This heralds the beginning of the breeding season for most of the life in the sea. The young of the fish, crabs and smaller invertebrates feed on this new life. This also means that a visit to a rocky shore in March can be the most interesting of the year, because rock pool fish like the Common Blenny, Lipophrys pholis, and the sea slugs venture onto the shore to breed, laying eggs on rocks in pools and on the underside of rocks immersed in water on the lower shore.

The tides in late March are amongst the largest of the year. The largest variations in the month occur over the weekend of the 8th and 9th. Four days after the Vernal Equinox that occurs on 20 March 1997 the low spring tides will recede a long way and if you are lucky with the weather, this is one of the most interesting times to explore the margins of land and sea for the short time that the tide is out.
 
 

A simple enough question on first thoughts of the Common Goldfish, but aquarist know that fish come in all sorts of shapes and sizes, and that many animals like the shellfish, crayfish and starfish are not fish in the normal use of the word. And I have heard that sharks are not really fish at all! This statement should not be dismissed out of hand, as the sharks, rays and related animals evolved into existence as a class of aquatic vertebrates called the Chondrichthyes, with a cartilaginous skeleton, about 360 million years ago. True fish have a skeleton made of bone and are classified in a class called Osteichthyes.

These are the fish kept by 99 per cent of aquarists.

The word 'fish' is not a technical scientific term, but a general word that can be defined as 'a limbless vertebrate with fins and gills that swims in water'. 
 

Corkwing Wrasse are fish of the kelp forest and weedy areas. They are experts at sliding through dense weed cover. Most of the time they search around rocks for hidden crustaceans, propelling themselves forward with gentle flicks of the pectoral (side) fins. When alarmed the pectorals beat rapidly and the long dorsal is folded down to reduce drag as the wrasse dives for the nearest cover. The books say the tail also swishes rapidly, but I have been unable to observe this in aquaria.

Shellfish is another non-technical term that includes a motley collection of molluscs and crustaceans that can be eaten. The crayfish is a crustacean and the starfish is an echinoderm and all these invertebrates are not really fish at all.
 
 

How does a fish swim? Now, this is really rather important to aquarists especially when they are compelled to explain or defend their hobby against detractors. Why is the aquarist not satisfied with a collection of glass-framed pretty pictures on the wall? The answer is again obvious although not often put in words.

The aquarist is interested in the movement and poetry of fishes, and other aquatic creatures, as they go about the normal day to day business.

Aquarists will know that some fish will swim at the surface, some in mid-water and many of the shore fish will spend most of their time on the bottom. The same is, of course, true in the great depths of the oceans. Bony fishes maintain their buoyancy by the use of an organ called a 'swim bladder' which they use to regulate the amount of gas and consequently adjust their position in the water column. This saves a lot of energy and has enabled the development of the diverse forms of bony fish. Sharks and rays lack a swim bladder and have to swim continuously. Also, many of the bottom dwellers also have no need for a swim bladder as they come to rest on the sea bed or the floor of the aquarium.
 
 

Actual moving about requires some form of propulsion. It is fascinating to watch how the different fish actually achieve this. Water is a very dense medium, about 830 times as dense as air, and has a viscosity about 60 times that of air. This means that marine life does not require the supporting skeletons of land dwellers, but water is more difficult to swim through than walking about on land. Because of this, fish and other marine animals have developed shapes that minimise this resistance. This is seen clearly in efficient streamlined fish like the Mackerel, Scomber scombrus, that cleave through the water at great speed. 

Fish are not rigid: the body flexes to some extent in all fishes. This is demonstrated at the most extreme by the wriggles of the eels. At the other end of the scale, the Sea-horses, do not undulate their body at all, and they swim by using rapid movements of the dorsal fin. This fin is used for hovering rather than actively swimming. Fish with rather limited swimming abilities, or very small fish, are the ones that are suitable for aquaria.
 
 

Measurement of the maximum speed of fish is usually achieved by placing a shoal of fish in a measured current and increasing the speed of the current until the fish are unable to maintain a position against the flow. A Trout can swim at about 6 mph. Generally, the larger a fish the faster it can swim. However, the large fish are not suitable for tank measurement and estimates have to be obtained from observations in the wild. The swordfishes could be the fastest with the sword actually serving to reduce turbulence and with speeds over short distances exceeding 60 mph. However, the largest species of tuna like the Bluefin, Thunnus thynnus, has been recorded at 43 mph.

FURTHER READING:
Fish Swimming by John J. Videler (Chapman & Hall), 1993. ISBN 0-412-40860-0.