
forming swarms
hundreds of metres long,

78
00:10:22,000 --> 00:10:26,119
which filter out all the small
planktonic animals in their path.

79
00:11:13,360 --> 00:11:17,991
Nevertheless, there are such
astronomic numbers of copepods

80
00:11:18,160 --> 00:11:22,712
that enough will survive
to form swarms of their own.

81
00:11:35,640 --> 00:11:38,314
A basking shark.

82
00:11:40,560 --> 00:11:45,840
This huge eight—tonne fish
feeds on plankton and nothing else.

83
00:11:46,000 --> 00:11:51,394
Every hour it filters 1,000 tonnes
of seawater through its gills.

84
00:11:55,840 --> 00:12:00,596
Numbers of these shark appear
when the bloom is at its peak

85
00:12:00,760 --> 00:12:05,198
and travel along currents
where the plankton is thickest.

86
00:12:07,480 --> 00:12:11,360
Even so, the supply of plankton
is so abundant

87
00:12:11,520 --> 00:12:14,638
that another group of animals
can feast —

88
00:12:14,800 --> 00:12:17,599
animals that live on the sea floor.

89
00:12:21,680 --> 00:12:26,709
Every centimetre of rock is covered
with invertebrates of some kind —

90
00:12:26,880 --> 00:12:29,952
anemones, sea squirts, gorgonians —

91
00:12:30,120 --> 00:12:34,080
all trapping plankton
from the passing water.

92
00:13:14,960 --> 00:13:19,398
Not all life in the Seasonal Seas
depends upon plankton.

93
00:13:19,560 --> 00:13:23,236
It is early spring
in British Columbia.

94
00:13:24,240 --> 00:13:28,996
Seaweed has started to grow slowly
in the cold water.

95
00:13:30,280 --> 00:13:34,194
As the hours of sunlight increase
and the water warms,

96
00:13:34,360 --> 00:13:38,400
these small plants turn into
great beds of bull kelp.

97
00:13:46,760 --> 00:13:49,992
The immense 30—metre long strands

98
00:13:50,160 --> 00:13:54,120
have small gas—filled floats
which keep them high in the water,

99
00:13:54,280 --> 00:13:57,352
within reach
of the energy—giving sunlight.

100
00:14:06,080 --> 00:14:10,359
Further south,
the sunshine is more powerful,

101
00:14:10,520 --> 00:14:13,433
so here,
on the coast of California,

102
00:14:13,600 --> 00:14:16,274
the biggest kelp of all can grow.

103
00:14:18,520 --> 00:14:22,116
This is giant kelp,
and by midsummer,

104
00:14:22,280 --> 00:14:27,594
each plant is growing in length
by nearly a metre a day.

105
00:14:34,280 --> 00:14:38,832
The main stem is fixed
to the bottom 35 metres below

106
00:14:39,000 --> 00:14:43,597
by a small
but immensely strong holdfast.

107
00:14:45,320 --> 00:14:46,959
Fully grown,

108
00:14:47,120 --> 00:14:51,751
these gigantic plants
are 100 metres long

109
00:14:51,920 --> 00:14:54,560
and they form vast marine forests.

110
00:14:56,000 --> 00:14:59,880
Great schools of fish
find sanctuary here.

111
00:15:06,040 --> 00:15:09,351
These are blacksmith fish.

112
00:15:09,520 --> 00:15:14,037
They are plankton feeders
and, whenever it seems safe,

113
00:15:14,200 --> 00:15:19,070
they head into open water to feed
where the plankton is thickest.

114
00:15:21,240 --> 00:15:24,472
But at the first hint of danger...

115
00:15:26,080 --> 00:15:29,710
..they rush back to the safety
of the forest.

116
00:15:32,440 --> 00:15:38,277
Fish are not the only creatures
that shelter here.

117
00:15:42,600 --> 00:15:47,038
Californian sea otters
gather in the kelp forest

118
00:15:47,200 --> 00:15:50,238
to rest and snooze in safety.

119
00:15:52,240 --> 00:15:55,039
To prevent themselves
being carried away

120
00:15:55,200 --> 00:15:59,114
into dangerous open water
where the big predators cruise,

121
00:15:59,280 --> 00:16:04,116
they anchor themselves by winding
straps of kelp around their body.

122
00:16:09,840 --> 00:16:13,436
Sooner or later,
they have to find food,

123
00:16:13,600 --> 00:16:17,389
and that lies on the seabed
a long way below them.

124
00:16:17,560 --> 00:16:21,679
Sea otters, however, can stay
underwater for up to ten minutes,

125
00:16:21,840 --> 00:16:25,834
ample time to find
the shellfish they need.

126
00:16:54,880 --> 00:16:58,430
Many of the smaller creatures
that live in these forests,

127
00:16:58,600 --> 00:17:00,751
such as urchins, graze on the kelp

128
00:17:00,920 --> 00:17:04,880
and can seriously damage it
if their numbers are unchecked.

129
00:17:05,040 --> 00:17:07,953
The sea otters
feed on some of these grazers

130
00:17:08,120 --> 00:17:10,919
and prevent them
getting too numerous.

131
00:17:11,080 --> 00:17:16,519
So, in effect, the sea otters
are the guardians of the forest.

132
00:17:19,600 --> 00:17:24,277
Garibaldi fish do not,
in fact, damage the kelp.

133
00:17:24,440 --> 00:17:26,511
If anything, they help it

134
00:17:26,680 --> 00:17:31,152
by picking off tiny animals
that encrust the leaves.

135
00:17:35,560 --> 00:17:38,997
They graze on bryozoans,
tiny colonial animals

136
00:17:39,160 --> 00:17:41,629
which build their colonies

137
00:17:41,800 --> 00:17:45,919
like a patchwork of white skins
on the leaves.

138
00:17:56,960 --> 00:18:01,591
When night falls, there are
fewer predatory fish around,

139
00:18:01,760 --> 00:18:05,959
and the bryozoans emerge
from their white shelters.

140
00:18:11,800 --> 00:18:13,792
Now, just like coral polyps,

141
00:18:13,960 --> 00:18:18,193
they start filtering out the
plankton under cover of darkness.

142
00:18:23,000 --> 00:18:27,756
They're not the only animals
to venture out at this time.

143
00:18:32,560 --> 00:18:36,270
This is an amphipod,
just two centimetres long,

144
00:18:36,440 --> 00:18:39,319
and it does eat kelp.

145
00:18:43,880 --> 00:18:45,519
In turn,

146
00:18:45,680 --> 00:18:49,196
it is excellent food
for many predators.

147
00:18:55,520 --> 00:19:00,037
To protect itself, it produces
silk, just like a spider,

148
00:19:00,200 --> 00:19:04,479
and uses it to sew together
two sides of a kelp frond

149
00:19:04,640 --> 00:19:06,518
and so form a shelter.

150
00:19:13,640 --> 00:19:17,475
This one is in particular need
of a secure home.

151
00:19:17,640 --> 00:19:19,632
She's a mother.

152
00:19:23,960 --> 00:19:27,636
There are over 50 youngsters
clustered on her abdomen,

153
00:19:27,800 --> 00:19:31,157
so her home is becoming
somewhat cramped.

154
00:19:32,160 --> 00:19:34,834
They will soon
be old enough to leave,

155
00:19:35,000 --> 00:19:37,720
and, whenever she can,
she kicks them out

156
00:19:37,880 --> 00:19:40,236
to get a taste of the outside world.

157
00:19:49,560 --> 00:19:51,199
Dawn.

158
00:19:52,320 --> 00:19:55,313
Beds of eel grass
grow in the shallows

159
00:19:55,480 --> 00:19:58,791
between the kelp forest
and the shore.

160
00:19:59,560 --> 00:20:03,031
A harbour seal
has found sanctuary here,

161
00:20:03,200 --> 00:20:07,319
and is sleeping
after a hard night's foraging.

162
00:20:08,320 --> 00:20:10,755
But not for long.

163
00:20:14,800 --> 00:20:17,520
A male seal gives a wake—up call.

164
00:20:22,640 --> 00:20:26,156
More seals are attracted
from all directions.

165
00:20:33,200 --> 00:20:37,797
It's June, the time
when young male harbour seals

166
00:20:37,960 --> 00:20:40,475
start their strange
mating displays.

167
00:20:43,240 --> 00:20:48,520
One listens attentively to the
grunting noises made by the other.

168
00:20:52,400 --> 00:20:55,916
These calls are almost certainly
a way of establishing

169
00:20:56,080 --> 00:20:58,276
which of the two will be dominant.

170
00:21:03,840 --> 00:21:08,596
But if the animals are closely
matched in size and experience,

171
00:21:08,760 --> 00:21:10,831
grunts won't settle the issue.

172
00:21:11,000 --> 00:21:14,038
The rivals
will have to come to blows.

173
00:21:18,560 --> 00:21:21,951
And seals can be
surprisingly violent.

174
00:21:32,920 --> 00:21:34,912
In a month's time,

175
00:21:35,080 --> 00:21:39,916
the breeding season will start
and then fights will be in earnest,

176
00:21:40,080 --> 00:21:43,869
but now, in midsummer,
these exchanges are harmless.

177
00:21:44,040 --> 00:21:45,793
They're just playing.

178
00:21:48,720 --> 00:21:51,758
Many of the creatures
that shelter in the kelp

179
00:21:51,920 --> 00:21:54,310
have to venture out
in order to feed.

180
00:21:55,320 --> 00:21:57,198
The bat ray for one.

181
00:22:01,080 --> 00:22:03,151
Unlikely though it seems,

182
00:22:03,320 --> 00:22:05,312
the sandy floor of the open sea

183
00:22:05,480 --> 00:22:08,234
is, for the bat ray,
a rich feeding ground.

184
00:22:22,800 --> 00:22:25,838
There is food
hidden within the sand,

185
00:22:26,000 --> 00:22:29,630
and the bat ray has
a special technique for finding it.

186
00:22:34,120 --> 00:22:36,874
It uses jets of water

187
00:22:37,040 --> 00:22:42,434
to blow the sand aside
and expose small invertebrates.

188
00:22:44,880 --> 00:22:49,875
A kelp bass hangs about alongside
waiting for scraps.

189
00:23:10,400 --> 00:23:13,313
Other hunters
are also on the prowl.

190
00:23:15,440 --> 00:23:17,716
The fan—tailed sole.

191
00:23:28,720 --> 00:23:33,158
There are mantid shrimps here,
living in tunnels.

192
00:23:36,880 --> 00:23:41,272
But, once again, hunger compels
them to venture into the open.

193
00:23:41,440 --> 00:23:44,194
That, of course, is a gamble.

194
00:23:46,960 --> 00:23:49,077
They will either eat...

195
00:23:50,800 --> 00:23:52,792
..or be eaten.

196
00:24:00,560 --> 00:24:03,075
A sea slug called Janolus.

197
00:24:03,240 --> 00:24:07,075
Its colours suggest
that it is poisonous,

198
00:24:07,240 --> 00:24:12,360
and so it is, to everything
except another kind of sea slug...

199
00:24:17,440 --> 00:24:20,478
..the predatory Navanax.

200
00:24:24,160 --> 00:24:27,915
Navanax pulls itself
along the trail of slime

201
00:24:28,080 --> 00:24:30,675
that Janolus leaves behind it.

202
00:24:45,240 --> 00:24:49,234
Once caught,
Janolus rolls into a ball.

203
00:24:54,120 --> 00:24:57,955
All Navanax gets
is a few yellow tentacles.

204
00:25:03,360 --> 00:25:07,559
And Janolus is swept to safety
by the current.

205
00:25:14,520 --> 00:25:19,754
It's now midsummer, and the sun is
shining at full strength.

206
00:25:23,160 --> 00:25:25,356
The increasing warmth is the cue

207
00:25:25,520 --> 00:25:29,434
for an Atlantic lobster
to start on a long journey.

208
00:25:29,600 --> 00:25:34,277
She's spent the winter 250 metres down,
far beyond the reach of the storms.

209
00:25:36,120 --> 00:25:41,479
But it was cold down there and now
she needs to find warmer water,

210
00:25:41,640 --> 00:25:45,031
so she's marching
towards the shallows.

211
00:25:45,200 --> 00:25:48,671
They, however,
are 150 kilometres away.

212
00:25:54,720 --> 00:26:00,398
After a month of walking, she arrives
at her favoured sand bank.

213
00:26:01,600 --> 00:26:03,876
But she is not the first here.

214
00:26:04,040 --> 00:26:08,831
Dozens of other lobsters
have already dug homes in the sand

215
00:26:09,000 --> 00:26:13,040
and they don't intend
to surrender them to newcomers.

216
00:26:45,080 --> 00:26:48,596
Size counts for everything
in these battles.

217
00:26:48,760 --> 00:26:50,991
The new arrival
is in need of a pit,

218
00:26:51,160 --> 00:26:57,509
and since she weighs a hefty seven
kilos, she'll probably get one.

219
00:27:10,480 --> 00:27:12,472
She has won.

220
00:27:18,200 --> 00:27:21,352
These battles continue
for the next two months,

221
00:27:21,520 --> 00:27:23,159
and they are crucial,

222
00:27:23,320 --> 00:27:27,553
for the females must have shelter
and warm water to raise young.

223
00:27:27,720 --> 00:27:29,313
For the last seven months,

224
00:27:29,480 --> 00:27:33,394
each female has been carrying
about 20,000 fertilised eggs,

225
00:27:33,560 --> 00:27:37,634
but their task
is approaching its end.

226
00:27:37,800 --> 00:27:42,158
The warmth of the shallows
is speeding the eggs' development.

227
00:27:43,840 --> 00:27:49,120
Two more months
and the eggs are ready to hatch.

228
00:28:06,440 --> 00:28:10,229
At first, they're not very good
at swimming.

229
00:28:10,400 --> 00:28:12,471
But within a few minutes,

230
00:28:12,640 --> 00:28:16,793
the babies are able to set off
in a purposeful way.

231
00:28:18,360 --> 00:28:23,719
At this time of the year,
the sea is full of larval animals.

232
00:28:23,880 --> 00:28:26,679
This one is a day old lobster.

233
00:28:28,080 --> 00:28:30,879
And this — a three—week—old crab —

234
00:28:31,040 --> 00:28:35,080
just ready to start its life
on the sea floor.

235
00:28:36,160 --> 00:28:39,995
Its feet touch the bottom
for the first time.

236
00:28:43,240 --> 00:28:48,634
At this stage, it's a vegetarian
with a taste for sea lettuce.

237
00:28:48,800 --> 00:28:51,599
As it grows,
it will repeatedly moult

238
00:28:51,760 --> 00:28:54,832
and expand into a bigger,
thicker skin.

239
00:28:55,000 --> 00:28:57,469
The chances are it will be eaten,

240
00:28:57,640 --> 00:29:02,795
but if it survives for five years,
it will become an armoured giant.

241
00:29:10,000 --> 00:29:12,469
Now it eats meat,

242
00:29:12,640 --> 00:29:17,157
and special adaptations
enable it to hunt in the dark.

243
00:29:20,600 --> 00:29:23,752
Its jointed feet
are covered in sensors

244
00:29:23,920 --> 00:29:29,200
which detect the slightest chemical
change in its surroundings.

245
00:29:29,360 --> 00:29:32,319
As it walks through the darkness,

246
00:29:32,480 --> 00:29:36,269
its feet can, literally,
taste the sand.

247
00:29:38,480 --> 00:29:40,949
As soon as it finds suitable food,

248
00:29:41,120 --> 00:29:44,750
it passes it forward
to its powerful crushing claws,

249
00:29:44,920 --> 00:29:47,754
which make light work
of the soft flesh.

250
00:29:47,920 --> 00:29:51,596
And those claws are also
very useful for defence.

251
00:29:57,040 --> 00:30:02,320
A one and a half metre long
common octopus glides by.

252
00:30:05,720 --> 00:30:10,715
The jet—propelled giant
is both powerful and very clever.

253
00:30:17,440 --> 00:30:22,595
With octopus about, it's risky
for even a crab to be in the open.

254
00:30:22,760 --> 00:30:25,912
Even in the dark,
the octopus's eyes are sensitive

255
00:30:26,080 --> 00:30:27,878
to the slightest movement.

256
00:30:37,840 --> 00:30:43,234
Against a hunter like this,
the crab's claws are useless.

257
00:30:52,000 --> 00:30:54,799
Late summer in South East Alaska.

258
00:31:00,880 --> 00:31:03,156
The water is still warming,

259
00:31:03,320 --> 00:31:06,392
and mysid shrimp
swarm near the surface.

260
00:31:11,920 --> 00:31:15,152
It's a final feast
for Pacific salmon

261
00:31:15,320 --> 00:31:19,519
returning to the coast
from the open Pacific.

262
00:31:25,400 --> 00:31:31,158
They are heading inshore to breed
and they arrive in huge numbers.

263
00:31:31,320 --> 00:31:35,997
They have to swim
far up the rivers to spawn,

264
00:31:36,160 --> 00:31:39,631
but this river's level
is still too low.

265
00:31:39,840 --> 00:31:43,629
They'll have to wait
till rain causes it to rise.

266
00:31:45,160 --> 00:31:49,313
For now, they are trapped
in the sea, close to the shore,

267
00:31:49,480 --> 00:31:51,199
the worst place to be.

268
00:31:52,560 --> 00:31:54,995
A three metre long salmon shark,

269
00:31:55,160 --> 00:31:59,234
a close relation
of the great white.

270
00:32:03,440 --> 00:32:05,033
This one has sensed

271
00:32:05,200 --> 00:32:08,477
minute electrical signals
from the salmon nearby.

272
00:32:08,640 --> 00:32:11,951
Shark can maintain
their blood temperature

273
00:32:12,120 --> 00:32:15,591
at a higher level
than the surrounding water.

274
00:32:15,760 --> 00:32:20,437
And that means they have the energy
to be quick.

275
00:32:25,920 --> 00:32:27,798
Quicker than salmon.

276
00:32:47,120 --> 00:32:53,469
Early autumn in Vancouver Island
in Canada, 600 miles to the south.

277
00:32:53,640 --> 00:32:57,395
The ocean temperature
is slowly dropping.

278
00:32:57,560 --> 00:32:59,756
40 metres below the surface,

279
00:32:59,920 --> 00:33:05,314
this year's baby herring feed
on the last of the summer plankton.

280
00:33:05,480 --> 00:33:10,475
Their movements attract attention
from the skies above.

281
00:33:18,320 --> 00:33:19,959
Gulls can't dive,

282
00:33:20,120 --> 00:33:23,955
so for now,
the fish are still safe.

283
00:33:25,200 --> 00:33:29,399
But there are birds which CAN dive.

284
00:33:32,160 --> 00:33:37,474
Auklets and murres swim with ease
down beneath the school.

285
00:33:37,640 --> 00:33:42,032
The panicked herring are forced
towards the surface.

286
00:33:51,160 --> 00:33:56,030
They gather into a giant
defensive ball of swirling fish.

287
00:33:57,040 --> 00:34:01,159
The commotion attracts
yellowtail rockfish.

288
00:34:03,400 --> 00:34:06,074
They, too, are hunters.

289
00:34:14,160 --> 00:34:17,676
The marauding fish
scatter the herring.

290
00:34:26,960 --> 00:34:31,591
Repeated attacks split the ball
into numerous smaller groups.

291
00:34:37,680 --> 00:34:39,672
Now, it's easier for the divers

292
00:34:39,840 --> 00:34:43,390
to keep the confused fish
penned at the surface.

293
00:34:43,560 --> 00:34:45,791
Even the gulls can get at them.

294
00:35:03,760 --> 00:35:08,676
Attacked from all sides, the little
fish have virtually no chance.

295
00:35:10,080 --> 00:35:13,517
More and more divers
are attracted to the scene.

296
00:35:17,360 --> 00:35:20,512
They harry the shrinking numbers
of herring

297
00:35:20,680 --> 00:35:23,593
right down
to the very last individual.

298
00:35:39,840 --> 00:35:43,436
Far bigger predators
cruise here, too.

299
00:35:44,400 --> 00:35:47,677
Pacific white—sided dolphin.

300
00:35:50,680 --> 00:35:54,196
But the dolphin
are mainly nocturnal hunters.

301
00:35:54,360 --> 00:35:57,512
During the day,
they concentrate on socialising.

302
00:35:57,680 --> 00:36:02,516
They display by releasing streams
of bubbles

303
00:36:02,680 --> 00:36:04,876
and they play games.

304
00:36:07,560 --> 00:36:11,270
Games like pass the seaweed,
for example.

305
00:36:44,400 --> 00:36:48,952
Exactly eight months ago,
in the winter off Western Scotland,

306
00:36:49,120 --> 00:36:54,275
an egg was laid and securely fixed
to a strand of kelp.

307
00:36:54,440 --> 00:36:58,514
Inside, a tiny embryo
started to develop.

308
00:37:06,480 --> 00:37:09,200
Protected by the tough egg case,

309
00:37:09,360 --> 00:37:12,876
it endured
the worst of the winter storms.

310
00:37:21,080 --> 00:37:23,754
By summer, it was half—grown.

311
00:37:35,600 --> 00:37:40,880
Now, at last, in late autumn,
it's nearly ready to hatch.

312
00:37:49,040 --> 00:37:52,078
A fully—formed miniature shark
swims free.

313
00:37:52,240 --> 00:37:55,039
This year's plankton will soon die,

314
00:37:55,200 --> 00:37:57,669
but the dogfish is so well developed

315
00:37:57,840 --> 00:38:01,277
that it can hunt immediately
for larger prey.

316
00:38:03,080 --> 00:38:05,914
The days are getting shorter now.

317
00:38:07,800 --> 00:38:12,033
In British Columbia,
the water begins to chill.

318
00:38:12,200 --> 00:38:16,558
This bizarre—looking creature
is searching the kelp for food.

319
00:38:18,280 --> 00:38:22,399
This is Melibe —
the hooded sea slug.

320
00:38:25,720 --> 00:38:30,078
It catches plankton
with its net—like head.

321
00:38:39,040 --> 00:38:42,750
As winter approaches,
plankton is becoming scarce,

322
00:38:42,920 --> 00:38:46,914
but Melibe
is an assiduous searcher.

323
00:38:49,640 --> 00:38:51,791
It can swim.

324
00:38:56,760 --> 00:38:59,912
It flaps away to look
for a better feeding spot,

325
00:39:00,080 --> 00:39:03,756
but its search is becoming
increasingly difficult.

326
00:39:03,920 --> 00:39:08,836
Within the next two weeks, most of
the plankton will have disappeared.

327
00:39:17,000 --> 00:39:19,959
As the sunlight
becomes ever more feeble,

328
00:39:20,880 --> 00:39:25,432
the kelp starts to die,
gradually rotting away to nothing.

329
00:39:58,160 --> 00:40:00,311
Soon, it will be winter.

330
00:40:04,840 --> 00:40:10,234
But 9,000 miles to the south,
the sun is rising on a new spring day.

331
00:40:10,400 --> 00:40:15,520
The southern hemisphere, too,
has temperate regions.

332
00:40:15,680 --> 00:40:19,754
And now the plankton is beginning
to bloom around Tasmania.

333
00:40:19,920 --> 00:40:22,992
Just as in the north,

334
00:40:23,160 --> 00:40:27,996
the Southern Seasonal Seas
have areas of rich green water...

335
00:40:28,800 --> 00:40:32,476
..with their own kelp forests...

336
00:40:33,600 --> 00:40:37,640
..and their own swarms of plankton.

337
00:40:45,040 --> 00:40:47,475
Some of the inhabitants
of these seas

338
00:40:47,640 --> 00:40:50,314
live only
in the southern hemisphere.

339
00:40:50,480 --> 00:40:53,791
This is one of them —
the handfish —

340
00:40:53,960 --> 00:40:58,193
that strolls around
on modified fins.

341
00:41:01,000 --> 00:41:05,870
But, when needs must,
it can resort to tail power.

342
00:41:10,280 --> 00:41:14,559
Every summer, visitors come
to the shallows around Tasmania.

343
00:41:14,720 --> 00:41:19,431
These are Australian squid,
about half a metre long.

344
00:41:22,520 --> 00:41:24,716
They are here to breed.

345
00:41:29,960 --> 00:41:34,318
The larger males compete
for the attentions of a female,

346
00:41:34,480 --> 00:41:37,314
displaying towards her
and to one another,

347
00:41:37,480 --> 00:41:41,997
by putting on a ballet,
continually changing costume colour.

348
00:42:10,880 --> 00:42:12,473
Eventually,

349
00:42:12,640 --> 00:42:14,996
they form pairs.

350
00:42:20,360 --> 00:42:24,593
A male passes a packet of sperm
across to the female.

351
00:42:28,520 --> 00:42:31,319
After the eggs
have been fertilised,

352
00:42:31,480 --> 00:42:35,520
they are deposited
in tough, rubbery egg cases

353
00:42:35,680 --> 00:42:39,071
that other creatures find poisonous.

354
00:42:39,240 --> 00:42:44,952
Within three weeks,
the babies are ready to hatch out.

355
00:42:50,680 --> 00:42:53,752
They are already
able to change colour,

356
00:42:53,920 --> 00:42:56,435
but aren't so good at swimming.

357
00:43:13,520 --> 00:43:18,720
There is another animal here
that is a rather more devoted parent.

358
00:43:18,880 --> 00:43:23,397
This is a male leafy sea dragon,

359
00:43:23,560 --> 00:43:27,315
an exquisitely—decorated relative
of the sea horse.

360
00:43:27,480 --> 00:43:33,158
He is carrying his partner's eggs
around with him.

361
00:43:33,320 --> 00:43:37,553
They would be a nutritious snack
for any predator that found them.

362
00:43:37,720 --> 00:43:41,475
But that's not easy
because the eggs are attached

363
00:43:41,640 --> 00:43:45,395
to their father's
perfectly—camouflaged body.

364
00:43:45,560 --> 00:43:49,031
They could scarcely be
in a safer place.

365
00:44:13,720 --> 00:44:17,919
By November,
10,000 miles to the north,

366
00:44:18,080 --> 00:44:20,311
winter has arrived.

367
00:44:20,480 --> 00:44:24,440
Norway now gets less than
five hours of daylight in every 24.

368
00:44:24,600 --> 00:44:27,479
The temperature is falling rapidly.

369
00:44:30,080 --> 00:44:34,233
But, despite the cold,
the sea is far from deserted.

370
00:44:34,400 --> 00:44:38,360
Every winter, 500 million tonnes
of adult herring

371
00:44:38,520 --> 00:44:41,479
seek shelter in these deep waters.

372
00:44:43,960 --> 00:44:46,873
They will stay here
for four months,

373
00:44:47,040 --> 00:44:50,078
living entirely
on the fat they accumulated

374
00:44:50,240 --> 00:44:52,960
during the summer feasts
of plankton.

375
00:44:53,120 --> 00:44:55,396
But they are not alone.

376
00:44:56,800 --> 00:44:58,439
Orca.

377
00:44:58,600 --> 00:45:03,800
This pod is part of a population
of some 500 killer whales

378
00:45:03,960 --> 00:45:08,193
that specialise in hunting
North Atlantic herring.

379
00:45:09,960 --> 00:45:12,031
(WHALE SONG)

380
00:45:20,880 --> 00:45:25,193
Using their echo location,
they have detected a shoal of herring

381
00:45:25,360 --> 00:45:27,431
50 metres below them.

382
00:45:27,600 --> 00:45:30,877
With enough air
for a ten—minute dive,

383
00:45:31,040 --> 00:45:33,874
they swim deep below the herring

384
00:45:34,040 --> 00:45:36,236
and drive the fish upwards.

385
00:45:41,440 --> 00:45:44,353
Bubbles stream from the rising fish

386
00:45:44,520 --> 00:45:49,037
as gas in their swim bladders
expands and escapes.

387
00:45:53,440 --> 00:45:59,038
Even an orca finds it difficult
to catch a healthy herring.

388
00:45:59,200 --> 00:46:03,353
But they have a devastating weapon
all their own.

389
00:46:10,600 --> 00:46:14,992
They use their tails to club the fish
with waves of water pressure.

390
00:46:22,520 --> 00:46:28,198
Then it's simply a matter
of collecting the stunned casualties.

391
00:46:42,000 --> 00:46:43,992
The herrings have no chance.

392
00:46:44,160 --> 00:46:47,597
Both orca and gulls
will eat as much as they can

393
00:46:47,760 --> 00:46:50,434
every day for the next four months.

394
00:46:55,120 --> 00:46:59,114
But there are so many fish
wintering here,

395
00:46:59,280 --> 00:47:01,670
over five billion individuals,

396
00:47:01,840 --> 00:47:04,992
that the losses
are almost unnoticeable.

397
00:47:12,680 --> 00:47:15,514
Violent as this winter weather
may be,

398
00:47:15,680 --> 00:47:20,630
it's essential for the renewal
of the riches of the Seasonal Seas.

399
00:47:20,800 --> 00:47:22,473
Out in the open oceans,

400
00:47:22,640 --> 00:47:26,759
the surging waters stir up
nutrients from the depths.

401
00:47:30,960 --> 00:47:32,599
By the end of winter,

402
00:47:32,760 --> 00:47:36,231
the Seasonal Seas will be
full of minerals once more,

403
00:47:36,400 --> 00:47:42,590
ready for the return of the sun
and the next great plankton bloom.