International Council for Shellfish Committee the Exploration of the Sea CM 1986/K:4 Ref. Fish Capture Committee

CATCH AND SIZE-SELECTION OF PANDALUS BOREALIS IN A BOTTOM TRAWL AND IMPLICATIONS FOR POPULATION DYNAMICS ANALYSES by E.M. Nilssen1 , R.B. Larsen2 & C.C.E. Hopkins1

1Aquatic Biology Group, Institute of Biology and Geology, University of Troms0, 9001 Troms0, . 2Institute of Fisheries, University of Troms0, 9001 Troms0, Norway.

ABSTRACT

The catch and size-selection characteristics of a prawn trawl with 20 mm mesh cod-end have been investigated in two north Norwegian fjords, and (ca. 7cPN), by placing fine-meshed bags at various strategie positions on the wings, belly and cod-end. An additional fine-meshed bag was placed under the mouth of the trawl, between the fishing-line and coco (gear-toggle distance = 30 cm). The chief area of size-selection of P. borealis was the cod­ end; negligible loss occurred through the wings or belly. The 50% retention point in the cod-end was at ca. 12.5 mm cpx. length (SF = ca. 0.6). A selection curve has been constructed, and the loss of • prawns of different size-groups and year/age-classes are quantified. Comparisons of the catch in the trawl itself with that of the fine- meshed bag under it showed that the latter caught on average 27 times more (litres) of prawns per unit volume swept. The volume and demography of the catches could differ significantly; different diel, depth distributions of small and large prawns, on a small vertical scale close to the bottom, are likely contributors to heterogeneity of catches in and under the trawl.

Stock surveys of ~ borealis using trawls fishing with relatively large bottom-clearances may provide highly inaccurate estimates of population size and demography. Nilssen, Larsen &Hopkins- Pandalus borealis Catch and Selection INTRODUCTION

The analysis of biological populations is first and foremost dependant on the principal of representative sampling of the population. Dependable estimates of the size and age/year-class composition of thc prawn Pandalus borealis Kr0yer are biased by the limitations and selectivity of the trawling gear used (Shumway et al., 1985). Both se1ectivity and availability may bias estimation by under-estimating the younger individuals, and features such as mean length-at-age for partly recruited age-groups tend to be over­ estimated (Frcchette & Parsons, 1982).

In 1978 a study of the population dynamies and ecological energetics of the prawn stock in Balsfjord, (69 0 22'N 19 0 23'E) was initiated (Hopkins, 1979; Seiring, 1982; Nilssen, 1984; Seiring • & Hopkins, 1985; Hopkins, in prep.). In order to compensate for selectivity (see Frechette &Labonte, 1981) it was considered desirable to conduct aseries of tests on the trawl used in the Balsfjord study to better determine how representatively we were sampling the population.

In this paper we present some preliminary results from aseries of 6 stations carried out in Balsfjord and the neighbouring fjord of Ma1angen. Fine-meshed bags were attached at various strategie positions on the the wings and belly, as weIl as covering the cod­ end. An additional fine-meshed bag was placed under the mouth of the trawl, between the fishing-line and coco (gear-toggle distance = 30 cm) to examine the quantity and demography of prawns passing und er • the trawl itself. Further data, using more stations at Malangen, with an additional trawl with reduced bottom clearence will be presented at a later date.

MATERIALS AND METHODS

Trawling for Pandalus borealis was carried out in and Malangen during the day between 20 and 26 February 1986. The trawl was towed by the 15.5 m, 210 h.p. R.V. "Ottar" of the University of Troms0.

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Nilssen, Larsen &Hopkins- Pandalus borealis Catch and Selection A standard Norwegian commercial, demersal prawn trawl was used in the investigations. Five trawl hauls were made on 20 and 21 February 1986 in Balsfjord (stations 1-5), and a single trawl haul was made on 25 February 1986 in Malangen (station 7). The characteristics of the 1050# (35 mm) PE prawn trawl are as follows: - The lengths of the head-line, fishing-line and the coco ground-rope are 34.35 m, 40.15 m and 34.65 m respectively. The numbers of floats on the head-line are 6 (0 = 0.200 m) + 16 (0 = 0.152 m). 3 * 5 kg of chain are used on the ground-rope. A pair of rectangular WD + FE doors (1.5 m~ and 240 kg) are used, and the brid1e (PP 0 = 0.020 m) lengths are 27.45 m (+ 5.75 m and 2.55 m).

Four fine-meshed bags were strategically placed about the trawl in order to catch those prawns passing through the meshes of the trawl itself: • Bag 1) Attached on the starboard lower wing-tips where the mesh-size is 44 mm Bag 2) Attached on the upper belly (forward funnel section) where the mesh-size is 35 mm Bag 3) Attached on the lower belly (funnel section) where the mesh-size is 30 mm Bag 4) Attached to the upper part of the cod-end; the latter being constructed of 30 mm mesh netting with a 20 mm mesh inner net. In addition a fifth bag was used Bag 5) Attached between the fishing-line and coco, centrally placed, to examine passage of prawns under the trawl itself •

Various technical specifications for the trawl, as weIl as the • varied positioning of bag 5 are illustrated in Figs. 1 and 2.

All the bags (1-5) were made of 20 mm mesh PA. In addition, bag 5 had a 1 mm mesh (20 GG monofilament nylon) inner net in its own "cod-end". Further details of bags 1-3 and bag 5 are shown in Figs. 3 and 4 respectively. Bags 1-3 euch covered 1 m2 of the trawl, but the effective openings during fishing were estimated from the angles of attack given in Fig. 1. Before mounting, bag 5 had a width (d) of 1.0 m and a height of 0.5 m, hut during fishing the height was reduced to 30 cm (gear-toggles'distance) while the width remained unaltered. The effective opening of bag 5 during fishing is determined by the angle of the bag opening relative to the direction

2 Nilssen, Larsen & Hopkins- Pandalus borealis Catch and Selection of towing (see Fig. 1). In the case when this angle is 60?, then the "fishing width" (d') of bag 5 will be d' = 1.0 * cos(90-60) 0.87 m thus reducing the fishing area to 0.26 m~ (1.0 * cos30 * 0.3).

At three stations Scanmar type DT-300 sensors were mounted on the trawl to measure the height (head-line to bottom) and width (between wing-tips) of the mouth opening. The mouth-opening (A m:2) of the trawl was calculated using the formula for an elipse A = (a + b)/4 * /2 where a is the distance between the two wings, and b is the maximum height of.the trawl opening •

Where possible the trawl was towed for 1.0 nautical mile (1852 m) • along the bottom. Trawled distance was estimated by following the passage of the research vessel relative to the bottom using radar fixes of suitab1e convenient topographieal features ashore. Distance sailed from the ship's log was also reeorded for back-up but not used in the trawled distance estimates.

After the trawl was taken onboard the catches (litres) of prawns in the cod end and in each of the fine meshed bags were measured and recorded. Sub-samples of between two and eight litres were removed from the cod-end for detailed analyses in the laboratory ashore, while the complete catches in each of the small meshed bags were removed for analyses ashore. These prawns were deep-frozen until • they were thawed in the laboratory. In the laboratory ashore the prawns were thawed at room temperature, and carapace length measured to the nearest 0.1 mm using digital calipers. Length-frequency distributions of prawns and preliminary statistics were performed using "Minitab" (Ryan et a1. , 1985). Separation of year-classes was carried out on length-frequency polymodal groups using techniques based on normal distribution theory (see Harding, 1949; Cassie, 1954). A computer program, based on some of the same principles described by Abramson (1971), MaeDonald &Pitcher (1979) and Schnute &Fournier (1980), was used in the present study to separate age-groups from length-frequency distributions. This program (Nilssen, 1984) is based on the BMDP

3 Nilssen, Larsen &Hopkins- Pandalus borealis Catch and Selection computer program PAR (Dixon, 1981) and uses the princip1es of "derivative-free nonlinear regression" (Ra1ston &Jennrich, 1978). The presumptive age of prawns within a particu1ar 1ength mode was prescribed using combined know1edge of the date of sampling and average date of egg-hatching in Balsfjord and Ma1angen of 15 April.

The selection/retention of prawns of a given size is the probability that they are retained by the trawl after it has entered it. It is generally expressed as the percentage of each size-group retained in the cod-end, and in practice takes the form of an approximately symetrical sigmoid curve. The size frequency distribution of those retained in the trawl is compared with that of the total population entering the trawl. The length where half of the individua1s are retained is cal1ed the 50% se1ection length (1. 50%). The selection factor (SF) is obtained by dividing the 1. 50% by the mean mesh size • in the cod end (Pope et ale 1975).

Differences between two independant samp1e means (eg. amounts of prawns caught in bag 5 and cod-end, 1engths of prawns of a given year-c1ass at Ba1sfjord and Ma1angen) were tested by "Minitab" two­ samp1e t-test, with the "alternative" cOlIUlland using separate (Le. non-poo1ed) standard deviations from the two samp1es; it is a conservative test, reducing both the degrees of freedom avai1ab1e and the chance of cOlIUllitting a Type I error (Ryan et a1. 1985).

RESULTS

Trawl characteristics

The trawl was found to have a mean maximum height and width of 3.4 m and 21 m respectively when towed with the vesse1's motor recording 1100 r.p.m, ca. 1.8 knots ship's speed.

Bulk catches in the trawl and fine-meshed bags

The vo1ume of prawns recorded in the cod-end of the trawl as well as in the collection bag mounted under the trawl (between the fishing- 1ine and the coco) in Balsfjord (stations 1-5) and in Ma1angen

4 Nilssen, Larsen &Hopkins- Panda1us borea1is Cateh and Se1eetion (station 7) are shown in Tab1e I. The mean eateh of prawns reeorded in the eod-end of the trawl in the six stations was 0.08 1itres per 103 m.3 whereas the mean eateh in the fine-meshed bag 5 was 1.77 1itres per 103 m3 , the 1atter eateh being signifieant1y grcater than the former (t = -3.42, d.f. =-5, P = 0.019). The eoeffieient of variation for the eatehes in the eod-end and bag 5 were simi1ar at 65% and 68% respeetive1y. No prawns were reeorded in the fine­ meshed bags 1-3 p1aeed on the wings and upper and 10wer bellies of the trawl from the Balsfjord stations, while a few seattered individua1s were reeorded from station 7 in Ma1angen.

Length-frequeney distributions in the eod-end and fine-meshed bags

The pereentage 1ength-frequeney distributions of prawns retained by • the eod-end, and eo11eeted by the fine-meshed bag 4 on the eod-end when eompared with that in bag 5 under the trawl in stations 1-5 in Ba1sfjord and station 7 in Ma1angen (Fig. 5) emphasize some very interesting simi1arities and differenees. First and foremost it is evident that the 1ength distribution of prawns eaught in hag 5 least resemb1e the others, the main differenee being the greater proportional representation of the 1985 year-elass « ca. 10 mmepx. 1ength). Considering that on1y a total of 24 prawns were reeorded from bag 4 from all the five stations in Ba1sfjorden, that no losses of prawns were registered from the areas of the trawl where bags 1-3 were mounted, and bearing in mind that the highest eatehes per relative swept-vo1ume were eaught in bag 5, it is elear that the majority of prawns (espeeia11y the 1985 O-group) were distributed in the 30 em immediately over the bottom.

There are a number of pertinent differenees between the 1ength­ frequeney distributions in Ba1sfjorden and Ma1angen (Fig. 5). First1y, it is evident that the 1985 year-e1ass (O-group) provides a proportionately greater eontribution to the prawn stock in Ma1angen than in Ba1sfjord. Seeond1y, a1though the eateh per relative swept­ vo1ume in Ma1angen is still greater in bag 5 than in the eod-end (as in Balsfjorden) prawns of > ca. 14.5 mm epx. 1ength are absent from the eateh in bag 5. These large prawns are nevertheless weIl represented in the eod-end. Third1y, the fine-meshed bag 4 eontained both high numbers and 1arge proportions of the 1985 year-e1ass «

5 Nilssen, Larsen &Hopkins- Pandalus borealis Catch and Selection ca. 12.5 mm cpx. length), indicating relatively poor retention of these prawns in the cod-end.

Retention and selection estimates

An attempt has been made at compensating for the effects of loss of prawns through the meshes of the trawl (cod-end/bag 4 around cod­ end) and for the different catches of prawns relative to bag 5 under the trawl using the cumulative length-frequency . distributions of prawns found the respective catches (Fig. 6A for Balsfjord; Fig. 6B .for Malangen). Selection can be examined by plotting the difference between the cumu1ative percentage distributions of prawns retained in the cod relative to those passing through the meshes and being retained in bag 4; this can be achieyed for Ma1angen where large • numbers of sma11 (1985 year-c1ass) prawns were present (Fig. 5), but is not possible for Balsfjord where the sma11 prawns that were present were restricted to bag 5 (ca. 30 cm above bottom). Based on the data plotted in Fig. 6B for Ma1angen it is evident that the 50% retention point in the cod-end is reached at ca. 12.5 mm cpx. length. This indicates that the se1ection factor (SF) for this trawl is ca. 0.6 (12.5/20).

Variation in carapace length in cod-end and bag 5

Thc mean carapace lengths recorded from the combined catch in the cod-end together with bag 4 covering the codend (a theoretical measure of the population composition actually entering the mouth of the trawl) with that in bag 5 from Balsfjord and Ma1angen are shown in Fig. 7. Similarities and differences between the means were tested using t-tests. In Balsfjord there was a tendency for the mean cpx. length to be larger in the combined cod-end/bag 4 catch than in bag 5 in all the year-classes (1982-1985); t-tests carried out revealed that there was no significant difference in the case of the 1982 year-c1ass «t = 1.52, d.f. = 213, P = > 0.05), while highly significant differences were detected in the case of the 1983 (t = 9.67, d.f. = 791, P < 0.001), 1984·(t = 3.19, d.f. = 253, P = 0.002), and 1985 (t = 5.48, d.f. =~38, P = < 0.001). In Malangen the absence of large prawns C> ca. 14.5 mm) from bag 5 prevented t-

6 Nilssen, Larsen &Hopkins- Pandalus borealis Cateh and Seleetion tests being earried out, but in the ease of the 1985 year-elass there was no signifieant differenee (t = 0.85, d.f.-8, P > 0.05). T-tests earried out to eompare the epx. lengths of Balsfjord prawns with those from Malangen revealed that there was no signifieant differenee in the ease of the 1982 year-elass (t = 1.29, d.f. = ~ 148, P = > 0.05), while highly signifieant differenees were deteeted in the ease of the 1983 (t = 16.80, d.f. =-190, P = < 0.001), 1984 (t = 9.58, d.f. ='707, P = < 0.001), and 1985 (t = 11.60, d.f. =­ 35, P = < 0.001) year-elasses.

DISCUSSION

Size seleetion through the meshes of a trawl ehiefly oeeurs in the eod-end (Pope et al. 1975). The present investigations earried out • in Balsfjorden and Malangen indieate that negligible pereentage lass of 'prawns occurs through the meshes of the wings (bag 1) or upper and 10wer bellies (bags 2 and 3 respeetive1y) of the trawl. Nevertheless, when p1entifu1 numbers of small prawns entered the trawl these were mainly lost through the meshes of the eod-end (eovered by bag 4). Our findings are thus in elose general agreement with the results of Sakhno & Sadokhin (1980) eondueted in the Barents Sea whieh suggest that ca. 95% of the trawls total mesh seleetion oeeurs in the eod-end, espeeially in it's upper half.

Construetion of a seleetion eurve in the traditiona1 manner by eomparing retention eharaeteristies of the eod-end also raises a number of pertinent points. Firstly, in the ease of Balsfjorden a traditional seleetion eurve eouldn't be eonstrueted as so few small prawns were availab1e above the height of the fishing-line (Fig. 6A), whereas in Ma1angen the avai1abi1ity of prawns to the trawl proper allowed the 50% retention size to be estimated at ea. 12.5 mm epx. l~ngth. Our results using a 20 mm eod-end mesh are elose1y eomparable with the 50% seleetion of 12 mm epx. length with 19 mm eod-end of Sakhno & Sadokhin (1980). Thomassen & Ulltang (1975) found a 50% seleetion size of 12-13 mm epx. length with a 30 mm mesh size. The 1atter authors suggested that biases and distortions ean be eaused by masking effeets when using a eod-end eover in se1eetion experiments. On the other hand, the limitations of the eovered/ partly eovered eod-end approach as used here haveto be eontrasted

7 Nilssen, Larsen &Hopkins- Pandalus borealis Cateh and Seleetion with the requirements for greater sampling intensity to obtain equal preeision in alternate hau1 experiments (Pope et al. 1975). Further experiments, eomparing eod-end sleetivity when using different fine­ meshed eover eonstruetions, need to be performed with this trawl to resolve this question.

Besides emphasizing the above well-known eharaeteristies of prawn trawls our investigations examine another aspeet of prawn trawls that doesn't appear to have been highlighted, _ namely the amount of prawns that ean pass under the trawl, below the fishing-line. When the gap under the trawl is ea. 30 em as used here, the ratio of eateh m-3 in the fine-meshed bag plaeed under the trawl- eompared with that reeorded in the eovered eod-end was on average 27.66 greater. Bearing in mind that little if any loss of prawns was demonstrated through. the wings or belly of the trawl, this is • indieative of mueh higher population densities of prawns on or elose to the bottom itself and/or a tendeney for prawns to attempt to evade eapture swimming towards the bottom.

Although it is weIl known that the population density of ~ borealis varies diurnally, with redueed trawl eatehes at night, eomparatively little is known about the relative variations in the near-bottom strata of this speeies (see review by Shumway et al. 1985). Having already emphasized that the greatest population density was found in the bottom 30 em, a eomparison of the eatehes (total amount of prawns and relative size distribution) in the eod-end, bag 4 and bag 5 in Balsfjord (stations 1-5) further indieates that the smaller • prawns were proportionately mueh better represented there than larger prawns. As the absolute density of large prawns (> 14 mm epx. length) eolleeted in bag 5 was still several orders of magnitude greater per unit volume swept than eolleeted in the trawl proper it is obvious that differenees in the sizes of prawns eaught are not aeeountable simply on the basis of relative evasion eapabilities of large or small prawns. This point of view is supported by the Malangen data (Fig 5) where hardly any large prawns were eaught in bag 5. Large prawns were proportionately better represented in the eod-end eatehes taken relatively late in the day in Balsfjorden (Nilssen &Hopkins, unpubl.) and at the Malangen station (1555-1655 hrs).

8 Nilssen, Larsen & Hopkins- Pandalusborealis Catch and Seleetion The differenee in eateh rate between hours of daylight and darkness is eharaeteristie of many fisheries for ~ borealis (see review by Shumway et al. 1985). Our data from Balsfjord for both the trawl itself and the bag p1aeed below the trawl itself elearly suggest that part of a diel rhythm is present, with maximum eatch rates elose to the midd1e of the day. Given that nearly all size-selection of prawns occurred within the eod-end, and that the average cateh per unit volume swept was orders of magnitude greater in the bag below the trawl compared with the eod-end, it ean be then deduced that small prawns aeeounted for the majority of the difference in catch. The partieularly low ratio of bag 5 to eod-end eatch seen at station 4 (1629-1708 hrs) in Balsfjord, coupled with the especially low amount of the eatch at this time, suggests that the majority of small prawns had moved weIl clear of the bottom and of the trawl itself. The amount (litres) of prawns remaining on the bottom was • still, on a unit volume basis, over twiee the maximum caught by the trawl itself. Based on these preliminary results from a winter situation, it is eoneluded that stock surveys of ~ borealis using trawls fishing with re1atively large bottom-elearances may provide highly inaccurate estimates not only of population size of up to several orders of magnitude, but also of population structure (demography).

Acknowledgements

We are grateful to skipper Kare Bendiksen of RV "Ottar" for his help and eneouragement in eonducting this research at sea. We also thank students at the Institute of Fisheries for their assistance in this research project both at sea and ashore, and Lis Olsen for illustrative expertise.

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Nilssen, Larsen &Hopkins- Pandalus borealis Catch and Selection REFERENCES

Abramson, N.J. 1971. Computer programs for stock assessment. FAO Fish. Tech. Pap. 101. 149 pp. Cassie, R.M. 1954. Some uses of probability paper in the analysis of size frequency distributions. Aust. J. Mar. Freshw. Res. 5(3): 515-522. Dixon" W.J. (ed.) 1981. BMDP Biomedical Computer Programs. University of California Press, Berke1ey. 727 pp. Frechette, J. & S.S.M. Labonte 1981. Biomass estimate, year­ c1ass abundance and mortality rates of Panda1us borealis in the NW Gulf of St. Lawrence. - Pp. 307-329 in: Frady, T. (ed.). Proceedings of the international pandalid shrimp symposium, Kodiak Alaska, Feb.' 1979. Sea Grant Rept. 81-31 • Frechette, J. &D.G. Parsons (eds.) 1982. Report of the shrimp • (Panda1us borealis) Ageing Workshop, , Quebec, 11-14 May, 1981 and Dartmouth, Nova Scotia, 20-21 November 1981. NAFO SCS Doc. 82/VI/14 Sero No. N543. 58 pp. Harding, I.P. 1949. The use of probability paper for the graphical analysis of polymodal frequency distributions. J. mare biol. Ass. UK 28: 141-153. Hopkins, C.C.E. 1979. Ecobiochemical studies on Panda1us borealis Kroyer from Balsfjorden, northern Norway: Preliminary results from a winter situation. Cons perm. int. Explor. Mer CM 1979/K:31. 26 pp. MacDonald, P.D.M. &J. Pitcher 1979. Age-groups from size­ frequency data: A versatile and efficient method of analyzing distribution mixtures. J. Fish. Res. Bd. Can. 36 (2): 987-1001. Nilssen, E.M. 1984. A comparison of population structure, survival and fecundity in the deep-water prawn Pandalus borealis (Kroyer) on two tr~ing grounds in Balsfjord, northern Norway (In Norwegian). Cand. real. thesis, University of Tromso. 195 pp. Pope, J.A., A.R. Margetts, J.M. Hamley &E.F. Akyuz 1975. Manual of methods for fish stock assessment. Part 111. Se1ectivity of fishing gear. FAO Fish. Tech. Pap. 41 (Rev. 1): 65 pp. Ralston, M.L. &R.I. Jennrich 1978. Derivative-free nonlinear regression. - Pp. 312-322 in: Hogben, D. &D. Fife (eds.). Proc. Comp. Sci. Stat.: Tenth annual symposium of the interface. National Bureau of Standards Special Publication 503.

10 Nilssen, Larsen &Hopkins- Pandalus borealis Catch and Selection Ryan, T.A., B.L. Joiner &B.F. Ryan 1985. Minitab, Handbook (2nd edition). Duxbury Press, Boston. 379 pp. Pennsylvania State University, 154 pp. Sakhno, V.A. & K. Sadokhin 1980. Investigation on bottom trawl selectivity in relation to the Barents sea shrimp (Pandalus borealis). Cons perm. int. Explor. Mer CM 1980/B:7, 11 pp. Schnute, J. & D. Fournier 1980. A new approach to length­ frequency analysis: growth structure. Can J. Fish. Aquat. Sei. 37: 1337-1351. Seiring, J.V. 1982. Seasonal variations in oxygen uptake in the prawn Pandalus borealis Kr0yer from Balsfjord, northern Norway, in relation to age, biochemical composition and starvation, measured in the laboratory (In Norwegian). Cand. real. thesis, University of Troms0. 133 pp • Seiring, J.V. &C.C.E. Hopkins 1985. An analysis of starvation-· • stress on respiration rates, and body weight and composition in the deep-water prawn Pandalus borealis from Balsfjorden, northern Norway, measured during mid-surnrner in a laboratory experiment. Pp. 287-298, In: Marine biology of polar regions and effects of stress on marine organisms (J.S. Gray &M.E. Christiansen eds.). Wiley, Chichester. Shumway, S.E., H.C. Perkins, D.F. Schnick & A.P. Stickney 1985. Synopsis of biological data on the pink shrimp, Pandalus borealis Kr0yer, 1838. FAO Fish. Syn. 144. 57 pp. Thomassen, T. &0. Ulltang 1975. Report from mesh selection experiments on Pandalus borealis in Norwegian waters. Cons. perm. int. Explor. Mer CM 1975/K:51 , 6 pp.

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Nilssen, Larsen & Hopkins- Pandalus borealis Catch and Selection

Tables

Table I. Relevant details for the prawn trawls taken at the stations in Balsfjord (1-5) and Malangen (7). Distance towed over bottom in km, trawled volume nf· 'l< 103, cod-end and bag 5 catches in litres per 103 m3 , bag 5 swept volume in m3. St. No./Time Distance Trawl Cod-end Bag 5 Bag 5 Catch ratio volume catch volume catch Bag 5/cod-end Balsfjord

1 1122-1209 1.85 104 0.16 480 2.28 14.3 2 1326-1408 1.85 104 0.11 480 3.53 32.1 • 3 1451-1534 1.85 104 0.04 480 2.49 62.3 4 1629-1708 1.85 104 0.09 480 0.41 4.6 5 0757-1000 5.56 311 0.04 1445 1.38 34.5

Malangen

7 1555-1655 2.78 229 0.03 722 0.55 18.3 MEAN 0.08 1.77 27.6

95% CL 0.03-0.13 0.50-3.05 6.4-48.9 CV% 65.3 68.4 73.4

12 I I

___I--- Y~1~~m_~ 2B=Door spread I B r i die an 9 I e, a: I L=Bridle length h I 2X=Wing spread tana="l=O.237: 2S=Fishing line I 0 I a=13.3 I I ===:~ : I

I:I

I I • , :. r I : ....a~ 1 O.5m.l~ h_' /------a~~~;lo 14--*

Fig. 1. Trawl geometry (door and wing spreads, and angle of sweap)

.. _---,~----~ - _._--- .. - _.... _--- used in the prawn sampling investigations in Balsfjord and Malangen, northern Norway. Asterisks indicate the approximate attachment positions of the fine-meshed bags used to examine size-selection of prawns. r--~----==::: f- I , ( \ ( I ( ( ./ I( r( . 'l ((. ( { ( -- ( ------\ " , I I ( ( \ I ( ! ' 11. . ( (

Fig. 2. Sketch of attachment positions of fine-meshed bags (1-5) mounted on the prawn trawl used in the investigations in Balsfjord and Malangen, northern Norway. A

MAT Rtex mm

PA

35 20 50 1260

PA 1260

50

50

50 • B

,.~ ------...... ---''-----

TRAWL BELLY

Fig. 3. A) Specifications of fine-meshed nylon bags 1-3 used in the selection investigations in Balsfjord--and Malangen, northern Norway. B) sketch of bag 3 in position on the trawl's upper belly. • A MAT MAT Rtex mm Ti Rtex ------mm 100 10 30 PA 20 70 1260

30 50 PA 20 ------1260 50

50

50 ------

B

- Fig. 4. A) Specifications of the fine-meshed bag 5 (between the fishing-line and coco) used in the selection investigations in Balsfjord and Malangen, northern Norway. The "cod-end" of bag 5 had an inner net of 1 rnrn mesh. B) Sketch of bag in position. •

SALSFJORD MALANGEN ST.'s 1-5 ST. 7 eOD-END n=2357

1: " 85 t r n = 110 30

n= 24 ~20 z w Ü cr10 w 0..

0 SAG 5 30 n =10

20 n=936

10

o I, , 4 12 12 16 20 24

Fig. 5. Pandalus borealis, Balsfjord (stations 1-5) and Malangen (station 7). The percentage length-frequency distributions of prawns retained by the cod-end, collected by the fine-rneshed bag 4 on the cod-end, and in bag 5 under the trawl. •

BALSFJORD 100 0,0 x-x:I:I-··· 0/ .x'x;-'· x,x .,. X' I 80 x'" ,- 0/ Je' ,. cf' x' .,- 0-0' // 60 0' ii 40 oI ,l / 0' )(,lt • lt lt >- ••"./x. • • / ~ 20 .X_X·X-X-X-X~-~:X.::1S.o - tu .lt 0---- - ./ :::> ;x 0'" ...... o lC 0' ••- lC' (')' •.•- ~ 0 ltf-/_---lI....lx!t:/:...:/c:.°::-r.L....6J-...-...d-.b-....:lI.'-=dd..~=.II.l.:!.•~·.:..._-lI__-L..__-'-__L-_-'_ *- tu ~ 100 o bag 4 < • cod-end ..J ~ lC bag 5 ::;: ... cumulative % ~ 80 difference o

60

40

20

o ' II 6 8 10 12 14 16 18 20 22 24 CARAPACE LENGTH (mm)

Fig. 6. Pandalus borealis. Cumulative pereentage-frequeney distributions of prawns found in the eod-end, bag 4 (fine-meshed bag on eod-end), and in bag 5 (fine-meshed bag plaeed between fishing­ line and eoeo) for Balsfjord (Fig. 6A) and Malangen (Fig. 6B). The differenee between the eumulative pereentage distributions of prawns retained in the eod-end and those passing through the meshes and being retained in bag 4 are also platted far Malangen. BALSFJORD MALANGEN BALSFJORD MALANGEN STN's 1-5 STN 7 STN's 1-5 STN 7 . (CE4 L51 (CE 4 L 51 ~ (CE L 1 16.0 22.5 f I 15.5 22.0 T f f 1 15.0 f E 21.5 !f E fI f :r: 20.0 14.5 I- C) z w 19.5 I f 11.0 -l w '! u « 19.0 10.5 11 0.. « Cl: « 18.5 10.0 u ! !

18.0 ! 9.5 17.5 1 9.0 1" • 82 year-class 8.5 I f ... 83 -"- T 84 -11- • 85 -,,- 8.0 CE cod-end 4 bag 4 7.5 L sum (CE+4) 01. 5 bag 5

Fig. 7. Pandalus borealis. Mean carapace lengths with 95% confidence limits for the various age-groups/year-classes found in the cod-end and the fine-meshed bag 5 placed under the trawl for stations 1-5 (Balsfjord) and station 7 (Malangen).