Susanne Lindegarth

Massive settlements of the Pacific oyster, Crassostrea gigas, in Scandinavia

The Pacific oyster (Crassostrea gigas) is an important aquaculture species world-wide. Due to its wide environmental tolerance and high growth rate, it has also become a successful invader in many areas, leading to major ecosystem changes. Low water temperatures were previously believed to restrict the establishment of Pacific oysters in Scandinavia. However, recent surveys reveal that the Pacific oyster is now established in many areas in Scandinavia. We present data on the current distribution, abundance and age-structure in Denmark, Sweden and Norway. The biomass of oysters in the Danish Wadden Sea increased from 1,056 to 6,264 tonnes between 2005 and 2007. Massive settlements were observed along the Swedish west coast in 2007, with densities >400 oysters per m−2. In Norway, populations are established on the southern coast, and specimens have been found as far north as 60°N. The potential impacts and probable causes of this recent large-scale establishment are discussed.

Massive settlements of the Pacific oyster, Crassostrea gigas, in Scandinavia : Invasion note

The Pacific oyster (Crassostrea gigas) is an important aquaculture species world-wide. Due to its wide environmental tolerance and high growth rate, it has also become a successful invader in many areas, leading to major ecosystem changes. Low water temperatures were previously believed to restrict the establishment of Pacific oysters in Scandinavia. However, recent surveys reveal that the Pacific oyster is now established in many areas in Scandinavia. The biomass of oysters in the Danish Wadden Sea has increased dramatically between 2005 and 2007, large numbers were observed along the Swedish west coast from settlement in 2006, and in Norway, populations are established along the southwest coast to 60°N.

Differential Retention of Okadaic Acid (OA) Group Toxins and Pectenotoxins (PTX) in the Blue Mussel, Mytilus edulis (L.), and European Flat Oyster, Ostrea edulis (L.)

ABSTRACT This paper presents the results from an experiment, where uptake and elimination of diarrhetic shellfish toxins (DST) of the okadaic acid (OA) and pectenotoxin (PTX) groups were compared between blue mussels (Mytilus edulis) and European oysters (Ostrea edulis). Caged mussels and oysters were suspended in the water column and exposed to a dense bloom of Dinophysis acuta (500–2000 cells L-1) for 4 wk, which was followed by detoxification in the laboratory during 7 wk. Weekly sampling and analysis of OA-group toxins including fatty acid esters (‘DTX-3’) as well as PTX in individual shellfish and plankton samples were performed. The results showed that mussels rapidly accumulated OA-group toxins to levels about 10 times above the regulation limit (160 µg OA kg-1 mussel) whereas concentrations never reached this limit in oysters during the field exposure. Overall, levels were 10–50 times greater in mussels. The OA-group toxins were mainly in the form of esters (>90%) in oysters, whereas in mussels, the esters constituted only a minor proportion of total OA toxin levels. Reduction rates were estimated for each OA toxin to evaluate if faster elimination could explain the lower toxin retention in oysters. However, no consistent species-specific difference in reduction rates were observed, but esters of OA appeared to be reduced at a faster rate in oysters (t1/2 = 23 days) compared with mussels (t1/2 = 35 days). In both species, the free form of OA was eliminated at a faster rate (t1/2=15–17 days) compared with free DTX1 (t1/2 = 23–31 days) and DTX2 (t1/2 = 28–33 days). Slightly slower elimination rates were estimated for the ester forms (t1/2 = 23–42 days). Regarding PTX, PTX2 seco acid (PTX2 SA) was the major PTX detected in both species, but small amounts of PTX2, PTX12 and PTX12 SA were also found. As for the OA-group toxins, oysters generally contained lower total amounts of PTX compared with mussels, but the difference was much less apparent. Estimation of reduction rates of the different PTX compounds showed that these toxins were rapidly eliminated in both oysters and mussels (t1/2 = 6–13 days). We propose that differential rates of gut assimilation and/or biotransformation of the OA-group and PTX explain some of the observed differences in retention and toxin profiles between the bivalves, rather than differences in elimination rates. However, models related to differences in feeding rates, particle selection and behavioural response to toxic algae should be tested in future experiments to evaluate the importance of preingestive mechanisms to the differential toxin retention in these bivalves.From the industrial perspective, our results suggest that O. edulis may be regarded as a low-risk species for DST contamination, which should be taken into consideration by regulatory authorities in charge of sampling frequencies and monitoring programs for shellfish toxins.

Experimental Effects of Temperature and Photoperiod on Synchrony of Gametogenesis and Sex Ratio in the European Oyster Ostrea edulis (Linnaeus)

ABSTRACT Insufficient knowledge about physiological processes underpinning reproduction in the European flat oyster, Ostrea edulis (Linnaeus), presents a number of unique challenges when designing a large-scale selective breeding program for hatchery production. The European flat oyster is a larviparous, protandric hermaphrodite wherein asynchronous spawns and variable sex ratios can lead to loss of genetic diversity and inbreeding depression within hatchery populations as a result of disproportionate gametic contributions in mass matings. A better understanding of O. edulis physiology in relation to environmental factors has the potential to improve hatchery processes for synchronization of gametogenesis, thus increasing fertilization potential and maximizing breeding population size of broodstock. This study sought to determine whether specific conditioning procedures—notably, manipulation of temperature and photoperiod—could be used to improve synchrony of gametogenesis and predictability of sex ratios in O. edulis. Two trials of accelerated yearly variations of temperature and photoperiod were conducted to uncouple the specific role of these factors in the timing of gametogenesis and sex of gametes over several spawning cycles. Histological analysis and quantitative analysis of gonad using a gonad filling index allowed for determination of sex ratios and staging of gametogenesis. Temperature was a controlling factor, although experimental results suggested that the effects of photoperiod, uncoupled from temperature, had no effect on the timing or rate of gametogenesis, or on sex ratios during a breeding season. A better understanding of factors affecting sex ratio and synchrony in timing of gamete release will allow for improved conditioning and broodstock handling protocols in the selective breeding of O. edulis.

Profiles and levels of fatty acid esters of okadaic acid group toxins and pectenotoxins during toxin depuration, Part I: Brown crab (Cancer pagurus)

In 2002, two outbreaks of diarrhetic shellfish poisoning (DSP) occurred in Norway, which was later confirmed to be caused by the consumption of brown crab (Cancer pagurus) contaminated predominantly by esters of okadaic acid (OA) after feeding on toxic blue mussels (Mytilus edulis). In addition to OA-group toxins, pectenotoxins (PTXs) are commonly detected in the toxin-producing algae (i.e. Dinophysis). In this paper, an experiment was set up to study the fatty acid ester profiles and depuration rates of OA-group toxins and PTXs from C. pagurus after feeding on M. edulis containing these toxin groups. OA, DTX1, DTX2 and PTX2 SA were all detected primarily in the form of fatty acid esters in the crab hepatopancreas (HP). Crabs preferentially assimilated toxins of the OA group after feeding on the mussels for 1 week. Detailed analysis of the fatty acid ester profile in crabs and mussels showed that the ester profiles in the crabs differed slightly from profiles of the fatty acid esters in M. edulis, but neither ester profile nor ester to free toxin ratio appeared to change in the crabs during the first 2 weeks of depuration. Calculations of depuration rates of the free forms of toxins resulted in similar reduction rates for OA and DTX2, whereas the depuration rate of DTX1, PTX2 and PTX2 SA was considerably faster. From the industrial perspective, the PTX-compounds are of minor importance compared to the OA group toxins in crabs, considering (1) the uncertainty regarding the oral toxicity of the PTXs, (2) the preferential ingestion of OA-group toxins compared to PTXs and (3) the faster depuration of PTXs.

Modeling and predicting the growth of the mussel, Mytilus edulis: implications for planning of aquaculture and eutrophication mitigation

Abstract The increased pressure on the marine ecosystems highlights the need for policies and integrated approaches for sustainable management of coastal areas. Spatial planning based on geographic information of human activities, ecological structures and functions, and their associated goods and services is a fundamental component in this context. Here, we evaluate the potential of predictive modeling to provide spatial data on one ecosystem function, mussel growth for use in such processes. We developed a methodology based on statistical modeling, spatial prediction, and mapping for the relative growth of the blue mussel, Mytilus edulis. We evaluated the performance of different modeling techniques and classification schemes using empirical measurements of growth from 144 sampling sites and data on biological, chemical, and physical predictors. Following comparisons of the different techniques and schemes, we developed random forest models to predict growth along the Swedish west coast. Implemented into GIS the best model produced in this study predicts that low, intermediate, and high growth rates can be expected in 53%, 32%, and 15% of modeled area, respectively. The results of this study also suggest that the nature and quality of predictor data hold the key to improving the predictive power of models. On a more general note, this study exemplifies a feasible approach based on measuring, modeling, and mapping for obtaining scientifically based spatial information on ecosystem functions and services affected by a complex set of factors. Such information is fundamental for maritime spatial planning and ecosystem‐based management and its importance is likely to increase in the future. Because of its close link to nutrient assimilation and production yield, site‐specific information of soft tissue growth such as the map of predicted growth rate developed in this study can be used as a tool for optimizing actions aimed at mitigating eutrophication and aquaculture operations and in maritime spatial planning processes of coastal areas.

Monitoring Recruitment Patterns of Mussels and Fouling Tunicates in Mariculture

ABSTRACT Methods to increase the precision of spat collection and strategies to mitigate fouling are greatly needed in aquaculture production. As such, larval recruitment of mussels and a common tunicate species was investigated. Recruitment was measured in shallow (1–2 m) and deeper (4–5 m) water at three sites during the summer of 2012. In addition, to evaluate the importance of timing in deployment of mussel ropes, differences in mussel yield were examined. The settlement plates provided a good description of the settling community with high temporal resolution. Peaks in recruitment were observed for both mussels and tunicates but recruitment rates and the timing of peaks differed among sites. Although mussel larvae preferred shaded substrates at some sites and times, these substrates were consistently preferred by tunicates. Mussels preferred to settle at shallow depths, whereas tunicates were consistently more abundant deeper. In contrast to predictions, there was no positive relationship between the yield of mussels on ropes and settlement rates on corresponding weeks. Somewhat surprisingly, the final abundance of mussels and tunicates were not related to the length of the recruitment and growth period. These results indicate that not only initial recruitment, but also mortality and repeated recruitment events are important processes shaping these dynamic assemblages. Combining the results, a minimum recommendation for monitoring larval settlement is to use, at two depths, one monitoring unit with several dark-surfaced sampling plates. Considering the fact that timing of deployment of mussel ropes in relation to mussel and tunicate settlement has been identified as a problem, it is believed that such methods can be used to optimize production of mussels. Thus, studies like these can also contribute to optimize farming techniques and practices in a broader context.

Developing methods for assessing abundance and distribution of European oysters (Ostrea edulis) using towed video

Due to large-scale habitat losses and increasing pressures, benthic habitats in general, and perhaps oyster beds in particular, are commonly in decline and severely threatened on regional and global scales. Appropriate and cost-efficient methods for mapping and monitoring of the distribution, abundance and quality of remaining oyster populations are fundamental for sustainable management and conservation of these habitats and their associated values. Towed video has emerged as a promising method for surveying benthic communities in a both non-destructive and cost-efficient way. Here we examine its use as a tool for quantification and monitoring of oyster populations by (i) analysing how well abundances can be estimated and how living Ostrea edulis individuals can be distinguished from dead ones, (ii) estimating the variability within and among observers as well as the spatial variability at a number of scales, and finally (iii) evaluating the precision of estimated abundances under different scenarios for monitoring. Overall, the results show that the can be used to quantify abundance and occurrence of Ostrea edulis in heterogeneous environments. There was a strong correlation between abundances determined in the field and abundances estimated by video-analyses (r2 = 0.93), even though video analyses underestimated the total abundance of living oysters by 20%. Additionally, the method was largely repeatable within and among observers and revealed no evident bias in identification of living and dead oysters. We also concluded that the spatial variability was an order of magnitude larger than that due to observer errors. Subsequent modelling of precision showed that the total area sampled was the main determinant of precision and provided general method for determining precision. This study provides a thorough validation of the application of towed video on quantitative estimations of live oysters. The results suggest that the method can indeed be very useful for this purpose and we therefor recommend it for future monitoring of oysters and other threatened habitats and species.