Jan Dierking

Isotopic signatures of eelgrass (Zostera marina L.) as bioindicator of anthropogenic nutrient input in the western Baltic Sea.

Eutrophication is a global environmental problem. Better management of this threat requires more accurate assessments of anthropogenic nitrogen (N) inputs to coastal systems than can be obtained with traditional measures. Recently, primary producer N isotopic signatures have emerged as useful proxy of such inputs. Here, we demonstrated for the first time the applicability of this method using the widespread eelgrass (Zostera marina) in the highly eutrophic Baltic Sea. Spatial availability of sewage N across a bay with one major sewage outflow predicted by eelgrass δ(15)N was high near and downstream of the outflow compared to upstream, but returned to upstream levels within 4 km downstream from the outfall. General conclusions were corroborated by traditional eutrophication measures, but in contrast to these measures were fully quantitative. Eelgrass N isotope ratios therefore show high potential for coastal screens of eutrophication in the Baltic Sea, and in other areas with eelgrass meadows.

Detection of Ciguatoxin in Fish Tissue Using Sandwich ELISA and Neuroblastoma Cell Bioassay

The applicability of a new enzyme‐linked immunoassay (ELISA) for detecting ciguatoxin (CTX) in fish tissue was evaluated by testing three fish species commonly implicated in ciguatera fish poisoning in Hawaii. A total of 164 individual almaco jack (Seriola rivoliana) and greater amberjack (S. dumerili) and a total of 175 individuals of the blue‐spotted grouper (Cephalopholis argus) were caught at various locations in the Hawaiian Islands. Muscle tissue from each individual was assessed for the presence of CTX using two methods: a semi‐quantitative ELISA that was recently developed for detecting picogram levels of CTX in fish extract and a neuroblastoma (NB) cell assay commonly used to screen for marine toxins in fish. Results of the tests were highly correlated, with the ELISA indicating the presence of CTX in 9.4% of all fish samples, and the NB assay indicating toxicity in 6.8% of the fish samples. We conclude that the ELISA produces reliable and accurate results that are consistent with those provided by the accepted NB assay and that the ELISA has potential for future applications in screening fish populations for CTX. J. Clin. Lab. Anal. 22:246–253, 2008.

Ciguatera in the Introduced Fish Cephalopholis argus (Serranidae) in Hawai'i and Implications for Fishery Management

Abstract: The Peacock gouper (Cephalopholis argus) was introduced to Hawai‘i in 1956 to establish a new fishery. It has become abundant, but the fishery failed due to concerns about ciguatera fish poisoning, a neurological disease in humans caused by ingestion of fish containing ciguatoxin. The aim of this study was to provide better understanding of geographic patterns of ciguatoxicity in C. argus and of the correlation of toxicity with morphometric characters of this species, with the goal to assess the possibility of a safe fishery. Overall, 18.2% of C. argus specimens from sites around O‘ahu and Hawai‘i Island contained ciguatoxin in concentrations potentially harmful to humans. This was higher than the rate of occurrence in Hawaiian reef fishes in general, and on the scale of ciguatoxicity in species banned from sale in fish markets. Toxicity was high around both analyzed islands. However, toxic individuals were significantly less common around O‘ahu than around Hawai‘i Island (8% versus 24%). Regular geographic patterns in toxicity within islands (e.g., gradients along coastlines) were not present, and variability in toxicity within each sample site was high. Toxicity was significantly but weakly positively correlated with C. argus length but not with fish condition (measured by length at weight). In conclusion, high prevalence of toxic individuals, variability in toxicity on all analyzed spatial scales, and low explanatory power of morphometric characters make the avoidance of ciguatoxic C. argus individuals difficult. A safe fishery for this species in Hawai‘i therefore does not appear feasible at present.

Oxygen depletion in coastal seas and the effective spawning stock biomass of an exploited fish species

Environmental conditions may have previously underappreciated effects on the reproductive processes of commercially exploited fish populations, for example eastern Baltic cod, that are living at the physiological limits of their distribution. In the Baltic Sea, salinity affects neutral egg buoyancy, which is positively correlated with egg survival, as only water layers away from the oxygen consumption-dominated sea bottom contain sufficient oxygen. Egg buoyancy is positively correlated to female spawner age/size. From observations in the Baltic Sea, a field-based relationship between egg diameter and buoyancy (floating depth) could be established. Hence, based on the age structure of the spawning stock, we quantify the number of effective spawners, which are able to reproduce under ambient hydrographic conditions. For the time period 1993–2010, our results revealed large variations in the horizontal extent of spawning habitat (1000–20 000 km2) and oxygen-dependent egg survival (10–80%). The novel concept of an effective spawning stock biomass takes into account offspring that survive depending on the spawning stock age/size structure, if reproductive success is related to egg buoyancy and the extent of hypoxic areas. Effective spawning stock biomass reflected the role of environmental conditions for Baltic cod recruitment better than the spawning stock biomass alone, highlighting the importance of including environmental information in ecosystem-based management approaches.

The Baltic Sea as a time machine for the future coastal ocean

Science-based, multinational management of the Baltic Sea offers lessons on amelioration of highly disturbed marine ecosystems. Coastal global oceans are expected to undergo drastic changes driven by climate change and increasing anthropogenic pressures in coming decades. Predicting specific future conditions and assessing the best management strategies to maintain ecosystem integrity and sustainable resource use are difficult, because of multiple interacting pressures, uncertain projections, and a lack of test cases for management. We argue that the Baltic Sea can serve as a time machine to study consequences and mitigation of future coastal perturbations, due to its unique combination of an early history of multistressor disturbance and ecosystem deterioration and early implementation of cross-border environmental management to address these problems. The Baltic Sea also stands out in providing a strong scientific foundation and accessibility to long-term data series that provide a unique opportunity to assess the efficacy of management actions to address the breakdown of ecosystem functions. Trend reversals such as the return of top predators, recovering fish stocks, and reduced input of nutrient and harmful substances could be achieved only by implementing an international, cooperative governance structure transcending its complex multistate policy setting, with integrated management of watershed and sea. The Baltic Sea also demonstrates how rapidly progressing global pressures, particularly warming of Baltic waters and the surrounding catchment area, can offset the efficacy of current management approaches. This situation calls for management that is (i) conservative to provide a buffer against regionally unmanageable global perturbations, (ii) adaptive to react to new management challenges, and, ultimately, (iii) multisectorial and integrative to address conflicts associated with economic trade-offs.

Literaturrecherche, Aus- und Bewertung der Datenbasis zur Meerforelle (Salmo trutta trutta L.) : Grundlage für ein Projekt zur Optimierung des Meerforellenmanagements in Schleswig-Holstein ; Endbericht (28.06.2013)

ENDBERICHT (28.06.2013) Gefordert durch das Landesamt fur Landwirtschaft, Umwelt und landliche Raume (LLUR) Schleswig-Holstein, Flintbek, aus der Fischereiabgabe Schleswig-Holstein Forderzeitraum 01.10.2012 – 15.03.2013

Effect of fish length and nutritional condition on the fecundity of distressed Atlantic cod Gadus morhua from the Baltic Sea: Potential fecundity of Baltic G. Morhua

The disappearance of larger individuals and the decrease in individual body condition suffered by Atlantic cod Gadus morhua in the eastern Baltic during the past two decades can be expected to affect the stock reproductive output. To investigate this, female G. morhua were collected during the spawning and pre-spawning period in 2015-2016. The current individual potential fecundity (FP ) of eastern Baltic G. morhua was estimated and analysed in relation to total length (LT ) and indices of nutritional status such as body condition (K) and hepato-somatic index (IH ) using generalized linear models. In addition, the current prevalence of atresia and its potential relation to K were investigated. Moreover, a calibration curve to estimate FP from oocyte diameter, based on the autodiametric oocyte counting method, was established for the first time for eastern Baltic G. morhua and can be used for future fecundity studies on this stock. The results showed that FP was mainly positively related to fish length, but K and IH also contributed significantly to the variation in FP . The model predicted that fish with K = 1·2 have a FP 51% higher than fish of the same LT with K = 0·8. The prevalence of fecundity regulation by atresia was 5·8%, but it was found only in fish in the pre-spawning maturity stage and with low K. Temporal changes in biological features such as the length composition and individual body condition of eastern Baltic G. morhua, should be accounted for when estimating stock reproductive potential.

Sustainable use of Baltic Sea resources

The BONUS symposium “Science delivery for sustainable use of the Baltic Sea living resources” held in Tallinn, Estonia, in October 2017 was an opportunity for the presentation and discussion of 107 papers that examined the state and dynamics of living resources of the Baltic Sea, and associated management challenges. The symposium included a half-day stakeholder panel discussion that addressed the main challenges related to sustainable management and matching research and policy/management needs. Based on the five symposium papers published in this Special Issue as well as the stakeholder panel discussion, it can be concluded that (i) new observations about the feeding ecology of clupeids supports a more complete understanding of trophic interactions in the pelagic realm and improved calibration of ecosystem models, (ii) to safequard sustainable and diverse fisheries resources, one should take into account the specific local characteristics of the fish community, (iii) to safeguard sustainable use of marine resources and mitigate cross-sectoral and transboundary conflicts, a risk-based approach should be adopted, and (iv) incorporation of scientific advice into management faces several obstacles including the reality that not all readily available knowledge is currently being incorporated into the decision-making process.