CM_1976_Special_meeting_No_01.pdf (311.8Kb)

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Population Assessments of Shellfish Stocks No. 1

Catch and effort data of the lobster fishery in south eastern Norwegian waters during 1928-1975

by

Stein Tveite

Statcns Biologiske Stasjon Fl~devigen

4800 Arendal, Norway

Introduction

The'lobster (Homarus ^u gammaEus L,) fishery along the NorvJ'egian ' Skagerrack coast takes place during a short autumn season, mainly.

The spring catches has always b~en small, and during the last years they have been negligible. The lobster is quite stationary (Dannevig 1936, Tveite 1970) in these waters, and variations between local

stocks may occur. In this paper, however, mainly general trends

applicable f6r the whole coast are looked for. In order to have some measures of year to ^ye~!

collecting catch

ns of the stock, a program for data was started in 1928', Length measurements of samples~~ve alsq been made. Parts of this material have earlier been dealt with by Dannevig (1936, 1~51).

Material and Methods

Between .22 and c34 fishermen have ,yearly .. been ans"leringa_ question-=:o ⁰ naire. From 1928 to 1939 i t was sent to fishermen between area

Krager~ and Mandal. From 1940 onwards the Lista area was included and after 1962 two fishermen at Hvaler have been answering the

questionnaire (Fig. 1')', The informations collected were: Location of fishing, duration, no. of traps, no. of traps lost, no. of lobsters caught, weight og lobster, and bait used. The fishermen were also asked to judge the abundance of undersized lobsters and edible crabs.

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Length measuI'emcnts of both und~rsized and marketable lobsters has been made regularly since 1949.

The ten~erature of the sea water~pumped from 15-19 m depth at

Fl~devjgen,has been lDeasured every day. The averages for the t

mont]ls May to October are calculated (Fig. 3).

R(='sults

T1)e tot,'31 C'"tch of lob::;ters along the NorvJegian Skagc=orrack coast incredsed IJ urn 1928 to 1932 vJher'ealtcr the catclles became smallep unt i l 194 I+. Since 19¹47 lJH=:l'e ha s been a fa lling trend in t11e

commercial catches.

The catch per unit effort has been calculated as number of lobsters per trap pep day. By regression analysis this measure is found to be si;:snifj cantly dependent of t]je number of tr'aps us(::,d ( Jd of t]je length of the periode fished, in the way that in long se~sons the chance of catching lobsters is gradually reduced and many traps :ceduce the Plficiancy per trap. The l'cla-tionship bct,,7cen nlJmber of days fished and catch per trap per day can be seen from Fig. 2 where the data for one fisherman pal'tj cipating in the program from 1928 t i l l 19/¹+ al'e shoHn. However, Hhen the data from all f:i'Jshermen from the ,'Jhole area are added, the average number of traps and days did noi vary much from year to year. The effect of these factors should

therefore be,> at least partly elirilin,;ted. From 1928 onl,Jards the c. p. u. ( deci'eased t i l l 1935 (Fig. 3). Then the values were increasing until 19¹+ 5 .. After 19^t+ 5 the1'e .. has been a falling tr'end Ullt i l -the miniJ;)um Has obser'ved in 1971.

The weight per trap per day has decreased at a lower rate than the numbers during the last 15 years (Fig. 3). This is due to an

increased average w~ightof the lobsters, especially after 1964 when the minimu legal size was raised to 22 cm.

When the material is split in smaller areas,'random variations are dominating. To elucidate the trends of the -local stocks, successive five year lDeans for c.p.u.e. have been calculated and reproduced in Fig. 4. The same trerids as for the total material can be seen, more or less pronounced. In the eastern areas the average c.p.u.e. is higher than in the western (Table 1).

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"

Table 1. Average c.p.u.e. by number, for 11 years before and 11 years after the minimum legal size was raised.

Area 1 1952-63 .35 1964-75 .24

2

.22 .14

3

.28 .20

4

.18 .13

5

.17 .10

6

.19 .10

7

.12 .09

8

.19 .08

From 1928 there was an increase in the effort until 1935. It resulted in increased catches until 1932 (Fig. 3). Although quite high number of traps were used in 1933, -34 and -35 the catches went down. This is, however, the only periode when there is great discrepancy between the catch and the effort. From 1960 the catch went down to approximately one fourth in 1971 whereas the c.p.u.e. decreased to nearly the half.

The informations collected about the stock of edible crab have been given the valu~s 1, 2 or 3 according to whether the fishermen considered the stock to be small, medium ^o~big. A mean of these are calculated (Fig. 3). In 1962 and 1975 all fishermen considered the

~tock to be above "average". This average is simply judged by the fishermen and is therefore dependent of the catch the nearest years before. An average catch might therefore" be different in different years. Now and then information on the actual numbers of crabs

caught are given. These informations indicate that the raise in the late fifties should be even more pronounced than shown in Fig. 3.

AJ)undance of sub-legal sized lobst~rs has been judged the same way as crabs. ,From 1928 to 36 the fis"hermen consideT'ed the small _lobsters to be quite numerous. From 1937 to45 there were few in spi~e of

increasing c.p.u.e. of the legal sized lobsters. Then the numbers increased t i l l 1951. From 1951 onwards the stock showed a decreasing trend with quite big fluctuation~ (Fig. 3).

The length measurements (Fig. 51 confirms the observations on

average weight from the questionnaires. From 1949 onwards the length distribution w~s fairly wide, then gradually the maxima came closer to the minimum size." During the later years there again has been a wider distribution. The vari~tions were not equeal in all areas. In the Krager~ (2) area the length distribution has always been more even than in others. At Arendal (4) and partly at Mandal (7) the

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distributions were closer to the minimum size during the 1949 to 54 pe0iode than in the two other areas and at Hvaler (1) lobsters

bigger than 27 cm are comperatively rare. During the years 1930 to 1931 Dannevig (1936) reported length distributions clos~ to the

. . .

mlnllIlUlIl Slze.

Discussion

The data presented in this paper are found suitable to describe the general trends of the lobster fishery within the periode concl'rned.

However, for details and local variations a larger program would be necessary.

From 1928 onwards the lobsters were quite abundant. D~ring those years an increasing riumber of unemployed people started fishing. The lncrease in effort resulted in higher catches up to 1932 when in spite ~f further increased effort the total catch went down. The lobster stock was most heavily exploited during the periode from 1932 to 1938. The reduction in effort because of the war, lead to increased catch per unit effort in spite of poor recruitment to the stock. FI'om 1945 -to the eal'ly fifties tJle fishery were quite good, due to the positive recruitment situ~tion. The falling trend of the fishery the last_tvJenty_years has mainly _ been caused by low recrui t=

ment. This is shown bbth by the low index for sublegal sized lobsters and by the increasing weight of marketable lobsters. The effort shlwn on Fig. 3 has been almost as low during the last years as i t was

during the' war. The benefitial effect, however, has not re~ched the same level as during t~~se years, most probably due to a relative high ^lluno fficia1¹¹ effort during the later years.

Except for the fishery itself, the only relevant factors available for explanation of the variations are temperature and abundance of crabs. Perhaps the relative high summer tempel'atures from 1943 to 1950 have made some successful broods of lobsters possible. And the low temperatures after 1962 ~ight have been unfavorab1e for breeding.

The abundance of crabs has increased considerably since 1960. Although crabs and 10bste~s have somewhat different preferences for food and habitat, there might be competition when one of the stocks is big.

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In 1964 the minimum legal size was raised from 21 cm to 22 cm. This has made the Height of the catches lligher than they otheJ'wise would have been, because the recapture rate is very high around this size

(Tveite 1970). HovJever, whether the new minimum size have had any

f

influence on the reproduction is impossible ~o say from the present material.

J\(~ f erences

DANNEVIG, A. 1936.

DAW\lEVIG, A. 1951.

TVEITE, S. 1970.

Hummer og hummerkultur I

Fisk.Dir.Skr.Ser.HavUnders. 4 (12): 1-60

~---~---~~-~~---~---~---~--

Lobster and oyster in Norway.

R~_pp ~ P. -v. Reun. cons. perm. int. Explor. Her. 128:·

92·-96.

S~mmenliknende fors~k med to merkemetoder for hummer

Fisket~ Ga~g 56 (12): 21+7--248

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