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EUROPEAN CETACEAN BYCATCH CAMPAIGN |
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Part of FANTARED undertaken in Portuguese waters |
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Introduction |
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There is a general need to identify and quantify the total impact of gill nets. The increased use of such nets has led to concern in some quarters that there must be a concurrent increase in the rate of loss or abandonment of nets. The extent of loss and the potential fate of nets so lost has never been examined systematically. From three separate proposals to study the subject area, presented by Portugal, Spain and United Kingdom, results this project. |
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ˇ to quantify the amount of lost gill nets and trammel nets and rates of gear loss in Portuguese waters (South of Portugal) by métier, and to obtain information on the principal factors responsible for these losses |
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ˇ to study the evolution, degradation, and movement of lost nets |
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ˇ to describe and quantify the impacts of gill nets and trammel nets on living species |
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Methods |
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ˇ four sets (100 m gill net and 100m trammel net) of experimental "lost" nets, fixed at one end, loose at the other, deployed in 18m depths |
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ˇ monitoring with two pairs of divers (video and photographic records, etc..): daily catch rate (by tagging catches), total length of organisms caught, state of decomposition, daily catch in weight, net height (5 m intervals), net area, net movement, net degradation, net colonisation |
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ˇ interviews and questionnaires of fishing boat owners and captains in order to estimate the amount of gear lost in Algarve waters and reasons for losses |
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Main Results |
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ˇ net height, net area decreased rapidly within the first 1 to 3 weeks, levelling off afterwards while net visibility increased dramatically within the first few weeks due to accumulation of debris and detritus |
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ˇ daily catches decreased exponentially over time, becoming negligible after 3-4 months (Figure ) |
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ˇ after one year, nets were completely destroyed and/or colonised and incorporated into the natural bottom |
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ˇ it was estimated that each 100m of gill net caught between 300 and 400 fish during 120 days, while the trammel nets caught half as much during the same period |
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ˇ the questionnaire surveys carried out showed that the main causes of gear loss are bad weather and interference by other fishing gears or activities. Some differences were found between the local and the coastal fleet. For the more inshore, local fleet, the main factor was bad weather (60%), clam dredges ("ganchorra") were second in importance (30%). For the coastal fleet, trawls accounted for 54% of the losses and bad weather was the second most important factor (26%) |
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ˇ estimates of the number of gill nets and trammel nets that could potentially be lost along the south coast of Portugal (1 net = approximately 50 m): |
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Conclusions |
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ˇ at the depths at which this study was carried out (20 m) monitoring by divers is an adequate methodology |
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ˇ two consecutive days of diving is sufficient in terms of sampling, in order to quantify net and catch characteristics and to obtain a video record |
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ˇ under these specific conditions of depth, location and season, experimental nets should be monitored with greater frequency in the initial phases. Therefore we recommend weekly monitoring during the first month, every two weeks in the second month, and monthly sampling thereafter |
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ˇ under these specific experimental conditions, both gill nets and trammel nets continue to fish for a maximum of 15 to 20 weeks; with an exponential decline in catches (numbers and biomass) over time |
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ˇ the major factors affecting gear loss were identified for the "local" and the "coastal" fleet |
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ˇ preliminary estimates of the amount of netting lost in the Algarve were made; however we do not have much confidence in these estimates due to difficulties in obtaining precise information from the fishermen. More interviews with fishermen should be carried out in order to improve the estimates concerning gear loss |
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