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Changes in the length compositions of some fish species as a consequence of alterations in the groundgear of the GOV-trawl
Fisheries Research 49 (2000) 39±50
Changes in the length compositions of some ®sh species as a consequence of alterations in the groundgear of the GOV-trawl E. Dahm* Federal Research Board for Fisheries, Institute for Fishery Technology, Palmaille 9, 22767 Hamburg, Germany Received 21 July 1999; received in revised form 5 January 2000; accepted 3 March 2000
Abstract Experiments using bagnets under the groundgear have shown that the ef®ciency of sampling trawls for different species and length groups of ®shes varies with the type of groundgear. Critics of the results claim that properties of the sampling gear change due to the attachment of the collection bags. The present contribution addresses these criticisms by comparing the length distributions of selected species obtained from catches with the same net but different rollergear. The results generally con®rm the original bagnet experiments. In agreement with earlier bagnet results for some bottom dwelling ®shes, the same changes in ef®ciency of the sampling trawl depending on ®sh length could be detected. For other species Ð also in agreement with existing knowledge Ð there was no effect when groundgear was changed. Furthermore, the present results hint at an increased ef®ciency for capture of larger ®sh of some species in the case of ®shing lines lifted by groundgear. Possible reasons for this phenomenon are discussed. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Survey trawls; GOV-trawl; Ef®ciency; Groundgears; Fish reactions on groundgear types
1. Introduction The ®shing ef®ciency of sampling trawls has been a matter of interest for a number of years. This concern was noted particularly in the early 1970s, when scientists working with underwater cameras detected that large numbers of ®sh observed in front of the groundrope were not caught but escaped through the gaps between the large components of roller groundgear. Early attempts to quantify this effect were made by changing the rollergear (Ehrich, 1987). An effective procedure to catch the escaping ®sh was developed simultaneously with such experiments by Norwegian scientists (EngaÊs and Godù, 1986), using small ¯at
* Tel.: 49-40-3890-5188; fax: 49-40-3890-5264. E-mail address: [email protected] (E. Dahm).
trawls, the so-called bagnets, fastened beneath the groundrope and behind the rollergear. This method has been used on a number of sampling trawls (Walsh, 1989; Godù and Walsh, 1992; Dahm and Wienbeck, 1992, 1996; Dahm, 1997; Munro et al., 1997). The results obtained were of great concern to stock assessment biologists. They suggest that some species enter a sampling trawl only by chance and that with most other species there is a serious lengthrelated bias. Even worse, serious differences in ef®ciency were observed on different ®shing grounds and at different times of the day. It has been attempted to refute these results by a criticism of the method. It has been claimed that attaching the bagnets at the trawl changes its water resistance and subsequently its ground contact and performance. However, accompanying net geometry measurements did not support this hypothesis and
0165-7836/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 7 8 3 6 ( 0 0 ) 0 0 1 9 2 - 2
40
E. Dahm / Fisheries Research 49 (2000) 39±50
Fig. 1. Area of operation of FRV ``Walther Herwig III'' during the experiment.
E. Dahm / Fisheries Research 49 (2000) 39±50
Fig. 2. Rollergear configurations tested.
41
42
E. Dahm / Fisheries Research 49 (2000) 39±50
Fig. 3. Reaction of haddock to changes in rollergear configuration at the GOV-trawl: (A) with Scottish rollergear; (B) with experimental rollergear.
lifting of the ®shing line, which could have caused this, would be hard to prove with underwater television due to the masking effect of sandclouds. Hence, effort was required to extend evidence that supported the bagnet results by an independent method, as, e.g. the experiment of Ehrich (1987). Particular emphasis should be laid this time on controlled conditions and comparability and also on other species than those which he had investigated.
2. Methodological approach The experiment was carried out from 22 to 29 November 1998 in a con®ned area of the Viking Bank, southwest of Norway (Fig. 1). A GOV bottom trawl with a codend of 50 mm mesh opening was used. The rollergear beneath this trawl was changed every second day. Three con®gurations were tested (see Fig. 2):
E. Dahm / Fisheries Research 49 (2000) 39±50
43
Fig. 4. Reaction of herring to changes in rollergear configuration at the GOV-trawl: (A) with Scottish rollergear; (B) with experimental rollergear.
1. The standard rollergear (Groundgear A) as described in detail by HagstroÈm et al. (1992), in eight hauls with average catches of 400 kg per haul. 2. The rollergear type for the GOV-trawl (Groundgear B) as used by Scottish scientists on hard and otherwise untrawlable ®shing grounds. It consists of nine ``spoked'' rubber discs of 30.5 cm diameter in the bosom kept in position by spacers of 80 mm diameter. The ®rst wing section on each side was of similar design except that the discs
were replaced by ``spoked'' rubber hemispheres of equal diameter. The second, third and fourth wing sections each had one single hemisphere with the rest covered by the mentioned spacers of 80 mm diameter. This con®guration was tested on 10 hauls with average catches of 460 kg per haul. 3. An experimental rollergear (Groundgear C) composed of rubber discs of 200 mm diameter cut out of used truck tyres. Spacers kept the rubber discs 50 cm apart. The bosom, ®rst and second wing
44
E. Dahm / Fisheries Research 49 (2000) 39±50
Fig. 5. Reaction of grey gurnard to changes in rollergear configuration at the GOV-trawl: (A) with Scottish rollergear; (B) with experimental rollergear.
sections were of similar design, whereas the third and fourth wing sections were as for the Groundgear B. Seven hauls were performed with this con®guration and average catches of 820 kg per haul obtained. Trials were started with con®guration 1. After 2 days con®guration 2 was used. Experiments with con®guration 3 ®nished the series after 4 days.
Catches were sorted to species, weighed and measured. Large catches were subsampled. For evaluation of con®gurations 2 and 3, the pooled length frequency distributions of each species having signi®cant numbers were normalised by multiplying the catch of each length class by the ratio of the total catch of the respective con®guration to that of the standard con®guration. The percentage deviation in each length class from the standard was then determined. A
E. Dahm / Fisheries Research 49 (2000) 39±50
45
Fig. 6. Reaction of horse mackerel to changes in rollergear configuration at the GOV-trawl: (A) with Scottish rollergear; (B) with experimental rollergear.
weighed least-squares quadratic regression was calculated for every species and compared with the standard to determine general trends. Compared to a linear regression, the quadratic regression yielded a better ®t in all cases. A test for goodness of ®t of the quadratic regression to the single points demonstrated signi®cant correlation (r-value) at the 99% level for all seven species except haddock and grey gurnard in the case of the experimental groundgear (Groundgear C).
3. Results Twenty-®ve hauls of 1 h average duration were made. A total of 85 465 ®sh was caught, composed by 29 species. Only seven of the species, as illustrated in Figs. 3±9, showed suf®cient abundance for subsequent evaluation by the method described. The ®gures demonstrate that lifting the groundrope off the bottom and creating escape holes in the
46
E. Dahm / Fisheries Research 49 (2000) 39±50
Fig. 7. Reaction of mackerel to changes in rollergear configuration at the GOV-trawl: (A) with Scottish rollergear; (B) with experimental rollergear.
rollergear offers escape possibilities which particularly the young ones of certain species (grey gurnard, whiting, interestingly also saithe and horse mackerel) apparently use. They ®t, thus, to the reaction type 3 (``stay ahead of the danger and look for an escape opportunity'') as classi®ed in Dahm (1997). On the contrary, however, other young ®sh (haddock, herring) seem not to be affected or even to be caught more easily than without lifting elements in the
groundgear. Their type of reaction to the trawl is more in accordance with No. 2 (``Jump as high as possible and escape'') of Dahm (1997). 4. Discussion Some of this research con®rms the ®ndings of the bagnet investigations discussed in the introduction,
E. Dahm / Fisheries Research 49 (2000) 39±50
47
Fig. 8. Reaction of saithe to changes in rollergear configuration at the GOV-trawl: (A) with Scottish rollergear; (B) with experimental rollergear.
even though a completely different approach has been used to investigate the possible in¯uence of rollergear design on the same trawl. In addition, this investigation hints at a hitherto unknown phenomenon. For bigger ®sh in at least some length classes, the results suggest an increased ef®ciency of the nets with groundropes lifted off bottom depending on the groundgear used. This was also
shown for cod by Ehrich (1987), but has not attracted attention. There is, apparently, a considerable length-dependent increase of catchability with groundgear rigs B and C for horse mackerel, saithe and grey gurnard. But the opposite can be observed, too: a decreasing positive ef®ciency from smaller to bigger size (haddock, mackerel).
48
E. Dahm / Fisheries Research 49 (2000) 39±50
Fig. 9. Reaction of whiting to changes in rollergear configuration at the GOV-trawl: (A) with Scottish rollergear; (B) with experimental rollergear.
Does the changing impact of other rollergear con®gurations on the sea bottom, or changes in the underwater noise created, also in¯uence the behaviour of bigger ®sh? Though this is an interesting hypothesis which, if veri®ed, would have considerable consequences on comparisons of different con®gurations of the same sampling gear or on the sampling procedure, there are still doubts.
One argument against the observed phenomenon is that ®shes sometimes show a variation in size distribution with water depth. Though the data were collected in a relatively con®ned area (126 square nautical miles) this possible source of variance was not taken into account, and thus, the catches made with the last con®guration (con®guration 3) are concentrated over the eastern, 40 m deeper parts of the area. Fortunately, however, the hauls with the Scottish
E. Dahm / Fisheries Research 49 (2000) 39±50
49
Table 1 Results of chi-square test on differences in length composition of catches taken with the Scottish rollergear (Groundgear B) in the western and eastern part of the fishing ground Species
w2 (sum)
DF
Significance threshold at 99% level
Difference significant
Haddock Herring Grey gurnard Horse mackerel Mackerel Saithe Whiting
12.76598 15.47466 43.19372 7.992981 84.88295 7.959528 12.77422
32 13 27 12 22 35 32
62.49 34.53 55.48 32.91 48.27 66.62 62.49
No No No No Yes No No
rollergear (Groundgear B) were made over both depths and thus allow for a study of possible depth effects. All size distributions from the west (hauls 23± 29) were compared to those from the east part of the area (hauls 30±33) by means of a chi-square test (Table 1). In most cases, no signi®cant difference of the size distributions could be detected. Obviously, however, there is only one case with diverging length composition. This is mackerel, where 26 specimens were caught in the east giving the length distribution a high degree of uncertainty. In conclusion, there seems to be no evidence that length distributions vary with depth of the ®shing ground, so this cannot explain changes in ef®ciency. The observed phenomena may still be due to the fact that it was not possible to ®sh all three con®gurations synchronously. One could speculate that increased creation of sandclouds caused by discs dragged sideways over the sea bottom as in the Scottish and the experimental rollergear could possibly increase the stimulus to rise and get caught and that bigger ®sh are more likely to be caught. There is, however, also the other contradictory explanation, that after 2 days of ®shing on a con®ned area only the big ®sh able to escape several times from increased ®shing effort are left and available to the net. Hence it should not be surprising if the trawls used to catch the bigger ®sh later. But then why is this valid only for distinct species? Could different degrees of immigration into the ®shing ground and consequent replacement of the thinned-out population explain this, or is the different swimming ability of species an acceptable explanation?
5. Conclusion The data presented here suggest that results of the bagnet investigations on young ®sh are valid, and support the idea that lifting the groundrope off bottom can have a positive effect on the catchability, at least for bigger ®sh of some species. To exclude time and spatial effects completely, however, a repetition of the experiment with a double or triple net con®guration enabling the synchronous comparison of different con®gurations of the same net might be helpful. A double net con®guration, however, would not be the true sampling gear in question, and therefore, might cause a new discussion on the validity of such comparison.
Acknowledgements The author wishes to express his sincere thanks to his colleagues Klaus Lange and Dr. Siegfried Ehrich for helpful criticism of earlier versions of this manuscript and to Dr. Richard Ferro for further helpful suggestions and a language check of the ®nal version.
References Dahm, E.,1997. Escape strategies of different marine species under the footrope of survey trawls. ICES A.S.C./W:22. Dahm, E., Wienbeck, H., 1992. Escapement of fish underneath the groundrope of a standard bottom trawl used for stock assessment purposes in North Sea. ICES C.M./B:20.
50
E. Dahm / Fisheries Research 49 (2000) 39±50
Dahm, E., Wienbeck, H., 1996. New facts on the efficiency or total gear selectivity of German survey bottom trawls Ð possible effects on stock assessment and stock protection. ICES C.M./ B:8. Ehrich, S., 1987. The portion of young cod escaping under the GOV-trawl rigged with a heavy bobbin footrope. ICES C.M./B:28. EngaÊs, A., Godù, O.R., 1986. Preliminary results of investigations on escapement of fish under the fishing line of a Norwegian sampling trawl. ICES C.M./B:30. Godù, O.R., Walsh, S., 1992. Escapement of fish during bottom
trawl sampling Ð implications for resource assessment. Fish Res. 13, 281±292. HagstroÈm et al., 1992. Evaluation of sources of variability in fishing power of the GOV trawl. Report from FTFB Subgroup. ICES C.M./B:39. Munro, P.T., Weinberg, K.L., Somerton, D., 1997. Estimating sizedependent capture rates at the footrope of two different kinds of survey trawls. ICES A.S.C./W:06. Walsh, S., 1989. Escapement of fish underneath the footgear of a groundfish survey trawl. ICES C.M./B:21.
Changes in the length compositions of some ®sh species as a consequence of alterations in the groundgear of the GOV-trawl E. Dahm* Federal Research Board for Fisheries, Institute for Fishery Technology, Palmaille 9, 22767 Hamburg, Germany Received 21 July 1999; received in revised form 5 January 2000; accepted 3 March 2000
Abstract Experiments using bagnets under the groundgear have shown that the ef®ciency of sampling trawls for different species and length groups of ®shes varies with the type of groundgear. Critics of the results claim that properties of the sampling gear change due to the attachment of the collection bags. The present contribution addresses these criticisms by comparing the length distributions of selected species obtained from catches with the same net but different rollergear. The results generally con®rm the original bagnet experiments. In agreement with earlier bagnet results for some bottom dwelling ®shes, the same changes in ef®ciency of the sampling trawl depending on ®sh length could be detected. For other species Ð also in agreement with existing knowledge Ð there was no effect when groundgear was changed. Furthermore, the present results hint at an increased ef®ciency for capture of larger ®sh of some species in the case of ®shing lines lifted by groundgear. Possible reasons for this phenomenon are discussed. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Survey trawls; GOV-trawl; Ef®ciency; Groundgears; Fish reactions on groundgear types
1. Introduction The ®shing ef®ciency of sampling trawls has been a matter of interest for a number of years. This concern was noted particularly in the early 1970s, when scientists working with underwater cameras detected that large numbers of ®sh observed in front of the groundrope were not caught but escaped through the gaps between the large components of roller groundgear. Early attempts to quantify this effect were made by changing the rollergear (Ehrich, 1987). An effective procedure to catch the escaping ®sh was developed simultaneously with such experiments by Norwegian scientists (EngaÊs and Godù, 1986), using small ¯at
* Tel.: 49-40-3890-5188; fax: 49-40-3890-5264. E-mail address: [email protected] (E. Dahm).
trawls, the so-called bagnets, fastened beneath the groundrope and behind the rollergear. This method has been used on a number of sampling trawls (Walsh, 1989; Godù and Walsh, 1992; Dahm and Wienbeck, 1992, 1996; Dahm, 1997; Munro et al., 1997). The results obtained were of great concern to stock assessment biologists. They suggest that some species enter a sampling trawl only by chance and that with most other species there is a serious lengthrelated bias. Even worse, serious differences in ef®ciency were observed on different ®shing grounds and at different times of the day. It has been attempted to refute these results by a criticism of the method. It has been claimed that attaching the bagnets at the trawl changes its water resistance and subsequently its ground contact and performance. However, accompanying net geometry measurements did not support this hypothesis and
0165-7836/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 7 8 3 6 ( 0 0 ) 0 0 1 9 2 - 2
40
E. Dahm / Fisheries Research 49 (2000) 39±50
Fig. 1. Area of operation of FRV ``Walther Herwig III'' during the experiment.
E. Dahm / Fisheries Research 49 (2000) 39±50
Fig. 2. Rollergear configurations tested.
41
42
E. Dahm / Fisheries Research 49 (2000) 39±50
Fig. 3. Reaction of haddock to changes in rollergear configuration at the GOV-trawl: (A) with Scottish rollergear; (B) with experimental rollergear.
lifting of the ®shing line, which could have caused this, would be hard to prove with underwater television due to the masking effect of sandclouds. Hence, effort was required to extend evidence that supported the bagnet results by an independent method, as, e.g. the experiment of Ehrich (1987). Particular emphasis should be laid this time on controlled conditions and comparability and also on other species than those which he had investigated.
2. Methodological approach The experiment was carried out from 22 to 29 November 1998 in a con®ned area of the Viking Bank, southwest of Norway (Fig. 1). A GOV bottom trawl with a codend of 50 mm mesh opening was used. The rollergear beneath this trawl was changed every second day. Three con®gurations were tested (see Fig. 2):
E. Dahm / Fisheries Research 49 (2000) 39±50
43
Fig. 4. Reaction of herring to changes in rollergear configuration at the GOV-trawl: (A) with Scottish rollergear; (B) with experimental rollergear.
1. The standard rollergear (Groundgear A) as described in detail by HagstroÈm et al. (1992), in eight hauls with average catches of 400 kg per haul. 2. The rollergear type for the GOV-trawl (Groundgear B) as used by Scottish scientists on hard and otherwise untrawlable ®shing grounds. It consists of nine ``spoked'' rubber discs of 30.5 cm diameter in the bosom kept in position by spacers of 80 mm diameter. The ®rst wing section on each side was of similar design except that the discs
were replaced by ``spoked'' rubber hemispheres of equal diameter. The second, third and fourth wing sections each had one single hemisphere with the rest covered by the mentioned spacers of 80 mm diameter. This con®guration was tested on 10 hauls with average catches of 460 kg per haul. 3. An experimental rollergear (Groundgear C) composed of rubber discs of 200 mm diameter cut out of used truck tyres. Spacers kept the rubber discs 50 cm apart. The bosom, ®rst and second wing
44
E. Dahm / Fisheries Research 49 (2000) 39±50
Fig. 5. Reaction of grey gurnard to changes in rollergear configuration at the GOV-trawl: (A) with Scottish rollergear; (B) with experimental rollergear.
sections were of similar design, whereas the third and fourth wing sections were as for the Groundgear B. Seven hauls were performed with this con®guration and average catches of 820 kg per haul obtained. Trials were started with con®guration 1. After 2 days con®guration 2 was used. Experiments with con®guration 3 ®nished the series after 4 days.
Catches were sorted to species, weighed and measured. Large catches were subsampled. For evaluation of con®gurations 2 and 3, the pooled length frequency distributions of each species having signi®cant numbers were normalised by multiplying the catch of each length class by the ratio of the total catch of the respective con®guration to that of the standard con®guration. The percentage deviation in each length class from the standard was then determined. A
E. Dahm / Fisheries Research 49 (2000) 39±50
45
Fig. 6. Reaction of horse mackerel to changes in rollergear configuration at the GOV-trawl: (A) with Scottish rollergear; (B) with experimental rollergear.
weighed least-squares quadratic regression was calculated for every species and compared with the standard to determine general trends. Compared to a linear regression, the quadratic regression yielded a better ®t in all cases. A test for goodness of ®t of the quadratic regression to the single points demonstrated signi®cant correlation (r-value) at the 99% level for all seven species except haddock and grey gurnard in the case of the experimental groundgear (Groundgear C).
3. Results Twenty-®ve hauls of 1 h average duration were made. A total of 85 465 ®sh was caught, composed by 29 species. Only seven of the species, as illustrated in Figs. 3±9, showed suf®cient abundance for subsequent evaluation by the method described. The ®gures demonstrate that lifting the groundrope off the bottom and creating escape holes in the
46
E. Dahm / Fisheries Research 49 (2000) 39±50
Fig. 7. Reaction of mackerel to changes in rollergear configuration at the GOV-trawl: (A) with Scottish rollergear; (B) with experimental rollergear.
rollergear offers escape possibilities which particularly the young ones of certain species (grey gurnard, whiting, interestingly also saithe and horse mackerel) apparently use. They ®t, thus, to the reaction type 3 (``stay ahead of the danger and look for an escape opportunity'') as classi®ed in Dahm (1997). On the contrary, however, other young ®sh (haddock, herring) seem not to be affected or even to be caught more easily than without lifting elements in the
groundgear. Their type of reaction to the trawl is more in accordance with No. 2 (``Jump as high as possible and escape'') of Dahm (1997). 4. Discussion Some of this research con®rms the ®ndings of the bagnet investigations discussed in the introduction,
E. Dahm / Fisheries Research 49 (2000) 39±50
47
Fig. 8. Reaction of saithe to changes in rollergear configuration at the GOV-trawl: (A) with Scottish rollergear; (B) with experimental rollergear.
even though a completely different approach has been used to investigate the possible in¯uence of rollergear design on the same trawl. In addition, this investigation hints at a hitherto unknown phenomenon. For bigger ®sh in at least some length classes, the results suggest an increased ef®ciency of the nets with groundropes lifted off bottom depending on the groundgear used. This was also
shown for cod by Ehrich (1987), but has not attracted attention. There is, apparently, a considerable length-dependent increase of catchability with groundgear rigs B and C for horse mackerel, saithe and grey gurnard. But the opposite can be observed, too: a decreasing positive ef®ciency from smaller to bigger size (haddock, mackerel).
48
E. Dahm / Fisheries Research 49 (2000) 39±50
Fig. 9. Reaction of whiting to changes in rollergear configuration at the GOV-trawl: (A) with Scottish rollergear; (B) with experimental rollergear.
Does the changing impact of other rollergear con®gurations on the sea bottom, or changes in the underwater noise created, also in¯uence the behaviour of bigger ®sh? Though this is an interesting hypothesis which, if veri®ed, would have considerable consequences on comparisons of different con®gurations of the same sampling gear or on the sampling procedure, there are still doubts.
One argument against the observed phenomenon is that ®shes sometimes show a variation in size distribution with water depth. Though the data were collected in a relatively con®ned area (126 square nautical miles) this possible source of variance was not taken into account, and thus, the catches made with the last con®guration (con®guration 3) are concentrated over the eastern, 40 m deeper parts of the area. Fortunately, however, the hauls with the Scottish
E. Dahm / Fisheries Research 49 (2000) 39±50
49
Table 1 Results of chi-square test on differences in length composition of catches taken with the Scottish rollergear (Groundgear B) in the western and eastern part of the fishing ground Species
w2 (sum)
DF
Significance threshold at 99% level
Difference significant
Haddock Herring Grey gurnard Horse mackerel Mackerel Saithe Whiting
12.76598 15.47466 43.19372 7.992981 84.88295 7.959528 12.77422
32 13 27 12 22 35 32
62.49 34.53 55.48 32.91 48.27 66.62 62.49
No No No No Yes No No
rollergear (Groundgear B) were made over both depths and thus allow for a study of possible depth effects. All size distributions from the west (hauls 23± 29) were compared to those from the east part of the area (hauls 30±33) by means of a chi-square test (Table 1). In most cases, no signi®cant difference of the size distributions could be detected. Obviously, however, there is only one case with diverging length composition. This is mackerel, where 26 specimens were caught in the east giving the length distribution a high degree of uncertainty. In conclusion, there seems to be no evidence that length distributions vary with depth of the ®shing ground, so this cannot explain changes in ef®ciency. The observed phenomena may still be due to the fact that it was not possible to ®sh all three con®gurations synchronously. One could speculate that increased creation of sandclouds caused by discs dragged sideways over the sea bottom as in the Scottish and the experimental rollergear could possibly increase the stimulus to rise and get caught and that bigger ®sh are more likely to be caught. There is, however, also the other contradictory explanation, that after 2 days of ®shing on a con®ned area only the big ®sh able to escape several times from increased ®shing effort are left and available to the net. Hence it should not be surprising if the trawls used to catch the bigger ®sh later. But then why is this valid only for distinct species? Could different degrees of immigration into the ®shing ground and consequent replacement of the thinned-out population explain this, or is the different swimming ability of species an acceptable explanation?
5. Conclusion The data presented here suggest that results of the bagnet investigations on young ®sh are valid, and support the idea that lifting the groundrope off bottom can have a positive effect on the catchability, at least for bigger ®sh of some species. To exclude time and spatial effects completely, however, a repetition of the experiment with a double or triple net con®guration enabling the synchronous comparison of different con®gurations of the same net might be helpful. A double net con®guration, however, would not be the true sampling gear in question, and therefore, might cause a new discussion on the validity of such comparison.
Acknowledgements The author wishes to express his sincere thanks to his colleagues Klaus Lange and Dr. Siegfried Ehrich for helpful criticism of earlier versions of this manuscript and to Dr. Richard Ferro for further helpful suggestions and a language check of the ®nal version.
References Dahm, E.,1997. Escape strategies of different marine species under the footrope of survey trawls. ICES A.S.C./W:22. Dahm, E., Wienbeck, H., 1992. Escapement of fish underneath the groundrope of a standard bottom trawl used for stock assessment purposes in North Sea. ICES C.M./B:20.
50
E. Dahm / Fisheries Research 49 (2000) 39±50
Dahm, E., Wienbeck, H., 1996. New facts on the efficiency or total gear selectivity of German survey bottom trawls Ð possible effects on stock assessment and stock protection. ICES C.M./ B:8. Ehrich, S., 1987. The portion of young cod escaping under the GOV-trawl rigged with a heavy bobbin footrope. ICES C.M./B:28. EngaÊs, A., Godù, O.R., 1986. Preliminary results of investigations on escapement of fish under the fishing line of a Norwegian sampling trawl. ICES C.M./B:30. Godù, O.R., Walsh, S., 1992. Escapement of fish during bottom
trawl sampling Ð implications for resource assessment. Fish Res. 13, 281±292. HagstroÈm et al., 1992. Evaluation of sources of variability in fishing power of the GOV trawl. Report from FTFB Subgroup. ICES C.M./B:39. Munro, P.T., Weinberg, K.L., Somerton, D., 1997. Estimating sizedependent capture rates at the footrope of two different kinds of survey trawls. ICES A.S.C./W:06. Walsh, S., 1989. Escapement of fish underneath the footgear of a groundfish survey trawl. ICES C.M./B:21.