Knowledge on how fishers exploit their fisheries resources is important for understanding how fishing affect the population dynamics of the exploited species and how the fishery may affect the ecosystem. The introduction of a new gear may affect the way fishers deploy their gear in space and time. Here we present the results of a study of the spatial dynamics of pulse trawl vessels when exploiting local aggregations of sole. The behaviour of pulse trawl vessels is compared to the behaviour of traditional beam trawl vessels. Because the pulse logbook monitoring is still ongoing, the results are preliminary and will be updated when the complete data set will become available. The logbook data set analysed comprised catch and effort information per tow collected between 1 January 2017 and 30 September 2018. The results were compared with an analysis of logbook data of traditional beam trawl vessels collected between 2000 - 2005. The study showed that pulse trawl (PT) and traditional beam trawl (BT) vessels had similar fishing patterns with alternating periods of searching, or sampling, for fishing grounds and exploitation of fishing grounds. The catch rate of sole during exploitation of a fishing ground was on average 22% (PT) and 23% (BT) higher than while searching for fishing grounds. PT deploy 73% of their tows while exploiting a fishing ground and 27% while searching or sampling, as compared to 69% and 31% in BT. The number of tows taken on a fishing ground by PT (large vessels: median = 16.4; small vessels: median = 18.8) was higher than by BT (median = 13.0). During an exploitation event – the period of successive tows made at a fishing ground – the sole catch rate declined over successive tows. Although the rate of decline varied substantially among the different fishing grounds, the statistical analysis showed that on average the rate of decline was faster for BT than for PT. Of the pulse fishing grounds distinguished during the study period 61% were exploited by a single vessel and 39% were exploited by two or more vessels. Vessels differ in the proportion of fishing grounds shared with other vessels. Fishing effort on shared fishing grounds is higher than on the fishing grounds exploited by a single vessel only. The logbook data provide detailed information on what happens on the local fishing grounds which is fundamental to assess the impact of the pulse trawl fishery and beam trawl fishery on the fisheries resources and on the benthic ecosystem. The study of the total pulse fleet provides a unique data set to study not only the dynamics of the whole fleet, including the interactions among pulse vessels, but also provides a solid basis to study competitive interactions with other fisheries. |
