Fishery biologists seek to accumulate knowledge covering the entire life of a species from egg to adult, including details of spawning areas and seasons, eggs and larval stages, fecundity, the factors that influence brood survival, nursery areas of young fish, their growth and maturation, feeding habits throughout life, changes in distribution at different stages of life and throughout the annual cycles of breeding and feeding, predators, diseases and all causes of mortality. The techniques of investigation vary according to the habits of particular fish, the nature of the area and the research facilities available.
If the fish spawn satisfactorily in aquaria, the early stages of life can usually be investigated in the laboratory. The eggs and larval stages are studied so that they may be readily identified when found at sea, and the effects on development of environmental factors such as salinity or temperature can be experimentally determined.
To discover areas and seasons of spawning, the landings of the commercial fishery may be examined. When the catch includes fish with fully ripe gonads, the regions from which such fish were obtained may be learnt from the fishermen. This gives a general indication of the probable sites of spawning, but for precise information it is usually necessary for research vessels to investigate the area. If the eggs are demersal, an attempt may be made to find them by dredging. If they are pelagic, tow net hauls are made at a large number of stations to determine precisely in which areas and at what depths the main concentration of eggs is found.
Standardized techniques of net hauling and egg counting have been devised for the calculation of the number of eggs in unit volume of water, or beneath unit surface area. The concentration of eggs at each sampling station can be plotted on a map, and lines drawn joining the points where equal numbers of eggs have been found. These 'egg density contours' indicate the zones and limits of the spawning area. As the spawning period proceeds, the areas of main spawning may shift, so these investigations need to be repeated at intervals. Such contour maps, based on the distribution of three-day-old eggs near the peak of the spawning process, are given in Lee and Ramster (1981; see Figure 9.18) for British commercial species.
Studies are also made of the fecundity of the fish of different ages, and in different areas. These may have a bearing on recruitment rates, and also provide some data which may be used for calculations of total population (see page 365).
During the pelagic phase of the life history, distribution and development of larvae and young fish can usually be studied from tow net hauls, but once the fish leave the surface water it is often difficult to find them, especially when they occur in rocky areas where fine-mesh trawls cannot easily be used. The biology of some species is not at all well known during this intermediate period between the end of the pelagic phase and the time when they become large enough to be caught by commercial nets.
Some information on feeding habits may be gained from observation of fish in aquaria, but this may not give a true picture of food preferences under natural conditions. The most reliable information on feeding habits comes from studies of the gut contents of fish captured at sea, but new techniques of underwater observation, such as free diving or underwater television, now open up new possibilities for the study of feeding and many other features of fish behaviour in natural surroundings.
Growth rates can seldom be determined by direct observation. The growth of fish in aquaria is not a reliable guide to growth in natural surroundings. Where tagging is thought to have no detrimental effect on the fish, measurements of tagged fish before release and after recapture provide some information about growth in the sea. In some cases, the width of growth zones between seasonal marks on meristic structures such as scales or otoliths is proportional to the overall growth of the fish. For example, the seasonal rings of herring scales give a close indication of the length of the fish in previous years (Figure 9.30). In most cases, however, average growth rates are determined by correlating size with age in an analysis of the stock composition (see below).
It is evident that individual fish differ greatly in their rates of growth. Intrinsic differences may be partly responsible for this, but growth rates also vary with locality due to environmental conditions, particularly the water temperature and the availability of food.
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