Population Composition

Good weather and continuous fishing activities allowed the examination of large samples in January, February, July, August, and September. In April, fishing is suspended, since only very small catches are obtained. This is probably due to moulting, copulation, and egg-extrusion at this time. In late March and early May, no fishing is done for the same reason, Consequently, in the two years of this study, page 6 few specimens were examined in March, none in April, and few in May. A total of 2,126 crayfish were measured and sexed, and size-frequency graphs were constructed each month (see Fig. 4).

Since the samples are taken at random, these analyses must approximate the true composition of the population each month. The size-groupings are in 2 cm. of total length as measured from the tip of the rostral spine to the end of the telson. Fig. 5 is the analysis of the whole sample for the two years.

Examination of Figs. 4 and 5 shows:—

(a) Size ranges: The samples are distributed over different size-ranges at different times. The ranges in the smaller samples will only roughly approximate the true size-range of the population in those months. For the months in which larger samples have been obtained, the size-range given must be fairly close to the true size-range of the population. It is apparent that the population in the fishing area is composed of males up to about 48 cm. and females up to about 40 cm. The maximum size in the males may be greater than this, because the entrance aperture in the fishing baskets may be too small to allow the entry of crayfish larger than about 48 cm. This cannot be so with the females, since 40 cm. must be near the maximum size, for, if males of 48 cm. can enter the pots, it would also be possible for females larger than 40 cm. to do so.

The conclusion is that few, if any, females larger than 40 cm. are found in this area and that males larger than 48 cm. may occur, but, if they do, they are rarely caught. Trawler fishermen operating in deep water report that their catches consist entirely of "outsize males," and no females are ever caught. It may be said, then, that males grow to a greater size than females.

Hickman (1945) examined 1,058 crayfish in Tasmania over a period of five years. Of these, 485 were males and 572 were females. The largest male was 15.9 cm. in carapace length (about 40 cm. total length), and the largest female was 12.4 cm. in carapace length (about 32 cm. total length). This indicates that in Tasmania also males reach a greater size than females.

It appears that the largest males taken off Wellington are caught between July and September. This suggests the possibility of an in-shore movement of large males at this time. This is further discussed on pages 7 and 9. The smallest crayfish caught was 17 cm. The mesh of the pots ranged from 2.5 cm. to 5 cm. As it is most unlikely that specimens even considerably smaller than 17 cm. could escape from as small a mesh as this, it appears that small crayfish are either absent from the fishing grounds or are not attracted by the bait used. It is interesting to note that the smallest specimen reported by Hickman (1945) from the examination of 1,058 specimens in Tasmania was 6.3 cm. in carapace length—i.e., about 17 cm. total length. He does not state the size of the mesh used.

(b) Size-range of greatest frequency: Different months see different size-groups taking prominence in the catches. In males, the size most often caught is larger in September than at any other time. The increase from August to September and page 7 the decrease from September to October is very sudden and striking. This suggests a September movement of large males towards the fishing grounds. The return of the mode back to 20.1 cm. to 22 cm. in October would indicate that the large males have moved away again. Consequently, their stay is very temporary. It may be connected with some food becoming temporarily available, possibly as a result of the warming of the water which begins in late August (see Fig. 3).

The size of greatest frequency in the catches of females does not differ significantly from month to month. This indicates a stable female population.

(c) Proportion of males to females: The ratio of the sexes in the catches varies from month to month. Table 1 gives the percentage of females in each month's sample.

Table 1

Month	Size of Sample	Percentage of Females
January	256	79.7
February	375	79.2
March	49	65.3
May	29	27.6
June	124	52.4
July	421	49.2
August	266	51.5
September	270	24.8
October	83	39.8
November	83	45.8
December	170	65.3

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page 9

Males and females often segregate temporarily in small, distinct, and adjacent areas. Since the pots are usually distributed over a considerable area each day, there is just as much chance of trapping crayfish from the male areas as from the female. Accordingly, this segregation would not greatly affect the ratio of the sexes in the catch; some other phenomena must account for the different sex ratios shown in Table 1. The high percentage of females in January and February is probably due to the moulting of the males, which, according to Von Bonde and Marchand (1935, p. 23), "does not correspond completely with the female season in any particular area, and therefore during the male ecdysis the crawfish population consists of a majority of females." Predator pressure during the soft-shelled stage sends the crayfish into hiding. Consequently, they do not enter the pots at this time. Wellington fishermen report having caught an occasional soft-shelled crayfish in December and January. Unfortunately, the sex of these is unknown, but, since males are known to moult before females (Von Bonde, 1936, p. 9), which moult in or about April, it is probable that they were males.

Although the small size of the May sample cannot give a very true picture of the population in this month, there is an indication of an increase in the percentage of males. This is probably due to the moulting and egg-extrusion occurring in the female population. Hickman (1945, p. 29) says: "In April the catches consist largely of males. This is no doubt due to the fact that the females are preparing to lay their eggs, and therefore do not enter the pots." Since moulting in females immediately precedes egg-extrusion (Von Bonde, 1936, p. 10), moulting must also play a part in reducing the number of females caught in or about April.

September sees a sudden increase in the proportion of males, the result of the appearance of large numbers of large males indicating a possible movement of larger males towards the fishing grounds at this time (see pages 6 and 7). Moulting in males is known to occur in December and January (see above), and may begin before this. This would account for the decreasing percentage of males from October onwards shown in Table 1.