Along the Italian coasts Sepia officinalis (Linnaeus, 1758) represents one of the most important target species for the artisanal fishery which utilises several gears for the exploitation of this cephalopod depending on the different habitats. The impact of three gears (fyke net, pot and trammel net) commonly used by fishermen to catch cuttlefish was investigated in two coastal areas (central Adriatic Sea and south-eastern Ligurian Sea) in 1999 and 2000, during the period of maximum occurrence of cuttlefish (from late winter to early summer). The impact was evaluated considering the efficiency of each gear both on the target species (S. officinalis) and on the bycatch (kept and discard). Comparison among the total catch rates, length-frequency distribution and sex ratio of the target species, according to the different gears and months, were ac-complished through statistical tests (ANOVA, Kolmogorov-Smirnov test, chi-square test). A census at field on the local fishing fleets, landings and activities was also performed to get a picture of the importance assumed by the cuttlefish fishery on the overall activity of the small-scale fishermen and to draft a cost/benefit ratio related to the use of the different gears in the two areas. In each area the sampling was carried out either on a sandy and a sand-rocky bottom in order to evaluate the influence of the type of bottom on the gear efficiency and the suitability of the different fishing methods. Moreover, to compare the yields of the three gears differing from each other in technical characteristics and catch modality, an experimental approach of using in each area gangs of the same length and a same permanence at sea was chosen. Therefore, taking into account the difference abundance of the target species in the two areas, in the Adriatic Sea for each gear was used a gang having a length of 300 m (20 units for each type of trap), while in Ligurian Sea a bigger amount of fishing gears was employed: 500 m of trammel net, 32 fyke nets and 32 traps. The gears were lowered into the water at dawn and pulled in at dawn of the subsequent day for about 24 h at sea. Moreover, in 2000 two additional sets of traps (one of pots and the other one of fyke nets) characterised by a longer fishing time (48 hours) were utilised to verify eventual changes in the catch efficiency of the gears due to the set time at sea. The geomorphologic and hydrological characteristics of the two areas affected the quali-quantitative composition of the catches and the efficiency of the three considered gears. In fact, independently from the type of bottom, all the gears gave the highest yields in the central Adriatic Sea, both in total and in terms of target species, likely because of a higher fish abundance in this area in respect to the south-eastern Ligurian Sea. Moreover, in the Adriatic Sea all the gears caught the highest number of species and gave the highest yields in weight on the sand-rocky bottom, while more abundant catches in number of specimens were recorded at the sandy site. On the contrary, in the south-eastern Ligurian Sea all the gears showed their lowest efficiency on the sand-rocky seabed, both in qualitative and in quantitative terms; this was particularly evident for the traps, whose catches were characterised by the complete absence of cuttlefish and by a very few number of species. Most of fyke net and pot catches were constituted by cuttlefish in both areas, while trammel net always showed the lowest selectivity, being its catches generally characterised by high percentages of bycatch. In the Ligurian Sea this last fraction included several commercial species of fish, while in the Adriatic Sea it mainly consisted of non com-mercial discard. In both areas discard was dominated by crabs belonging within the food chains of the coastal zones; these benthic decapods were generally dead when rejected at sea and this might alter the local macrozoobenthic communities so creating an impact that is very difficult to quantify. The yields of S. officinalis obtained with the different gears evidenced the highest efficiency of trammel net in respect to the traps on the sand-rocky seabed of the Adriatic Sea, while no substantial differences were recorded in the effectiveness of the three gears on the sandy bottom. Differently, in the Ligurian Sea the trammel net resulted to be the most efficient gear to catch cuttlefish both on the sandy and on the sand-rocky seabed. A general decrease of S. officinalis catches from April to June was observed for pots and trammel net in the Adriatic Sea and for trammel net on the sand-rocky bottom in the Ligurian Sea. On the contrary, fyke net fishing yields remained either constant or increased, according with the fact that a longer permanence at sea and siltation transform this type of trap in a suitable shelter for cuttlefish. Fishing yields obtained with the fyke nets and pots kept at sea for 48 h in both areas did not show appreciable differences with the correspondent gears having a 24 h fishing time. In the Adriatic Sea the cuttlefish caught on the sand-rocky bottom showed a higher mean mantle length and a wider size range in respect to the specimens collected on the sand, while no differences were recorded in the south-eastern Ligurian Sea. Comparison of the size-frequency distributions of catches obtained with the three gears evidenced that in the Ligurian Sea the cuttlefish collected with trammel net were bigger than those obtained with the two traps, while in the Adriatic Sea fyke nets caught smaller specimens than the other gears. In both areas a general significant reduction of the mean mantle length of the caught cuttlefish was recorded during the sampling period but, in spite of this, all the specimens recorded in the Adriatic Sea and most of those collected in the Ligurian Sea were bigger than the length of maturity. This confirms that in the period late winter-early summer the exploitation of the cuttlefish in the coastal zone is mainly focused on adults which migrate inshore for spawning. Therefore, this fishing activity might influence the reproductive potential of S. officinalis, but this impact is likely to be very scarce considering that this species seems to spawn only once in its life cycle and that the traps act as suitable substrates for the egg attachment. An additional analysis based on the frequency of only male or only female catch as well as on the sex-ratio of specimens inside each trap was conducted to clarify the catching mechanism of fyke nets and pots. The results showed that in both areas males were attracted by the presence of a female inside the pots, while the entry of females seemed to be accidental and probably related to their spawning. No sexual attraction seemed to affect the entry of cuttlefish into the fyke nets. Finally, the investigation performed at the landing sites showed that cuttlefish fishery was carried out in the two areas with vessels having similar technical characteristics, but in the Ligurian Sea this activity was conducted all year round, while in Adriatic Sea it showed a strong seasonal pattern limited to the period late winter - early summer. Nevertheless, the estimated total landings were only slightly higher in the Ligurian than in the Adriatic Sea. The cost/benefit analysis based on the main factors affecting the artisanal fishery targeting cuttlefish suggested that, independently for the type of bottom, the use of fyke nets in the Adriatic Sea may represent a good compromise among different factors, such as purchase, maintenance and storage costs, handiness, yields and environmental impact. On the contrary in the Ligurian Sea trammel net seems to be the only suitable gear because of the negligible catches of traps.
Sepia officinalis: impact of three set gear fishing techniques in the Adriatic and the Ligurian Sea. Study contract n. 98/069. Final Report
Fabi G;
2001
Abstract
Along the Italian coasts Sepia officinalis (Linnaeus, 1758) represents one of the most important target species for the artisanal fishery which utilises several gears for the exploitation of this cephalopod depending on the different habitats. The impact of three gears (fyke net, pot and trammel net) commonly used by fishermen to catch cuttlefish was investigated in two coastal areas (central Adriatic Sea and south-eastern Ligurian Sea) in 1999 and 2000, during the period of maximum occurrence of cuttlefish (from late winter to early summer). The impact was evaluated considering the efficiency of each gear both on the target species (S. officinalis) and on the bycatch (kept and discard). Comparison among the total catch rates, length-frequency distribution and sex ratio of the target species, according to the different gears and months, were ac-complished through statistical tests (ANOVA, Kolmogorov-Smirnov test, chi-square test). A census at field on the local fishing fleets, landings and activities was also performed to get a picture of the importance assumed by the cuttlefish fishery on the overall activity of the small-scale fishermen and to draft a cost/benefit ratio related to the use of the different gears in the two areas. In each area the sampling was carried out either on a sandy and a sand-rocky bottom in order to evaluate the influence of the type of bottom on the gear efficiency and the suitability of the different fishing methods. Moreover, to compare the yields of the three gears differing from each other in technical characteristics and catch modality, an experimental approach of using in each area gangs of the same length and a same permanence at sea was chosen. Therefore, taking into account the difference abundance of the target species in the two areas, in the Adriatic Sea for each gear was used a gang having a length of 300 m (20 units for each type of trap), while in Ligurian Sea a bigger amount of fishing gears was employed: 500 m of trammel net, 32 fyke nets and 32 traps. The gears were lowered into the water at dawn and pulled in at dawn of the subsequent day for about 24 h at sea. Moreover, in 2000 two additional sets of traps (one of pots and the other one of fyke nets) characterised by a longer fishing time (48 hours) were utilised to verify eventual changes in the catch efficiency of the gears due to the set time at sea. The geomorphologic and hydrological characteristics of the two areas affected the quali-quantitative composition of the catches and the efficiency of the three considered gears. In fact, independently from the type of bottom, all the gears gave the highest yields in the central Adriatic Sea, both in total and in terms of target species, likely because of a higher fish abundance in this area in respect to the south-eastern Ligurian Sea. Moreover, in the Adriatic Sea all the gears caught the highest number of species and gave the highest yields in weight on the sand-rocky bottom, while more abundant catches in number of specimens were recorded at the sandy site. On the contrary, in the south-eastern Ligurian Sea all the gears showed their lowest efficiency on the sand-rocky seabed, both in qualitative and in quantitative terms; this was particularly evident for the traps, whose catches were characterised by the complete absence of cuttlefish and by a very few number of species. Most of fyke net and pot catches were constituted by cuttlefish in both areas, while trammel net always showed the lowest selectivity, being its catches generally characterised by high percentages of bycatch. In the Ligurian Sea this last fraction included several commercial species of fish, while in the Adriatic Sea it mainly consisted of non com-mercial discard. In both areas discard was dominated by crabs belonging within the food chains of the coastal zones; these benthic decapods were generally dead when rejected at sea and this might alter the local macrozoobenthic communities so creating an impact that is very difficult to quantify. The yields of S. officinalis obtained with the different gears evidenced the highest efficiency of trammel net in respect to the traps on the sand-rocky seabed of the Adriatic Sea, while no substantial differences were recorded in the effectiveness of the three gears on the sandy bottom. Differently, in the Ligurian Sea the trammel net resulted to be the most efficient gear to catch cuttlefish both on the sandy and on the sand-rocky seabed. A general decrease of S. officinalis catches from April to June was observed for pots and trammel net in the Adriatic Sea and for trammel net on the sand-rocky bottom in the Ligurian Sea. On the contrary, fyke net fishing yields remained either constant or increased, according with the fact that a longer permanence at sea and siltation transform this type of trap in a suitable shelter for cuttlefish. Fishing yields obtained with the fyke nets and pots kept at sea for 48 h in both areas did not show appreciable differences with the correspondent gears having a 24 h fishing time. In the Adriatic Sea the cuttlefish caught on the sand-rocky bottom showed a higher mean mantle length and a wider size range in respect to the specimens collected on the sand, while no differences were recorded in the south-eastern Ligurian Sea. Comparison of the size-frequency distributions of catches obtained with the three gears evidenced that in the Ligurian Sea the cuttlefish collected with trammel net were bigger than those obtained with the two traps, while in the Adriatic Sea fyke nets caught smaller specimens than the other gears. In both areas a general significant reduction of the mean mantle length of the caught cuttlefish was recorded during the sampling period but, in spite of this, all the specimens recorded in the Adriatic Sea and most of those collected in the Ligurian Sea were bigger than the length of maturity. This confirms that in the period late winter-early summer the exploitation of the cuttlefish in the coastal zone is mainly focused on adults which migrate inshore for spawning. Therefore, this fishing activity might influence the reproductive potential of S. officinalis, but this impact is likely to be very scarce considering that this species seems to spawn only once in its life cycle and that the traps act as suitable substrates for the egg attachment. An additional analysis based on the frequency of only male or only female catch as well as on the sex-ratio of specimens inside each trap was conducted to clarify the catching mechanism of fyke nets and pots. The results showed that in both areas males were attracted by the presence of a female inside the pots, while the entry of females seemed to be accidental and probably related to their spawning. No sexual attraction seemed to affect the entry of cuttlefish into the fyke nets. Finally, the investigation performed at the landing sites showed that cuttlefish fishery was carried out in the two areas with vessels having similar technical characteristics, but in the Ligurian Sea this activity was conducted all year round, while in Adriatic Sea it showed a strong seasonal pattern limited to the period late winter - early summer. Nevertheless, the estimated total landings were only slightly higher in the Ligurian than in the Adriatic Sea. The cost/benefit analysis based on the main factors affecting the artisanal fishery targeting cuttlefish suggested that, independently for the type of bottom, the use of fyke nets in the Adriatic Sea may represent a good compromise among different factors, such as purchase, maintenance and storage costs, handiness, yields and environmental impact. On the contrary in the Ligurian Sea trammel net seems to be the only suitable gear because of the negligible catches of traps.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
