Coping with storms
story and photos by Roger Grace
Storms are dangerous. Violent weather puts under stress the physical environment in which we live. Our bodies are at risk from structures collapsing on us, or from materials moving around unpredictably and colliding with our fragile frames. At sea, a normally calm surface can become a boiling turmoil of huge waves and foam, smashing strong boats on the rocks or sinking them to the bottom.
Humans can often avoid being caught in storms. A large amount of modern technology is aimed at predicting weather patterns, and those of us who go to sea for our living take heed of weather warnings if we are wise.
But what of marine animals and plants? They have no weatherman to warn them of approaching storms. Even if they did, a lot of marine organisms are attached to the seabed, or can move only a very short distance. Only the fortunate few are capable of getting up and leaving at a rate which will allow them to move out of the path of the storm into deeper, quieter waters.
For marine animals and plants, the most violent sea conditions occur where the sea meets the land. Within a few metres of the shoreline is where all the energy stored up in waves and swells, which may have been wind-driven over hundreds of kilometres, is finally dissipated in one last crashing blow on the rocky shore or a surf beach. Yet this is where there is the greatest variety of marine life. The shoreline and shallow seabed just offshore house more species than any other marine habitat. And they all have to cope with what the sea can throw at them. They cope, or they perish.
Nature has devised an extraordinary range of mechanisms and techniques to help animals and plants cope with storms. It is really the frequency and violence of waves reaching a shore which determine the types of animals and plants which can live there.
On a rocky shore, the more frequent and violent the wave action, the tougher and more firmly anchored the animals and plants must be. On a wave-exposed sandy shore, animals must be able to burrow quickly when big waves churn up the sand and rip them from the security of their sub-surface existence.
The thinking observer will notice quite a difference in types and shapes of animals and plants living on wave-exposed shores, compared to sheltered ones. In sheltered areas, marine life can afford to be somewhat flamboyant or delicate in shape, because there are no violent waves to rip complex-shaped organisms from the rocks.
Big, complicated and flimsy clumps of oysters attach, not too firmly, to the mid-tide level on sheltered inner-harbour rocky shores. On a wave-exposed shore, such structures would be smashed to pieces in a moment. Instead you will find sheets of barnacles, firmly cemented and offering a low profile to the waves. Low flat cones of limpets hold firmly to the rock with a powerful foot. In some species the shape of the edge of the shell grows to fit precisely the shape of the rock surface on their permanent home site, so the waves cannot get under the shell and rip the animal from the rock.
Evolution has moulded shore animals and plants to forms that are appropriate to the degree of wave action experienced on a particular shore. A marine biologist can âreadâ a shore by looking at the animals and plants, and judge the degree of exposure to wave action experienced by that shore. He does not have to visit on a stormy day. The organisms themselves give a clear indication, simply by their shape and the way they occupy the shoreline.
A parallel situation exists in diving depths just offshore. Organisms living in shallow rocky areas subject to frequent heavy wave action are designed to attach very firmly to the rocks. The seaweeds are stuck to the rock by a strong holdfast, and their fronds are leathery and strong, able to rub over rocks and each other without risk of being torn away.
Move to a sheltered area behind a reef, and you will see a different range of seaweeds. Tall, delicately branching clumps grow up from the seabed, perhaps forming forests a couple of metres high.
Increasing depth tempers the violent movement of water, so that fine, delicate branching organisms become more common. Some of the most beautiful marine life lives in maximum diving depths at places like the Poor Knights. Tall yellow finger sponges and fan-shaped pink gorgonians cover the deep rock faces, with the fine branches of tiny moss-like bryozoans and hydroids filling the spaces between.
In contrast, up in the wave-swept shallows the organisms are either specially adapted like the leathery fronds of flap-jack weeds and bull-kelp, or are low encrusting forms like turfing coralline seaweeds, paint-like pink encrusting weeds, or thin sheet-like films of sponges and colonial ascidians carpeting the rock surface with a very low profile of life.
What of fishes? Are they too specialised for coping with waves? Some years ago I witnessed a graphic illustration of how the shape and behaviour of different fishes influence their ability to survive in a violent storm.
In the winter of 1978, a deep depression marched down the east coast of Northland and through the Hauraki Gulf. The resulting southeasterly storm devastated the coast. Some seven metres of sand was ripped away from the foredune at Mimiwhangata. A fishing boat was pulverised into matchsticks as it was forced through a narrow gap between two rock stacks on the end of Takatu Point. Waves surging high up the cliffs on the end of the point sluiced hundreds of flax plants into the sea. Flax fibre ended up tangled in everything. The spines of kina were coated in thick mats of it. Big boulders were overturned in three metres of water.
The farmer at Tawharanui collected several john dory and kahawai off his front lawn for breakfast. The upper beach at Mimiwhangata was littered with hundreds of goatfish or red mullet, numerous leatherjackets, kahawai, and john dory. Conspicuous by their absence, however, were the crevice and hole-dwelling reef fish. There were no red moki, no scorpionfish, no kelpfish or marblefish. These species survived by wedging themselves into their holes and crevices.
After the storm died away, I dived in areas I was familiar with, and all the reef fish were still there, but many of them showed signs of the storm. Red moki in particular suffered many broken dorsal fin spines. Big scrapes on their sides had lost a lot of scales. With the protective coating of mucus gone from damaged areas, small green seaweeds were starting to grow on the broken spines and damaged sides. One red moki looked so green along one side I dubbed him âGarden-gutsâ.
This incident clearly illustrated the importance of bottom topography to reef fish. Where a rocky bottom is flat or undulating with no crevices, reef fish are generally present only in low numbers. In highly broken rocky bottom areas, where there is an abundance of caves, holes, tunnels and crevices, reef fish are usually far more abundant. They need holes not only to escape predators, but to wedge themselves into when storms rage above. Without their cave and crevice homes, they would soon become exhausted battling the storm, and perish on the beach like their sand-dwelling counterparts.