CM_1961_18.pdf (313.8Kb)

International Council for -ehe

Exploration of the Sea COrr'.parati ve F~s hing C ornmi ttee No. 18

The Selectivity of Halibut Gill Nets by

SQ Olsen & J. Tjemsland

Introduction

In 1936 gill nets were introduced in the Norwegian halibut fisheries, and in a short time a successful gill-net fishery for large pre-spavming and spayming; halibut developed in all major spawning areas.

Prel7'iously, the nets \fvere made of hemp, but in recent years a change to polyamide fibres has taken place. Until noYv the mesh size has been regulated by law not to be less than 20.88 cm between the knots (about lS!; mesh si.'Ze), but L.'1. Ju.."le -r..:l1is·

year this regulation 'Vvas changed to prohibit the use of mesh sizes less than about 18^ti• T'ne fishing is carried out in the ,!\linter and the nets are set in long fleets at the bottom in 240 to 300 fathoms depth.

Material ~'1.d Methods

During the years 1957 to 1960 experimental fishing 'With nets of differont mesh sizes and different materials 'Were conducted ;n January-Februar;:v in one parti!;cular area in. northern Norway. The nets used 'Were hemp and nylon nets of about lSH mesh size and nylon nets nith mesh sizes of approximately 19!! and 23!l. Measured under a tension of 4 kilos the LlGsh

sizes averaged 42.8 cm, 49.6 cm and 61.9 cm respectively" for the three different types of nylon nets.

The nets 'Were spread in groups of" tvvo to fiv·e nets of each -type throughout the fleet, and 'with changing positions of th:.; groups during the se2.son.· The type of nets for each individual fish Was recorded and the total length YJas measured. In 1959 and 1960 girth measurements 'Were also taken.

The data collected in 1957 and 1958 showed that observations on methods of

attacbment -VIlere needed and for the two last years ~ experiments records of such observations were provided. For this reason only the material collected in 1959 and 1960 has been used L.'1. this treatment.

In analysing the data the methods of attachment were grouped in 1) meshing by the operculum and point of greatest girth, and 2) all other methods of attachQent (i.oo meshed by the maxillae, attached by the teeth, entangled by the tail, ccmpletely embedded

in the net etc.).· Table ¹ below giV'es the numbers caugh"c as PGI' type of net and

attacr~ent method.

Table 1. Nurnbers of halibut caught in 1959 and 196c; specifi8d by type of net and method of s"ttachIllent.

-, ---~i ---~---~---~--- ---

, ! ____________ -.JJumbers caught :

1 I - - - -~--I----------~----,

i Type of net i Attacbment l i Attachment 2 T-~~mp 1~--·----r---·---~~---t---2;---+-

I

Nylon 16!!

I

145 44

Nylon 19^{11 j} 92 29

__ ~ylon ~3T! ^______

1 ______

~_

-_______ .l __________

25 _ _ _ _ .1.

Calculation of selection curv"es

For the meshed fish (attacbment group 1) selection curv'es for the "Gnree different mesh sizes of nylon nets were calculated following the method described by Holt (1957)~-

nl .::-:~ .. (~_. ~xp. - (1 - ID) 2 / 02

'Where: n

l is the number of £'ish of length 1 caught and

\n

is the mean selection 18ngth.

Table 2 ( page 4,) gives the length distribution and log. ratios TIhen adjusted for differences in effort (no. of nets fished)~ and in Figure 1 the log. ratios are plotted against length and lines fitted by the method of' least squares are dravm.

From the lines of best fit v'alues for K, Im and ()2 Viere computed~-

K = 2 b

1 <'l K· 9

Am A

and ^{er 2 -, 2}

a

1

=

K . Be Bm

where K is the ratio between mean se0;6ction length, 1 and mesh size, 8, and a and b are coefficients of the equation y

=

0.1 + b describin~ the line of best fit for the log.

ratios.

The following values for K were found~-

KBA ::: 3.325, KCA

=

3.133 and KCB = 3.154

The indices A, B and C refer to the mesh sizes 16!!, 19^t1 and 23!l respectiv'ely.

Ehe arit-hmeticr mean of these,

K ""

3.204 gives:

A~m

=

136.96 cm, Blm

=

158.72 cm, variance ()2

=

2886.

The selection curves established in this way for meshing by the operculum or point of' greatest girth were then used to find the total selection C¹Lrv'es for all methods of attacbment, folloWing the procedure described in the Appendix.

As/first step a free hand curve Was' fitted to the ratios betvveen the numbers of fish meshed and those attached in other ways. This curv'e has a parabolic shfl.po with a

minimum approximately at the length of greatest selection by meshing (Figure 2). This show<

that other methods of attachllent are of significance mainly for the small and very large fish and thus tend to increase the efficient selection range of the net, i.e. make the selection curve more flat-topped.

To find the selection curv'e for the 16¹¹ hemp net the length distribution of the meshed fish taken by this type of net was'compared by that of the 16¹¹ nylon net as

adjusted for the effect of mesh selection. Thsreafter the total selsetion curve was found in the same way as for the nylon nets.

Discussion

From Figure 1 it appears that the plots of the log. ral:;lOS deviate considerably from linearity, as would be expected because of the heterogeneity with regard to attacbment method and the relatively small number of observations considering the very great sizo

range. Nevertheless, there is no great difference bet'tiJeen the three indepenJent values of K, and they compare fairly well with the ratio of ho.lf' the girth to the total length, which was found to equal 3.04.

To further test the validity of the assumption that the L1eo.n selection length is proportional to the mesh size, the lengths at which the log. ratios equal zero, were plotted against the sum of the mesh size as described by Olsen (1959). The plots fit fairly well to a straight line through the origin, with a slope of L599. This corresponds to a value

of K ~ 3.198, against i ~ 3.204. -

The total selection curv'es for the four dif'ferent types of nets used are shown in Figure 3. The cUI'lres are fairly flat-topped, :l!.e. the halibut gill nets are effective over a great range of fish size. TIms the nylon nets have an eff'iciency of 50% or more, as referred to~that of the mean selection length, over'a range of about 104 cm, 110 cm and 110 cm for the 16!!, 19!! and 23^Tl uesh size respectively.

The selection curve for the hemp net is more peaked and the' 50% selection range eqUB.ls about 83 cm, which is 20% less tho,n -that of the 19!1 nylon net.

- 3 -

It is a general experience in all gill-nej fisheries that nets made of polyamide fibres are more efficient than similar nets madejRatural fibres~ as for instance stated by Sffitersdal (1957). The results of these experiments would suggest that at least part of this-difference in fishing power is caused by the extended selection range of polyamide nets.

Summary

Mesh selection data collected by experimental fishing with halibut gill nets of different mesh sizes were analysed by the method described by Holt (1957). The ratio between the mean selection length for meshed fish and the mesh size Was found to be fairly low, v~z. K

=

^3.204.

A method for finding an empirical selection curve describing both the selection by meshing and all other kinds of attachment was dev-eloped and applied to the data. The

resulting total selection curV'e for halibut gill nets was found to be relatiV'ely flat- topped, but less pronounced in nets made of hemp than of nylon. It is suggested that this difference in selection range accounts for at least part of the difference in efficiency generally found between gill nets made of natural fibres and polyamide nets.

Halt, S. J. 1957

Olsen, Steinar 1959

Sffitersdal, G. 1957

References

itA method of determining gear-selectiV'ity and its application!!. Paper No. S.15, Jo~nt ScL Meeting ICNAF /ICES/FAO, Lisbon, 1957. (Mli:m.eo- graphed) •

UMesh selection in herring gill nets!!. J.Fish.Res.

Bd. Canada, 16(3), 1959.

!lOn the fishing power of nylon gill netsTl. Paper No. E.7, Joint ScL Meeting-ICN.A.F/ICES/FAO, Lisbon, 1957 (Mimeographed).

Table 2. Halibut meshed by operculum and point of greatest girth. Length distribution and log.ratios when _ ________________ adjusted for differences in effort.

-- ~--·1=171 no~;l~~~[~ =1--=l-lj~~~-~i~hOdF 1~~~!~s2;~Sh:t- --1- -.... -- -I ---I,i Range in om r-- ... ~~-- ~ --1 . -nl-~~~f. f~: .. ] -<~~r i~~ - illOge

B/A

I logo

C IB

I logo

CIA _____________

~

_________ L effort effort, 1 I -1---f---~---___________

L_ -- ---

---j 70-89 79.5

I

5 4 5 84

I I

1

I

90-109 99.5

i

13 5 7.30

i

2 3.22 1 -0.57'11

I I

-1.3956 11

I

6 9.66 -l.Do79 I -0.5084

I I

5 8.05 0.0296 -1.4708

I

110--129 119.5 44 11 16.06 139.5 34 24 35.04 130-149

-1. 5162 -1.4407 150-169 159.5

i

24 21 30.66 7 11.27 0.2450 -1.0009 -0.7559 170-189 179.5

I

14 11 16.06 18 28.98 0.1371 0.5902 0.7275 190-209 199.5

I

5 8 11.68 19 30.59 0.8475 0.9628 1.8112 219.5

I

4 7 10.22

I

12 19.32 0.9381 0.6369 230-249 239.5

i

2 1 1,16 I

I

i

________ != __ 2~

___________

l_________ ____ _ ________

j __________________________ , _______

1 _______ ~ _______ ~~~~ ____ 1. _______________ J ____________________ . ________ _

210-229 7 11.27

1. 5748

- 5 -

APPENDIX

Method to find an empirical total selection curve describin~ all means of attachment.

If the selection cun'e for one method of attacbment (Le. meshing at the maxillae, operculum, point of maximum girth etc.) is known, an emp,irical selection

curv'e describing all kinds of attachment can' be found, although no simple mathematical expression for this curve may be established.

Let Yl be the ordinate of the known selection cunre at length 1, nl the catch in number of fish caught by this method of attacbment. Similarly Yl'': and nl^l refer to the selection cun'e and catch of all other methods of attachment.

Then,

P

and P • Y t

1

where N_ is the total number of fish of length 1, 1.

encountered by the net fu~d P is a constant.

We hail'e now

and

n Y

1

Y ^t

1

Y ~

1

n !

1 n_ J.

and the ordinate for the total selection

it. "1 t == Yl ⁺ Y 1 t ==

or, if we chose Y

l as unity == 1.

m

Yl

y 1

Y r

Im

(1 +

(1 +

curve nl

!

) . . . 0 0 . . . 0 0 0 .

n_ 1.

) 0 . 0 0 • • 0 . . . 0 0 0 0

(1) (2)

(3)

(4)

(5)

(6)

·

-l'-

·

·

·

·

+

·

~~~

loge

dO~!/

1.8

~

^{A ) /}

1.6 1.4 1.2 1.0 m.8 0.6 0.4 0.2 0.0 0.2 004

0.6 0.8 Lo 1.2 1.4 1.6

r ^{lA S/}

L / j~:~::'/

i _:

1/

^J'."

! '--

U

100.

::)

~ ^, _/."IJt,i:~,

^I

! ! 1 LJ f " i . ! ""'1 j I

ii-;;o----12o---i4;---:':~:,'"~---"":T-"--·_;.71SO_-·-·-'2oo----Z2"6

~/// .«,.ff"

L. """""""""" ./~.,-,!~

^"

"~"._

⁰ """""",,()"""""" f",·,,/f

!

"'91.~""'"

:

"""

i-).~~"er

! "'"""""".

L

_;

I

log log e

I

e

i - L~ () log

e

r-

I

^j~~

! ./

.1 !

Figure 1. Plots of log. ratios ~gainst length

(

I,enttii--\"cm)

BI-f\.

C/I'!.

C/B

- 7 -

1.6

... -... : ... -... -... " ... -... .

"',)...}

() ---'

'··'o"''''''''''''''''',,,,,,.,Q,,,,,,,,,,,,,,

i.

""",,,,,,,,,,,,,,,,,,,,,,,,,""t''.t''''''/'"''''''''

, I

i f !1~'I"'II'II!1110Ht"'nll'II\I'\t\\t\H\\"'''\\'\\1 f

--L-_i_, _ _ ~I

_---1_

^j 1 ___ 1_---1 ! .-1-_1 ___ ,_

80 loo 120 140 160 180 200 220 240 Length

i

lod- i

!

0.9t-

,

. ! i

008

1

_i

0.71~

0.6

I

1-

Figure 2. Ratios between numbers of fish meshed and those attached in other ways, 16!! - nylon nets ⁰

(cm)

N 23

! j"" , / H 16 ""'" "'>,.

:: f""·,,,/'/'/

""""''''''''t'''''''''''''''''''''''''''''''''/''/''// """,.,''''''''''"."."."".".", •• "", """""""''''''''''''''''''''''.Jl: 19

o 0 ] - : " , _{I •} .... " " ... , , " " ' " ' ' ' ' ' ' ' ' ' ' : _: """"""',,£;f. 16

t , t I ~ i : ~ l

L_L-- . __ l ____ i __ .. .J .. _,_---L.---.J _ _ _ i ____ L. ______ I ___

1-__

L,_--'-_ _ ;

80 loo 120 140 160 180 200 --=2-':2-0-

Figure 3. Relativ'e selection curv'es describing all methods of attacbment,

240 Length (cm) H 16 - hemp 16!!, }IT 16 - nylon 16T!, N 19 - nylon Hill, N 23 - nylon 231!.