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A Study of the Marine Spiny Crayfish Jasus lalandii (Milne-Edwards) Including Accounts of Autotomy and Autospasy

Discussion

Discussion

Before this study was carried out, the only published information available on Jams lalandii in New Zealand waters was that of Parker (1889), on the skeleton, and Young (1926), on the growth rate of a small crayfish in captivity. It is considered that, although the results presented here are based on limited data and are likely to be considerably modified by future work, an increase in the knowledge of several aspects of the crayfish population has been achieved.

Most of the crayfish caught in Wellington waters were taken in supplejack pots baited with fish-heads. In Australia, pots and bait of a similar kind are used. Iron hoops with netting bags suspended from them are used in South Africa. The bait is placed in the centre of the ring, and when the hoop settles on the bottom it lies on the top of the net and in the middle of the ring. Crayfish crawl over the bait, and when the net is raised they are held in the netting bag.

In the Wellington area, crayfish are caught mainly in-shore on the rocky parts of the coast, but trawlers occasionally take large catches in the trawl nets in deeper water. The in-shore fishery is centred mainly at Island Bay. Motor-launches carrying two or three men are used, and about twenty pots are employed by each boat. There is considerable variation in the size of the catches throughout the year, the heaviest catches being taken in September, when large males appear in considerable numbers in the catch. There appears to be some correlation between the increasing water temperature from August on and the release of larvae which begins about the end of September.

page 26

The sex and size composition of the catches in 1948 was approximately the same as in 1947. The catches consist of males from 17.9 cm. to 48 cm. (total length) and of females from 17 cm. to 40 cm. As the mesh of the basket seems to be small enough to hold crayfish smaller than 17 cm., it appears that specimens of this size are either absent from the fishing grounds or are not attracted to the bait used. Since females do not become mature until at least 19 cm. in total length, individuals between 17 cm. and 19 cm. must be protected by law to ensure the conservation of stocks. The smallest crayfish found by Hickman (1945) in a study of 1,058 specimens in Tasmania over a period of five years were a male and a female 17 cm. in total length, which agrees with the above. The largest female was 32 cm. and the largest male was 40 cm. It is to be concluded that both males and females in Wellington waters reach a greater size than they do in Tasmania, and that, in this species, males reach a greater size than females.

The size of males most often caught varies from month to month. This is probably due in part to the different localities fished, but the large number of very large males taken in September leads to the conclusion that large males, absent from the fishing grounds at any other time, move into the fishing areas in September. Few large males are taken in October, which indicates that their stay is very temporary. This habit may be related to some food becoming temporarily available, possibly as a result of the warming of the water which begins in late August. It cannot be due to movement for moulting, since, for males, this occurs in December, January, and February. In any case, if males were moulting, they would not enter the pots very readily, since in the soft-shelled condition they remain in seclusion to avoid predators. The size-range of the females most often caught does not vary greatly throughout the year, which indicates a fairly stable population.

The ratio of the sexes in the catches varies considerably from month to month. Females predominate in the December, January, and February catches. This is probably due to the moulting of males which occurs at this time, for, in a soft-shelled condition, crayfish are inactive. Von Bonde and Marchand (1935) found that moulting in males occurs mainly between November and February, and they consider that this is the reason for the high proportion of females at this time. The percentage of females in May is small. Since "berried" females first appear in May and since moulting precedes egg-extrusion by a few days (Von Bonde, 1936), it is considered that these two phenomena are responsible for the low percentage of females in the May catches. In April, so few crayfish may be obtained that fishing is not worth while. This also is probably a consequence of moulting, copulation, and egg-extrusion in females and copulation in males. In June, July, and August, there are approximately equal numbers of males and females caught. In September, the proportion of males increases, probably as a result of an in-shore movement of large males, as suggested above. Another possible contributing factor is an inactive condition of females just before and during the larval release. In October, when larval release is complete in many specimens, females page 27 appear in large numbers, and in November, when larval release is complete in almost all of the females and when males are beginning to moult, females are present in even larger numbers. In December, when all females have finished larval release and when many males are moulting, females predominate in the catches.

Although the present legal size-limit of 9 in. in total length appears to have been based on little research, it appears to have been a reasonable choice. Firstly, some females become mature when about 7½ in. in total length, and all are mature when they reach 10 in., so that the present restriction, although not protecting all the immature specimens, does protect most. This will tend to conserve stocks. The amount of usable meat in specimens below the limit is very small compared with the amount in larger specimens. One of the main reasons for this is that leg meat is too difficult to remove from small crayfish and consequently is used by few people. The leg meat of crayfish slightly larger than 9 in. can readily be extracted and comprises one-third of the usable meat. Small crayfish have relatively less tail meat than larger ones. For these two reasons it is wasteful to take crayfish smaller than about 9 in. in total length. It is far more economical to leave them to grow to a size where they will provide a reasonable amount of meat. At the same time this must not be carried to an extreme, for small crayfish make the best use of the available food as far as converting it to meat is concerned. The other important factor to be taken into consideration is the effect that size restrictions have on the fisherman's income. Under the 9 in. limit, the fisherman has to reject 8 per cent. by weight of his catch, which is not a very great hardship. It must be noted, however, that raising the limit another inch would necessitate the rejection of 15 per cent. of the catch.

Tagging operations in South Africa have not pointed to any purposive migration. Considerable movement occurs, but of the haphazard type to be expected from random wandering in search of food. The limited results of the present investigation (nine specimens returned) indicate a similar type of movement, and it is probable that no definite migration occurs. At the same time, the fact that two females, tagged and released on the same date, eluded recapture for a year and then were caught in September within a few days of each other suggests that they moved initially to some area not fished (possibly deep water) and then returned to the fishing area (in-shore) in preparation for the larval release which begins in September. This is, however, based on only two returns, and may not be confirmed by future work.

Some females become mature when 7 cm. in carapace length; others remain immature until 9.7 cm. in carapace length. The smallest mature female reported by Hickman (1945) was 7.2 cm. in carapace length. The agreement of these two sizes is to be expected, since Tasmanian and Wellington waters have similar temperatures. The smallest mature female reported by Von Bonde (1936) was 4.4 cm. in carapace length. This indicates that in South Africa, where the water is warmer, maturity is reached at a smaller size. This agrees with Temple page 28 man's finding (1936) that maturity in the North American lobster is attained at a smaller size in warm waters than in cold. The largest females caught appear to be still mature, and there is no evidence of loss of fertility in old females.

No females carrying eggs are found in March, but in May nearly 100 per cent, of the females are "in berry." No fishing is done in April, so that no data for this month are available, but since nearly all the females are "in berry" in May, it is probable that egg-extrusion begins in April. It occurs in Tasmania at the same time. Most of the females release larvae in October, but a few are still found "in berry" in December. All are free of eggs by the end of December. Hickman (1945) found that most of the females became free of eggs by the end of September and that all were free by the end of December, which is in agreement with my results. Larval release may be correlated with the period of increasing water temperature which begins in late August. There is evidence to indicate that large females extrude their eggs before small ones, since they are the first ones found "in berry."

The number of eggs produced increases approximately as the cube of the length. Von Bonde reached this conclusion, but the number of eggs increases more rapidly with length in South Africa than in New Zealand. For the size-range 8.3 cm. to 13.5 cm. carapace length, the number of eggs ranges from 86,000 to 549,000. This is in agreement with Hickman's figures (1945) of 65,170 to 413,220 for the carapace size-range of 7.4 cm. to 12.4 cm., but is considerably different from Von Bondes' results of 3,000 in the smallest to 200,000 in the largest (no sizes given). The conclusions of all three workers are at a variance with Challenger, who states that an average specimen carried 850,000 eggs. This "average" specimen was actually very large (12.7 cm. in carapace length), yet it is 1.2 cm. smaller than the largest examined in this investigation and which carried only 549,000 eggs. It is only 0.3 cm. larger than the largest examined by Hickman and which carried only 413,220. Challenger does not describe his method of counting in any detail, and his figure is much higher than those of other workers, so that the accuracy of his method may be in doubt.

One hundred and sixty-eight crayfish were tagged and released at various points around the coast. Since tags attached to the exoskeleton are lost during moulting, tags which were inserted through the exoskeleton and embedded in the deeper tissues were used; 5.4 per cent. have been returned. Eight of these were tagged with the xylonite tag described in the text, and one of these had passed through a moult. In all those that had been liberated for more than about six weeks, a chitinous pocket had formed around the tag. This appears to protect the surrounding tissues. Accordingly, this type of tag seems to be satisfactory. Only one of the specimens tagged with the wire type of tag has been returned. This had moulted, and the tag was embedded in chitin so that, judging from this single return, this type of tag is also satisfactory. At the same time, it is inferior to the xylonite type because its position on the ventral aspect makes it difficult to see. Von Bonde found that this tag was retained in moulting in aquaria. In both types page 29 of tag, the presence of the chitinous sheath indicates that the tag may be cast off with this chitin at a later moult. These tags seem to be efficient, but proof that they are retained for more than a few moults is wanting.

The tagging results have given data on migration, which is discussed above, and on growth rate. A male, 21.5 cm. in total length when released, grew 2.1 cm. in total length in one year. A female, 24.8 cm. in total length when released, grew 1.9 cm. in the same time. Von Bonde has shown that specimens of about this size moult twice a year, so that the average growth per moult for specimens of this size is about 1 cm. in total length. Templeman has found that, in the North American lobster, males and females grow at about the same rate until the female becomes mature, when the rate of growth of females becomes less than males. The female mentioned above was mature, so that it is not surprising to find that it did not grow as much as the male. Young observed that a specimen 4.2 cm. in total length moulted eight times in three years and two months in an aquarium. The size achieved at the end of this time was 14 cm. The average growth a year was about 3 cm. The fact that the yearly growth of this specimen was greater than that of the tagged specimens described above is to be expected, since small animals grow rapidly, but as they grow older the rate of growth decreases.

The amount of usable meat in Wellington crayfish increases from about 47 gm. in the smallest (6.7 cm. carapace length) to 1,449 gm. in the largest (19.6 cm. carapace length). The weight of usable meat per unit carapace length ranges from 6.7 gm. to 74 gm. over the same size-range. Leg meat is so difficult to extract from specimens smaller than about 9 cm. in carapace length that few, if any, people would take the trouble to extract it. In specimens slightly larger than this (9.1 cm. carapace length), 78gm. can be extracted quite easily, which represents one-third of the total meat available. Accordingly, it is wasteful to take specimens smaller than about 9 cm. carapace length. This is about the same as the legal limit, so that one effect of the size restrictions is to prevent the wastage of potential leg meat.

One half of the crayfish caught are between 9 cm. and 13 cm. in carapace length, and these provide one-half of the usable meat in the catch. This is the most important size-group in an economic fishery, and the fact that specimens in this size-range comprise 50 per cent, of the catch indicates that the fishery is an economic one.

In the event of legal action being taken under the minimum-size regulations on the basis of measurements of a catch after cooking, it could be argued that the crayfish were above the minimum size when caught and had shrunk in cooking. Accordingly, it is of value to know whether any appreciable shrinkage does occur. A slight shrinkage was found, but it is too small to be considered. The total length may either increase slightly or decrease slightly, depending on the state of relaxation at death of the muscles connecting the abdomen with the cephalothorax. The loss of weight is considerable, and increases with size.

Some of the information essential for the proper exploitation of the crayfish page 30 population has been presented for the first time. Being based on limited data, it is only a start, and it is hoped that further work will be done in the near future so that the New Zealand crayfish industry may be placed on a sound basis.