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ICES STATUTORY MEETING 1993 Shellfish Committea K:39

SIZE SELECTION OF OCTOPUS VULGARIS CUVIER, SEPIA OFFICINALIS HIERREDDA RANG AND SEPIELLA ORNATA RANG IN BOTTOM TRAWLS OFF THE COAST OF GUINEA-BISSAU.

by

Joäo M. da F. Pereira • Instituto Nacional de Investiga<;äo das Pescas, Av. de Brasnia - 1400 Lisboa, Portugal.

ABSTRACT

The results presented refer to three species of caught in two consecutive bottom trawl hauls, shot off the coast of Guinea-Bissau, during a research cruise of the portuguese RN "NORUEGA", in June 1991. Selectivity experiments were performed using the covered cod-end method, with a 61.4 mm cod-end mesh size. The selection factors (SF) estimated for the two species of (0.85 and 0.84) fall between the

• range of values found in the literature for fish and crustaceans. The values of L50'l' were, respectively, 55.5 and 54.5 for Sepia officinalis hierredda and Sepiella omara.

The characteristics of the size selection observed in the two species of Sepiidae and the lack of size selection obtained for the Octopodid (Ocropus vulgaris) suggest a behaviour­ dependent selection which departs from the pattern usually found in fish.

Kay words: , trawling, selectivity, selection factor, Octopus vulgaris, Sepia officinalis hierredda, Sepiella amata. 2

INTRODUCTION

In the context of the scientific assistance programme which INIP has undergone in the waters of Guinea-Bissau since 1988, selectivity experiments have taken place, which aimed to identify the characteristics of the size selection of a cod-end similar in construction material and mesh size to the ones used commercially in bottom trawl nets in this country.

Primarily, selectivity experiments undergone by INIP in Guinea-Bissau were directed at • shrimps, particularly Penaeus notialis and Parapenaeopsis at/antiea since these are the main target species of the commercial trawl fishery in the area. In 1991, however, trawl selectivity experiments were also directed to tish and cephalopods, the latter being an important secondary target tor the local (multinational) fleet of trawlers.

This study presents and analyses selectivity results concerning the catches of Oetopus vulgaris, Sepia offieinalis hierredda and Sepiella omata, the most coinmon cephalopod species in the area. Oetopus vulgaris and Sepia officinalis hierredda are the two cephalopod species most sought after by commercial trawlers in the area•

• MATERIAL AND METHODS

Sampling methods

Fishing hauls were performed by the portuguese RN "NORUEGA", a 47.5 m (495 GRT) stern trawler with approximately 900 HP. 3 The fishing gear employed was the standard crustacean bottom trawl net used at INIP (FGAV020), with approximate vertical and horizontal openings of, respectively, 2.0 and 31.5 m at 3.0 Knots. These and other technical specifications of the net are described in Leite et sI. (1990).

The cod-end was constructed of a polyethylene thread tied in a diamond shaped mesh of 61.4 mm (± 1.34 mm) as measured from the inside with a pair of callipers applying a strength of 4 Kg to each individual mesh. Two randomly chosen longitudinal rows of • 31 consecutive meshes were measured. The cod-end cover was applied overthe cod-end with a distance between the two bottoms of approximately 2 m and had a mesh size of between 20 and 25 mm.

Two 60 minute hauls were performed during daylight at an average speed of 3.0 Knots•

. All specimens of the three species studied were measured to the nearest lower 1/2 cm and the measure taken was the standard Dorsal Mantle Length (DML) (Roper et al., 1984).

Oat8 analysis methods

• The specimens of each species captured in the cod-end and cover were grouped in 5 mm size classes. For each size class (L), the proportion in numbers (Pl) of the total catch (nl ) retained (rd in the cod-end was calculated by:

(1 )

The total catch" (Nl ) was assumed to be: 4

(2)

where r and n are the catches (in numbers) in the cod-end and cover, respectively, for L L size class L, assuming no escapes from the cover bag.

The proportion of each size class retained in the cod-end was assumed to fit a logistic distribution of the type:

(3) • _ 1 PL - l+e-(a+,bL)

Pope (1966).

To allow for a linear (least squares fit) regression, expression (3) is log natural transformed into:

(4) Ln{~) = a+bL • l-PL where Ln is the natural logarithm and a and bare, respectively, the intercept and the slope of the Une which relates the first term of (4) with L (in size class units).

The length at which 50% of the specimens were retained in the cod-end (L50..) was calculated by: 5

(5)

The selection range (A) is defined as:

(6) ~s,-~s, • and can be estimated from: (7) A = 2 Ln3 b

where l2S'I and l7S'I are the lengths at which, respectively, 25% and 75% of the specimens are retained by the cod end, and may be calculated using:

(8)

• and

(9)

The selection factor (SF), a value which theoretically depends upon the (physical) characteristics 01 the species (shape and size) and the size (and presumably type) of the

mesh, by relating l50'1 with the mesh size was calculated from: 6 (10) SF = Lsot Mesh Size

RESULTS

Table 1 summarizes the results of the catches obtained in the two trawling hauls.

.Table 1 - Summary of the catches in each experimental haul and percentage of catch • corresponding to each of the three species studied. Fishing station 1 Fishing station 2

Cod-end Cover Cod-end Cover

Octopus vulgaris 0.6 2.2 1.0 2.8

Sepia officinalis 5.1 0.3 34.0 1.0 hierredda

Sepiella ornata 0.8 0.5 1.4 1.3

Other cephalopods 0.3 0.1 1.9 2.6 • Other species 157.2 482.4 214.8 1192.2 Total 164.0 485.5 253.1 1199.9

OV% 0.4 0.5 0.4 0.2

SOH % 3.1 0.1 13.4 0.1

SO% 0.5 0.1 0.6 0.1 OV = Octopus vulgaris; SOH =.Sepia officina/is hierredda; SO = Sepiella ornata 7

Tables 2 to 4 present size classes, numbers in the cod end and cover bag as weil as the corresponding percentage of retention for, respectively, Octopus vulgaris, Sepia officinalis hierredda and Sepiel/a ornata.

Table 2 - Oata obtained from the size selection experiments with Octopus vulgaris.

Length Groups Cod-end Cover , Retained (mm)

25- 29 1 1 50.00 30- 34 2 4 33.33 • 35- 39 2 6 25.00 40- 44 2 9 18.18 45- 49 3 13 18.75 50- 54 1 14 6.67 55- 59 3 8 27.27 60- 64 4 6 40.00 65- 69 0 3 0.00 70- 74 1 4 20.00 75- 79 1 0 100.00 80- 84 0 2 0.00 85- 89 0 0 90- 94 0 2 0.00 95- 99 0 1 0.00 100-104 0 0 105-109 0 0 110-114 0 0 • 115-119 1 0 100.00 8

Tabl. 3 - Data obtained from the size selection experiments with Sepia officinalis hierredda.

Length Groups Cod-end Cover , Retained (DIll> 25- 29 0 1 0.00 30- 34 0 5 0.00 35- 39 0 3 0.00 40- 44 0 12 0.00 45- 49 1 11 8.33 50- 54 2 10 16.67 55- 59 9 4 69.23 • 60- 64 19 3 86.36 65- 69 17 2 89.47 70- 74 24 0 100.00 75- 79 23 0 100.00 80- 84 14 0 100.00 85- 89 13 1 92.86 90- 94 26 0 100.00 95- 99 16 1 94.12 100-104 9 0 100.00 105-109 17 0 100.00 110-114 12 0 100.00 115-119 17 0 100.00 120-124 16 0 100.00 125-129 12 0 100.00 130-134 9 0 100.00 135-139 9 0 100.00 140-144 7 0 100.00 145-149 1 0 100.00 150-154 1 0 100.00 • 155-159 2 0 100.00 160-164 3 0 100.00 165-169 1 0 100.00 170-174 0 0 175-179 1 0 100.00 180-184 1 0 100.00 9

Table 4 - Data obtained from the size selection experiments with Sepiella ornata.

Lenqth Groups Cod-end Cover , Retained (mm) 25- 29 1 5 16.67 30- 34 1 10 9.09 35- 39 1 16 5.88 40- 44 6 25 19.35 45- 49 7 19 26.92 50- 54 10 21 32.26 55- 59 20 10 66.67 60- 64 18 6 75.00 65- 69 4 3 57.14 • 70- 74 5 1 83.33

Figure 1 presents retention values for Octopus vulgaris plotted against DML, resulting in 2 8 patternless scatter. A squared correlation coefficient (r ) of 0.107 indicates 8 significant lackof fit (d.f. = 7; ANOVA: F-Ratio .. 0.841, p .. 0.390) ofthe data to the model.

• 10

•. 1

•• 4 •

-... • - '.3 -.. • '"'. • -.. - •• r • -.. • G-.. • ...-

•. I •

II

DorSlI 1I •• llt Lugt' (111

Figur. 1 _Scatter diagram of the proportion of specimens retained in the cod-end in each length group against length group for Octopus vulgaris. • Figures 2 and 3 depict the fit obtained with (4) tor, respectively, Sepia o"icina/is

hierredda and Sepiella ornata. 11

• AllIlI

flllt •

• •

1..

'0... c: I.i ... a: ..c: .. t . 4 ..0- Q..-

I.Z

Dorsal lIantle length (u)

Figura 2 - Regression line and scatter diagram of the proportion of specimens retained in the cod-end in each length group against length group for Sepia officinalis hierredda. 12

•' •• u' - I,,, ••

1.1 ..... c

I.' -• Cl<.. 0 .- I .• ca. 0 Q,-

I.l

IS

DOIS" M,nllt Ltngtb (li)

Figura 3 - Regression line and scatter diagram of the proportion of specimens retained in the cod-end in each length group against length group for Sepiella ornata. 13 Table 5 summarizes the selectivity parameters caleulated for the two speeies of Sepiidae, as weil as the regression parameters obtained tor Octopus vulgaris.

Table 5 - Selectivity parameters.

Parameter O. vulgar!-s S. o. hierredda S. ornata a -0.368 -5.125 -4.824 b -0.017 0.092 0.089 r 0.107 0.675 0.820 e ~" (nun) 43.617 42.055 t.so" (nun) 55.518 54.458 ~5" (mm) 67.419 66.861 A (nun) 23.802 24.806 SF 0.854 0.838

DISCUSSION

The magnitude of the selection factor obtained tor each speeies and trawl mesh size depends primarily upon the measure taken from the , as it relates the size of the mesh to the measure of the animal's size. If the total length of the animal is measured (as usually happens in fish), then the value of SF, refleets the physical charaeteristics of the latter and the way in which it comes into contaet with the fishing gear.

In fish, seleetion faetors are thus greater than 1, refleeting the shape of the , where length is usually greater than breadth or height. It is then one of the latter two eharaeteristics which really influences the ability of the fish to escape from the net.

In crustaceans such as, for example, the Norway lobster (Nephrops norvegicus) the measure taken is usually the carapace length (Briggs, 1966), which is a small proportion 14 of the totallength of these animals. The values of SF encountered are then usually less than 1, reflecting the proximity of the magnitude of this measure to the maximum breädth or height of the animal and consequently to the mesh size through whieh it is able tö eseape.

in cephalopods, whieh are mostly soft-bodied it is harder to have preconeeived ideas as to the magnitude of SF whieh eould be expeeted. The standard measure taken tröm these animals, DML, usually represents the whole extent of the animal's hard parts. In Sepiids, DML represents a measure of the internat shell, leaving the head and tentaeles, which are soft and may vary in shapa. In Oetopodids DML represents the least size variable portion of the animal's body, it's mantle eavity and head, although it often eorresponds tö lass than a third of the totallength of the animal. It is thus reasonabla to say that the measures of the length of the speeimens in any of these families are a good representation of their size, and one would thus expeet to see a seleetion faetor greater than 1. Cur results show that this is not so.

The seientifie literature is searee on selectivity experiments on eephalopods, partieularly with otter-trawls. For Sepiids, experiments undertaken by Sanders and Bouhlel (1983a) on Sepia pharaonis, converted to the same mesh size used in our experiments, present a value for SF of 0.824 and for the same specil:!s fröm Sanders and Bouhlel (1983b) . another SF of0.628. These authors used the alternate haul method in their experiments, whieh produces values of Lso'l' greater by 4 to 5% than the <>nes obtairied with the

, .'\ . " eovered cod-end method (Pope '966). Thus, to obtain a value of SF trom our experiments comparable 10 that which would be obtained using the alternate haul method, the following expression: ..

15 (11 ) SF= Lsoi+O•045.

must be used, resulting in a SF tor Sepia officinalis hierredda ot 0.95 arid tor Sepiella 'ornata of 0.93, both slightly larger than the ones in Sanders and Bouhlel (1983a and .. 1983b) but less than 1.

Our results tor Octopus vuiiJaris indicate that the species may not be self3cted for size with the mesh type and size used in these experiments, which is a different conclusion trom the results in the literature. Ariz and Fernandez (1980) experimented with a mesh sizB öf 60.9 mm and obtained a SF of i .0; Idelhaj (1982) obtained a SF of 0.925 for a rriesh size of 61 mm. It does not seem possible that the results obtained in aUf experiments may iri any way be a reflection of some miss-performance of the gear, since those obtained for the two Sepiids in the same experiments are comparable to the ones in the literature for similar species and the number of specimens of Octopus vulgaris captured in each trawl are similar to those used by other authors.

The values of SF obtained for the two Sepiids are iower than would be expected~ This " is probably an indication of a different behaviour from that seen in tish and crustaceans • and which could also explain the lack of ci size selection obÜ!ined tor Octopus vulgaris. Cephalopods possess suckers with which they grab their prey. and it is possible that when tirider stress they may not be able to avoid holding to the net and thus get caught when they might otherwise escape, or squeeze through mesh sizes smaller than thei,. body measure such as might be the case with Octopus vulgaris.

it would seem advisable to pursue these experiments with other cephalopod species under varying coriditions. 1I results such as the ones obtai~ed here hold, trawling with ....

16

mesh sizes which might allow escapes of fish and crustaceans of a given size may catch cephalopods of the same size. It may thus be wise to exercise caution in managing cephalopod stocks under the assumption that they are escaping any given legal mesh size.

ACKNOWLEDGEMENTS e I wish to thank all my colleagues who helped me with the hand work in Guinea-Bissau, during this eventful trip where just about everything prevented me from being physically operational ••• they were the best I To my friend and colleague, Ms. F~tima Cardador, my special thanks for the helpful criticism in revising the manuscripts.

REFERENCES

ARIZ, J.; FERNANDEZ, M.A.R., 1980. Selection on octopus (Octopus vulgaris) and sea breams of the spanish cephalopods bottom trawl off North West Atrica. ICES Doe. e.M. 1980/K:35, 19 pp•

• BRIGGS, R.P., 1986. A general review of mesh selection tor Nephrops norvegicus (L.) Fish.Res., 4(1): 59-73~

IDELHAJ, A. 1986. Etude de la selectivite du chalut de fond type espagnol utilise dans la pecherie des cephalopodes du Sahara marocain. Trav.Doc. Dev.Peche Marit., 33: 27pp. ...

17

LEITE, A.; FERREIRA, C.; FONSECA, P.; HENRIQUES, V., 1990. Teste de redes da arrasto pelo fundo. Campanha da pesca experimental NIE "NORUEGA" _02080889,14-16 Agosto 1989. Relat.INIP, 26: 31pp.

POPE, J.A., 1966. Manual of methods for fish stock assessment. Part 111. Selectivity of fishing gear. FAO Fish.Tech.Pap., 41: 50pp.

ROPER, C.F.E.; SWEENEY, M.J. and ~AUEN, C.E., 1984. FAO species catalogue. Vol.3. e Cephalopods of the world. An annotated and iIIustrated catalogue of species of interest to fisheries. FAD Fish. Synop., (125) Vol.3: 277pp.

SANDERS, M.J.; BDUHLEL, M., 1983a. Mesh selection study conducted in the People's Democratic Republic of Yemen on the Sepia pharaonis. FAD Fish.Tech.Pap., 231: 181-203.

SANDERS, M.J.; BOUHLEl, M., 1983b. Third report of mesh selection studies conducted in the People's Democratic Republic of Yemen on the cuttlefish Sepia pharaonis. FAO Fish.Tech.Pap., 231: 22p• •