Selectivity on young Hake in the nursery ground of the Pomo Pit was studied in summer 1970 and spring 1973. In 1970's experiments a French trawl was tested ...
FERRETTI M. and C. FROGLIA
Results of selectivity experiments, made with different trawls, on more in1portant adriatic demersal fish
Estratto da: Quaderni del Laboratorio di Tecnologia della Pesca
ANCONA 1975 VOL. 11 • N. 1
RESULTS OF SELECTIVITY EXPERIMENTS, MADE WITH DIFFERENT TRAWLS, ON MORE IMPORTANT ADRIATIC DEMERSAL FISH Mario Ferretti and Carlo Froglia Laboratorio di Tecnologia della Pesca (C.N.R.) - Ancona
INTRODUCTION The Laboratorio di Tecnologia della Pesca, since 1969, inserts selectivity studies in its research programs with the aim of improving management of the Italian fishing industry. The aim of a selectivity research is to evaluate the increase in the age of recruitment to fishery consequent to an increase in the minimum mesh size, and successively to evaluate changes in yield. In the Mediterranean sea many species are involved together in trawl fishing and a mesh that is effective in preserving one species, may be ineffective for another which is present in the same area. Therefore minum mesh size can be only a compromise obtained from knowledge of biology and population dynamics of the most important species in every fishery. Due to lack of data on length/age composition of commercial catches and imprecision of data on total landings, such ,evaluation, for what we know, was never made in Mediterranean countries, at least at regional level. Many countries fixed at 40 mm. stretch the minimum size for trawl codends meshes. Countries, such as Italy, also fixed a minimum size for marketable fish, but usually the last measures don't agree with selectivity of permitted gear. In some areas, the Adriatic sea for example, in spite of laws in tforce, meshes smaller than permitted are used. On nursery grounds for red mullet, and hake, age of recruitment to bottom life and age of recruitment to fishery practically coincide when trawls with codend meshes of 24-30 mm. are used. Quad. Lab. Tecnol. Pesca, 2 (1), 3-16
4
M. Ferretti and C. Froglia
METHODS For reasons discussed above we began investigations on the mor.e important species for Adriatic fishery, i.e.: Red Mullet (Mullus barbatus), Hake (Merluccius merluccius), Poor Cod (Trisopterus mitus capelanus) and Sole (Solea vulgaris), and we gathered data also for other species when there were enough speciemens in catches to draw a selection curve. The gear tested was Italian bottom 'trawl (the type used by Ancona fishermen), French trawl (chalut a grande ouverture verticale) and « Rapido » (a kind of beam trawl used in pleuronectid fishery and towed at a speed of more than five knots). All the cod-ends used were in knotless Polyammide ( PA); only in some experiments, made with a French trawl, the cod~end was in knotted plaited Polyamide. The cover cod-end technique was the most used in our experiments, but to evaluate selectivity for Hake of Italian bottom trawl we used alternate hauls, and to evaluate selectivity of « Rapido » we used parallel hauls, towing two gears simultaneously, both of which were 3.5 m. large. For all, but the October 1969 experiments the research stern-trawler « Luiciotta » (over all length 30,7 m., tonnage 130 GRT) was used. RESULTS Part of data in this report has already been published and discussed at a meeting of the C.F.G.M. working party on Demersal Resources Appraisal and Exploitation. To those length data 2.5 mm. had to be added as length measures were taken at a half centimeter lower. The data still unpublished are discussed here in a preliminary way, because for some species further investigations are needed. Red Mullet (Mull us barb,atus) Selectivity on red mullet was extensively investigated in a coastal area where in summer time it is the most common species and goes to make up about half of the commercial catch. Italian and French trawls with cod-ends of knotless Polyamide were tested. Meshes were a little bigger or smaller than regular. Changes in selectivity due to cod-end circumference, age compo-
Selectivity Experiments on Adriatic Fish
5
sition of exploited population and quantity of trash in cod-end were observed. From the 34 hauls made with Italian trawls it was possible to evaluate (TRESCHEV 1966) the mean weighted selection factor for knotless PA codends. It was found to be 2.1. This value is a little smaller than those reported in litterature (GILLAT 1961, LARRANETA, SUAU & SAN FELIU 1969). Different flexibility of netting yarn and a higher trawling speed can be responsible for this. Selection range is also very narrow (minimum 1.5 cm., maximum 3.0 cm.). and we think it results from mesh shape in fishing (strongly rhomboid), lower variance of mesh size and from morphological characteristics of Red Mullet. Hake ( Merluccius nzerluccius) Selectivity on young Hake in the nursery ground of the Pomo Pit was studied in summer 1970 and spring 1973. In 1970's experiments a French trawl was tested with cover technique; in 1973 selectivity was obtained from alternate hauls made to compare yields of Italian trawls with different cod-end meshes ( 42 mm. and 29 mm.). Average selection factors are: 2.8 for French gear and 2.6 for Italian. It's noteworthy that in the Pomo Pit a circadian fluctuation in young Hake catchability, probably due to feeding behaviour, could be a source of error in alternate hauls experiments. We plan to get more extensive data on Adriatic Hake selectivity in the future and to evaluate the mean weighted selection factor for this species. Selection, factors close to ours were found for knotless PA by, LARRANETA, SUAU & SAN FELIU (1969) whereas higher factors are reported by JUKIC (1972). but perhaps netting yarn tested by last Author was very flexible. Foor Cod (Trisopterus minutus capelanus) Poor Cod represents the main species in catches of some areas of the Middle Adriatic. Selectivity of knotless Poylamide for Poor Cod was evaluated with a French trawl in the Pomo Pit. Selection factor was 2.9 and selection range was 2.9 cm., with a dissimetry in the shape of the curve, i.e.: for lengths greater than 50% retention length a sharp increase in retention percentage was noticed.
6
M. Ferretti and C. Froglia
Whiting ( Merlangius merlangus) Poor cod is replaced by Whiting in Northern Adriatic fishery. Whereas spawning areas are located in North Adriatic, great quantities of larvae are drifted southward and, in late spring and summer, all along italian coasts, from the mouths of the Po to the Conero promontory, trawlers catch juveniles on muddy bottoms with depths of 20-40 m. Selectivity of Polyamide for Whiting was tested in spring 1971. Selection factor obtained for knotted PA is significantly greater (3.1) that for knotless PA with higher resultant linear density (2.6). Our selection factors for both experiments are lower than those reported in litterature for North sea Whiting (see: ICES Cooperative research report n. 2, 1964), but we think it is normal that juveniles and adults have different selection factors. Horse Mackerel (Trachurus trachurus) Mesh selection experiments for Whiting enabled us to draw selection ogives for Horse Mackerel also; two other hauls on this species were made earlier in September 1970. In all experiments a French trawl was used. Selection factors obtained were 2.4 and 2.7 for cod-end in knotless PA and 3.2 for codend in knotted PA. Again as for Whiting, knotted PA appeared to be more selective than knotless PA. Common Sole (Sol ea vulgaris) Rapido, a kind of beam-trawl, an extensive description of which is given by BINI, is widely used by Italian coastal trawlers and nearly all the italian catch of Adriatic Sole is taken with this gear. Selectivity of a knotless PA cod-end with meshes a little bigger than regular was evaluated for Common Sole and Scaldfish. Two rapidos, only differing in cod-end mesh size ( 42 mm.-22 mm) were towed at the same time at a speed of 4.6-6.3 knots. In this way we obtained parallel hauls working with one boat, and catch of gear with smaller meshes was considered as sum of cod-end plus cover catches in selectivity experiments with cover. Large fluctuations in selction factors were noticed and it is noteworthy that performance of gear and mesh opening can be greatly influenced by bottom sediments and quality and or quantity of epifauna (shells, crabs, echinoderm etc.). In the period October 1973-February 1974, four experiments to
Selectivity EXiperiments on Adriatic Fish
7
evaluate selectivity for Sole were made. In computations a total of 33 hauls, made at night and by day, were considered. Selection factors obtained range from 3.4 to 4.0. It is not surprising that selection factors are higher than those for non-flat fish, if we only remember Sole is a fish with high vitality, strong skin with abundant mucus and a section that adapts well to a strongly rhomboid mesh. Scaldfish ( Arnoglossus laterna) First data on selectivity for Scaldfish were obtained from May 1971 experiments when a French trawl with knotted or knottless PA cod-ends was tried. Knotted material proved to be less selective for this species. Selection ogives were also drawn from data collected in January and February 1974 experiments with rapidos. Also if catches were much lower than those obtained with the net, and data are not conclusive, selection factors of rapidos cod-end seem to be higher than those of net cod-end of same material. REMARKS ON SOME FACTORS CONDITIONIG SELECTIVITY In Report of the 11th Session of GFCM, possibility that fishermen lower trawl selectivity by hanging slyness was pointed out. Experiments made in August 1972 on Red Mullet gave evidence that an increase in the number of meshes in a cod-end was responsible for a significant decrease to the value of 50% retention lenght, and Selection factors changed from 2.0 to 1.8. In the 1972 experiments a decrease of Selection factors when in exploited population younger speciemens predominate was also noticed. Presently we are unable to give satisfactory explanations for this. Quantities of trash can also influence cod-end selectivity. CASSIE (1955), for example, reported a decrease in selection factor in presence of algae, of the genus Carpophylum, blocking lumen of meshes. In summer and early fall 1973 in coastal waters of Ancona, huge quantities of Prosobranch Mollusc Philine quadripartita wer.e present. Increase in 50% retention lenght was observed in connection with big catches of those molluscs, probably as a result of increase in mesh openings. In fig. 1 50% retention lenghts are plotted against different
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8
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M. Ferretti and C. Froglia
quantities of trash in cod-end and the regression line that best fits experimental data is drawn.
E u
.. •
50
100
150
200
kg. trash
FIG. 1 . Correlation between quantity of " trash , in the cod-end and 50% retention length for « Mullus barbatus "·
Properties of netting material, as everybody knows, greatly influence selectivity. From our experiments, also if data is not conclusive, results that polamide knotless net is less selective than polyamide knotted net for fish such as Whiting and Horse Mackerel, whereas it's more selective for flat-fish. It is noteworthy that in rapidos experiments the inside perimeter of cod-end meshes was lower than natural maximum body girth for Soles of a length equal to SO% retention length.
CONCLUSIONS As said in Introduction, selectivity experiments are only the first step in a study f9r a better management of sea ressourc.es. We have begun to collect data also on length-weight relationships and on growth of some speces, but until data on landings, fishing effort and lenght composition of catches are r.ot available, every law for minimum mesh size will be more empiric than scientific.
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Selectivity Experiments on Adriatic Fish
9
ACKNOWLEDGMENTS We thank all the Collegues of the Laboratory that helped us in the work at sea as well as in the analysis of data. Special thanks are due to th crew of the R/V Luiciotta for their invaluable cooperation.
SUMMARY Results of selectivity experiments with different trawls on the following species: Mullus barbatus, Merluccius merluccius, Trisopterus minutus capelanus, Merlangius merlangus, Trachurus trachurus, Solea vulgaris and Arnoglossus laterna are discussed and compared with data available from literature. Netting materials cod-end size and quantity of trash in cod-end proved to influence selectivity of gear tested. Six tables summarizing results of all experiments are added.
RIASSUNTO Vengono illustrati i risultati degli esperimenti di selettivita eseguiti con reti a strascico di tipo italiano e francese e con « rapidi » sulle piu importanti specie ittiche presenti in Adriatico: Mullus barbatus, Merluccius merluccius, Trisopterus minutus capelanus, Merlangius merlangus, Trachurus trachurus, Solea vulgaris, Arnoglossus laterna. I risultati ottenuti vengono brevemente comparati con quelli noti dalla letteratura. Per la totalita delle specie, esclusi i Pleuronettiformi, si e evidenziata una maggiore selettivita delle reti in poliammide annodato rispetto a quelle in poliammide senza nodo. Si e altres'i. evidenziata l'influenza sui fattore di selettivita della quantita di organismi non commerciali (molluschi, echinodermi etc.) presenti nella cattura. Vengono infine fornite sei tabelle riassuntive degli esperimenti di selettivita finora eseguiti.
10
M. Ferretti and C. Froglia
BIBLIOGRAPHY BINI G.- Ricerche sui comportamento in mare delle reti a strascico per sog!iole. Memoria Minist·ero Marina Mercantile, 2, 32 pagg. + 1 tab. CASSIE M.R., 1955 - The escapement of small fish from trawl nets. N. Zealand Marine Department Fishery Bulletin, 11, 99 pgg. FERRETTI M., FROGLIA C., COSIMI G. & ANTOLINI B., 1973 - Osservazioni su alcuni fattori che influenzano la selettiviH:t di una rete a strascico. Quad. Lab. Teen. P.esea, 1 (5), 129-141. FROGLIA C., GALLI B., 1970 - Selettivita e capacita di cattura di una rete a strascico di tipo italiano su popolazioni di Mullus barbatus L. Quad. Lab. Teen. Pesea, 1 (1 ), 18 pgg. GILLAT E., 1961 - On the selection of Upeneus mollueensis and Mullus barbatus by trawl cod-ends in the Israeli fisheries. Proe. gen. Fish. Coun. Medit. 6, 93-106. KOURA R., 1969 - Codend mesh size effect on italian otter trawl efficiency. Stud. Rev. gen. Fish. Coun. Medit., 39, 13-21. LARRANETA M.G., SUAU P. & SAN FELIU J.M., 1969 - Experiencias de selectividad en la pesqueria de arrastre en el Levante espanoL Inv. Pesq. 33 (1), 15-53. LEVI D., FROGLIA C. & SCORCELLETTI R., 1971 - Selettivita di una rete di tipo relingato (chillut a grande ouverture verticale). Quad. Lab. Teen. Pesca, 1 (2), 23-37. TRESCHEV R.I., 1966 - Selectivity of trawls with reference to the redfish. Trudy vses. nauehno-issled. Inst. morsk. ryb. Khoz. Okeanogr. 41, 17-61.
TAB. 1 SUMMARY OF RESULTS OF MESH SELECTION EXPERIMENTS FOR RED MULLET (Mullus barbatus) DATE Depth (m.) Trawl Cod-end material Resultant linear density (Rtex) Mean mesh size (mm.) Gauge Method N. of hauls Haul duration (minutes) 50% retention length (cm.) Selection factor Selection range (25%-75%) (cm.) N. of fishes in selection range (total of all hauls) Average total cod-end catch (Kg/h) Average cod-end trash (Kg/h) Towing speed (Knots)
October '69
August September '70
September '70
August
August
September
'72
September October '74
14-16 Italian* 90HP PA knotless Unknown
12-16 French* 28,2/35,2 PA knotless 3450
14-15 Italian* 300-400HP PA knotless 3050
10-11 Italian1' 300-400HP PA knotiess 3050
10-11 Italian* 300-400HP PA knotless 3050
10-11 Italian'' 300-400HP PA knotless 3050
12-16 Italian* 300-400HP PA knotless 3050
38 Triangular 5 Kg. Cover 3 90 7,7 2,0
35,5 Triangular 5 Kg. Cover 5 90 8,5 2,3
42,0 ICES
42,7 ICES
42,7 ICES
42,7 ICES
41,8 ICES
Cover 4 90 8,3 2,0
Cover 4 60 7,8 1,8
Cover 4 60 8,5 2,0
Cover 3 60 7,0 1,6
Cover 16 60 8,8 2,1
'72
'72
7,0-8,4
7,6-9,2
7,4-9,3
6,4-9,0
7,5-9,5
5,9-8,1
7,3-10,2
954
1919
3393
2885
1887
1370
3562
12 Not weighed 4,0-4,3
28
26
11,5
18
33,5
44,5
72
88 3,8-4,2
67 3,9-4,0
87,7 3,9-4,4
42"'4
27 not measured
-------
3,8-4,2
* Cod-end circumference greater than circumference of aft part of belly.
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M. Ferretti and C. Froglia
12
TAB. 2 SUMMARY OF RESULTS OF MESH SELECTION EXPERIMENTS FOR HAKE (M erluccius merluccius)
August September 1970
May 1973
200-240
200
Trawl
French 28,2/35,2
Italian 300-400HP
Cod-end material
PA knotless
PA knotless
Resultant linear density (Rtex)
3450
3050
Mean mesh size (mm.)
35,5
42,0
Gauge
Triangular 5 Kg.
ICES
Method
Cover
Alternate hauls
8
5
DATE Depth (m.)
N. of hauls Haul duration (minutes)
120
120
50% retention length (cm.)
9,0
11,0
Selection factor
2,8
2,6
Selection range (25%-75%) (cm.)
7,7-11,1
9,5-12,5
N. of fishes in selection range (total of all hauls)
4292
2442
Average total cod-end catch (Kg/h)
18
54
Average cod-end trash (Kg/h)
14
8
Towing speed (Knots)
Not measured
Not measured
Selectivity Experiments on Adriatic Fish
13
TAB. 3
SUMMARY OF RESULTS OF MESH-SELECTION EXPERIMENTS FOR POORCOD (Trisopterus minutus capelanus) AND WHITING (Merlangius merlangus) SPECIES
POOR-COD
DATE
August September '70
May '71
May '71
200-240
25-35
25-35
Trawl
French 28,2/35,2
French 28,2/35,2
French 28,2/35,2
Cod-end material
PA knotless
PA knotless
PA knotted
Resultant linear density (Rtex)
3450
3450
1770
Mean mesh size (mm.)
35,5
34,0
33,7
Gauge
Triangular 5 Kg.
ICES
ICES
Method
Cover
Cover
Cover
9
4
5
Haul duration (minutes)
120
90
90
50% retention length (cm.)
10,4
8,7
10,6
Selection factor
2,9
2,6
3,1
Selection range (25%-75%) (cm.)
8,4-11,3
7,0-10,4
7,2-14,0
N. of fishes in selection range (total of all hauls)
4481
656
740
Average total cod-end catch (Kg/h)
18
not weighed
not weighed
Average cod-end trash (Kg/h)
14
not weighed
not weighed
not measured
not measured
not measured
Depth (m.)
N. of hauls
Towing speed (Knots)
WHITING
14
M. Ferretti and C. Froglia
TAB. 4
SUMMARY OF MESH SELECTION EXPERIMENTS FOR HORSE MACKEREL (Trachurus trachurus)
September 1970
May 1972
May 1972
12-16
25-35
25-35
Trawl
French 28,2/35,2
French 28,2/35,2
French 28,2/35,2
Cod-end material
PA knotless
PA knotless
PA knotted
Resultant linear density (Rtex)
3450
3450
1770
Mean mesh size (mm.)
34,0
34,0
33,7
Gauge
ICES
ICES
ICES
Method
Cover
Cover
Cover
N. of hauls
2
4
5
Haul duration (minutes)
90
90
90
50% retention length (cm.)
8,2
9,2
11,0
Selection factor
2,4
2,7
3,2
Selection range (25%-75%) (cm.)
6,9-9,5
8,0-10,8
8,4-13,5
N. of fishes in selection range (total of all hauls)
781
444
769
Average total cod-end catch (Kg/h)
12
not weighed
not weighed
Average cod-end trash (Kg/h)
20
not weighed
not weighed
not measured
not measured
not measured
DATE Depth (m.)
Towing speed (Knots)
Selectivity Experiments on Adriatic Fish
15
TAB. 5
SUMMARY OF RESULTS OF MESH SELECTION EXPERIMENTS FOR COM MON SOLE (Solea vulgaris) October November '73
Dicember 1973
January 1974
February 1974
12-15
12-15
12-14
12-14
Trawl
Rapido
Rapido
Rapido
Rapido
Cod-end material
PA knotless
PA knotless
PA knotless
PA knotless
Resultant linear density (Rtex)
3050
3050
3050
3050
Mean mesh size (mm.)
43,0
42,5
41,6
41,5
Gauge
ICES
ICES
ICES
ICES
Method
Parallel hauls
Parallel hauls
Parallel hauls
Parallel hauls
N. of hauls
12
9
7
5
Haul duration (minutes)
120
60
90
90
50% retention length (cm.)
17,2
14,6
16,2
15,5
Selection factor
4,0
3,4
3,9
3,7
Selection range (25%-75%) (cm.)
15,4-19,1
13,7-15,5
14,9-17,6
12,9-18,1
N. of fishes in selection range (total of all hauls)
1144
151
122
279
Average total cod-end catch (Kg/h)
5,1
not weighed
2,6
7,2
Average cod-end trash (Kg/h)
20
not weighed
36
34
Towing speed (Knots)
4,6-5,8
5,3-5,9
5,0-5,8
5,0-6,3
DATE Depth (m.)
16
M. Ferretti and C. Froglia
TAB. 6
SUMMARY OF RESULTS OF MESH SELECTION EXPERIMENTS FOR SCALDFISH ( Arnoglossus laterna) DATE
May 1971
May 1971
January 1974
February 1974
25-35
25-35
12-14
12-14
Trawl
French 28,2/35,2
French 28,2/35,2
Rapido
Rapido
Cod-end material
PA knotted
PA knotless
PA knotless
PA knotless
Resultant linear density (Rtex)
1770
3450
3050
3050
Mean mesh size (mm.)
33,7
34,0
41,6
41,5
Gauge
ICES
ICES
ICES
ICES
Method
Cover
Cover
Parallel hauls
Parallel hauls
N. of hauls
5
5
7
5
Haul duration (minutes)
90
90
90
90
50% retention length (cm.)
6,9
8. 2
11,1
10,0
Selection factor
2,0
2,4
2,7
2,4
Selection range (25%-75%) (cm.)
5,5-8,3
6,6-9,9
10,0-12,4
9,0-11,0
N. of fishes in selection range (total of all hauls)
1276
900
216
95
Average total cod-end catch (Kg/h)
not weighed
not weighed
2,6
7,2
Average cod-end trash (Kg/h)
not weighed
not weighed
36
34
Towing. speed (Knots)
not measured
not measured
Depth (m.)
5,0-5,8
5,0-6,3
