Danish Waters Research Articles

Phocine distemper virus (PDV) seroprevalence as predictor for future outbreaks in harbour seals

Phocine distemper virus (PDV) infections caused the two most pronounced mass mortalities in marine mammals documented in the past century. During the two outbreaks, 23,000 and 30,000 harbour seals (Phoca vitulina), died in 1988/1989 and 2002 across populations in the Wadden Sea and adjacent waters, respectively. To follow the mechanism and development of disease spreading, the dynamics of Morbillivirus-specific antibodies in harbour seal populations in German and Danish waters were examined. 522 serum samples of free-ranging harbour seals of different ages were sampled between 1990 and 2014. By standard neutralisation assays, Morbillivirus-specific antibodies were detected, using either the PDV isolate 2558/Han 88 or the related canine distemper virus (CDV) strain Onderstepoort. A total of 159 (30.5%) of the harbour seals were seropositive. Annual seroprevalence rates showed an undulating course: Peaks were seen in the post-epidemic years 1990/1991 and 2002/2003. Following each PDV outbreak, seroprevalence decreased and six to eight years after the epidemics samples were tested seronegative, indicating that the populations are now again susceptible to new PDV outbreak. After the last outbreak in 2002, the populations grew steadily to an estimated maximum (since 1975) of about 39,100 individuals in the Wadden Sea in 2014 and about 23,540 harbour seals in the Kattegat area in 2013. A re-appearence of PDV would presumably result in another epizootic with high mortality rates as encountered in the previous outbreaks. The current high population density renders harbour seals vulnerable to rapid spread of infectious agents including PDV and the recently detected influenza A virus.

Ichthyotoxicity of the microalga Pseudochattonella farcimen under laboratory and field conditions in Danish waters.

Blooms of the marine dictyochophyte Pseudochattonella farcimen have been associated with fish kills, but attempts to verify ichthyotoxicity of this microalga under experimental conditions have not been successful. In the early spring of 2009 and 2011, P. farcimen bloomed in the inner Danish waters. The blooms occurred at a seawater temperature of ~2°C and correlated with extensive kills of farmed salmonid fish (2009) and wild populations (2011). Several strains of P. farcimen were isolated from the 2009 bloom. However, exposure of rainbow trout Oncorhynchus mykiss to laboratory-grown P. farcimen cultures did not reveal any toxic effects. During the 2011 bloom, fish were exposed to bloom water under both laboratory and field conditions. While no clinical effect was observed on fish incubated in bloom water in the laboratory trial, a remarkable difference was seen in the field trial between rainbow trout kept in tanks supplied with a continuous flow of filtered versus non-filtered bloom water. Histological examination of the gill tissue revealed karyorrhexis and epithelial loosening in the affected fish. Microscopy analysis of algal cell morphology suggested that mucocysts detected on the cell surface only in freshly sampled bloom water might be associated with ichtyotoxicity.

The use of shellfish for eutrophication control

Effects of excess loading of nutrients to the marine environment can be mitigated by mussel cultures, basically through nutrient removal from the marine environment when shellfish are harvested. Shellfish farming also provide other goods and services to the marine environment primarily through the impact on water transparency caused by shellfish filtration. There is an increasing awareness of the mitigation potential in mussel culture in relation to eutrophication, but so far practical examples of culture on full scale devoted to mitigation are few. Further, impact of mussel farming on nutrient cycling, e.g. in the sediments below the culture units, has raised concerns. In this review, we clarify concepts in relation to nutrient mitigation and discuss goods and services delivered by mussel mitigation cultures and their impact on an ecosystem scale based primarily on results from studies in heavily eutrofied estuaries. A multi-criteria approach for site selection is presented based on experiences from Danish waters, and economic aspects of mitigation cultures are analysed in relation to use of the produced mitigation mussels. Future perspectives for extractive cultures are discussed in relation to source of excess nutrients.

Sensitivity of the air–sea CO<sub>2</sub> exchange in the Baltic Sea and Danish inner waters to atmospheric short-term variability

Abstract. Minimising the uncertainties in estimates of air–sea CO2 exchange is an important step toward increasing the confidence in assessments of the CO2 cycle. Using an atmospheric transport model makes it possible to investigate the direct impact of atmospheric parameters on the air–sea CO2 flux along with its sensitivity to, for example, short-term temporal variability in wind speed, atmospheric mixing height and atmospheric CO2 concentration. With this study, the importance of high spatiotemporal resolution of atmospheric parameters for the air–sea CO2 flux is assessed for six sub-basins within the Baltic Sea and Danish inner waters. A new climatology of surface water partial pressure of CO2 (pCO2w) has been developed for this coastal area based on available data from monitoring stations and on-board pCO2w measuring systems. Parameterisations depending on wind speed were applied for the transfer velocity to calculate the air–sea CO2 flux. Two model simulations were conducted – one including short-term variability in atmospheric CO2 (VAT), and one where it was not included (CAT). A seasonal cycle in the air–sea CO2 flux was found for both simulations for all sub-basins with uptake of CO2 in summer and release of CO2 to the atmosphere in winter. During the simulated period 2005–2010, the average annual net uptake of atmospheric CO2 for the Baltic Sea, Danish straits and Kattegat was 287 and 471 Gg C yr−1 for the VAT and CAT simulations, respectively. The obtained difference of 184 Gg C yr−1 was found to be significant, and thus ignoring short-term variability in atmospheric CO2 does have a sizeable effect on the air–sea CO2 exchange. The combination of the atmospheric model and the new pCO2w fields has also made it possible to make an estimate of the marine part of the Danish CO2 budget for the first time. A net annual uptake of 2613 Gg C yr−1 was found for the Danish waters. A large uncertainty is connected to the air–sea CO2 flux in particular caused by the transfer velocity parameterisation and the applied pCO2w climatology. However, as a significant difference of 184 Gg C yr−1 is obtained between the VAT and CAT simulations, the present study underlines the importance of including short-term variability in atmospheric CO2 concentration in future model studies of the air–sea exchange in order to minimise the uncertainty.

Examination of six commonly used laboratory fixatives in HAB monitoring programs for their use in quantitative PCR based on Taqman probe technology

Due to the need for more rapid and reliable detection, quantification and enumeration of harmful algal species the use of molecular methods are increasingly being used in monitoring and field studies. However, many studies often require sample fixation to allow for transportation before analyses are conducted. Here, we describe the effects of six fixatives (acidified Lugol's iodine with or without sodium thiosulphate, glutaraldehyde, paraformaldehyde (PFA), formalin and ethanol) on quantitative real-time polymerase chain reaction (qPCR) amplification with Taqman probes. We applied extracted total genomic DNA from four harmful algal species from Danish waters, representing three dinoflagellates (Alexandrium tamarense, Karenia mikimotoi, Karlodinium veneficum and a haptophyte (Prymnesium parvum). The Cq values generated on the qPCR amplification plot were compared to those of an unfixed sample that acted as a control. For all species positive amplifications were achieved from DNA templates from all preserved samples. However, amplification efficiencies between fixatives and species varied. Yet it was found that Lugol's iodine was the most ideal short-term fixative for enumeration of cells by qPCR as well as being the safest to handle. The effect of age on Lugol's iodine fixed samples was also addressed. Samples were fixed and stored at 5°C in the dark and total genomic DNA extracted after 24h, 72h, 1 week, 2 weeks, 1 month and 2 months. Samples remained stable for 1 month for A. tamarense and K. veneficum and 2 months for K. mikimotoi and P. parvum.

Contrasting response to nutrient manipulation in Arctic mesocosms are reproduced by a minimum microbial food web model.

A minimum mathematical model of the marine pelagic microbial food web has previously shown to be able to reproduce central aspects of observed system response to different bottom-up manipulations in a mesocosm experiment Microbial Ecosystem Dynamics (MEDEA) in Danish waters. In this study, we apply this model to two mesocosm experiments (Polar Aquatic Microbial Ecology (PAME)-I and PAME-II) conducted at the Arctic location Kongsfjorden, Svalbard. The different responses of the microbial community to similar nutrient manipulation in the three mesocosm experiments may be described as diatom-dominated (MEDEA), bacteria-dominated (PAME-I), and flagellated-dominated (PAME-II). When allowing ciliates to be able to feed on small diatoms, the model describing the diatom-dominated MEDEA experiment give a bacteria-dominated response as observed in PAME I in which the diatom community comprised almost exclusively small-sized cells. Introducing a high initial mesozooplankton stock as observed in PAME-II, the model gives a flagellate-dominated response in accordance with the observed response also of this experiment. The ability of the model originally developed for temperate waters to reproduce population dynamics in a 10°C colder Arctic fjord, does not support the existence of important shifts in population balances over this temperature range. Rather, it suggests a quite resilient microbial food web when adapted to in situ temperature. The sensitivity of the model response to its mesozooplankton component suggests, however, that the seasonal vertical migration of Arctic copepods may be a strong forcing factor on Arctic microbial food webs.

Commercial cultivation and bioremediation potential of sugar kelp, Saccharina latissima, in Danish waters

Several seaweed species have been successfully tested for their biofilter potential for integrated multi-trophic aquaculture (IMTA). In this study, Saccharina latissima bioremediation potential was assessed over 12 months with respect to the yield, phosphorous (P) and nitrogen (N) content and removal. The experiment took place at two commercial cultivation areas: in close proximity to a blue mussel and fish farm (IMTA) and at a reference site, both situated outside Horsens Fjord in Denmark. The maximum biomass yield over the first growing season was achieved in August (1.08 ± 0.09 and 1.51 ± 0.13 kg fresh weight (FW) m−1) and September (0.92 ± 0.18 and 1.49 ± 0.16 kg FW m−1). Yield was significantly higher at the IMTA compared to the reference site in August (P < 0.05). A second growing season did not improve biofiltration efficiency. The highest N and P removal was achieved in August and September. Again, the IMTA location showed better N and P removal compared with the reference site in August: 5.02–7.02 g N and 0.86–1.23 g P m−1 of cultivation line (P < 0.05). S. latissima shows potential for assimilation and removal of nutrients, particularly nitrogen. Seasonal variations of seaweed biofilter efficiency, condition, and potential applications should be taken into account when evaluating the best suited harvest time. For Horsens Fjord, our results showed that the harvest time should take place in August–September in order to achieve maximum biofiltration efficiency (including N and P in epiphytes). However, for human consumption, it is better to harvest in May when the seaweed is free of epiphytes.

Environmental Factors and Seasonal Variation in Density of Mussel Larvae (&amp;lt;i&amp;gt;Mytilus edulis&amp;lt;/i&amp;gt;) in Danish Waters

Mussel larval densities may fluctuate considerably on both small spatial and short temporal scales. So far, only few and scattered data on the occurrence of mussel larvae have been reported from Danish waters. However, seasonal variation in density of blue mussel (Mytilus edulis) larvae as related to environmental factors (temperature, salinity, phytoplankton biomass) is basic information of relevance for future line-mussel farming in Danish waters. Here we report on the density of mussel larvae in a number of potential farming sites in the inner Danish waters. The mussel larval density measured in Skive Fjord, an eutrophicated inner branch of Limfjorden, during a period of 20 years, from 1989 to 2009, along with corresponding temperatures and chlorophylla, makes up the most important series of data reported here. In most years, a pronounced spring density peak and a subsequently lower autumn peak could be seen in Skive Fjord, but most conspicuous in the period 1993 to 2002 where the mean maximum spring larval density was 319 ± 260 ind·l-1. Further, data on mussel larval densities have been recorded on 4 locations in the Great Belt region: Kerteminde Bay in 2008 to 2011, and in 2008 at 3 other locations: Musholm Bay, Svendborg Sund, and Horsens Fjord. The maximum spring densities in the studied waters were observed in Skive Fjord, typically in May, whereas 10 to 100 times lower peak densities were found at the other locations studied. The reported observations show that mussel larvae are omnipresent in the studied areas and it is suggested that the larval density is sufficient forrecruitment to future line-mussel farms.

Reduced Cover of Drifting Macroalgae Following Nutrient Reduction in Danish Coastal Waters

Based on a large dataset from the national Danish monitoring programme, we analysed the temporal variability of drifting algae cover in shallow (1–2 m) water during a period of reduced nutrient loadings. Algal cover was analysed both in absolute terms and relative to eelgrass, Zostera marina, cover to test the hypotheses that (1) the cover of drifting algae and the relative dominance of algae versus eelgrass in shallow waters have declined in parallel to reductions in nutrient levels during the last decades, and (2) spatio-temporal differences in algal cover can be related to differences in nutrient conditions and environmental characteristics. The cover of drifting algae was positively related to total nitrogen concentration and Secchi depth but negatively related to exposure, salinity and mean summer temperature. The cover of drifting macroalgae showed significant declines over the past two decades, paralleling the reduction in nutrient concentrations. The present cover of drifting algae is low (<10 %) and probably pose little threat to the general distribution of eelgrass in shallow Danish waters though local accumulations may still be harmful. However, the ratio between drifting algae cover and eelgrass cover showed no significant trend, reflecting that eelgrass cover had not increased despite the reduced levels of nutrients and drifting algae. This ratio also showed no consistent relationship to water quality probably because different regulation mechanisms govern drifting algae and eelgrass, and feedback mechanisms may delay eelgrass recovery. Reduced drift algal cover may be an early sign of improved ecological status, while further improvements in terms of recovery of eelgrass meadows have longer perspectives.

Do larger tag packages alter diving behavior in harbor porpoises?

Table S1. Basic data for the harbor porpoises tagged with a satellite tag and data loggers (experimental animals) or just with a satellite tag (control animals) in Danish waters, 2009–2011. Histogram column: ST = Surface Time, DD = Dive Duration, TAD = Time At Depth. Figure S1. Harbor porpoise with secondary data logger unit mounted on the right side and the satellite tag mounted on the left side of the dorsal fin just before releasing the animal back into the water. Figure S2. Satellite tracks of the 12 harbor porpoises during the study period. Left panel: 7 experimental animals, right panel: 5 control animals. The large black dots mark the tagging sites and individual porpoises are distinguished by different colors. The bold lines are the Periods 1 while the Periods 2 are shown with thin dashed lines. Each position transmitted by the porpoises is indicated by a small black dot along the track. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Jellyfish and Ctenophores in Limfjorden (Denmark)—Mini-Review, with Recent New Observations

Limfjorden is a major Danish water system that connects the North Sea via Thyborøn Kanal in the west and to the Kattegat in the east. Limfjorden is heavily eutrophicated and certain areas suffer from oxygen depletion each summer. Bottom-dwelling fish have disappeared as the number of jellyfish and ctenophores has increased. The abundance and predation impact of jellyfish and ctenophores in Limfjorden have been described in a number of studies conducted during the last decade, and a mini-review of this literature is given here in Section 2. The common jellyfish Aurelia aurita may, in a few years, be very abundant and exert a considerable predatory impact on zooplankton and fish larvae. Abundance, species composition, and population dynamics of A. aurita may, at irregular time intervals, be strongly influenced not only by the water brought into Limfjorden from the North Sea, but also by competition with the invasive ctenophore Mnemiopsis leidyi that occurred for the first time in extremely high numbers in 2007, brought into the fjord system with North Sea water. It has been suggested that Limfjorden may function as an incubator for M. leidyi with the potential to further seed M. leidyi into the Kattegat and adjacent Danish waters, and recent observations seem to support this hypothesis. In Section 3, we report on two bloom events of ctenophores, Pleurobrachia pileus and M. leidyi, along with their predators (Beroe spp.) in Limfjorden in the autumns of 2012 and 2013, when the usually dominating A. aurita was absent. However, the present observation of B. ovata being M. leidyi’s native predator and a new species in Limfjorden may in the future reduce the abundance of M. leidyi.

Review of environmental exposure concentrations of chemical warfare agent residues and associated the fish community risk following the construction and completion of the Nord Stream gas pipeline between Russia and Germany

This paper compiles all the measured chemical warfare agent (CWA) concentrations found in relation to the Nord Stream pipeline work in Danish waters for the past 5 years. Sediment and biota sampling were performed along the pipeline route in four campaigns, prior to (in 2008 and 2010), during (in 2011) and after (in 2012) the construction work. No parent CWAs were detected in the sediments. Patchy residues of CWA degradation products of Adamsite, Clark I, phenyldichloroarsine, trichloroarsine and Lewisite II, were detected in a total of 29 of the 391 sediment samples collected and analyzed the past 5 years. The cumulative fish community risk quotient for the different locations, calculated as a sum of background and added risk, ranged between 0 and 0.017 suggesting a negligible acute CWA risk toward the fish community. The added risk from sediment disturbance in relation to construction of the pipelines represents less than 2% of the total risk in the areas with the highest calculated risk. The analyses of benthic infauna corroborate the finding of CWA related low risk across the years. There was no significant difference in CWA risk before (2008) and after the pipeline construction (2012).

The role of subsidence in a weakly unstable marine boundary layer: a case study

Abstract. The diurnal evolution of a cloud free, marine boundary layer is studied by means of experimental measurements and numerical simulations. Experimental data belong to an investigation of the mixing height over inner Danish waters. The mixed-layer height measured over the sea is generally nearly constant, and does not exhibit the diurnal cycle characteristic of boundary layers over land. A case study, during summer, showing an anomalous development of the mixed layer under unstable and nearly neutral atmospheric conditions, is selected in the campaign. Subsidence is identified as the main physical mechanism causing the sudden decrease in the mixing layer height. This is quantified by comparing radiosounding profiles with data from numerical simulations of a mesoscale model, and a large-eddy simulation model. Subsidence not only affects the mixing layer height, but also the turbulent fluctuations within it. By analyzing wind and scalar spectra, the role of subsidence is further investigated and a more complete interpretation of the experimental results emerges.

First report on Beroe ovata in an unusual mixture of ctenophores in the Great Belt (Denmark)

Abstract Between mid-December 2011 and mid-January 2012 an unusual mixture of ctenophores was observed and collected at Kerteminde harbor (Great Belt, Denmark). In addition to native zooplanktivorous species Pleurobrachia pileus (O.F. Muller, 1776) and Bolinopsis infundibulum (O.F. Muller, 1776), non-native zooplanktivorous Mnemiopsis leidyi A. Agissiz, 1865 and their predator Beroe cucumis Fabricius, 1780 that had earlier been recorded in the area, two more predators on zooplanktivorous ctenophores were recorded in inner Danish waters: Beroe ovata sensu Mayer and Beroe gracilis Kunne, 1939. The identity of B. ovata was confirmed using nuclear marker 18S. Identification of B. gracilis was based on morphology. The presence of the observed mixture of ctenophores in Great Belt may predict future faunal changes in Danish and perhaps other temperate coastal waters. Key words: genetic analyses, morphology, non-native species, Danish waters Introduction The occurrence of the invasive ctenophore

Somatic growth of mussels Mytilus edulis in field studies compared to predictions using BEG, DEB, and SFG models

Prediction of somatic growth of blue mussels, Mytilus edulis, based on the data from 2 field-growth studies of mussels in suspended net-bags in Danish waters was made by 3 models: the bioenergetic growth (BEG), the dynamic energy budget (DEB), and the scope for growth (SFG). Here, the standard BEG model has been expanded to include the temperature dependence of filtration rate and respiration and an ad hoc modification to ensure a smooth transition to zero ingestion as chlorophyll a (chl a) concentration approaches zero, both guided by published data. The first 21-day field study was conducted at nearly constant environmental conditions with a mean chl a concentration of C=2.7μgL−1, and the observed monotonous growth in the dry weight of soft parts was best predicted by DEB while BEG and SFG models produced lower growth. The second 165-day field study was affected by large variations in chl a and temperature, and the observed growth varied accordingly, but nevertheless, DEB and SFG predicted monotonous growth in good agreement with the mean pattern while BEG mimicked the field data in response to observed changes in chl a concentration and temperature. The general features of the models were that DEB produced the best average predictions, SFG mostly underestimated growth, whereas only BEG was sensitive to variations in chl a concentration and temperature. DEB and SFG models rely on the calibration of the half-saturation coefficient to optimize the food ingestion function term to that of observed growth, and BEG is independent of observed actual growth as its predictions solely rely on the time history of the local chl a concentration and temperature.

Resolved complex coastlines and land–sea contrasts in a high-resolution regional climate model: a comparative study using prescribed and modelled SSTs

We configured a coupled model system, comprising a regional climate model (RCM) and a regional ocean model, for the North Sea and Baltic Sea region at 6 nm resolution. A two-way nested fine-grid (1 nm) ocean domain is for the first time included for the Danish coastal waters in coupled RCMs to resolve the water exchange between the two regional seas. Here, we (1) assess the sensitivity of the near-surface atmosphere to prescribed sea surface temperatures (SSTs) from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim (ERAI) reanalysis and those modelled by the coupled system, and (2) examine different ocean responses in coarse and fine grids to atmospheric forcing. The experiments were performed covering the years 1990–2010, both using ERAI lateral boundary conditions. ERAI SSTs generally agree well with satellite SSTs in summer with differences within 1°C, but the ERAI overestimates the ice extent by 72% in winter due to the coarse resolution in the Baltic Sea. The atmosphere in the Baltic land–sea transition was more sensitive to high-resolution modelled SSTs with a significant improvement in winter, but it also provided a cold bias in summer as a combination of errors from both atmospheric and ocean models. Overall, the coupled simulation without observational constraints showed only minor deviations in the air–sea interface in the Baltic coastal region compared to the prescribed simulation, with seasonal mean differences within 2°C in 2 m air temperatures and 1°C in SSTs. An exception was in the Danish water, where the fine-grid ocean model yielded a better agreement with SST measurements and showed a smaller difference between the two simulations than the coarse-grid ocean model did. In turn, the modification on the atmosphere induced by modelled SSTs was negligible. The atmospheric–ocean–ice model in this configuration was found capable of reproducing the observed interannual variability of SST and ice extent in the Baltic Sea as well as the monthly extreme wind speeds and sea levels on a local scale for Denmark during the period 1990–2010. This article provides the first results in an attempt to resolve the Danish coasts with this accuracy in an RCM as a first step towards a fully coupled system for the region of interest.

Effects of noise and by-catch on a Danish harbour porpoise population

Ships and wind turbines generate noise, which can have a negative impact on marine mammal populations by scaring animals away. Effective modelling of how this affects the populations has to take account of the location and timing of disturbances. Here we construct an individual-based model of harbour porpoises in the Inner Danish Waters. Individuals have their own energy budgets constructed using established principles of physiological ecology. Data are lacking on the spatial distribution of food which is instead inferred from knowledge of time-varying porpoise distributions. The model produces plausible patterns of population dynamics and matches well the age distribution of porpoises caught in by-catch. It estimates the effect of existing wind farms as a 10% reduction in population size when food recovers fast (after two days). Proposed new wind farms and ships do not result in further population declines. The population is however sensitive to variations in mortality resulting from by-catch and to the speed at which food recovers after being depleted. If food recovers slowly the effect of wind turbines becomes negligible, whereas ships are estimated to have a significant negative impact on the population. Annual by-catch rates ≥10% lead to monotonously decreasing populations and to extinction, and even the estimated by-catch rate from the adjacent area (approximately 4.1%) has a strong impact on the population. This suggests that conservation efforts should be more focused on reducing by-catch in commercial gillnet fisheries than on limiting the amount of anthropogenic noise. Individual-based models are unique in their ability to take account of the location and timing of disturbances and to show their likely effects on populations. The models also identify deficiencies in the existing database and can be used to set priorities for future field research.

Empirical Ship Domain based on AIS Data

In this paper the minimum ship domain in which a navigator feels comfortable is estimated. That is, we estimate the free space surrounding a ship into which no other ship or object should enter. This is very useful when estimating the maximum flow through a channel or a bridge span. The paper benefits from the introduction of Automatic Identification System (AIS) data as it is now much easier to conduct studies involving a large number of observations. Our observations are based on ships sailing in southern Danish waters during a four year period, and from the observations we estimate how closely ships pass each other and fixed objects in open sea navigation. The main result is the establishment of an empirical minimum ship domain related to a comfortable navigational distance.

Possible causes of a harbour porpoise mass stranding in Danish waters in 2005.

An unprecedented 85 harbour porpoises stranded freshly dead along approximately 100 km of Danish coastline from 7–15 April, 2005. This total is considerably above the mean weekly stranding rate for the whole of Denmark, both for any time of year, 1.23 animals/week (ranging from 0 to 20 during 2003–2008, excluding April 2005), and specifically in April, 0.65 animals/week (0 to 4, same period). Bycatch was established as the cause of death for most of the individuals through typical indications of fisheries interactions, including net markings in the skin and around the flippers, and loss of tail flukes. Local fishermen confirmed unusually large porpoise bycatch in nets set for lumpfish (Cyclopterus lumpus) and the strandings were attributed to an early lumpfish season. However, lumpfish catches for 2005 were not unusual in terms of season onset, peak or total catch, when compared to 2003–2008. Consequently, human activity was combined with environmental factors and the variation in Danish fisheries landings (determined through a principal component analysis) in a two-part statistical model to assess the correlation of these factors with both the presence of fresh strandings and the numbers of strandings on the Danish west coast. The final statistical model (which was forward selected using Akaike information criterion; AIC) indicated that naval presence is correlated with higher rates of porpoise strandings, particularly in combination with certain fisheries, although it is not correlated with the actual presence of strandings. Military vessels from various countries were confirmed in the area from the 7th April, en route to the largest naval exercise in Danish waters to date (Loyal Mariner 2005, 11–28 April). Although sonar usage cannot be confirmed, it is likely that ships were testing various equipment prior to the main exercise. Thus naval activity cannot be ruled out as a possible contributing factor.

Field data and growth model for mussels Mytilus edulis in Danish waters

Growth rates of blue mussels, Mytilus edulis, in suspended net-bags were measured in field experiments conducted in Limfjorden and Great Belt, Denmark, for extended periods of time (22–69 days) with up to 8 intermediate data samplings during the period. The resulting time series of growth of different overlapping size-classes of M. edulis were analysed individually and after being assembled to cover the full range of sizes, as full time series in terms of weight specific growth rate (µ=(1/W) dW/dt,% day−1) as a function of dry weight of soft parts (W, g). The results were compared to a simple bioenergetic growth model for mussels (µ=aW b , a=0.871×C−0.986, b=− 0.34), and observed power-law relations of growth data were in good agreement with the model, which takes into account the prevailing average chlorophyll a (chl a) concentration (C, µg chl a l−1) at field sites. Thus, the b-values of data were close to the predicted −0.34 while the a-value was in one case lower than estimated due to suboptimal conditions during part of the growth period. As a supplementary interpretation the growth data have been expressed in terms of the time to double the dry weight of soft parts for a given size of mussel and at a given experimental site.