Introduction

In a previous paper (Cassie 1955, Table 28) a table has been given of the estimated mean growth rate of the snapper in the Hauraki Gulf. This has been derived from a series of data collected principally during the years 1949 to 1953. All estimates of the size-age relationship were plotted on a common graph, and a smooth curve was drawn by eye to give what appeared to be the best fit, making due allowance for the varying degrees of reliability of different sets of information. While it is believed that this table is sufficiently accurate for the purposes for which it has been employed (a tentative estimate of optimum mesh size) it is evident that a more critical appraisal must be made both of the methods of age determination and of the results obtained. The figures already given will doubtless form a convenient reference until better are obtained, but are of little value in more critical work where the comparison of growth between different regions, seasons, races or year classes is desired. In the present communication therefore the main objects are: to assess the techniques employed, indicating the errors inherent in them; to present those estimates for which the reliability has been reasonably well established; to indicate the lines along which future investigations might profitably proceed.

Four methods are commonly used for the determination of age in fish:

Since each of the methods is subject to its own peculiar sources of error, it would be desirable to employ all four, but in the present case it has only been possible to obtain results from the first two A number of snapper have been marked in the Hauraki Gulf and other areas, but no returns have yet been obtained. This may be due in part to the fact that the snapper seems to be particularly susceptible to damage by the trawl, which is otherwise the most convenient method of taking these fish in adequate numbers. Even in shallow water and with very short tows there is

a tendency, particularly among the larger fish, for the body cavity to become distended with gas so that the fish has difficulty in diving to the bottom again Even those which are not obviously incapacitated are susceptible to attack by sea birds and (presumably) by predatory fish. No facilities are at present available for stocking and maintaining a suitable aquarium in the Auckland region for the keeping of captive snapper.