Beam Trawl Research Articles

МОРФОЛОГИЧЕСКИЕ ХАРАКТЕРИСТИКИ И ОСОБЕННОСТИ БИОЛОГИИ БАРБУСА СМИТА PUNTIOPLITES PROCTOZYSTRON (BLEEKER, 1865) В ДЕЛЬТЕ РЕКИ МЕКОНГ

Background. Fisheries in the Mekong River are characterized by high pressure and extremely low selectivity towards fishing targets, which greatly complicates the conservation and possible restoration of the region's biodiversity. Relevance. Knowledge of fish population structure and biology is essential for fishery management and rational use of resources; however, the fisheries of the region are predominantly artisanal, for which the information on catches is scarce. This work is aimed at a study of the morphological characteristics of Smith's barb Puntioplites proctozystron (Bleeker, 1865) and the distribution of its quantitative parameters. Methods. The data have been collected from the bottom trawl catches in the Mekong River Delta that were conducted using a beam trawl in the low water and flood periods of 2018–2022. Results. The average abundance and biomass of this species in fresh waters were 8.3±20.8 ind./ha and 114.7±229.4 g/ha, and in the estuary they were 0.6±2.7 ind./ha and 12.1±63.5 g/ha. These quantitative parameters for Smith's barb in the relatively shallow and low-flow Hau River and its estuarine channels were statistically significantly higher than in the rest of the delta estuary system (8.5±22.2 ind./ha and 110.7±248.7 g/ha as compared to 1.4±6.5 ind./ha and 25.0±90.1 g/ha). Examination of morphological characteristics showed a very low variability of traits in mature individuals; the coefficient of variation was less than 10 %. Conclusion. The obtained results indicate the absence of sexual dimorphism and different morphotypes in the population, provide the ground for minimum landing size recommendations, and can serve as a foundation for the further population research of this species.

Bionomic study of the detritic bottoms dominated by macroalgae from the southern coast of Mallorca (Balearic Islands)

Abstract This bionomic study of the detritic bottoms dominated by macroalgae from the south of Mallorca (Balearic Islands, Western Mediterranean) includes a quantitative description of the algal communities found in the area, as well as their bathymetric and geographical distribution. The results presented here are based on data collected in two oceanographic campaigns conducted in July 2012 and September 2014, using a Jennings beam trawl. A hierarchical group average agglomerative clustering, accompanied by the SIMPROF test, allowed the identification of seven different macroalgal communities, of which two are described here for the first time: the Cryptonemia longiarticulata fields and the Maërl beds of indeterminate rhodoliths. Depth and rhodolith abundance were the two main features driving the distribution of these communities. We found that seven species contributed 70% of the similarity between samples (SIMPER test), with the indeterminate species of rhodoliths (23.6%) and the encrusting fleshy red alga, Agissea inamoena (15.6%) being the most important. The methodology used for the sample selection and quantification processes turned out to be very efficient and faster than other methods used for the characterization of macroalgal communities from detritic bottoms, suggesting that this study could serve as a baseline for similar studies and for future management and conservation actions.

Türkiye'nin Ege Denizi kıyılarında Holothuria tubulosa (Gmelin, 1790) ve Holothuria polii (Delle Chiaje, 1823) popülasyon yoğunluğunun av yapılan ve yapılmayan alanlar arasındaki karşılaştırılması

Increasing demand from the Asian market first led to the collapse of economically valuable sea cucumber stocks in the Indo-Pacific, and then demand shifted to lower value Mediterranean species. The effect of this change in sea cucumber stocks in Türkiye started to be seen after the 2010s. To address it, we carried out a study to compare the population density of the most caught Holothuria tubulosa and Holothuria polii species between exploited and non-exploited areas in the Aegean Sea coast of Türkiye. The study was carried out between September 2018 and March 2020 at 4 stations, two of which are in Çandarlı Bay, where sea cucumber fishing is free, and two in İzmir Bay, where it is prohibited. Samplings between 0-20 m depths were carried out with underwater transect line technique, and at depths deeper than 20 m, with beam trawling operations. A total of 6 sea cucumber species were identified, including Holothuria mammata, Holothuria sanctori, Parastichopus regalis and Holothuria forskali species, apart from the target species H. tubulosa and H. polii. Only one P. regalis individual was recorded in the samplings in waters deeper than 20 m. The mean densities of H. tubulosa and H. polii were found to be higher in İzmir Bay stations than in Çandarlı Bay (p<0.05). The results of this study clearly reveal that the population density of sea cucumber in sampling areas has decreased tens of times compared to the last 5 years.

Results of trawl surveys in Khanka lake in 2018 and 2020

Since 2022, the possibility of trawl fishing on Khanka Lake has been opened. In 2018 and 2020, accounting work was carried out using a beam trawl. Studies have shown that the highest biomass in trawl catches was far eastern freshwater shrimp (Palaemon modestus), noodle fish, sharpbelly, bagrid catfish and humpback. According to the trawl survey, the stocks of shrimp and small fish species amounted to 3,5–4,6 thousand tons. Taking into account the underestimation of these biological resources during the performance of work, the resource base can be at the level of 14,0–15,0 thousand tons.

Selectivity Characteristics of the Sorting Grid in Shrimp Beam Trawls used to Reduce Bycatch in the Sea of Marmara

Considerable by-catch problems have occurred in the commercial beam trawl fishery targeted at deep water pink shrimp in the Sea of Marmara. In this study, the grid systems were used to reduce the untargeted catch and improve the species' selectivity. By-catch reduction ratios of the beam trawls deployed with and without the grid were compared by the parallel hauling method. Selectivity values of 20 mm bar spacing grid and 24 mm mesh size cod-end were also analyzed by SELNET program. In general, by-catch reduction ratios of the grid beam trawl were determined as 50.6% and 56.9% in terms of number and weight, respectively. The reduction of target catch (Parapenaus longirostris) in the same gear was found to 23.1% in terms of number and 23.2% in terms of weight. In the model for rose shrimp, obtained according to the lowest AIC value, the selectivity parameters of the grid for L50 were 13.51 cm and for SR 4.76 cm. Selectivity parameters of the same species for 24 mm mesh size cod-end were; L50 = 8.99 cm and SR = 5.56 cm. These results indicated that grid usage benefits and contribute to sustainable fishery in the target catch-oriented fishing applications.

Infaunal and epifaunal secondary production in the Barents Sea, with focus on snow crab (Chionoecetes opilio) prey resources and consumption

AbstractSince the first observation of snow crab (Chionoecetes opilio) in the Barents Sea in 1996, the population has increased significantly, supporting a commercial fishery on the Norwegian shelf since 2012. To investigate whether the availability of benthic prey organisms may support a continued geographical snow crab expansion, benthic invertebrate production was studied across the central parts of the Barents Sea and around Svalbard, where snow crabs are currently absent or at low densities. Annual productivity (P/B ratio) from 66 stations collected by grab and beam trawl was estimated using a multiparameter artificial neural network model. Mean infaunal productivity and production were 0.43 yr−1 and 38.4 g ww m−2 yr−1, respectively, while the epifaunal production was considerably lower with 2.5 g ww m−2 yr−1. The proportions of epi- and infaunal production suitable as prey for snow crab were 98 and 96%, respectively. Areas close to the Polar Front represent the most attractive snow crab foraging region, having the highest benthic secondary production, high estimated primary production, and bottom water temperatures within the snow crab’s preferences. At snow crab densities of 12800 ind. km−2, high enough to support commercial fishing, their mean consumption rate was estimated to be around 1.5 g ww m−2 yr−1, which amounts to 4% of mean infaunal prey production. Food availability is, therefore, not expected to be a hindrance to further population expansion of the snow crab in the Barents Sea.

Complementary Sampling Methods to Improve the Monitoring of Coastal Lagoons

Monitoring the ecological status of marine coastal lagoons requires the integration of multiple indices. However, the efficacy of monitoring programs is complicated by the diverse array of habitats that conform coastal lagoons. In this study, we compared four sampling methods (25-m and 50-m beach seines, beam trawl and Riley push net) in the Ria Formosa coastal lagoon (South Portugal) for assessing fish assemblage and diversity. We compared species richness and assemblage structure with species accumulation curves and multivariate analysis, and assessed diversity patterns using taxonomic, phylogenetic and functional diversity indices. Variation in fish assemblage structure was mostly explained by gear type, and almost all diversity metrics varied not only according to sampling method but also depending on habitat characteristics and season. Based on operational costs and diversity patterns captured by each gear, we conclude that the combined use of 25-m beach seine and beam trawl is the preferred approach. The proposed methodology will provide the data necessary for assessment of ecological status of coastal lagoons.

Different life strategies of the three commercially exploited scallop species living under the same environmental conditions

To understand the response of marine species to a changing environment, it is crucial to have deep insight into their main biological traits. This study used a multi-species approach to comparatively analyse the reproductive and growth strategies of three commercially important scallop species. Target taxa were larger sized Mediterranean scallop Pecten jacobeaus, and two smaller sized species Aequipecten opercularis and Flexopecten glaber. Specimens were collected at approximate monthly intervals from February 2017 to October 2018 from a commercial beam trawl in the northern Adriatic area (west coast of the Istria peninsula, 25-35 m depth). Three different complementary methods were applied to obtain comprehensive knowledge about the reproductive cycle including assessment of the gonadosomatic index (GSI), histological analysis of gonad tissue, and measuring oocyte size. Growth strategies of three target species were analyzed using high-resolution stable oxygen isotope data that were temporally aligned on sea water temperature data obtained by AdriSC ROMS model. Results indicate diverse strategies employed by these taxonomically related species exposed to the same environmental conditions. Pecten jacobaeus spawned in the late summer and early fall, while shell growth slowed down during warmer season. Aequipecten opercularis spawned in the winter and slowed down shell deposition process during the cold season. Spawning of F. glaber occurred during early to mid-summer and growth slowed down during warmer season. This study provides an important scientific baseline for sustainable management and future aquaculture attempts of scallops.

Fishing tactics and the effect of resource depletion and interference during the exploitation of local patches of flatfish

Abstract The fine-scale exploitation pattern of fishers and the interactions among fishing vessels determine their impact on exploited populations, habitats, and ecosystems. This study used a unique combination of high resolution data of fishing tracks (positions recorded at 1 and 6 min intervals) and catch rates of sole (Solea solea) and plaice (Pleuronectes platessa) per tow, to study how pulse trawl (PUL) and tickler chain beam trawl (TBT) fishers exploit patches of concealed flatfish. PUL and TBT fishers had similar tactics. Effort was concentrated in the core of the patch. PUL fishers trawled in a systematic manner with successive tows segments placed parallel to each other at a median distance of ∼200 m. In 45% of the cores, simultaneous trawling by multiple PUL vessels occurred. A total of 40% of the cores were revisited in the following week, of which 50% were re-exploited. Catch rate in the core was ∼50% higher than the background catch rate and decreased over time due to resource depletion and interference related to the response of flatfish to the fishing activities. Interference contributed up to 67% to the decline in catch rate and was larger in TBT than in PUL.

A Transdisciplinary Approach Towards Studying Direct Mortality Among Demersal Fish and Benthic Invertebrates in the Wake of Pulse Trawling

Between 2009 and 2020, beam trawlers in the North Sea switched to electrical stimulation to target sole (Solea solea). The transition to pulse trawling raised widespread concern about possible adverse effects of electrical stimulation on marine organisms. Environmental NGO’s and small scale fishers claimed that it would electrocute marine life and create a ‘graveyard’ in the wake of pulse trawlers. This paper uses realistic field experiments to investigate the ‘graveyard’ hypothesis. In cooperation with fishers, a field experiment was designed where we simultaneously sampled marine organisms in the wake of pulse trawlers and in untrawled control areas. The impact was quantified by estimating the direct mortality among three dominant fish species and four dominant invertebrate species. In total, nine experimental tows were conducted in two years. Direct mortality among fish and invertebrates was low (0-10%) and did not differ between the pulse trawl track and the untrawled controls. Equally, no impact of the pulse trawl was found on external damages and vitality scores. The limited effects observed are likely due to the mechanical impact of the pulse and the sampling gear. The results of experiment do not support the claim that pulse trawling results in mass mortality among marine organisms in the trawl track. Throughout the research period, the concerns of small-scale fishers on pulse fishing shifted from being focused on biological effects to political and managerial issues. This can partly be attributed to the engagement in and the results of our research and has increased its credibility and salience. By integrating fishers’ knowledge and examining their perceptions through transdisciplinary research, we were able to show the importance of untangling the intricate relation between perceived knowledge gaps and political or management related concerns.

Energy audit and carbon footprint in trawl fisheries

The combustion of fossil fuels is considered a major cause of climate change, which is why the reduction of emissions has become a key goal of the Paris climate agreement. Coherent monitoring of the energy profile of fishing vessels through an energy audit can effectively identify sources of inefficiency, allowing for the deployment of well-informed and cost-efficient remedial interventions. We applied energy audits to a test fleet of ten vessels, representing three typical Mediterranean trawl fisheries: midwater pair trawl, bottom otter trawl, and Rapido beam trawl. Overall, these fisheries use approximately 2.9 litres of fuel per kilogram of landed fish, but the fuel consumption rate varies widely according to gear type and vessel size. This amount of fuel burned from capture to landing generates approximately 7.6 kg∙CO2/kg fish on average. Minimising impacts and energy consumption throughout the product chain may be another essential element needed to reduce the environmental costs of fishing. Our results provided a set of recognised benchmarks that can be used for monitoring progress in this field.

Grid-net design that successfully reduces discarded catch and damage to benthic species in the veined rapa whelk beam trawl fishery

ABSTRACT In this study, a grid-net design (GND) with three different bar spacings was compared with the commercial beam trawl used in the veined rapa whelk fishery to minimize the discarded catch amount and the loss of commercial product. In addition, the damage and mortality rates of the species that entered the tested fishing gear were determined to understand whether the tested fishing gear has a possible negative effect on benthic species. In the grid-net design, a rectangular metallic grid positioned 6 cm off the ground substituted half of the codend to allow the escape of discard species. Sea trials were carried out in the south-eastern Black Sea between 13 and 24 October 2021. Results showed that the discarded catch amount was significantly reduced by each GND (P < 0.05). Compared with the commercial fishing gear, the reductions in the discarded catch amounts (in terms of number) were 91.6%, 92.2%, and 95.0% for 22, 25, and 28 mm GNDs, respectively. The minimum commercial product loss was determined at 22 mm GND. In addition, it was seen that each GND compared to commercial gear caused statistically significantly less damage and mortality on the discarded species. In conclusion, 22 mm GND was the most successful design among the tested gears in minimizing commercial product loss and reducing the discarded catch amount. Recommendations are given for this design to be adopted by the commercial fleet.

Demersal and epibenthic communities of sedimentary habitats in the Avilés Canyon System, Cantabrian Sea (NE Atlantic)

The aim of this study was to describe the epibenthic and demersal communities of the Avilés Canyon System (ACS) in relation to the environmental variables that characterize their biotope. ACS (Cantabrian Sea, NE Atlantic) was recently included in the Natura 2000 network as a Site of Community Importance (SCI). Data of faunal biomass derived from 6 surveys carried out using beam trawl and otter trawls in 2009 and 2010 within INDEMARES and ERDEM projects. Data were divided into two groups to obtain information about the two ecological compartments: benthic and demersal. The total number of species used in this analysis ascended to 116 in the case of benthic organisms and 110 in the case of demersal. Hierarchical clustering analysis was applied to obtain groups of samples similar in terms of species composition for the two ecological components. Depth was the main discriminating factor for grouping hauls, showing high consistency of bathymetric range independently from the compartment examined. Six groups were identified by depth as follows: medium shelf (∼100–200 m), external shelf (∼150–300m), shelf break (∼300–400 m), upper-slope (∼500–700 m), upper middle-slope (∼700–1100 m), and lower-middle-slope (1200–1500 m). SIMPER analysis on biomass values was performed to determine the structure of the faunal assemblages observed for each group in both compartments. Using biomass values for the analysis allowed for the exploration of groups playing important roles in ecosystem functioning and energy fluxes taking place on the sedimentary bottom of this SCI. Finally, CCA analysis revealed that the main environmental drivers were depth, broad scale bathymetric position index (BPI), near-bottom salinity, sedimentary type, and dynamics related variables (Q50 phy and So). This study gives an inventory of the soft bottom assemblages along a very wide depth range (100–1500 m) inside a SCI, linking both epibenthic and demersal communities with the biotope preferences. This study contributes to fundamental knowledge on soft-bottom communities as a pre-requisite, necessary for the next steps in terms of management framework in the SCI.

Türkiye’nin Batı Karadeniz Kıyılarından 14 Balık Türü Için Boy-Ağırlık İlişkisi

In this study, the length-weight relationship (LWR) of 14 fish species including Merlangius merlangus (Linnaeus, 1758),Trachurus trachurus (Linnaeus, 1758), Mullus barbatus Linnaeus, 1758, Gobius niger Linnaeus, 1758, Scorpaena porcus (Linnaeus, 1758), Arnoglossus kessleri Schmidt, 1915, Uranoscopus scaber Linnaeus, 1758, Serranus hepatus (Linnaeus, 1758), Sygnathus thyphle Linnaeus, 1758, Aphia minuta (Risso, 1810), Trachinus draco Linnaeus, 1758, Parablennius tentacularis (Brünnich, 1768), Illionymus risso Lesueur 1814, Entomacrodus marmoratus (Bennett, 1828) caught from the western Black Sea coasts of Turkey was investigated. The sampling was carried out seasonally in 2017-2018 using beam trawling nets with 2 m aperture. The depths of the sampling points varied from 10 m to 30 m. The minimum and maximum lengths and weights, length-weight relationships, parameters of a and b, ± 95% CI of b, r2, growth type (isometric or allometric) of samples, and statistical analyses of the relationship were determined. Estimates for parameter b of the length–weight relationship ranged between 2.58 and 3.586.

Offshore wind farms affect the spatial distribution pattern of plaice Pleuronectes platessa at both the turbine and wind farm scale

Abstract We investigated how the distribution of plaice Pleuronectes platessa, a typical soft-sediment fish species, has been affected by the introduction of hard substrate [turbines and scour protection layer (SPL)] at both turbine and wind farm scale in two Belgian offshore wind farms (OWFs). Diving transects (40 m) at 11 monopiles revealed four times higher plaice abundances on the sandy patches of the SPL (average radius 16.5 m) compared to the surrounding sand. We suggest that the configuration of the SPL, i.e. an open rock field, offering increased food and shelter opportunities, with sandy patches in between, facilitating the natural burrowing behaviour of plaice, forms the basis for the increased plaice abundances at the turbine scale. At the wind farm scale, beam trawl catches in between the turbines and in reference zones revealed significantly increased plaice abundances in one OWF, which suggests that wind farms can act as refuge areas for plaice, at least under specific conditions. Differences in environmental conditions, turbine foundation type, and surrounding fishing pressure may explain the equivocal findings between both OWFs, whereas low statistical power could have hampered the detection of general refuge effects. Next to the integration of different spatial scales (turbine/wind farm) within one study, longer-term monitoring and including extra life history parameters (e.g. length and sex ratio) might enhance the detectability of potential refuge effects.

Annotated checklist and biodiversity analysis of benthic fauna at&amp;nbsp;Sylt Outer Reef and Borkum Reef Ground (North Sea)

Benthic fauna caught by ring dredge and 2 m beam trawl at the NATURA 2000 Sylt Outer Reef (SAR) and Borkum Reef Ground (BRG) sites in the North Sea are examined in relation to the intensity of mobile bottom-trawling fisheries. Samples were taken from 33 stations in the two areas, and the collected benthic fauna, consisting of infauna, epifauna, and demersal fish was determined. A total of 123 species were found, consisting of the phyla Chordata, Mollusca, Arthropoda, Echinodermata, Annelida, Cnidaria, and Bryozoa, with Chordata and Mollusca being the most species-rich phyla. The species compositions of BRG and SAR are relatively clearly separated. There was greater species diversity at BRG, likely due to lower fishing pressure from mobile bottom trawling than at SAR. Long-term data acquisition and analysis will be needed to visualize past and future changes in biodiversity

Avilés Canyon System: Increasing the benthic biodiversity knowledge

Macro and megafauna were studied in the Avilés Canyon System (ACS), southern Bay of Biscay (Cantabrian Sea), during several oceanographic cruises carried out from 2009 to 2017. The biodiversity of ACS is summarized and its description is herein updated after sampling surveys of several programmes (ECOMARG, INDEMARES, SponGES, INTEMARES) conducted by the Spanish Institute of Oceanography (IEO).This study has updated previous knowledge in the canyon area from past national and international projects, their reports and publications as well as data collected in the context of regional projects designed to gain new insight into the diversity of marine invertebrates and fishes from the ACS. Samples were taken using a range of sampling gears (Rock dredge, Beam trawl, Trawl gear GOC-73, Suprabenthic sledge, Box corer and Remoted operated vehicle), from 55 to 2291 m in depth. A total of 1015 species were identified at the ACS: 98 Porifera, 153 Cnidaria, 14 Brachiopoda, 22 Bryozoa, 97 Mollusca, 151 Annelida, 315 Arthropoda, 74 Echinodermata and 91 Chordata. New records for the Bay of Biscay fauna include 13 Porifera species, 17 Cnidaria, 7 Mollusca, 8 Arthopoda, 3 Echinodermata and 4 Chordata. Also the bathymetric range of some species has been extended. As a result of the research projects carried out in the area in the last fifteen years, important information is now available which suggests that the ACS houses a large number of species with a high ecological value, that it represents a biodiversity hotspot in terms of the presence of sponge aggregations and coral reefs in certain regions, and that it sustains important fisheries due to the abundance of comercial species. Given the relevance of the species and habitats occurring in the ACS, there is a need to implement a conservation and management plan of the area in order to maintain habitats in good state of preservation.

Trawling effects on biogeochemical processes are mediated by fauna in high-energy biogenic-reef-inhabited coastal sediments

Abstract. Dynamic, sandy environments are generally less vulnerable to mechanical stress compared to silty, low-energy habitats. Biogenic reef communities, however, may provide an exception to this. This study explores the physical, biological, and biogeochemical effects of bottom trawl fishing on a coastal ecosystem dominated by the tube-building polychaete Lanice conchilega. Two specific gear types, both used to exploit North Sea sole (Solea solea), were compared: electric pulse trawls and tickler chain-rigged beam trawls. We detected a ∼1 cm bathymetric deepening after trawling associated with significant losses in benthic chlorophyll a caused by both fishing gears. Tickler chain trawls significantly reduced sediment oxygen consumption (57 %), total organic matter mineralization (56 %), denitrification (61 %), nitrification (60 %), and total benthos densities (52 %), while pulse trawls had no statistically significant impact on these parameters. Before trawling, significant relationships could be found between L. conchilega and very fine sand fractions, oxygen and nitrate fluxes, macrobenthos densities, and species richness; however, the trawl disturbances from both gears disrupted these connections. Our results suggest a reduced average effect for pulse compared to tickler chain beam trawls for several ecological and biogeochemical characteristics, though their impact was still significant for L. conchilega and associated species. This study also suggests that faunal-mediated ecosystem functions in habitats dominated by L. conchilega may be sensitive to relatively shallow sediment penetration from trawl gears and should be considered when assessing habitat vulnerability.

Effect of electrical stimulation used in the pulse trawl fishery for common sole on internal injuries in sandeels

Abstract Electric stimulation was used in the North Sea beam trawl fishery for common sole to reduce its environmental impact. Because electrical stimulation may cause internal injuries in fish, a laboratory experiment was conducted to study the effect of pulse exposure on lesser sandeel (Ammodytes tobianus) and greater sandeel (Hyperoplus lanceolatus), important mid-trophic species in the North Sea ecosystem. We exposed 244 sandeels between two electrodes to a pulsed bipolar current for 2 s in an experimental cage with 5 cm sediment; 221 control fish were handled similarly but not exposed. The occurrence of spinal injuries and internal haemorrhages were scored using X-radiography and dissection. None of the sandeels exposed to a field strength of up to 600 V m–1 showed spinal injury or haemorrhage. Equal numbers of minor spinal abnormalities were found in exposed and control fish. In the absence of spinal injuries, we estimated by bootstrapping the field strength below which spinal injuries are unlikely to occur, i.e. the lower limit threshold, and the corresponding limit dose–response relationship between field strength and injury probability. We conclude that it is unlikely that pulse trawl fishery will have an ecologically significant adverse effect on the population abundance of sandeels, because of the low probabilities of exposure and injury.

Unexpected high discard mortalities of juvenile brown shrimp (Crangon crangon) in the North Sea shrimp fishery

The brown shrimp Crangon crangon is the target species of one of the largest crustacean fisheries in the North Sea, producing considerable amounts of bycatch every season. Mortality rates of bycaught juveniles have traditionally been considered negligible, but this assumption was based on studies with short haul durations and short observation times. We investigated shrimp mortality due to trawling and handling under conditions typical of the commercial shrimp fishery in the German Bight. A total of 1440 juvenile shrimps from 48 hauls with commercial beam trawls was analyzed in discard survival experiments, yielding insights into the effects of haul duration and two prevalent sieving methods (riddle and drum) on mortality rates. Shrimps were observed daily for three weeks to test for delayed mortality and moult performance as an indicator of fitness. On average, ca. 70% of the juvenile shrimps moulted successfully during the observation period, most of them (44.5%) in the first week. On average 6% of the shrimp were already dead in the pit. Mortalities of those transferred alive into the aquarium without passing the sieve reached mortalities over three weeks of up to 22% (June) in the 120 min hauls leading to a total mortality of 28%. Passage though a sieve resulted in a 67% (riddle sieve) or 100% (drum sieve) increase of mortality compared to the level that was observed from samples from the pit before sieving. In May 2016, a special situation was encountered with very high immediate (7.0–35.0%) and increased longer-term (19.4–31.9%) mortalities of juvenile shrimps regardless of treatment, indicating an unknown source of additional mortality. We hypothesize that these higher mortalities may be explained by undersized shrimp experiencing a repeated passage of the catch and sorting process of different vessels. In fact, our study emphasizes that bycatch mortalities of undersized shrimps estimated in earlier studies have been severely underestimated.