The selectivity of a traditional trammel net, a monofilament trammel net and a gill net was investigated in two coastal areas (central Adriatic sea and northern Tyrrhenian sea) on samples collected during one year. Three mesh sizes for each set net were tested: 45, 70 and 90 mm (stretched mesh size). Catch efficiency, modality of capture and selection parameters for each net and each mesh size were estimated on the following species: annular sea-bream (Diplodus annularis), thin-lip grey mullet (Liza ramada), striped sea-bream (Lithognathus mormyrus), red mullet (Mul¬lus barbatus), axillary sea-bream (Pagellus acarne), pandora (Pagellus erythrinus), common cuttlefish (Sepia officinalis), common sole (Solea vulgaris) and tub gurnard (Trigla lucerna). Selectivity was estimated through an indirect and a direct method. The former was the Sechin model, consisting in the estimation of the selection parameters on the basis of maximum girth and head girth data of the considered species. The direct method compared the length-frequency distributions of set net catches with the length-frequency distributions of the fish population at sea sampled through a bottom trawl net. An overall of 68 and 87 species were caught by set nets in the Adriatic and in the Tyrrhenian sea respectively. In both areas the two trammel nets were more efficient than the gill net, catching the highest number of species and of individuals. As regards the target species, the highest yields (n° individuals/1,000 m of net) of L. mormyrus, L. ramada, M. barbatus, S. flexuosa, S. vulgaris, T. lucerna and S. officinalis were recorded in the Adriatic sea, while the highest catches of D. annularis, P. erythrinus and P. acarne were obtained in the Tyrrhenian sea. The three nets tested showed the same catch efficiency for P. acarne, M. barbatus and S. flexuosa. The two trammel nets gave similar catches for P. erythrinus, higher than those obtained with the gill net. Gill net was the most efficient gear for D. annularis in both areas and for T. lucerna in the Adriatic sea, while the three nets were equally efficient for this last species in the Tyrrhenian sea. The monofilament trammel net resulted the most suitable gear only for L. ramada. The standard trammel net gave the highest catches of S. officinalis and S. vulgaris in both areas and for L. mormyrus in the Tyrrhenian sea. The 45 mm mesh size of each type of net showed the highest efficiency for most of the target species, with the exception of S. vulgaris and S. officinalis, for which the 70 mm mesh size was either similar or more efficient than the smallest mesh. The catches obtained with the 90 mm mesh size of each net were very low for most of the species. The three set nets showed a similar selectivity only for S. flexuosa. As regards the other species, a percentage of individuals caught by tangled/pocket effect generally occurred in the catches of gill net and trammel nets. This portion was generally negligible in gill net catches and gradually increased going to the monofilament and to the standard trammel net, causing a gradual widening of the size catch range and confirming this last type of net as the least selective. Tangling on the great specimens was generally more evident in the Adriatic sea, while the small tangled individuals were more abundant in the Tyrrhenian sea, as a consequence of the different demographic structure of the populations at sea in the two areas. However, gilling and/or wedging resulted to be the most important ways of capture for most of the target species, while L. ramada, T. lucerna and S. officinalis were mainly caught by tangling and/or pocket. For management purposes, the 45 mm mesh size appeared the most suitable among the three mesh sizes tested for the exploitation of most of the target species considered in the present study, because it caught the highest number of individuals but, at the same time, allowed to protect the juveniles. On the other hand, the 70 mm mesh size seemed more adequate only for S. vulgaris, L. ramada, T. lucerna and also for S. officinalis.
Trammel and gill net selectivity in the Adriatic and Tyrrhenian sea. Study contract n. 94/086 - Final Report
Fabi G;
1998
Abstract
The selectivity of a traditional trammel net, a monofilament trammel net and a gill net was investigated in two coastal areas (central Adriatic sea and northern Tyrrhenian sea) on samples collected during one year. Three mesh sizes for each set net were tested: 45, 70 and 90 mm (stretched mesh size). Catch efficiency, modality of capture and selection parameters for each net and each mesh size were estimated on the following species: annular sea-bream (Diplodus annularis), thin-lip grey mullet (Liza ramada), striped sea-bream (Lithognathus mormyrus), red mullet (Mul¬lus barbatus), axillary sea-bream (Pagellus acarne), pandora (Pagellus erythrinus), common cuttlefish (Sepia officinalis), common sole (Solea vulgaris) and tub gurnard (Trigla lucerna). Selectivity was estimated through an indirect and a direct method. The former was the Sechin model, consisting in the estimation of the selection parameters on the basis of maximum girth and head girth data of the considered species. The direct method compared the length-frequency distributions of set net catches with the length-frequency distributions of the fish population at sea sampled through a bottom trawl net. An overall of 68 and 87 species were caught by set nets in the Adriatic and in the Tyrrhenian sea respectively. In both areas the two trammel nets were more efficient than the gill net, catching the highest number of species and of individuals. As regards the target species, the highest yields (n° individuals/1,000 m of net) of L. mormyrus, L. ramada, M. barbatus, S. flexuosa, S. vulgaris, T. lucerna and S. officinalis were recorded in the Adriatic sea, while the highest catches of D. annularis, P. erythrinus and P. acarne were obtained in the Tyrrhenian sea. The three nets tested showed the same catch efficiency for P. acarne, M. barbatus and S. flexuosa. The two trammel nets gave similar catches for P. erythrinus, higher than those obtained with the gill net. Gill net was the most efficient gear for D. annularis in both areas and for T. lucerna in the Adriatic sea, while the three nets were equally efficient for this last species in the Tyrrhenian sea. The monofilament trammel net resulted the most suitable gear only for L. ramada. The standard trammel net gave the highest catches of S. officinalis and S. vulgaris in both areas and for L. mormyrus in the Tyrrhenian sea. The 45 mm mesh size of each type of net showed the highest efficiency for most of the target species, with the exception of S. vulgaris and S. officinalis, for which the 70 mm mesh size was either similar or more efficient than the smallest mesh. The catches obtained with the 90 mm mesh size of each net were very low for most of the species. The three set nets showed a similar selectivity only for S. flexuosa. As regards the other species, a percentage of individuals caught by tangled/pocket effect generally occurred in the catches of gill net and trammel nets. This portion was generally negligible in gill net catches and gradually increased going to the monofilament and to the standard trammel net, causing a gradual widening of the size catch range and confirming this last type of net as the least selective. Tangling on the great specimens was generally more evident in the Adriatic sea, while the small tangled individuals were more abundant in the Tyrrhenian sea, as a consequence of the different demographic structure of the populations at sea in the two areas. However, gilling and/or wedging resulted to be the most important ways of capture for most of the target species, while L. ramada, T. lucerna and S. officinalis were mainly caught by tangling and/or pocket. For management purposes, the 45 mm mesh size appeared the most suitable among the three mesh sizes tested for the exploitation of most of the target species considered in the present study, because it caught the highest number of individuals but, at the same time, allowed to protect the juveniles. On the other hand, the 70 mm mesh size seemed more adequate only for S. vulgaris, L. ramada, T. lucerna and also for S. officinalis.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
