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Dredging for wild mussels Mytilus edulis Linnaeus, 1758 takes place in the marine protected areas (Natura 2000 sites) in Denmark. The fishery is strictly regulated to limit the impact of dredging to the benthic environment, and requires the use of modified lightweight mussel dredges. Nevertheless, the depletion of the benthic macrofauna associated with M. edulis beds after impact from such dredges is yet to be quantified. Here, dredging is shown to result in a significant decline in the density, species richness, and biological traits directly in the dredge track but no significant impact on the community composition. Species richness remained significantly affected 4 mo post dredging, although the positive trend after 4 mo may signal recovery. An effect of dredging was also detected in areas adjacent (approximately 5 m) to the dredge tracks, but needs further investigation to understand this impact. The results provide an estimate of benthic macrofaunal depletion in the dredge track as well as of the short-term recovery, which may be of use in the management of the fishery. Finally, it is suggested that species richness may be a more sensitive indicator than density in a eutrophic and species-poor system. KEY WORDS: Before-After-Control-Impact (BACI), biological traits, bivalve fishery, ecosystem based fisheries management, fisheries effects, mussel, Mytilus edulis, INTRODUCTION Globally, 15 million tons of bivalve molluscs are produced each year for human consumption, of which 11% are exploited from wild stocks (Wijsman et al. 2019). In Europe, scallops, [...]

ABSTRACT Shell growth, weight-specific growth of the soft tissue, and oxygen consumption were measured in native blue mussels, My thus spp., from different locations in Denmark, covering a salinity range [...]

Wild populations of the native European flat oyster (Ostrea edulis) are nowadays rare. Worldwide, flat oyster populations have declined due to overexploitation, diseases and pollution. Yet, in the Limfjorden, Denmark, a small but persistent population of wild flat oyster have endured for more than 165 years, which has sustained a unique fishery on wild oyster beds. Over time, fluctuations in the wild population size has had implications for the fishery, alternating between a large population with high yields, and too small to withstand a fishery. The flat oyster fishery has been alleged to cause these unpredictable fluctuations in the population. However, the combined effect of natural variation (e.g. water temperature) and the fishery has not been investigated. Here we indicate that summer water temperature is an important factor causing fluctuations in the flat oyster population, thought to be due to its influence on recruitment success. Whereas the fishery often constitutes a minor part of the net oyster mortality. We found a positive correlation between summer water temperature and increased oyster landings ≥5 years later. Our results demonstrate the importance of adaptive management for the oyster fishery in the Limfjorden, which has ensured the survival of the flat oyster population, even when the oyster population has been low. These results highlight that management based on annual stock assessments, closed areas with brood stock, and dynamic annual total allowable catch, can ensure the persistence of an endangered oyster species and support a sustainable fishery.

: Long-line mussel farming is well established in many coastal areas around the world, and the concurrent carbon enrichment below farms potentially can have local environmental impacts. The degradation of released mussel (Mytilus edulis) pellets was investigated by a number of complementary incubation approaches. After 140 h in suspension, most of the labile fraction of the pellet material had been degraded. However, because of the shallow water depth at our study site (8 m), only a minute fraction of carbon was released to the water column during sinking, of which ~50% was rapidly turned over. Pellets settling at the sediment in realistic concentrations immediately stimulated the benthic [O.sub.2] uptake. The elevated [O.sub.2] uptake gradually declined and reached the background level after 96 h, at which time ~25% of the added material had been degraded. The main fraction of pellets (75%) was more refractory and remained in the sediment where it was either retained or degraded on much longer time scales. The quantitative response of benthic pellet enrichment observed in the laboratory was confirmed by in situ trap measurements and incubations of sediment collected below and away from active farms. Resume: L'elevage des moules sur filieres est bien etabli dans plusieurs regions cotieres dans le monde et l'enrichissement en carbone sous les elevages peut potentiellement generer des impacts environnementaux locaux. Nous avons etudie la degradation des boulettes fecales liberees par les moules (Mytilus edulis) par plusieurs methodologies complementaires d'incubation. Apres 140 h en suspension, la plus grande partie de la portion labile du materiel des boulettes est degradee. Cependant, a cause de la faible profondeur de l'eau a notre site d'etude (8 m), seulement une fraction minuscule du carbone est liberee dans la colonne d'eau durant la sedimentation, dont ~50 % est rapidement recycle. Les boulettes qui se sont deposees sur le sediment en concentrations realistes ont immediatement stimule la consommation benthique de [O.sub.2]. La consommation accrue de [O.sub.2] a graduellement decline et atteint le niveau de base apres 96 h; a ce moment, ~25 % du materiel ajoute etait degrade. La fraction principale (75 %) des boulettes est plus refractaire et demeure dans le sediment ou elle est retenue ou se degrade sur des echelles temporelles beaucoup plus longues. La reponse quantitative a l'enrichissement benthique en boulettes observee en laboratoire a pu etre confirmee en nature par des mesures dans des collecteurs in situ et par l'incubation de sediments preleves sous les elevages actifs et loin de ces elevages. [Traduit par la Redaction], Introduction It is well established that long-line mussel farming can affect the local benthic environment (e.g., Grant et al. 1995). Mussel pseudofeces and fecal pellets increase local biogenic sedimentation, and [...]

ABSTRACT Limfjorden has a long tradition of mussel production based on wild stock fisheries. As a result of environmental concern of adverse effects of fishery, farming of mussels on off-bottom [...]

Starting in 1980s, the inner Danish waters have yearly been exposed to seasonal oxygen deficiency (hypoxia). Through spatial-temporal interpolation of monitoring data (1998-2005), we investigated oxygen deficiency impacts on suitable burying habitats for lesser sandeel (Ammodytes tobianus). Furthermore, the consequences of a predicted 4 [degrees]C temperature increase within this century were investigated. Maps of bottom oxygen deficiency (oxygen saturation [is less than or equal to][S.sub.crit] of sandeel) were overlaid on maps of sediment composition. Throughout the study period (1998-2005), about 8% of the suitable sediments were affected by oxygen deficiency during an average year and 23% in the most severe year. Regional differences underlay the interannual variations. The extent of oxygen deficiency in enclosed regions varied from 20% to 33% of the suitable seabed being affected, whereas in open-water regions oxygen deficiency problems were limited during average years. However, large areas of the open-water seabed experienced oxygen deficiency during severe years. In such years, under a 4.0 [degrees]C temperature increase scenario, the extent of oxygen deficiency on open-water suitable patches was predicted to increase from 25% to about 40%. Depuis les annees 1980, les eaux interieures du Danemark sont exposees chaque annee a une carence saisonniere en oxygene (hypoxie). En faisant des interpolations spatio-temporelles dans les donnees de surveillance (1998-2005), nous examinons les impacts de la carence en oxygene sur les habitats d'enfouissement du lancon equille (Ammodytes tobianus). Nous etudions les consequences d'un accroissement predit de temperature de 4 [degrees]C au cours du siecle actuel. Nous avons superpose des cartes de deficit d'oxygene au fond (saturation d'oxygene [is less than or equal to] [S.sub.crit] du lancon) sur des cartes de composition des sediments. Au cours de la periode d'etude (1998-2005), environ 8% des sediments appropries a l'enfouissement sont affectes par le deficit d'oxygene durant une annee moyenne et 23% durant l'annee la plus extreme. Des differences regionales sous-tendent les variations entre les annees. L'etendue du deficit d'oxygene dans les regions fermees couvre de 20% a 33% du fond de mer approprie a l'enfouissement, alors qu'en eau libre, les problemes de deficit d'oxygene sont limites durant les annees moyennes. Cependant, de grandes surfaces du fond de mer en eau libre connaissent un deficit d'oxygene durant les annees extremes. Durant ces annees-la, avec un scenario d'augmentation de la temperature de 4.0 [degrees]C, nous predisons que l'etendue du deficit d'oxygene sur les parcelles appropriees a l'enfouissement en eau libre augmentera de 25% a environ 40%. [Traduit par la Redaction], Introduction The marine coastal zone constitutes a minor part of the world's ocean but is nonetheless highly productive and plays an important role for many fish species (Beck et al. [...]

The EU Habitat Directive adopted in 1992, requires member states of the European Union to protect species and habitats considered to be of 'Community Interest' and listed in annexes to the directive. The appropriate environmental assessment of "plans and projects" is an important part of the conservation process. Despite several amendments and guidelines supporting the implementation of the Habitat Directive, science based operational procedures, indicators, and impact criteria for assessing potential negative impacts on marine Natura 2000 areas are still lacking. The lack of a generic and operational methodology complicates the management of plans and projects with potential impact on marine Natura 2000 areas. In this study, generic methods for the assessment of marine aquaculture in the inner Danish waters in relation to Natura 2000 areas was developed and applied for assessment of nine existing marine fin fish farms, in accordance with the latest methodological guidance on the provisions of Article 6(3) and (4) of the Habitat Directive. The applied methodology is based on high resolution 3D hydrodynamic- and ecosystem modelling (MIKE by DHI), that describes the dynamical physical, chemical, and biogeochemical processes and changes of marine ecosystems in time and space. To our knowledge, this is the first study that formulates operational biological quality elements, key indicators, concrete and generic impact criteria, and assessment procedures for operational assessment across several distinct marine habitat types. The method represents a generic, operational, transparent, and science-based assessment tool, that simplifies management, and is widely applicable for quantification of environmental impacts from various marine activities and eutrophication related pressures across geographical zones and different marine habitat types in marine Natura 2000 areas. • Science-based method for assessment of impact on habitats in Natura (2000) areas. • The method is applicable for assessment of impacts from various pressures. • The method is applicable for assessment of impacts across geographical zones. • The study defines biological quality elements, indicators, and impact criteria. • The study defines procedures for assessment of several distinct marine habitats. [ABSTRACT FROM AUTHOR]