| 1. |
- Calamnius, Linda
(författare)
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Behaviour of grey seals (Halichoerus grypus) and their prey in and near set traps
- 2017
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The seal populations of the Baltic Sea Area were at historically low levels in the 1970’s, due to two factors. The first was an extensive hunt and the second was emissions of organochlorines, which affected the reproductive abilities of the females. Laws and regulations were set in force to improve the marine environment and from the early 1990’s the populations of the grey seal (Halichoerus grypus), the harbour seal (Phoca vitulina) and the ringed seal (Pusa hispida) have recovered. It is a success for the management of the Baltic Sea Area environment that the seal populations have increased. Coupled with the increase of the populations, is also an increase of conflicts with the inshore fisheries. The seals frequenting the fishing gear takes fish and damage gear. It is mainly the grey seal which is the culprit and it is predominantly males. These males have been proven to be specialists. Two studies were conducted to learn more about the behaviour of seals and of their prey. The first study investigated the pattern of seal visits in the middle chamber of a herring pontoon trap. A camera filmed the seals which entered and the seals were identified. There were almost 1400 visits by 12 individuals. Of all visits, 84 % took place within 5 minutes of each other. Of all visits, 3.7 % were concurrent visits, i.e. two males inside the middle chamber at the same time. By studying these visits in detail, it could be concluded that there was a dominance hierarchy among the seals in the trap. A simulation of the visits was executed to examine whether the proportion of concurrent visits was random or non-random. The simulations used the same distribution pattern of the realized visits. If the visits had been random, then c 9.5 % of them would have been concurrent. This suggests that there is a pattern to their visits. The second study investigated the effect of a Seal Exclusion Device (SED) on seal visits and on catch. The experiment was conducted during two years. In 2012, using a SED with a diamond mesh and in 2016 using two SEDs; the diamond mesh and a square mesh - with the entire frame rotated 45°. The expectation was that the SEDs would reduce the number of seal visits, increase the catch and possibly deter larger fish from entering. In 2012, the diamond mesh had an effect on the size of trout, with larger trout entering the control trap, whereas large salmon were not affected by the presence of the SED. In 2016, larger salmon were caught in the traps with a SED. There was no significant result regarding the catch of trout, possibly due to small samples. The number of seal visits in both sets of experiments were too low to be able to draw any conclusions regarding presence of seals. The SEDs did not have any effect on the quantity of caught fish.
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| 2. |
- Calamnius, Linda, et al.
(författare)
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Pontoon trap for salmon and trout equipped with a seal exclusion device catches larger salmons
- 2018
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Ingår i: PLOS ONE. - : PLoS. - 1932-6203. ; 13:7
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Tidskriftsartikel (refereegranskat)abstract
- The growing seal populations of the Baltic have led to more frequent interactions with coastal fisheries. The motivation for seals to interact with fishing gear is high. It provides high densities of fish. A successful means of mitigating the conflict is the pontoon trap. Seal visits here have been frequent. Seals have access to most parts of the trap system including the middle chamber, which is an overhead environment. Concerns have been raised about seals possible entanglement in this specific part of the trap. As a means of keeping seals from entering the middle chamber, two different Seal Exclusion Devices (SEDs) were tested. A diamond mesh SED and a square mesh SED, which was rotated 45. The aim was to compare the functionality of the different SEDs with respect to seal deterrent abilities and catch composition. The hypothesis tested were (i) that seals would not be able to enter the middle chamber, (ii) that the catch would increase and (iii) that the SED would deter larger fish from swimming into the middle chamber. Catch data and underwater film were collected. Larger salmons were caught in traps equipped with SEDs. The SEDs did not affect the number of caught fish or the total catch per soak day. © 2018 Calamnius et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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| 3. |
- Calamnius, Linda
(författare)
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Salmonids and grey seals (Halichoerus grypus) : mitigation in pontoon traps
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the 1970’s, the seal populations of the Baltic Sea Area were at historically low levels. They have recovered and increased since then. The increase of the seal populations is a success for the management of the Baltic Sea Area environment. It has also meant an increase in number of interactions with coastal fisheries. Seals take fish and damage fishing gear. Three studies were carried out with the purpose of contributing to a sustainable fishery and fewer interactions between seals and fishers. The first study compared the effect two different Seal Exclusion Devices (SEDs) had on the catch and on seal visits. The SEDs used were a diamond mesh SED and a square mesh SED, with the frame rotated 45°. They were compared with a control, an open frame. The expectation was that using SEDs would reduce the number of seal visits, increase the catch and deter larger fish from entering. Larger salmons (Salmo salar) were caught in the traps with selection panels. For brown trout (Salmo trutta), there was no difference in size of fish between the SEDs. Neither of the SEDs had any effect on total catch or catch per unit effort. The number of seal visits were too low to be able to draw any conclusions regarding presence of seals. The second study examined the efficiency of selection panels in a pontoon trap for salmon and whitefish. One control and two experimental traps were used. The mesh in the control trap had 35 mm bar length. The selection panel was square mesh with 50 mm bar length. In one of the experimental traps, the selection panel covered 30 % of the inner netting. In the other, it covered 100 %. The results showed that proportionally more fish of commercial size were caught in traps with selection panels. Using selection panels contributes to a sustainable fishery. The third study analysed a series of visits by seals in the middle chamber of a herring pontoon trap. Visiting seals were filmed in the middle chamber. Roughly, 1 400 visits by 12 seals were recorded. Of all visits, 3.5 % were overlapping visits, i.e. two seals inside the middle chamber at the same time. Forty simulations of random visits were performed resulting in an average of 7.1 % overlapping visits. There was a significant difference between the actual overlapping visits and the simulated. This suggests that the seals avoided swimming in when another seal was present
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| 4. |
- Lundin, Mikael, et al.
(författare)
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Selection efficiency of encircling grids in a herring pontoon trap
- 2011
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Ingår i: Fisheries Research. - : Elsevier BV. - 0165-7836 .- 1872-6763. ; 111:1-2, s. 127-130
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Tidskriftsartikel (refereegranskat)abstract
- High bycatches of undersized herring constitute a major problem in the Baltic Sea herring trap fishery. In an attempt to reduce these bycatches, this field study evaluates the efficiency of rigid selection grids encircling a herring pontoon trap. The results show that 54-72% of the undersized herring were removed from the catch. The introduction of such grids would therefore represent a significant step towards a more efficient and sustainable herring fishery in the Baltic Sea. © 2011 Elsevier B.V.
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| 5. |
- Lundin, Mikael, et al.
(författare)
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Size selection of herring (Clupea harengus membras) in a pontoon trap equipped with a rigid grid
- 2011
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Ingår i: Fisheries Research. - : Elsevier BV. - 0165-7836 .- 1872-6763. ; 108:1, s. 81-87
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Tidskriftsartikel (refereegranskat)abstract
- A sustainable fishery in the Baltic and Bothnian Seas requires the development and introduction of fishing gear which fishes selectively and at the same time excludes raiding seals. The purpose of this study was twofold: firstly to test and evaluate rigid grids as a method for retaining only larger herring in a pontoon trap, and secondly to analyze which factors were influencing the selection process. The results demonstrate that it is indeed possible to sort herring by size in a pontoon fish chamber. The efficiency of excluding undersized herring was at best 27%, using a selection grid covering just over 0.1% of the fish chamber wall. The factors which have most effect on the selection were the quantity of fish in the trap, the season of the year, the time of day and the presence of seals. (C) 2010 Elsevier B.V. All rights reserved.
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| 6. |
- Lundin, Mikael, et al.
(författare)
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Size selection of whitefish (Coregonus maraena) in a pontoon trap equipped with an encircling square mesh selection panel
- 2015
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Ingår i: Fisheries Research. - : Elsevier BV. - 0165-7836 .- 1872-6763. ; 161, s. 330-335
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Tidskriftsartikel (refereegranskat)abstract
- Many fishing methods result in significant catches of non-target species or individuals that are too small to be retained for economic values or within quota regulations. In the Baltic Sea trap fishery, the major problem is the bycatch of juvenile and non-marketable whitefish (Coregonus maraena) which constitutes a threat to the sustainability of the fishery and a time-consuming problem for the fishers. This study evaluates the effectiveness of a modified fish trap in reducing such bycatch. An encircling selection panel consisting of 50 × 50 mm square mesh netting was installed in a pontoon fish chamber of a salmon/whitefish trap. Comparative fishing was conducted against a control trap without selection panel in the inshore waters of the Bothnian Sea. Comparisons of catch compositions between the traps showed that there was a 72% reduction in juvenile whitefish catch in the experimental trap. The length at 50% retention (30.1 cm) corresponded well to the minimum market size (>30 cm) of whitefish that are desired for the local market. Fishers are advised to use a netting panel of 50 × 50 mm square mesh for their traps to reduce undersized whitefish. The results are important for the sustainability of whitefish stocks in the Baltic Sea.
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| 7. |
- Lundin, Mikael, et al.
(författare)
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The efficiency of selection grids in perch pontoon traps
- 2015
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Ingår i: Fisheries Research. - : Elsevier BV. - 0165-7836 .- 1872-6763. ; 162, s. 58-63
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Tidskriftsartikel (refereegranskat)abstract
- In commercial fishing, minimizing the bycatch of undersized fish or non-target species is highly beneficial, to avoid unnecessary fish mortality and to save time for the fishers. Two pontoon traps developed for perch fishing were equipped with size selection grids, and the efficiency with which under-sized fish could escape was tested. Average size of perch, roach, and whitefish was larger in traps with selection grids compared to in control traps without grids. Selection efficiencies using these comparisons were 82–86% for perch, 33% for whitefish and 100% for roach. The selection grids were filmed with an underwater video camera over the daily cycle, to estimate timing, and total number of exits from the traps. Selection efficiencies, calculated by extrapolating number of escapes observed to the total time of trap submergence, were 94–100% for perch and 100% for roach. The discrepancy in the selection efficiency estimates for perch probably depends on an uncertainty in the extrapolation, because of the variation in escape rate across time periods. Perch and roach differed in time of day for escapes. For perch most escapees was seen in the evening, and for roach most fish escaped at night, probably reflecting the general activity cycles of the two species. Over a fishing season, several thousands of fish would be able to escape from each trap, and an increase in the use of size selection grids could potentially be an efficient tool for fish population management.
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