On the efficiency of a two-metre beam trawl for juvenile plaice (Pleuronectes Platessa)

Netherlands Journal of Sea Research 9 (1) : 69-85 (1975)

ON THE EFFICIENCY OF A TWO-METRE BEAM TRAWL FOR JUVENILE PLAICE (PLEURONECTES PLATESSA) by B. K U I P E R S (Wetherlands Institute for Sea Research, Texel, The .Netherlands)

CONTENTS I. Introduction . . . . . . . . . . . . . . . . . II. Methods . . . . . . . . . . . . . . . . . . 1. Beam trawl experiments . . . . . . . . . . . . . 2. Enclosure experiments . . . . . . . . . . . . . III. Results of b e a m trawl experiments . . . . . . . . . . . 1. Disturbance by boat and motor . . . . . . . . . .

2. Lateral escape . . . . . . . . . . . 3.. Escape underneath . . . . . . . . . 4. Calculation of total efficiency . . . . . . IV. Results of experiments with the enclosure . . . . V . Discussion .

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VI. Summary . VII. References.

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69 70 71 73 74

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74 76 78 79 79

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84 84

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I. I N T R O D U C T I O N On the Balgzand, an extensive area of tidal flats in the most western part of the W a d d e n Sea, large numbers of juvenile plaice are present during spring and early summer. An ecological study, aimed at obtaining estimates of the predation pressure exerted by these juvenile plaice of mainly O-, I- and II-group upon the benthic fauna in the area, was made in the years 1972 to 1974 (KulPERS, 1973, 1975). Reliable estimates of the numbers of young plaice present on the Balgzand throughout the seasons are of basic importance to this study. Contrary to the situation in the adult North Sea plaice, where absolute numbers can be estimated from catch statistics (e.g. GULLAND, 1965), the numbers of juvenile plaice in an area without a commercial fishery can only be calculated from experimental sampling (RILEY & CORLETT, 1966; EDWARDS& STEELE, 1968; LOCKWOOD, 1972). From earlier work on the Balgzand (KuIPERS, 1973) it appeared that the young plaice population in the tidal area carries out a feeding migration over the tidal flats every high tide. Especially for I- and II-group plaice which were found to disperse quite regularly over the area, a

70

B. K U I P E R S

limited number of hauls was found to be sufficient to calculate mean catches with a standard error of the mean of only 30%. Reliable estimates on the absolute abundance can be derived from the mean catch per unit of area only when adequate information on the efficiency of the fishing gear is available. Some authors have presented efficiency data for several types of small gear for juvenile flatfish (RILEY • CORLETT, 1966; EDWARDS~¢ STEELE, 1968; LOCKWOOD, 1972). However, direct use of this information is hampered by the restriction that apart from differences in the shape and size of the nets efficiency figures are only valid under ambient environmental conditions. For that reason experiments were carried out in the area to establish the efficiency of the standard beam trawl used in the Balgzand studies. Acknowledgements.--This study would have been impossible without the help of Mr R. Dapper, M r J . Marquenie (his work on the enclosure especially), Mr W. de K a n and certainly without that of Mr W. van Arkel who carried out the statistical computations. II. M E T H O D S The experiments were carried out on the Balgzand around high water at 1 to 2 m depth. The bottom in the area mainly consists of fine and medium fine sands (with a silt content of less than 5%/oJ,~successively flattened by the tidal streams and disturbed by the characteristic burrowing activities of great numbers of Arenicola marina. Usually the

Fig. 1. The 2 m beam trawl used in the Balgzand plaice investigations; in detail the connection of the netting with a ground chain of the same size as the single tickler chain.

BEAM TRAWL

71

water is very turbid due to large quantities of suspended silt (Secchi disc readings less than 1.00 m); water temperatures varied between 14 ° and 20 ° C during the experiments. The 2 m beam trawl used to sample the young plaice population is shown in Fig. 1. The net was made of knotless nylon netting with a mesh size of 10 ram, stretched mesh. The net was towed by a rubber dinghy with an outboard motor of 20 HP, the draught of boat and motor being about 40 cm. The length of the fishing line was 8 m and the towing speed was kept between 30 and 35 m/min, following the indications of RILEY & CORLETT (1966). T w o basically different methods were used to estimate the efficiency of the fishing unit of boat and net. At first the possible sources of disturbance and escape causing reduction of the catch were analysed. The factors which are supposed to be effective are listed in the next section. A second approach, in which catches by the standard gear were compared with an independently determined abundance estimate is described in a following section. 1.

BEAM TRAWL" EXPERIMENTS

Disturbance and escape caused by boat and m o t o r . - - I n trawl fisheries, in shallow water in particular, fish might be disturbed and chased away by the boat before the net arrives. In order to measure this disturbance, catches of standard hauls were compared to those made in the same area with a 400 m long floating fishing line. This floating line drifted with the wind over an undisturbed area as it was veered. T h a n the end was picked up by the dinghy and a haul of 400 m was made. The proportion of "catch of standard gear" over "catch with 400 m fishing line" per unit of area gives a measure of the efficiency in respect of disturbance (Ea). Lateral escape.--Fishes may escape from the approaching net by evading to the sides. This problem can be simplified as follows: a certain part of the net (the central part) takes all fishes in front of it whereas the fishes in front of the rest of the net (the side parts) all manage to escape laterally. This lateral escape was measured by comparing catches of the standard 2 m net to those of a net constructed out of 2 standard nets (Fig. 2). If indeed the width of the zone of escape, called b, is equal for both nets (a basic assumption) and the mean catches of the single net (width a) and double net are c and C, respectively, then: C 2a ~ 2b c a--2b

72

B. KUIPERS a

2~

I b

Fig. 2. The 2 m and 4 m beam trawl as used in measurements on lateral escape; b indicates the zones with supposed lateral escape of 100%. Since the mouth width of the standard 2 m trawl is 190 cm: b=

½C-- c C__c

~.

190 em

The efficiency with respect to lateral escape (El) is: 190 - - 2b E1 -190 .100%

(1)

(2)

Escape underneath.--Some fishes present in front of the net may escape underneath the ground rope. It may be expected that by raising the number of tickler chains, catches increase up to a level where this kind of escape is negligible. Assuming that disturbance by boat and motor and lateral escape do not change significantly with the increasing weight of the net, this type of efficiency (Eu) is: standard catch Eu = m a x i m u m catch " 100% Mesh-selection.--The standard gear was fitted with net material with a mesh size of 1 cm stretched, through which only the youngest stages of the O-group (plaice <2.5 cm, ANON., 1969) are able to pass. Since in the present study this part of the population was of minor importance no further attention was given to this factor. Herding.--Herding of fish in front of the ground rope during fishing might shorten the strip actually fished. In plaice, herding has been observed by day in clear water over a distance of about 5 m (BLAxTER, PARRISH & DICKSON, 1964). Since this type of escape is mainly based on vision, in the turbid waters of the Wadden Sea herding will probably take place over smaller distances than 5 m. Moreover, with a

BEAM TRAWL

73

standard haul-length of about 400 m, herding over even 5 m distance will not have an appreciable effect on the c a t c h . Escape u p w a r d s . - - G e a r studies, carried out in the Netherlands, indicate that flatfish do not evade gears by escaping over the top of the net (KoRBEE, personal communication). Therefore, no attention was given to this escape possibility. 2. E N C L O S U R E

EXPERIMENTS

In order to measure the real number of plaice per unit area, and to compare the figures with the catches by the standard beam trawl, a circular enclosure was built on the flat. This approach differs basically from the analysis of the factors that influence the beam trawl catches. The enclosure (Fig. 3), principally after HELLIER (1958), consisted of

Fig. 3. T h e enclosure, with the net down, at low water. Radial lines connect releasing mechanisms with a single trigger line; the detail shows the net tied up at 2 m above the ground.

a seine net of 85 m length held in position by a circle of iron stakes. The net, weighted with a ground chain, enclosed an area of about 500 m 2. It could be folded up at a height of 2 m above the ground, not touching the water surface, and released from a distance at high tide. As soon as the flat became dry the fishes enclosed could be collected. Trials with small mesh sizes (5 × 5 and 15 × 15 mm) failed because the tidal currents lifted the net or washed holes in the bottom.

74

B. K U I P E R S

A mesh size of 30 x 30 mm seemed to be the minimum for proper operation. Before the experiments were started the enclosure was tested several times on escape possibilities. The net was closed at low tide, preventing the plaice to enter with the rising water. At high tide a known number of measured plaice was released within the enclosure. After withdrawal of the water the number and sizes of the plaice retrieved gave an idea of the recovery rate and mesh selection (Fig. 4). A length of 14 cm numbers of ploice Io I

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Fig. 4. Mean results of 3 mesh selection measurements with the enclosure (moving averages of 3). Above 14 cm length numbers retrieved (O) are about equal to numbers introduced (0). was chosen as the minimum length at which the enclosure data could be used: of an original number of 31 plaice smaller than 14 cm, only 8 were retrieved; 81 plaice larger than 14 cm were all recovered at low tide. The main objection to this method may be that the construction of stakes and stays might have influenced the distribution of the plaice in the enclosure area. Although this possibility could not be evaluated these density measurements are considered to give the most reliable estimate available of the actual density. III. RESULTS OF BEAM TRAWL EXPERIMENTS 1.

DISTURBANCE

BY

BOAT

AND

MOTOR

At 3 occasions 6 pairs of hauls with a 400 m and an 8 m fishing line were made in the Balgzand area (June 1973, April t974 and J u n e 1974). The numbers caught and the size distribution of the fish were considered. In order to cope with the small numbers per length class (1 cm) and the varying size distribution between series the data for all hauls were taken together for each of the 2 line-lengths (Fig. 5a). Thus the weighted mean effect of disturbance by boat and motor could

BEAM

TRAWL

75

be measured. When comparing the catches by each of the 2 linelengths for 3 length-groups (Table I) it clearly appears that escape due to disturbance increases with increasing size of the fish. To estimate the relation between Ea and length x (Fig. 5b), the moving averages of 3 (as pictured in Fig. 5a) were used as the numbers present in each length class. At the smallest length classes values of Ed exceed 100% which probably is due to the absence of any significant disturbance at these lengths (Ea = 100%) in combination with a high variance between hauls for O-group plaice as compared to I- and II-group (KuIPERS, 1973). In order to locate the length at which disturbance becomes significant, the weighted double logarithmic regression line was com-

E 160-

numbers of )loice

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5. a . Catches by length class of the 2 m 4 0 0 m (broken line) fishing line (moving

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20

length in crn

beam trawl with 8 m (solid line) and with averages o f 3 ) . b . Efficiency in respect o f disturbance (Ed) against length of the plaice; surface of the circles indicates weight of the points; the curve represents weighted double logarithmic regression line; broken lines indicate confidence limits (see page 8 3 ) .

76

B.

KUIPERS TABLE

I

Numbers of plaice caught in the undisturbed situation (long line) and by the standard fishing method (short line) for 3 length groups (the last column gives the ratio Ed). Length

Numbers Short line

Long line

Ratio

(%) <7 7-15 >15

124 69 18

114 87 27

108 79 67

p u t e d several times, leaving out m o r e of the smallest length classes successively, to obtain the best fit. T h u s leaving out the d a t a for plaice smaller t h a n 5 c m - - s h o w i n g a m e a n efficiency o f 1 0 0 % - - t h e following relation was calculated: In E d - - 6.147 - - 0.538 l n x and presented in Fig. 5b (r = 0.54, p < 0.01). T h e linear regression line fits the d a t a even as well as the logarithmic one. H o w e v e r , the linear relation cuts the abscissa at a length of 19.7 cm whereas above this length the net still catches some plaice. W h y in general efficiencies will have an exponential relation with length will be a r g u m e n t e d on page 81.

2.

LATERAL

ESCAPE

T o measure the effect of lateral escape, 6 pairs of hauls were carried out in S e p t e m b e r 1973. By c o m b i n i n g the d a t a o f all 2 m a n d all 4 m hauls the m e a n result o f the experiments can be analysed. T h e total n u m b e r s o f plaice taken by the nets were calculated for the same length groups as used in T a b l e I ( T a b l e II). T h e total n u m b e r s of plaice per length class taken by the 2 gears TABLE

II

Numbers of plaice taken by the 2 m and 4 m beam trawl for 3 size classes. Length (cm)

<7 7-15 >15

Numbers 4 m trawl

2 m trawl

Ratio

164 316 237

79 108 72

2.07 2.92 3.29

BEAM T R A W L

77

are given in Fig. 6a. At each length class the size of the "non-fishing" part of the net and the efficiency Ex were calculated according the equations (1) and (2). Computation of the weighted double logarithmic regression line, again leaving out plaice smaller than 5 cm, resulted in the relation: In E1 = 6.535 -- 0.818 lnx which is shown in Fig. 6b (r = 0.52, p < 0.01). E1 reaches 100% at a length of 4.5 cm, indicating that no lateral escape occurred below that size. The efficiency of over 100% measured below 4.5 cm must, in view of the differences in the estimates between series, be contributed to the high variance in the O-group catches. ~umbers

~f )loice 160

30 140-

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25

120-

|

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20

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I00-

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i

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2'0 in cm Fig. 6. a. Catches by length class of the 2 m (solid line) and of the 4 m (broken llne) beam trawl (moving averages of 3). b. Efficiency in respect of lateral escape (El) against length of the plaice; surface of the circles indicates weight of the points; the curve represents weighted double logarithmic regression line; broken lines indicate confidence limits (see page 83). length in c m

;

,b

,;

length

78

B. K U I P E R S

ZIJLSTRA(1972) reports the catch of a 5.6 m beam trawl to be about 2.7 times that of a 2.8 m one, for larger O-group plaice (c. 10 cm). This would indicate an efficiency E1 of approximately 50% which is within the range of the present estimates for smaller sized trawls. 3. ESCAPE U N D E R N E A T H

Series of hauls with a changing number of chains were made in September 1972 and J u n e 1973. The experiments were planned in such a way that with each number of tickler chains an equal number of hauls was made at the same position. A number of 3 tickler chains proved to be the maximum for proper operation; with 4 chains the numbers o f )loice 40-

35

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i

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^ < ~

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54I !

ol 0

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20 length in c m

Fig. 7. Catch by length class of the standard 2 m beam trawl (heavy line) compared to catches by the same gear without tickler chain (thin line), with 2 (broken line) and with 3 (dotted line) tickler chains.

BEAM

TRAWL

79

TABLE I I I

Numbers of plaice of 3 length-groups caught with a different number of ticklerchains.

Length (~m)

<7 7-15 >15

Ni~mbers Wo chain

1 chain

2 chains

3 chains

101 42 9

164 71 12

163 69 8

140 70 9

net stuck too often in the sandy bottom. The numbers of plaice caught were taken together for all hauls with the same number of chains (Fig. 7) and divided over 3 length groups (Table III). The results demonstrate that catches already reach a maximum with the use of 1 tickler chain which, by the way, was the standard fishing method. Therefore, it is concluded that escape underneath the net did not play a significant part in the efficiency of the standard net. 4.

CALCULATION

OF TOTAL

EFFICIENCY

The experiments described indicate that net efficiency of the standard gear mainly is the result of 9 factors, the disturbance by the boat and the lateral escape from the mouth of the net. In order to combine the effect of the key factors the 2 efficiency estimates Ea and E1 were multiplied for each length class. The resulting efficiency E is thought to represent for each ½ cm length class the ratio mean number caught over mean number present in the area fished by the standard gear (Fig. 8). The weighted double logarithmic regression line was calculated; this curve must be considered as providing the best estimate of the efficiency-length relation for the standard 2 m beam trawl. The estimated relation between E and length is: In E = 8.387 -- 1.558 lnx where x is length measured to the ½ cm below; r = 0.61 (p < 0.01). IV. R E S U L T S OF E X P E R I M E N T S W I T H THE E N C L O S U R E In total 15 times an enclosure experiment was carried out. In all cases the net was closed around high water. Shortly thereafter 2 trawl hauls were made to compare the numbers caught per unit of area by the 2 methods. However, most of times when the water left the flat it appeared that the enclosure net had failed to work because the ground

80

B. K U I P E R S

chain did not t o u c h the b o t t o m over its full length due to for instance the presence o f seaweed or crabs. A l r e a d y a small opening, over 50 c m of the chain, a p p e a r e d to be sufficient for nearly all plaice to escape. A p p a r e n t l y these fish once enclosed by the net searched all along the g r o u n d rope for an opening. At only 3 occasions the device

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Io

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80

60"

40-

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"'"{2

rn

I

20-

I I

o o

I

;

,~

,;

2%

length in cm

Fig. 8. Efficiency of the 2 m beam trawl plotted against length of the plaice. Surface of the circles indicates weight of the points; solid part of the curve represents weighted double logarithmic regression calculated as E = Ea × E1 (moving averages from Figs 5a and 6a) ; confidence limits indicated (see page 83). Surface of the squares indicates weight of efficiency percentages measured with the enclosure method. Dotted part of the efficiency curve for plaice smaller than 5 cm calculated for an accepted selection factor and range of selection of the net of 2.2 and 1.6 cm. f u n c t i o n e d properly. T h e n u m b e r s of ptaice (between 14 and 20.5 cm) c a u g h t within the enclosure a n d those taken b y the pairs o f s t a n d a r d hauls are c o m p a r e d in T a b l e IV. T h e m e a n n u m b e r o f plaice c a u g h t by the enclosure at the o t h e r 12 occasions was 9.6, whereas b y the hauls (per 500 m 2) a m e a n o f 11.3 was obtained. These experiments, where i n d e e d openings in the enclosure were observed, give no indication w h a t e v e r o f the efficiency o f the trawl. At the m e a n length of all plaice involved in the enclosure experi-

BEAM T R A W L

81

TABLE IV Number of plaice between 14 and 20.5 cm caught by the enclosure net and by beam trawl hauls compared; data of 1973.

Gear

fifumbers June 22

Enclosure (500 ms) Hauls (per 500 ms) Mean efficiency

44 10.4 23.6%

ffune 26

41 12.5 30.4%

3une 27

40 12.5 31.2%

ments, 16.4 cm, an efficiency of only 20% was measured in the analysis of separate factors. When from the enclosure data for all length classes of 0.5 cm separately a mean trawl efficiency is calculated (Fig. 8) it appears that the difference in results between both methods is mainly caused by the plaice larger than 18.5 cm; at smaller lengths the 2 methods gave about equal efficiency percentages (Fig. 8). V. DISCUSSION The results of this study indicate the efficiency of a small 2 m beam trawl to be dependent on the length of the fish, in the sense that the efficiency declines when length increases. This result is not surprising since m a x i m u m swimming speed - - undoubtedly the most important factor for escape - - shows a positive linear relationship with length (BLAXTER& DICKSON, 1958). Furthermore, the susceptibility to disturbance (alertness) will develop with a g e - - b y for instance learning processes--and again a positive correlation between the effect of disturbance and length may be expected. Therefore, in fish, length affects the ability to evade an active fishing gear in at least 2 ways. In lateral escape, as well as in escape caused by boat and motor, swimming speed and alertness will be involved. This in fact is the reason why in the calculations the efficiencies E1 and Ed are expressed as exponential function~ of length. The same reasoning holds for the total efficiency E that moreover is the product of again 2 factors. The decline of the efficiency of the net is rather drastic; from approximately 100% in plaice under 7 cm it decreases to 15 to 30% in plaice over 15 cm; the low efficiency estimate for the larger plaice being obtained by 2 independent methods. Although the agreement between the results of the 2 methods used to estimate the efficiency of the beam trawl suggests that the actual efficiency might be approached fairly well, several fundamental assumptions have to be discussed. The enclosure technique presupposes the distribution inside and

82

B. K U I P E R S

outside the fence to be the same. As was stated already (page 74) the presence of the enclosure might have had the effect that density inside stayed lower than outside, and the efficiency would have been overestimated. Since larger plaice seem to be more susceptible to disturbance than smaller ones (cf. disturbance by boat) the discrepancy between the results of both methods for plaice larger than 18.5 cm may be explained by the disturbing presence of the fence. Also the analysis of separate factors could have led to an overestimated efficiency. In this case the standard hauls were compared to hauls in which certain alterations were introduced in order to exclude one specific possibility of escape or disturbance. However, it is questionable to what extend this approach was successful. With the floating fishing line technique disturbance might have been diminished but not excluded and Ea, therefore, overestimated. In comparing the catches of the 2 m and 4 m beam trawl, the latter catches might have been too small because of a greater disturbance by the larger net. This would have resulted in an overestimation of El. Lastly, the catch of the net with some extra tickler chains might, apart from having been increased because of a diminished escape underneath, at the same time have been negatively effected due to a greater disturbance. This, in fact, is indicated by the decrease in catch with 2 and 3 tickler chains (Table III). Therefore, the efficiency of the net will have been rather lower than higher as estimated. For O-group plaice (between 2.5 and 6 cm) the results of this study are less precise than for I- and II-group because of a high variance between hauls. The results indicate that the net took almost 100%. Moreover, mesh selection operated in the lower length ranges of the O-group (below a length of 3 to 3.5 cm with the mesh size as used). The possible effect of mesh selection, accepting a selection factor of 2.2 (ANON., 1969) is indicated in Fig. 8 which provides a picture of the efficiency of the standard net for all length classes encountered. Comparable data for similar types of gear cited in the literature, all referring to mean efficiency values for O-group plaice (Table V), TABLE

V

Effieiencies of comparable nets fbr juvenile plaice (all mesh sizes involved "90 rows] yard"). Gear

4 m beam trawl 2 m beam trawl Riley net

Length=group Efficiency (ram) (%)

O-group c. 15-35 10-100

33-57 25-35 30-60

Author

RILEY& CORLETT(1966) EDWARDS& STEELE (1968) LOCKWOOD(1972)

BEAM TRAWL

83

are low as compared with our data for O-group. This may be due to the different net design or, more likely, to the clearer water in the areas studied. Information on the reliability (confidence limits) of these estimates is scarce. Net efficiency has sometimes been applied as constant over the whole length range of the fish (EDWARDS • STEELE, 1968) or, in contrast to the present results, as increasing with length (LocKwooD, 1972). It is clear that the application of ill-defined net efficiencies will lead to errors in population and production studies. For instance, length distribution as calculated from the catch with the use of a constant efficiency, will not reflect that of the natural population and (cf. Fig. 8) mean lengths as well as biomass and food-intake will be underestimated. Also when efficiency decreases with increasing length of the fish, the use of a constant efficiency during the growth season (e.g. EDWARDS & STEELEfrom J u l y onwards) will result in an overestimated mortality rate and thereby in an underestimated fish production. As stated in the Introduction, the main aim of the study on the Balgzand plaice population, is an estimation of the overall predation pressure exerted on secondary producers. The efficiency curve presented here plays an important part in these calculations. Possibly the curve is not accurate for length classes under 5 cm. However, the poorer precision of the efficiency estimates for these O-group plaice is of minor importance for estimates on the 1973 Balgzand population since biomass and food-intake of this group appeared to be negligible as compared to those of the I- and II-group. The efficiency for I- and II-group will have been estimated most accurately for the numerically dominating length groups. These length groups have, at the same time, the greatest share in the predation under study. Furthermore, the fact that the efficiency figures might be too high will lead to underestimates of the exerted predation which are useful as they indicate the lower limits of this predation. The confidence limits of the exponential curves presented in Figs 5b, 6b and 8 were calculated as follows. At each length class of 0.5 cm the deviation of the observed percentage (as calculated from moving averages of 3) from the mean percentage read from the curve was used to calculate the standard error of the mean. The broken lines in Figs 5b, 6b and 8 indicate the average 95% confidence limits along the calculated curves for Ea, E1 and E, respectively. The reliability ranges of each point on the curves thus estimated are not directly based on the variance in the experimental data. However, the latter variance will not only be caused by fluctuations in efficiency of the net, but rather by fluctuations in density of the plaice during the experiments.

84

B.

KUIPERS

T h e confidence limits as presented in the figures, therefore, are actually overestimates in respect o f the net efficiency itself. I t might have been possible to estimate that p a r t o f the variance due to fluctuations in d e n s i t y - - u s i n g d a t a f r o m the s t a n d a r d sampling p r o g r a m carried out on the Balgzand at the same t i m e - - a n d thus to indicate to w h a t extent fluctuations in actual efficiency occurred. H o w e v e r , no attempts were m a d e to analyse the variance a n y further, as the overestimated reliability ranges o f the efficiency curve presented in Fig. 8 are considered to meet the r e q u i r e m e n t s o f future calculations. VI. S U M M A R Y Some measurements on the fishing efficiency o f a 2 m e t r e b e a m trawl used in plaice studies in the W a d d e n Sea are described. E x p e r i m e n t a l hauls were m a d e in o r d e r to measure the r e d u c i n g effect o f specific characteristics o f the fishing p r o c e d u r e on the catch. I n a second a p p r o a c h catches by the s t a n d a r d s a m p l i n g m e t h o d were c o m p a r e d to those o b t a i n e d by an enclosure net. Both methods led to a b o u t the same efficiency estimate for I I - g r o u p plaice ( a p p r o x i m a t e l y 2 0 % ) , whereas, as a result o f the c o m p a r a t i v e hauls, a negative correlation between fish length a n d efficiency was found. VII. REFERENCES

ANON., 1969. Report of the working group on assessment of demersal species in the North Sea. ICES Coop. Res. Rep. Series A, no. 9: 1-74. BLAXTER,J. H. S. & W. DIcacsoN, 1958. Observations on the swimming speed of fish.--J. Cons. perm. int. Explor. Mer 24 (1): 472-479. BL~.XTSR,J. H. S., B. B. PARRmH& W. DICKSON, 1964. The importance of vision in the reaction of ~ to drfftnets and trawls. Modern fishing gear of the world 2. Fishing News (Books), London. EDWARDS, R. & J. H. STEELE, 1968. The ecology of O-group plaice and common dabs at Loch Ewe. I. Population and food.--J, exp. mar. Biol. Ecol. 2t 215--238. GULLAND,J. A., 1965. Estimation of mortality rates. Annex to Arctic Fisheries Working Group Report. ICES, C.M. 1965, Doc. no. 3 (mimeo). HELLmR, T. R., 1958. The drop-net quadrat, a new population sampling device.-Pubis Inst. mar. Sci. Univ. Tex. 5" 165-167. KUmERS, B., 1973. On the tidal migration of young plaice (Pl~uronectes platessa) in the Wadden Sea.--Neth. J. Sea Res. 6: 376-388. , 1975. Experiments and field observations on the daffy food intake of juvenile plaice (Pleuror~aes ptatessa L.). In: H. BARNES. Proc. 9th Europ. mar. biol. Syrup. Aberdeen University Press: 1-12. LOCKWOOD, S. J., 1972. An ecological survey of an O-group plaice (Pleuron~aes platessa L.) population, Fiiey Bay, Yorkshire. University of East Anglia, England (thesis) : 1-167.

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RILEY, J. D. & J. CORLETT, 1966. The numbers of O-group plaice in Port Erin Bay, 1964--66.--Ann. Rep. mar. biol. Sm Port Erin 78: 51-56. ZXJLS'rRA,J. J., 1972. On the importance of the Waddensea as a nurserey area in relation to the conservation of southern North Sea fishery resources.---Syrup. zool. Soc. Lond. 29: 233-258.