Ludvig Ahm Krag

Inferring Fish Escape Behaviour in Trawls Based on Catch Comparison Data: Model Development and Evaluation Based on Data from Skagerrak, Denmark

During the fishing process, fish react to a trawl with a series of behaviours that often are species and size specific. Thus, a thorough understanding of fish behaviour in relation to fishing gear and a scientific understanding of the ability of different gear designs to utilize or stimulate various behavioural patterns during the catching process are essential for developing more efficient, selective, and environmentally friendly trawls. Although many behavioural studies using optical and acoustic observation systems have been conducted, harsh observation conditions on the fishing grounds often hamper the ability to directly observe fish behaviour in relation to fishing gear. As an alternative to optical and acoustic methods, we developed and applied a new mathematical model to catch data to extract detailed and quantitative information about species- and size-dependent escape behaviour in towed fishing gear such as trawls. We used catch comparison data collected with a twin trawl setup; the only difference between the two trawls was that a 12 m long upper section was replaced with 800 mm diamond meshes in one of them. We investigated the length-based escape behaviour of cod (Gadus morhua), haddock (Melanogrammus aeglefinus), saithe (Pollachius virens), witch flounder (Glyptocephalus cynoglossus), and lemon sole (Microstomus kitt) and quantified the extent to which behavioural responses set limits for the large mesh panel’s selective efficiency. Around 85% of saithe, 80% of haddock, 44% of witch flounder, 55% of lemon sole, and 55% of cod (below 68 cm) contacted the large mesh panel and escaped. We also demonstrated the need to account for potential selectivity in the trawl body, as it can bias the assessment of length-based escape behaviour. Our indirect assessment of fish behaviour was in agreement with the direct observations made for the same species in a similar section of the trawl body reported in the literature.

Size Selection of Antarctic Krill (Euphausia superba) in Trawls

Trawlers involved in the Antarctic krill (Euphausia superba) fishery use different trawl designs, and very little is known about the size selectivity of the various gears. Size selectivity quantifies a given trawls ability to catch different sizes of a harvested entity, and this information is crucial for the management of a sustainable fishery. We established a morphological description of krill and used it in a mathematical model (FISHSELECT) to predict the selective potential of diamond meshes measuring 5–40 mm with mesh opening angles (oa) ranging from 10 to 90°. We expected the majority of krill to encounter the trawl netting in random orientations due to high towing speeds and the assumed swimming capabilities of krill. However, our results indicated that size selectivity of krill is a well-defined process in which individuals encounter meshes at an optimal orientation for escapement. The simulation-based results were supported by data from experimental trawl hauls and underwater video images of the mesh geometry during fishing. Herein we present predictions for the size selectivity of a range of netting configurations relevant to the krill fishery. The methods developed and results described are important tools for selecting optimal trawl designs for krill fishing.

Quantification of species selectivity by using separating devices at different locations in two whitefish demersal trawls

This paper presents a quantitative approach to the study of fish behaviour in trawls with the aim of maintaining the catching efficiency of target species and reducing discards of unwanted bycatch. Differences in vertical distributions of species during passage through a trawl are used to sort the catch into separate compartments prior to size selection. It is demonstrated that behavioural differences may be utilized in separating species prior to size selection. Comparisons indicate that these patterns are consistent over replicate trials. Unlike cod (Gadus morhua), haddock (Melanogrammus aeglefinus), saithe (Pollachius virens), and whiting (Merlangius merlangus) do not change their vertical preference longitudinally in the trawl.

From Fishing to Fish Processing: Separation of Fish from Crustaceans in the Norway Lobster-Directed Multispecies Trawl Fishery Improves Seafood Quality.

Fishing gears have negative impacts on seafood quality, especially on fish in the mixed trawl fishery targeting Norway lobster (Nephrops norvegicus). In this fishery, which is worth about €80 millions in Denmark alone, the quality of fish can be significantly improved by simple gear changes. A trawl codend divided into an upper and lower codend was designed to separate fish from Norway lobster during the fishing process by encourage fish to swim into the upper codend by using a frame at the entrance of the lower codend. Separate codends for fish and Norway lobster in the same gear provide the opportunity to selectively reduce small low-value fish, which will reduce catch weight and sorting time onboard the vessel. For this horizontally divided test codend and a standard codend, in which the catch was mixed, quality assessments were performed on the same batches of fish during three steps of the value chain: i) aboard the fishing vessel; ii) at the Fishermen’s Collection Central, and iii) in the production plant. Four species of fish and fillets from fish caught in the upper codend of the test codend were of significantly better quality for several of the assessed parameters compared with those caught in the standard codend: i) newly caught fish showed significantly less scale loss and discolourations and had significantly better texture; ii) landed fish had significantly better skin appearance and texture and significantly fewer discolourations; and iii) fillets showed significantly fewer blood spots and had significantly better texture. There were no differences in injuries for newly caught fish or gaping and bruises for fillets between the test and standard codends. The decrease in catch-related damages in the test codend is explained by little contact between fish and animals with hard or spiny surfaces due to successful separation of fish and Norway lobster into the upper and lower codends, respectively, and by lower catch weight in the upper codend of the test codend compared with the standard codend. The decrease in damages may also improve quality indirectly by inflicting less stress to the fish and subsequently give better texture, which offers advantages such as pre-rigor filleting and fresher products for the market. Significant improvements in fish quality can potentially increase the catch value in nationally important fisheries.

Improving the effectiveness of escape windows in directed Norway lobster Nephrops norvegicus trawl fisheries

A substantial improvement in the bycatch selectivity of Norway lobster Nephrops norvegicus trawls is required, particularly with respect to cod Gadus morhua, whose stocks are at low levels in several areas. Conventional escape windows are not adequate to properly release cod and other bycatch species caught in the trawls. To address this issue, we developed a novel sorting box concept consisting of a four-panel section with a window on the top in order to improve the escape of cod and other bycatch species through an escape window while retaining the target catch of Norway lobster. The concept was tested on a commercial trawler in Kattegat and Skagerrak. Two different window mesh sizes and two different sorting box heights were tested using a traditional codend cover and a dual codend cover. We observed greatly reduced bycatches of both cod and other fish species compared to a standard codend. The reduction in bycatch decreased with decreasing mesh size and increasing height of the sorting box. Escape of Norway lobster through the escape window was limited. A modified version of the sorting box concept was implemented in the Kattegat fishery from 2009 onwards.

Investigating fish behavioural responses to LED lights in trawls and potential applications for bycatch reduction in the Nephrops-directed fishery

for bycatch reduction in the Nephrops-directed fishery DTU Orbit (07/11/2019) Investigating fish behavioural responses to LED lights in trawls and potential applications for bycatch reduction in the Nephrops-directed fishery Light-emitting diodes (LEDs) have been tested in trawl fisheries to reduce the bycatch of unwanted species through behavioural stimulation. Previous studies used LED lights to either highlight escaping routes or increase the contact rate with square-mesh panels. However, phototactic responses (moving towards or away from light sources) to LED lights could also be exploited to separate species during the catching process. We investigated if either positive or negative phototaxis can be used to improve fish vertical separation from Nephrops (Nephrops norvegicus) in the aft section of a horizontally separated trawl codend. The aim was to increase the proportion of fish entering the upper compartment. We conducted two different experiments in front of the separation into compartments, inserting green LED lights in the upper and lower netting panel, respectively. Species vertical separation was analysed and compared in two identical trawls towed in parallel, one equipped with lights and one without. We obtained significant changes in vertical separation, but no clear species-specific phototactic response was identified. Neither of the light positions improved fish separation from Nephrops. However, the potential of LED lights as behavioural stimulators is confirmed, and a more mechanistic understanding of light and fish vision may improve the results of future applications.

When is enough, enough? Quantifying trade-offs between information quality and sampling effort for fishing gear selectivity data

There is general pressure throughout the world’s fisheries for the industry to have greater involvement not only in the development of fishing gears but also in the testing and documentation of their effect. In the European Union, the Common Fisheries Policy of 2013, together with the proposed reform of the technical measures regulation, highlights the need for greater flexibility in fisheries through increased stakeholder involvement. To achieve this flexibility, there is a need for additional fishing gears available to the fishermen. A way to facilitate this is to have the industry take part in the development and testing of fishing gears, as well as collect data on their performance. However, to have a successful industry-collected data programme, fishermen have to be able to collect data on the length of a portion of the catch. In this study, we determine how many individuals need to be measured to correctly evaluate the relative selective performance of a new gear compared to a standard gear. The evaluation was carried out by analysing catch ratio curves, their associated uncertainties, and the trade-offs between uncertainties and sampling effort. Results show that with relatively small sample sizes (500 to 1000 individuals) it is possible to correctly evaluate the performance of a gear for a given species. By having the industry develop and test their own gears, as well as being involved in the collection of data, the number of potential gear solutions available to address the different issues emerging in the fisheries is increased.

Collecting size-selectivity data for Antarctic krill (Euphausia superba) with a trawl independent towing rig

For the development of efficient trawls to minimize catch loss, escape mortality and potential negative ecosystem impacts from the fishery, the understanding about trawl selectivity processes are crucial. Small crustaceans are regarded as being less motile than most fish species. Crustaceans also display low levels of active avoidance from trawl netting, which in turn may cause direct contact with netting on multiple occasions on their passage towards the codend increasing the probability for escapement. Full-scaled experiments to estimate gear selectivity are highly resource demanding and are highly technically challenging for several types of fisheries. In this study, we developed and tested a trawl-independent towed-rig construction designed to investigate size selectivity of Antarctic krill (Euphausia superba). The results indicate that valid selectivity estimates can be obtained using this method, but due to the small sample size, results are inconclusive. However, the findings of the current study show a potential for developing easier and more cost-effective ways of investigating and estimating size selectivity of Antarctic krill and other small crustacean species in trawls.

Developing and testing a computer vision method to quantify 3D movements of bottom-set gillnets on the seabed

Gillnets are one of the most widely used fishing gears, but there is limited knowledge about 1 their habitat effects, partly due to the lack of methodology to quantify such effects. A stereo 2 imaging method was identified and adapted to quantify the dynamic behavior of gillnets in3 situ. Two cameras took synchronized images of the gear from slightly different perspectives, 4 allowing to estimate the distance from the observation unit to the gear such as in the human 3D 5 vision. The sweeping motion on the seabed and the penetration into the sediment of the leadline 6 of light and heavy commercial bottom gillnets deployed in sandy habitats in the Danish coastal 7 plaice fishery were assessed. The direct physical disruption of the seabed was minimal as the 8 leadline was not penetrating into the seabed. Direct damage to the benthos could however 9 originate from the sweeping movements of the nets, which were found to be higher than usually 10 estimated by experts, up to about 2 m. The sweeping movements were for the most part in the 11 order of magnitude of 10 cm, and resulted in a total swept area per fishing operation lower than 12 any of the hourly swept area estimated for active fishing gears. Whereas the general perception 13 is that heavy gears are more destructive to the habitat, light nets were moving significantly 14 more than heavy ones. The established methodology could be further applied to assess gear 15 dynamic behavior in-situ of other static gears. 16

Estimating escapement of fish and invertebrates in a Danish anchor seine

11 The codend is generally presumed to be the place where the main selectivity of fish occurs in towed 12 fishing gears, but other parts of the net have been found to contribute to the selectivity process of several 13 invertebrate species. This means that conventional selectivity or survival studies may ignore the 14 selectivity of net parts other than the codend for certain species. By attaching 12 small meshed collecting 15 bags to different parts of a Danish anchor seine net and conducting normal commercial fishing activities, 16 this study showed that there is a substantial escapement of fish and (especially) invertebrates from the 17 forward parts of the seine net. For seven species of demersal fish, most fish escaped through the lower 18 panel close to the codend. All invertebrate species were found in higher numbers in the collecting bags 19 than in the codend where many organisms escaped in the lower panel of the wings or the belly. Mean 20 levels of visible damage ranged from 1.00 to 3.25 for collected invertebrates and were similar for all gear 21