1 00:00:43,551 --> 00:00:44,793 There are some seas 2 00:00:44,827 --> 00:00:49,344 where fish swarm in millions, 3 00:00:49,379 --> 00:00:53,896 and plankton blossoms in vast clouds. 4 00:00:56,137 --> 00:00:57,965 The sheer quantity of life here 5 00:00:58,000 --> 00:01:01,103 is unmatched anywhere in the oceans. 6 00:01:08,620 --> 00:01:11,620 These are the most productive seas on Earth. 7 00:01:17,034 --> 00:01:19,034 They are the seasonal seas. 8 00:01:24,000 --> 00:01:25,793 The seasonal seas border 9 00:01:25,827 --> 00:01:27,517 the temperate parts of the world, 10 00:01:27,551 --> 00:01:30,931 where conditions change through the year. 11 00:01:38,448 --> 00:01:41,862 The seasons also affect the underwater world. 12 00:01:44,517 --> 00:01:47,137 The power of the sun is constantly changing. 13 00:01:51,827 --> 00:01:53,758 In the far north, during the summer, 14 00:01:53,793 --> 00:01:55,827 there are long hours of sunlight. 15 00:01:55,862 --> 00:01:57,862 But in winter, that dwindles, 16 00:01:57,896 --> 00:02:00,758 and eventually, there can be weeks of darkness. 17 00:02:03,344 --> 00:02:06,275 The summers are warm and gentle, 18 00:02:06,310 --> 00:02:09,448 the winters racked by savage storms. 19 00:03:03,724 --> 00:03:05,310 In conditions like this, 20 00:03:05,344 --> 00:03:09,137 life of any sort has to struggle to survive. 21 00:03:13,965 --> 00:03:15,965 January on Sable Island, 22 00:03:16,000 --> 00:03:19,275 off the coast of Nova Scotia in North America. 23 00:03:19,310 --> 00:03:21,724 Grey seals have managed to get ashore 24 00:03:21,758 --> 00:03:23,793 through the crashing breakers. 25 00:03:32,896 --> 00:03:36,103 Gales here can blow for days on end. 26 00:03:55,482 --> 00:03:59,931 Sable Island has the world's largest colony of grey seals. 27 00:03:59,965 --> 00:04:02,931 100,000 come here to breed each year, 28 00:04:02,965 --> 00:04:05,482 just when the weather is at its worst. 29 00:04:11,862 --> 00:04:14,586 To add insult to injury, the pups, 30 00:04:14,620 --> 00:04:18,862 having suckled for only 18 days, are then abandoned. 31 00:04:18,896 --> 00:04:20,862 Their mothers must return to the sea 32 00:04:20,896 --> 00:04:22,862 to find food for themselves. 33 00:04:27,586 --> 00:04:30,931 As yet unable to dive, the pups are marooned, 34 00:04:30,965 --> 00:04:34,310 and sustained by nothing except their fatty blubber. 35 00:04:36,620 --> 00:04:40,724 It will be five weeks before they are strong enough to swim. 36 00:04:40,758 --> 00:04:43,137 But by then, it will be early spring, 37 00:04:43,172 --> 00:04:46,068 and the ocean will be teeming with food. 38 00:05:00,965 --> 00:05:05,448 By May, spring has reached the coasts of Scotland. 39 00:05:08,137 --> 00:05:11,206 Underwater, it arrived rather earlier. 40 00:05:11,241 --> 00:05:13,827 During March, the seas had warmed enough 41 00:05:13,862 --> 00:05:16,482 to trigger a magical transformation. 42 00:05:29,655 --> 00:05:33,586 These are phytoplankton-- tiny, floating algae, 43 00:05:33,620 --> 00:05:35,793 each much smaller than a pinhead. 44 00:05:35,827 --> 00:05:38,517 And they multiply with amazing speed 45 00:05:38,551 --> 00:05:40,827 to produce more annual growth 46 00:05:40,862 --> 00:05:43,517 than all the plants on land put together-- 47 00:05:43,551 --> 00:05:45,862 6 billion tons of it. 48 00:05:48,482 --> 00:05:52,758 This immense bloom spreads across the face of the ocean, 49 00:05:52,793 --> 00:05:54,689 and within a couple of months, 50 00:05:54,724 --> 00:05:58,482 turns vast areas of it a dense green. 51 00:06:03,344 --> 00:06:04,655 Let's begin. 52 00:06:04,758 --> 00:06:07,448 My dog is one of us. 53 00:06:07,551 --> 00:06:09,724 You and me, girl. 54 00:06:09,827 --> 00:06:12,931 Home protection? Hunting. 55 00:06:13,034 --> 00:06:15,344 Out here, most folks let things go. 56 00:06:15,448 --> 00:06:17,206 I guess I'm not most folks. 57 00:06:17,310 --> 00:06:18,241 I want to live. 58 00:06:18,344 --> 00:06:20,275 Are you prepared to kill? 59 00:06:20,379 --> 00:06:23,000 Yes. 60 00:06:23,103 --> 00:06:25,689 One of us is not leaving this island. 61 00:06:25,793 --> 00:06:27,241 You ready? 62 00:06:33,793 --> 00:06:37,000 Animal life reacts to the blooming sea. 63 00:06:37,034 --> 00:06:39,724 Although these polyps look like small anemones, 64 00:06:39,758 --> 00:06:42,482 they're about to change into something else. 65 00:06:42,517 --> 00:06:44,068 As they separate, 66 00:06:44,103 --> 00:06:47,724 they reveal themselves to be tiny common jellyfish. 67 00:06:50,931 --> 00:06:53,758 They're less than 3 millimeters across. 68 00:06:57,965 --> 00:06:59,275 Within a few months, 69 00:06:59,310 --> 00:07:02,931 they will have assembled into vast swarms. 70 00:07:07,827 --> 00:07:11,551 Minute copepods are part of their staple diet. 71 00:07:11,586 --> 00:07:14,551 These appear every spring, in vast numbers, 72 00:07:14,586 --> 00:07:17,551 and graze on the phytoplankton bloom. 73 00:07:21,310 --> 00:07:23,448 Their beating legs create currents 74 00:07:23,482 --> 00:07:24,965 that sweep the little algae 75 00:07:25,000 --> 00:07:27,896 into the basket-like filters around their mouths. 76 00:07:32,551 --> 00:07:34,862 On this microscopic scale, 77 00:07:34,896 --> 00:07:37,793 water is so viscous that the phytoplankton 78 00:07:37,827 --> 00:07:43,034 can't swim against the current. 79 00:07:43,068 --> 00:07:46,310 Whilst feeding, sensors on the copepod's antennae 80 00:07:46,344 --> 00:07:48,689 give warning of dangers ahead. 81 00:07:53,793 --> 00:07:57,137 Lighting by lasers reveals that feeding copepods 82 00:07:57,172 --> 00:08:01,275 leave wakes behind them, rather like jet trails in the sky. 83 00:08:20,068 --> 00:08:22,551 Slightly larger floating predators 84 00:08:22,586 --> 00:08:26,344 are able to use these trails to find their prey. 85 00:08:40,034 --> 00:08:41,827 A close call, 86 00:08:41,862 --> 00:08:46,206 but some predators are simply too large to avoid. 87 00:08:53,275 --> 00:08:54,586 Jellyfish. 88 00:08:58,655 --> 00:09:01,689 They may appear to be delicate as well as beautiful, 89 00:09:01,724 --> 00:09:05,655 but they are deadly hunters. 90 00:09:42,758 --> 00:09:45,724 With every pulse of the delicate bells, 91 00:09:45,758 --> 00:09:49,241 plankton-rich water is drawn into their lacy throats 92 00:09:49,275 --> 00:09:50,862 and pushed out again, 93 00:09:50,896 --> 00:09:54,034 leaving behind copepods stuck to the membranes. 94 00:10:02,620 --> 00:10:07,206 A single sea nettle jellyfish, only a few centimeters across, 95 00:10:07,241 --> 00:10:10,758 can collect thousands of copepods in a day. 96 00:10:22,482 --> 00:10:26,275 So the killing power of giants, like these, 97 00:10:26,310 --> 00:10:28,068 is hard to estimate. 98 00:10:32,827 --> 00:10:36,896 Each of these jellyfish weighs up to 30 kilos, 99 00:10:36,931 --> 00:10:40,793 and has tentacles stretching over 8 meters. 100 00:10:43,896 --> 00:10:49,103 But there are even greater dangers awaiting the copepods. 101 00:10:49,137 --> 00:10:50,620 By the late spring, 102 00:10:50,655 --> 00:10:55,827 the baby common jellyfish are fully grown. 103 00:10:55,862 --> 00:10:59,137 They gather in millions, forming immense swarms 104 00:10:59,172 --> 00:11:01,965 hundreds of meters long, which filter out 105 00:11:02,000 --> 00:11:05,137 all the small planktonic animals in their path. 106 00:11:53,379 --> 00:11:57,344 Nevertheless, there are such astronomic numbers of copepods 107 00:11:57,379 --> 00:12:01,172 that enough will survive to form swarms of their own. 108 00:12:16,965 --> 00:12:20,517 A basking shark. 109 00:12:20,551 --> 00:12:25,172 This huge 8-ton fish feeds on plankton, 110 00:12:25,206 --> 00:12:27,793 and nothing else. 111 00:12:27,827 --> 00:12:30,724 Every hour, it filters 1,000 tons 112 00:12:30,758 --> 00:12:32,551 of seawater through its gills. 113 00:12:36,551 --> 00:12:40,241 Numbers of these shark appear when the bloom is at its peak, 114 00:12:40,275 --> 00:12:42,379 and travel along those water currents 115 00:12:42,413 --> 00:12:44,137 where the plankton is thickest. 116 00:12:47,862 --> 00:12:51,482 Even so, the supply of plankton is so abundant 117 00:12:51,517 --> 00:12:54,482 that another group of animals is able to share in the feast-- 118 00:12:54,517 --> 00:12:57,034 animals that live on the seafloor. 119 00:13:02,137 --> 00:13:04,000 Every centimeter of rock 120 00:13:04,034 --> 00:13:06,862 is covered with invertebrates of some kind-- 121 00:13:06,896 --> 00:13:10,379 anemones, sea squirts, gorgonians-- 122 00:13:10,413 --> 00:13:15,103 all trapping plankton from the passing water. 123 00:13:55,448 --> 00:14:00,655 Not all life in the seasonal seas depends upon the plankton. 124 00:14:00,689 --> 00:14:03,827 It's early spring in British Columbia. 125 00:14:03,862 --> 00:14:09,241 Seaweed has started to grow slowly in the cold water. 126 00:14:09,275 --> 00:14:11,379 As the hours of sunlight increase, 127 00:14:11,413 --> 00:14:13,103 and the water warms, 128 00:14:13,137 --> 00:14:18,034 these small plants turn into great beds of bull kelp. 129 00:14:27,827 --> 00:14:30,482 The immense, 30-meter-long strands 130 00:14:30,517 --> 00:14:32,413 have small, gas-filled floats 131 00:14:32,448 --> 00:14:34,206 which keep them high in the water, 132 00:14:34,241 --> 00:14:37,000 within reach of the energy-giving sunlight. 133 00:14:47,448 --> 00:14:50,275 Further south, the sunshine is more powerful, 134 00:14:50,310 --> 00:14:52,758 so here, on the coast of California, 135 00:14:52,793 --> 00:14:55,551 the biggest kelp of all can grow. 136 00:14:59,448 --> 00:15:02,827 This is giant kelp, and by midsummer, 137 00:15:02,862 --> 00:15:07,103 each plant is growing in length by nearly a meter a day. 138 00:15:15,172 --> 00:15:19,379 The main stem is fixed to the bottom 35 meters below 139 00:15:19,413 --> 00:15:22,758 by a small, but immensely strong, holdfast. 140 00:15:27,103 --> 00:15:31,620 Fully grown, these gigantic plants are 100 meters long, 141 00:15:31,655 --> 00:15:36,413 and they form vast marine forests. 142 00:15:36,448 --> 00:15:39,517 Great schools of fish find sanctuary here. 143 00:15:46,620 --> 00:15:50,172 These are blacksmith fish. 144 00:15:50,206 --> 00:15:53,517 They are plankton feeders, and whenever it seems safe, 145 00:15:53,551 --> 00:15:55,379 they head off into open water 146 00:15:55,413 --> 00:15:57,896 to feed where the plankton is thickest. 147 00:16:02,241 --> 00:16:06,172 But at the first hint of danger, 148 00:16:06,206 --> 00:16:09,103 they rush back to the safety of the forest. 149 00:16:14,413 --> 00:16:17,896 Fish are not the only creatures that shelter here. 150 00:16:23,103 --> 00:16:26,310 Californian sea otters gather in the kelp forest 151 00:16:26,344 --> 00:16:29,413 to rest and snooze in safety. 152 00:16:33,000 --> 00:16:35,172 To prevent themselves being carried away 153 00:16:35,206 --> 00:16:39,103 into dangerous open water, where the big predators cruise, 154 00:16:39,137 --> 00:16:40,586 they anchor themselves 155 00:16:40,620 --> 00:16:44,000 by winding straps of kelp around their body. 156 00:16:50,655 --> 00:16:53,137 Sooner or later, they have to find food, 157 00:16:53,172 --> 00:16:57,000 and that lies on the seabed a long way below them. 158 00:16:57,034 --> 00:16:58,413 Sea otters, however, 159 00:16:58,448 --> 00:17:01,000 can stay underwater for up to 10 minutes, 160 00:17:01,034 --> 00:17:04,758 and that's ample time to find the shellfish they need. 161 00:17:35,689 --> 00:17:38,655 Many of the smaller creatures that live in these forests, 162 00:17:38,689 --> 00:17:41,379 such as urchins, graze on the kelp, 163 00:17:41,413 --> 00:17:45,172 and can seriously damage it if their numbers are unchecked. 164 00:17:45,206 --> 00:17:48,689 The sea otters, however, feed on some of these grazers, 165 00:17:48,724 --> 00:17:51,068 and prevent them getting too numerous. 166 00:17:51,103 --> 00:17:53,482 So, in effect, the sea otters 167 00:17:53,517 --> 00:17:55,793 are the guardians of the forest. 168 00:18:02,034 --> 00:18:04,724 Garibaldi fish do not, in fact, damage the kelp. 169 00:18:04,758 --> 00:18:08,275 If anything, they help it by picking off tiny animals 170 00:18:08,310 --> 00:18:10,931 that, in places, encrust the leaves. 171 00:18:16,896 --> 00:18:19,206 They graze on bryozoans, 172 00:18:19,241 --> 00:18:22,241 tiny colonial animals which build their colonies 173 00:18:22,275 --> 00:18:25,655 like a patchwork of white skins on the leaves. 174 00:18:39,517 --> 00:18:43,310 When night falls, there are fewer predatory fish around, 175 00:18:43,344 --> 00:18:46,862 and the bryozoans emerge from their white shelters. 176 00:18:52,551 --> 00:18:55,103 Now, just like coral polyps, 177 00:18:55,137 --> 00:18:57,413 they start filtering out the plankton 178 00:18:57,448 --> 00:18:59,275 under cover of darkness. 179 00:19:04,034 --> 00:19:07,586 They're not the only animals to venture out at this time. 180 00:19:14,137 --> 00:19:18,000 This is an amphipod, 2 two centimeters long, 181 00:19:18,034 --> 00:19:20,206 and it does eat kelp. 182 00:19:25,344 --> 00:19:29,517 In turn, it is excellent food for many predators. 183 00:19:36,827 --> 00:19:40,724 To protect itself, it produces silk, just like a spider, 184 00:19:40,758 --> 00:19:44,275 and uses it to sew together two sides of a kelp frond, 185 00:19:44,310 --> 00:19:46,103 and so, form a shelter. 186 00:19:55,137 --> 00:19:58,896 This one is in particular need of a secure home. 187 00:19:58,931 --> 00:20:00,206 She's a mother. 188 00:20:04,034 --> 00:20:07,655 There are over 50 youngsters clustered on her abdomen, 189 00:20:07,689 --> 00:20:10,413 so her home is becoming somewhat cramped. 190 00:20:14,275 --> 00:20:16,241 They will soon be old enough to leave, 191 00:20:16,275 --> 00:20:18,034 and now, whenever she can, 192 00:20:18,068 --> 00:20:21,103 she kicks them out, to get a taste of the outside world. 193 00:20:30,551 --> 00:20:33,793 Dawn. 194 00:20:33,827 --> 00:20:36,241 Beds of eelgrass grow in the shallows 195 00:20:36,275 --> 00:20:38,551 between the kelp forest and the shore. 196 00:20:41,241 --> 00:20:43,827 A harbor seal has found sanctuary here, 197 00:20:43,862 --> 00:20:47,034 and is sleeping after a hard night's foraging. 198 00:20:50,000 --> 00:20:51,620 But not for long. 199 00:20:55,655 --> 00:20:58,655 A male seal gives a wake-up call. 200 00:21:03,103 --> 00:21:06,275 More seals are attracted from all directions. 201 00:21:13,965 --> 00:21:17,827 It's June, the time when young male harbor seals 202 00:21:17,862 --> 00:21:23,172 start their strange mating displays. 203 00:21:24,862 --> 00:21:27,724 One listens attentively to the grunting noises 204 00:21:27,758 --> 00:21:29,137 made by the other. 205 00:21:33,068 --> 00:21:35,172 These calls are almost certainly 206 00:21:35,206 --> 00:21:38,620 a way of establishing which of the two will be dominant. 207 00:21:44,827 --> 00:21:46,827 But if the animals are closely matched 208 00:21:46,862 --> 00:21:50,862 in size and experience, grunts won't settle the issue. 209 00:21:50,896 --> 00:21:53,413 The rivals will have to come to blows. 210 00:21:59,931 --> 00:22:02,793 And seals can be surprisingly violent. 211 00:22:14,724 --> 00:22:17,586 In a month's time, the breeding season will start, 212 00:22:17,620 --> 00:22:19,758 and then, fights will be in earnest. 213 00:22:19,793 --> 00:22:23,448 But now, in midsummer, these exchanges are harmless. 214 00:22:23,482 --> 00:22:24,896 They're just playing. 215 00:22:30,896 --> 00:22:33,172 Many of the creatures that shelter in the kelp 216 00:22:33,206 --> 00:22:35,896 have to venture out in order to feed-- 217 00:22:35,931 --> 00:22:38,068 the bat ray, for one. 218 00:22:42,655 --> 00:22:44,310 Unlikely though it seems, 219 00:22:44,344 --> 00:22:46,793 the sandy floor of the open sea is, 220 00:22:46,827 --> 00:22:49,482 for the bat ray, a rich feeding ground. 221 00:23:03,551 --> 00:23:06,241 There is food hidden within the sand, 222 00:23:06,275 --> 00:23:10,000 and the bat ray has a special technique for finding it. 223 00:23:16,172 --> 00:23:19,689 It uses jets of water to blow the sand aside, 224 00:23:19,724 --> 00:23:22,000 and expose small invertebrates. 225 00:23:25,551 --> 00:23:28,793 A kelp bass hangs about alongside, 226 00:23:28,827 --> 00:23:30,379 waiting for scraps. 227 00:23:51,413 --> 00:23:55,931 Other hunters are also on the prowl. 228 00:23:55,965 --> 00:23:58,068 The fantailed sole. 229 00:24:09,655 --> 00:24:11,586 There are mantid shrimps here, 230 00:24:11,620 --> 00:24:13,000 living in tunnels. 231 00:24:17,724 --> 00:24:19,137 But, once again, 232 00:24:19,172 --> 00:24:22,517 hunger compels them to venture into the open. 233 00:24:22,551 --> 00:24:25,172 That, of course, is a gamble. 234 00:24:28,068 --> 00:24:29,793 They will either eat... 235 00:24:31,724 --> 00:24:33,103 ...or be eaten. 236 00:24:42,586 --> 00:24:45,275 A sea slug, called Janolus. 237 00:24:45,310 --> 00:24:47,482 Its colors suggest that it's poisonous, 238 00:24:47,517 --> 00:24:49,000 and so it is, 239 00:24:49,034 --> 00:24:52,586 to everything except another kind of sea slug. 240 00:24:59,068 --> 00:25:01,689 The predatory Navanax. 241 00:25:04,862 --> 00:25:08,310 Navanax pulls itself along the trail of slime 242 00:25:08,344 --> 00:25:10,724 that Janolus leaves behind it. 243 00:25:26,206 --> 00:25:30,379 Once caught, Janolus rolls into a ball. 244 00:25:35,103 --> 00:25:39,241 All Navanax gets is a few yellow tentacles. 245 00:25:43,551 --> 00:25:47,551 And Janolus is swept to safety by the current. 246 00:25:56,206 --> 00:25:57,586 It's now midsummer, 247 00:25:57,620 --> 00:26:00,793 and the sun is shining at full strength. 248 00:26:04,206 --> 00:26:06,620 The increasing warmth is the cue 249 00:26:06,655 --> 00:26:10,413 for an Atlantic lobster to start on a long journey. 250 00:26:10,448 --> 00:26:13,448 She's spent the winter 250 meters down, 251 00:26:13,482 --> 00:26:17,172 far beyond the reach of the storms. 252 00:26:17,206 --> 00:26:19,448 But it was cold down there, 253 00:26:19,482 --> 00:26:22,206 and now, she needs to find warmer water, 254 00:26:22,241 --> 00:26:26,620 so she's marching towards the shallows. 255 00:26:26,655 --> 00:26:30,689 They, however, are 150 kilometers away. 256 00:26:36,103 --> 00:26:38,379 After a month of walking, 257 00:26:38,413 --> 00:26:41,655 she finally arrives at her favored sandbank. 258 00:26:43,344 --> 00:26:45,586 But she's not the first here. 259 00:26:45,620 --> 00:26:47,241 Dozens of other lobsters 260 00:26:47,275 --> 00:26:49,724 have already dug themselves homes in the sand, 261 00:26:49,758 --> 00:26:52,793 and they don't intend to surrender them to newcomers. 262 00:27:26,206 --> 00:27:29,758 Size counts for everything in these battles. 263 00:27:29,793 --> 00:27:32,655 The new arrival is in urgent need of a pit, 264 00:27:32,689 --> 00:27:35,724 and since she weighs a hefty 7 kilos, 265 00:27:35,758 --> 00:27:38,137 she stands a good chance of getting one. 266 00:27:52,275 --> 00:27:53,793 She's won. 267 00:27:58,379 --> 00:28:01,379 These battles continue for the next two months, 268 00:28:01,413 --> 00:28:02,862 and they're crucial, 269 00:28:02,896 --> 00:28:05,103 for the females must have both shelter 270 00:28:05,137 --> 00:28:09,034 and warm water if they're to raise their young. 271 00:28:09,068 --> 00:28:12,241 For the last seven months, each of these females 272 00:28:12,275 --> 00:28:16,655 has been carrying around about 20,000 fertilized eggs. 273 00:28:16,689 --> 00:28:19,310 But their task is approaching its end. 274 00:28:19,344 --> 00:28:22,896 The warmth of the shallows is speeding the eggs' development. 275 00:28:25,931 --> 00:28:29,517 Two more months and the eggs are ready to hatch. 276 00:28:47,827 --> 00:28:52,551 At first, they're not very good at swimming, 277 00:28:52,586 --> 00:28:54,275 but within a few minutes, 278 00:28:54,310 --> 00:28:58,000 the babies are able to set off in a purposeful way. 279 00:29:00,551 --> 00:29:02,034 At this time of the year, 280 00:29:02,068 --> 00:29:04,620 the sea is full of larval animals. 281 00:29:04,655 --> 00:29:07,310 This one is a one-day-old lobster. 282 00:29:10,413 --> 00:29:12,896 And this, a three-week-old crab, 283 00:29:12,931 --> 00:29:18,068 just ready to start its life on the seafloor. 284 00:29:18,103 --> 00:29:21,206 Its feet touch the bottom for the first time. 285 00:29:23,689 --> 00:29:25,275 At this stage in its life, 286 00:29:25,310 --> 00:29:29,344 it's a vegetarian with a taste for sea lettuce. 287 00:29:29,379 --> 00:29:32,620 As it grows, so it will repeatedly molt, 288 00:29:32,655 --> 00:29:36,137 and expand into a bigger, thicker skin. 289 00:29:36,172 --> 00:29:38,827 The chances are it will be eaten. 290 00:29:38,862 --> 00:29:41,000 But if it survives for five years, 291 00:29:41,034 --> 00:29:44,034 it will have become a magnificent armored giant. 292 00:29:51,655 --> 00:29:53,586 Now, it eats meat, 293 00:29:53,620 --> 00:29:57,137 and special adaptations enable it to hunt in the dark. 294 00:30:03,068 --> 00:30:06,137 Its jointed feet are covered in sensors 295 00:30:06,172 --> 00:30:08,827 which detect the slightest chemical change 296 00:30:08,862 --> 00:30:11,275 in its surroundings. 297 00:30:11,310 --> 00:30:13,344 And as it walks through the darkness, 298 00:30:13,379 --> 00:30:19,310 its feet can literally taste the sand. 299 00:30:19,344 --> 00:30:21,517 As soon as it finds suitable food, 300 00:30:21,551 --> 00:30:25,000 it passes it forward to its powerful crushing claws, 301 00:30:25,034 --> 00:30:28,172 which make light work of the soft flesh. 302 00:30:28,206 --> 00:30:32,310 And those claws are also very useful for defense. 303 00:30:38,793 --> 00:30:43,551 A 1 1/2-meter-long common octopus glides by. 304 00:30:46,965 --> 00:30:51,103 The jet-propelled giant is both powerful and very clever. 305 00:30:58,620 --> 00:31:00,068 With octopus about, 306 00:31:00,103 --> 00:31:04,724 it's risky for even a crab to be in the open. 307 00:31:04,758 --> 00:31:06,448 Even in the dark, 308 00:31:06,482 --> 00:31:09,758 the octopus's eyes are sensitive to the slightest movement. 309 00:31:19,310 --> 00:31:23,655 Against a hunter like this, the crab's claws are useless. 310 00:31:34,206 --> 00:31:37,000 Late summer in Southeast Alaska. 311 00:31:43,206 --> 00:31:44,965 The water is still warming, 312 00:31:45,000 --> 00:31:48,551 and mysid shrimp are swarming near the surface. 313 00:31:54,137 --> 00:31:57,000 It's a final feast for Pacific salmon 314 00:31:57,034 --> 00:32:00,275 returning to the coast from the open Pacific. 315 00:32:07,724 --> 00:32:09,793 They're heading inshore to breed, 316 00:32:09,827 --> 00:32:13,965 and they arrive in huge numbers. 317 00:32:14,000 --> 00:32:17,068 They have to swim far up the rivers to spawn, 318 00:32:17,103 --> 00:32:20,310 but this river's level is still too low. 319 00:32:20,344 --> 00:32:26,517 They'll have to wait until rain causes it to rise. 320 00:32:26,551 --> 00:32:29,448 So, for now, they are trapped in the sea 321 00:32:29,482 --> 00:32:32,862 close to the shore-- the worst place to be. 322 00:32:35,517 --> 00:32:38,000 A 3-meter-long salmon shark, 323 00:32:38,034 --> 00:32:40,758 a close relation of the Great White. 324 00:32:45,000 --> 00:32:48,275 This one has sensed minute electrical signals 325 00:32:48,310 --> 00:32:51,793 from the salmon nearby. 326 00:32:51,827 --> 00:32:54,310 Shark can maintain their blood temperature 327 00:32:54,344 --> 00:32:58,620 at a higher level than the surrounding sea water, 328 00:32:58,655 --> 00:33:02,068 and that means they have the energy to be quick. 329 00:33:07,586 --> 00:33:09,896 Quicker, even, than salmon. 330 00:33:29,793 --> 00:33:33,000 Early autumn in Vancouver Island in Canada, 331 00:33:33,034 --> 00:33:36,172 600 miles to the south. 332 00:33:36,206 --> 00:33:39,586 The ocean temperature is slowly dropping. 333 00:33:39,620 --> 00:33:43,275 40 meters below the surface, this year's baby herring 334 00:33:43,310 --> 00:33:48,310 are feeding on the last of the summer's plankton. 335 00:33:48,344 --> 00:33:52,275 Their movements attract attention from the skies above. 336 00:34:00,206 --> 00:34:05,206 Gulls can't dive, so, for now, the fish are still safe. 337 00:34:08,482 --> 00:34:11,068 But there are birds which can dive. 338 00:34:14,068 --> 00:34:17,758 Auklets and murres swim effortlessly down 339 00:34:17,793 --> 00:34:20,620 beneath the school. 340 00:34:20,655 --> 00:34:24,137 The panicked herring are forced towards the surface. 341 00:34:32,655 --> 00:34:38,275 They gather into a giant defensive ball of swirling fish. 342 00:34:38,310 --> 00:34:41,344 The commotion attracts yellowtailed rockfish. 343 00:34:44,793 --> 00:34:46,551 They, too, are hunters. 344 00:34:56,275 --> 00:34:59,344 The marauding fish scatter the herring. 345 00:35:08,586 --> 00:35:13,482 Repeated attacks split the ball into numerous smaller groups. 346 00:35:20,172 --> 00:35:23,689 Now, it's easier for the divers to keep the confused fish 347 00:35:23,724 --> 00:35:25,103 penned at the surface. 348 00:35:25,137 --> 00:35:27,862 And there, even the gulls can get at them. 349 00:35:44,758 --> 00:35:46,517 Attacked from all sides, 350 00:35:46,551 --> 00:35:49,655 the little fish have virtually no chance. 351 00:35:52,310 --> 00:35:55,758 More and more divers are attracted to the scene. 352 00:35:59,793 --> 00:36:02,103 They harry the shrinking numbers of herring 353 00:36:02,137 --> 00:36:05,206 right down to the very last individual. 354 00:36:22,137 --> 00:36:26,620 Far bigger predators cruise here, too. 355 00:36:26,655 --> 00:36:29,310 Pacific white-sided dolphin. 356 00:36:32,137 --> 00:36:35,758 But the dolphin are mainly nocturnal hunters. 357 00:36:35,793 --> 00:36:40,310 During the day, they concentrate on socializing. 358 00:36:40,344 --> 00:36:43,827 They display by releasing streams of bubbles, 359 00:36:43,862 --> 00:36:45,551 and they play games. 360 00:36:48,655 --> 00:36:52,172 Games like Pass the Seaweed, for example. 361 00:37:25,551 --> 00:37:28,620 Exactly eight months ago, in the winter, 362 00:37:28,655 --> 00:37:30,689 off the west coast of Scotland, 363 00:37:30,724 --> 00:37:35,689 an egg was laid, and securely fixed to a strand of kelp. 364 00:37:35,724 --> 00:37:39,551 Inside, a tiny embryo started to develop. 365 00:37:48,344 --> 00:37:50,827 Protected by the tough egg case, 366 00:37:50,862 --> 00:37:54,689 it endured the worst of the winter storms. 367 00:38:03,000 --> 00:38:05,896 By summer, it was half grown. 368 00:38:17,068 --> 00:38:19,758 Now, at last, in the late autumn, 369 00:38:19,793 --> 00:38:23,413 it's nearly ready to hatch. 370 00:38:30,965 --> 00:38:34,448 A fully formed miniature shark swims free. 371 00:38:34,482 --> 00:38:36,827 This year's plankton will soon die, 372 00:38:36,862 --> 00:38:39,241 but the young dogfish is so well-developed 373 00:38:39,275 --> 00:38:42,482 that it can hunt immediately for larger prey. 374 00:38:45,689 --> 00:38:47,965 The days are getting shorter now. 375 00:38:50,000 --> 00:38:54,172 In British Columbia, the water begins to chill. 376 00:38:54,206 --> 00:38:55,965 This bizarre-looking creature 377 00:38:56,000 --> 00:38:59,862 is searching through the kelp for food. 378 00:38:59,896 --> 00:39:03,931 This is Melibe, the hooded sea slug. 379 00:39:07,620 --> 00:39:10,931 It catches plankton with its net-like head. 380 00:39:20,724 --> 00:39:25,103 As winter approaches, plankton is becoming scarce, 381 00:39:25,137 --> 00:39:28,068 but Melibe is an assiduous searcher. 382 00:39:31,551 --> 00:39:32,724 It can swim. 383 00:39:38,448 --> 00:39:42,137 It flaps away to look for a better feeding spot, 384 00:39:42,172 --> 00:39:45,310 but its search is becoming increasingly difficult. 385 00:39:45,344 --> 00:39:46,931 Within the next two weeks, 386 00:39:46,965 --> 00:39:49,551 most of the plankton will have disappeared. 387 00:40:01,241 --> 00:40:04,896 As the sunlight becomes ever more feeble, the kelp starts to die, 388 00:40:04,931 --> 00:40:08,758 gradually rotting away to nothing. 389 00:40:40,241 --> 00:40:42,551 Soon, it will be winter. 390 00:40:45,586 --> 00:40:48,310 But 9,000 miles to the south, 391 00:40:48,344 --> 00:40:52,103 the sun is rising on a new spring day. 392 00:40:54,206 --> 00:40:57,310 The Southern Hemisphere, too, has temperate regions, 393 00:40:57,344 --> 00:40:59,586 and now, the plankton is beginning to bloom 394 00:40:59,620 --> 00:41:01,137 around Tasmania. 395 00:41:03,655 --> 00:41:06,724 Just as in the north, the southern seasonal seas 396 00:41:06,758 --> 00:41:11,517 also have areas of rich, green water, 397 00:41:11,551 --> 00:41:14,275 with their own kelp forests... 398 00:41:16,655 --> 00:41:19,689 ...and their own swarms of plankton. 399 00:41:27,310 --> 00:41:29,344 Some of the inhabitants of these seas 400 00:41:29,379 --> 00:41:32,827 live only in the Southern Hemisphere. 401 00:41:32,862 --> 00:41:35,793 This is one of them-- the handfish-- 402 00:41:35,827 --> 00:41:39,000 that strolls around on modified fins. 403 00:41:44,586 --> 00:41:49,034 But when needs must, it can resort to tail power. 404 00:41:53,310 --> 00:41:54,758 Every summer, 405 00:41:54,793 --> 00:41:57,517 visitors come to the shallows around Tasmania. 406 00:41:57,551 --> 00:42:01,724 These are Australian squid, about half a meter long. 407 00:42:05,000 --> 00:42:06,931 They are here to breed. 408 00:42:12,586 --> 00:42:15,862 The larger males compete for the attentions of a female, 409 00:42:15,896 --> 00:42:18,413 displaying towards her and to one another 410 00:42:18,448 --> 00:42:20,724 by putting on a ballet, during which 411 00:42:20,758 --> 00:42:24,137 they continually change the color of their costumes. 412 00:42:53,793 --> 00:42:55,758 Eventually, they form pairs. 413 00:43:02,896 --> 00:43:07,137 A male passes a packet of sperm across to the female. 414 00:43:12,000 --> 00:43:14,103 After the eggs have been fertilized, 415 00:43:14,137 --> 00:43:17,379 they are deposited in tough, rubbery egg cases 416 00:43:17,413 --> 00:43:19,827 that other creatures find poisonous. 417 00:43:22,896 --> 00:43:24,344 Within three weeks, 418 00:43:24,379 --> 00:43:26,793 the babies are ready to hatch out. 419 00:43:32,689 --> 00:43:36,068 They are already able to change color, 420 00:43:36,103 --> 00:43:38,620 but they're not quite so good at swimming. 421 00:43:56,448 --> 00:43:58,275 There's another animal here 422 00:43:58,310 --> 00:44:01,551 that is a rather more devoted parent. 423 00:44:01,586 --> 00:44:05,137 This is a male leafy seadragon, 424 00:44:05,172 --> 00:44:08,793 an exquisitely decorated relative of the seahorse. 425 00:44:11,068 --> 00:44:14,965 He is carrying his partner's eggs around with him. 426 00:44:15,000 --> 00:44:17,310 They would be a highly nutritious snack 427 00:44:17,344 --> 00:44:20,793 for any predator that found them, 428 00:44:20,827 --> 00:44:23,965 but that's not easy, because the eggs are attached 429 00:44:24,000 --> 00:44:26,965 to their father's perfectly camouflaged body. 430 00:44:27,000 --> 00:44:29,586 They could scarcely be in a safer place. 431 00:44:56,000 --> 00:44:59,413 By November, 10,000 miles to the north, 432 00:44:59,448 --> 00:45:02,034 winter has arrived. 433 00:45:02,068 --> 00:45:05,448 Norway now gets less than five hours of daylight 434 00:45:05,482 --> 00:45:07,482 in every 24. 435 00:45:07,517 --> 00:45:10,275 The temperature is falling rapidly. 436 00:45:12,206 --> 00:45:16,655 But despite the cold, the sea is far from deserted. 437 00:45:16,689 --> 00:45:20,862 Every winter, 500 million tons of adult herring 438 00:45:20,896 --> 00:45:23,517 seek shelter in these deep waters. 439 00:45:26,482 --> 00:45:28,758 They will stay here for four months, 440 00:45:28,793 --> 00:45:31,275 living entirely on the fat they accumulated 441 00:45:31,310 --> 00:45:33,689 during the summer feasts of plankton. 442 00:45:36,034 --> 00:45:39,172 But they are not alone. 443 00:45:39,206 --> 00:45:40,517 Orca. 444 00:45:40,551 --> 00:45:43,241 This small pod is part of a population 445 00:45:43,275 --> 00:45:45,517 of some 500 killer whales 446 00:45:45,551 --> 00:45:49,275 that specialize in hunting North Atlantic herring. 447 00:46:03,344 --> 00:46:05,931 Using their echolocation, they have detected 448 00:46:05,965 --> 00:46:10,103 a shoal of herring 50 meters below them. 449 00:46:10,137 --> 00:46:12,655 With enough air for a 10-minute dive, 450 00:46:12,689 --> 00:46:15,241 they swim deep below the herring, 451 00:46:15,275 --> 00:46:17,310 and drive the fish upwards. 452 00:46:23,862 --> 00:46:26,310 Bubbles stream from the rising fish 453 00:46:26,344 --> 00:46:29,931 as gas in their swim bladders expands and escapes. 454 00:46:35,896 --> 00:46:41,448 Even an orca finds it difficult to catch a healthy herring, 455 00:46:41,482 --> 00:46:45,000 but they have a devastating weapon all their own. 456 00:46:52,448 --> 00:46:54,862 They use their tail to club the fish 457 00:46:54,896 --> 00:46:57,068 with waves of water pressure. 458 00:47:04,862 --> 00:47:06,448 Then, it's simply a matter 459 00:47:06,482 --> 00:47:09,344 of collecting the stunned casualties. 460 00:47:24,344 --> 00:47:26,586 The herrings have no chance, 461 00:47:26,620 --> 00:47:30,241 and both orca and gulls will eat as much as they can 462 00:47:30,275 --> 00:47:33,310 every day for the next four months. 463 00:47:38,344 --> 00:47:40,758 But there are so many fish wintering here-- 464 00:47:40,793 --> 00:47:43,068 over five billion individuals-- 465 00:47:43,103 --> 00:47:46,103 that the losses are almost unnoticeable. 466 00:47:54,862 --> 00:47:57,034 Violent as this winter weather may be, 467 00:47:57,068 --> 00:47:59,103 it's essential for the renewal 468 00:47:59,137 --> 00:48:02,724 of the riches of the seasonal seas. 469 00:48:02,758 --> 00:48:04,448 Out in the open oceans, 470 00:48:04,482 --> 00:48:09,000 the surging waters stir up nutrients from the depths. 471 00:48:14,103 --> 00:48:15,724 By the end of winter, 472 00:48:15,758 --> 00:48:18,689 the seasonal seas will be full of minerals once more, 473 00:48:18,724 --> 00:48:20,862 ready for the return of the sun 474 00:48:20,896 --> 00:48:23,931 and the next great plankton bloom.