Objective
The aim of the research is: to assess the survival rate and damage due to loss of scales experienced by fish when escaping from the codends of a trawl towed by a commercial trawler of over 500 horsepower; and to concurrently measure the selectivity, towing geometry and water flow in these codends in order to study the dependence of escape, survival and scale loss upon these parameters. Studies will be made of 3 different commercial codend configurations: 90 mm diamond mesh; 100 mm diamond mesh; and 110 mm diamond mesh; and, subject to their availability in the chosen test area, haddock, whiting, cod, saithe and plaice.
In 1992 and 1993 experiments took place on the west coast of Scotland to assess the survival rate and damage due to loss of scales experienced by fish when escaping from the codends of a trawl towed by a commercial trawler of over 500 HP. Studies involved different commercial codend configuration, 90 mm diamond mesh, 100 mm diamond mesh and 110 mm diamond mesh with respect to catches of whiting and haddock. In 1993 the survival rates of haddock and whiting controls were both 100%. The survival rates for the haddock and whiting experimental groups were 48%-67% and 52%-60% (70 mm codend), 79%-82% and 73%-78% (90 mm codend), 73%-83% and 67%-77% (100 mm codend) and 85%-89% and 83%-86% (110 mm codend) respectively. The haddock and whiting size ranges were 15 cm-38 cm and 17 cm-35 cm respectively. These percentages relate only to the numbers of survivors from the total escapes in a particular codend mesh size category, regardless of fish length. The apparent improvement in percentage survival, as the mesh size increases, may be due to the fact that more larger fish escape through the larger meshes.
Evidence indicates that age and fitness of a fish and not only its length may play a role in its ability to survive the traumatic experience of a codend escape. Analysis of the data also revealed, that there was no clear relationship between survival rate of an escaping fish of a given length and mesh size for this haddock and whiting population over the diamond mesh range of 70 mm-100 mm. The new fish damage assessment techniques worked well and analysis of the data showed that the mean total percentage appears to be independent of mesh size.
These findings, albeit from one fishery comprising of a fairly narrow size range of fish, demonstrate that the extra numbers of fish that can escape from a trawl's codend after its selectivity has been improved will be of a length where fairly high survival can be expected. The implication that a significant proportion of the smallest fish escaping die could influence future fisheries management decisions. These results can also help to improve the accuracy of fishing mortality estimations used in stock assessment calculations.
The research work comprises the following main phases.
Phase 1 is the site selection, initial survival and codend selectivity tests. A review will be made of the existing techniques for measuring the survival of fish escaping from codends and the results obtained to date. Cage design, techniques for transporting the fish from the trawl to the cages and fish sample size will be determined. A final choice will be made of the 3 different codend configurations to be tested. A pilot study of fish survival and codend selectivity will be made. Fish escaping from the trawlers' 3 test codends will be collected in a special small mesh cover. The fish will be transferred by the divers to the cages. They will be fed regularly and their survival compared to that of control fish caught using handlines and barbless hooks. Dead fish will be removed and analysed. Inspection of the cages will initially be made daily then on a weekly basis until there are no more fish deaths. Survivors will be measured at the end of the tests (length, girth, damage etc). When fish are not being collected for the survival tests the selectivity, geometry and water flow in the codends will be measured. A different small mesh cover technique with wide hoops which keep the cover clear of the large mesh codend has been developed and will be used for the selectivity tests. A towed underwater vehicle, FOCUS, will be used to observe fish behaviour in the codend, check that the codends and covers are functioning correctly and to measure the codend geometry in conjunction with a self recording instrument mounted on the codend itself.
Phase 2 is the full survival and scale damage tests. The survival tests will be extended to include plaice and series of hauls will be made in which the scale damage experienced by fish escaping through the codend meshes will be measured.
Phase 3 is the full codend selectivity and physical property tests. The work completed under phase 1 will, in all probability, be in the form of a preliminary study to sort out techniques with the main useful measurements being obtained in this phase. The data will be examined to determine if there is a correlation between codend mesh size, mesh opening and water flow and the resultant codend selectivity, degree of scale damage and survival rate for the different fish species investigated.
Fields of science (EuroSciVoc)
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques.
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques.
- agricultural sciencesagriculture, forestry, and fisheriesfisheries
- social sciencessociologydemographymortality
- natural sciencesbiological scienceszoologyichthyology
- natural sciencesmathematicspure mathematicsgeometry
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9850 Hirtshals
Denmark