Prehistoric versus modern Baltic Sea cod fisheries: selectivity across the millennia

Combining Stone Age and modern data provides unique insights for management, extending beyond contemporary problems and shifting baselines. Using fish chronometric parts, we compared demographic characteristics of exploited cod populations from the Neolithic Period (4500 BP) to the modern highly exploited fishery in the central Baltic Sea. We found that Neolithic cod were larger (mean 56.4 cm, 95% confidence interval (CI)+/-0.9) than modern fish (weighted mean length in catch =49.5+/-0.2 cm in 1995, 48.2+/-0.2 cm in 2003), and older (mean ages=4.7+/-0.11, 3.1+/-0.02 and 3.6+/-0.02 years for Neolithic, 1995, and 2003 fisheries, respectively). Fishery-independent surveys in 1995 and 2003 show that mean sizes in the stock are 16-17 cm smaller than reflected in the fishery, and mean ages approximately 1-1.5 years younger. Modelled von Bertalanffy growth and back-calculated lengths indicated that Neolithic cod grew to smaller asymptotic lengths, but were larger at younger ages, implying rapid early growth. Very small Neolithic cod were absent and large individuals were rare as in modern times. This could be owing to selective harvests, the absence of small and large fish in the area or a combination. Comparing modern and prehistoric times, fishery selection is evident, but apparently not as great as in the North Atlantic proper.     

Impact of 21st century climate change on the Baltic Sea fish community and fisheries

The Baltic Sea is a large brackish semienclosed sea whose species-poor fish community supports important commercial and recreational fisheries. Both the fish species and the fisheries are strongly affected by climate variations. These climatic effects and the underlying mechanisms are briefly reviewed. We then use recent regional – scale climate – ocean modelling results to consider how climate change during this century will affect the fish community of the Baltic and fisheries management. Expected climate changes in northern Europe will likely affect both the temperature and salinity of the Baltic, causing it to become warmer and fresher. As an estuarine ecosystem with large horizontal and vertical salinity gradients, biodiversity will be particularly sensitive to changes in salinity which can be expected as a consequence of altered precipitation patterns. Marine-tolerant species will be disadvantaged and their distributions will partially contract from the Baltic Sea; habitats of freshwater species will likely expand. Although some new species can be expected to immigrate because of an expected increase in sea temperature, only a few of these species will be able to successfully colonize the Baltic because of its low salinity. Fishing fleets which presently target marine species (e.g. cod, herring, sprat, plaice, sole) in the Baltic will likely have to relocate to more marine areas or switch to other species which tolerate decreasing salinities. Fishery management thresholds that trigger reductions in fishing quotas or fishery closures to conserve local populations (e.g. cod, salmon) will have to be reassessed as the ecological basis on which existing thresholds have been established changes, and new thresholds will have to be developed for immigrant species. The Baltic situation illustrates some of the uncertainties and complexities associated with forecasting how fish populations, communities and industries dependent on an estuarine ecosystem might respond to future climate change.     

First evidence of spawning of eastern Baltic cod (Gadus morhua callarias) in the Belt Sea,the main spawning area of western Baltic cod (Gadus morhua L.)

Two cod stocks (western Baltic cod, WBC, and eastern Baltic cod, EBC) are managed in the Baltic Sea which is characterized by different main spawning areas and different main spawning periods. In this study we analyse the spatial and temporal occurrence of spawning individuals of both cod stocks in the main spawning grounds of the Baltic Sea based on eight microsatellite loci. Our results suggest that EBC (Gadus morhua callarias) has formed currently temporally stable, substantially homogeneous population not only in the Bornholm Sea (ICES SD: 25) but also in the Arkona Sea (ICES SD: 24). The presented analyses proved that EBC (G. m. callarias) can temporarily also spawn in the Belt Sea.     

North Sea cod and climate change – modelling the effects of temperature on population dynamics

In order to examine the likely impacts of climate change on fish stocks, it is necessary to couple the output from large‐scale climate models to fisheries population simulations. Using projections of future North Sea surface temperatures for the period 2000–2050 from the Hadley General Circulation Model, we estimate the likely effects of climate change on the North Sea cod population. Output from the model suggests that increasing temperatures will lead to an increased rate of decline in the North Sea cod population compared with simulations that ignore environmental change. Although the simulation developed here is relatively simplistic, we demonstrate that inclusion of environmental factors in population models can markedly alter one's perception of how the population will behave. The development of simulations incorporating environment effects will become increasingly important as the impacts of climate change on the marine ecosystem become more pronounced.     

Fecundity of Baltic cod: temporal and spatial variation

Potential and relative potential fecundity of 701 cod Gadus morhua caught in the Bornholm Basin (ICES Sub-division 25) during 1987–1996 and 106 from the Gdansk Deep (ICES Sub-division 26) in 1995 and 1996 varied significantly between years but neither between different spawning grounds nor between different sampling months within one spawning season. Growth and water temperature are factors identified to influence the mean relative fecundity explaining 66% of the variance between years.     

Effects of freezing on length and mass measurements of Atlantic cod Gadus morhua in the Baltic Sea

An aggregated sample of 925 Atlantic cod Gadus morhua collected by four countries in different regions of the Baltic Sea during different seasons were measured (total length, L_T = 161–890 mm and weighed (mass, M = 45–6900 g) both before freezing and after defrosting. The cod were found to decrease significantly in both L_T and M following death and frozen storage. There was an average (±SD) change in L_T of −2.91% (±0.05%) following freezing, independent of starting L_T. Total M changed by −2.65% (±0.14%), independent of starting mass. Shrinkage of L_T and M did not differ significantly between 1 and 4 months frozen storage, though L_T shrinkage was significantly greater after 1 or 4 months in the freezer compared with after 5 days. There was significant variation in L_T and M shrinkage between regions of capture. A significant negative relationship between condition of cod and L_T or M change was also observed. Equations to back-calculate fresh L_T and M from thawed L_T, M and standard length (L_S), gutted L_T, gutted L_S and gutted M are provided.     

The forgotten feeding ground: patterns in seasonal and depth-specific food intake of adult cod Gadus morhua in the western Baltic Sea

This study presents the diet composition of western Baltic cod Gadus morhua based on 3150 stomachs sampled year-round between 2016 and 2017 using angling, gillnetting and bottom trawling, which enhanced the spatio-temporal coverage of cod habitats. Cod diet composition in shallow areas (<20 m depth) was dominated by benthic invertebrate species, mainly the common shore crab Carcinus maneas. Compared to historic diet data from the 1960s and 1980s (limited to depth >20 m), the contribution of herring Clupea harengus decreased and round goby Neogobius melanostomus occurred as a new prey species. Statistical modelling revealed significant relationships between diet composition, catch depth, fish length and season. Generalized additive modelling identified a negative relationship between catch depth and stomach content weight, suggesting reduced food intake in winter when cod use deeper areas for spawning and during peak summer when cod tend to avoid high water temperatures. The results of this study highlight the importance of shallow coastal areas as major feeding habitats of adult cod in the western Baltic Sea, which were previously unknown because samples were restricted to deeper trawlable areas. The results strongly suggest that historic stomach analyses overestimated the role of forage fish and underestimated the role of invertebrate prey. Eventually, this study shows the importance of a comprehensive habitat coverage for unbiased stomach sampling programmes to provide a more reliable estimation of top predator diet, a key information for food web analyses and multispecies models.     

Selectivity of fishing gears used in the Baltic Sea cod fishery

The Baltic cod (Gadus morhua) plays a very significant role in the Baltic Sea ecosystem being the major fish top predator and the most important commercial species for the Baltic Sea fishing industry. Consequently the management and understanding of the exploitation pattern of the stock is of major importance. Technical regulations, improving the selectivity of cod, have been a major management strategy and the Baltic Sea is likely to be the area where most fishing gear selectivity studies, focussing on size selectivity, have been conducted over time. The methodology for conducting and analysing selectivity data has been significantly improved in recent years. The subject is reviewed since the choice of methodology can have a significant effect on the interpretation of the outcome of selectivity experiments. Factors affecting the selectivity of trawl and gill nets are considered. Alternative ways to improve the size and species selectivity of trawls using selective devices are reviewed. Selectivity parameters from available literature are listed and the correlations of selectivity parameters to the mesh size for different gears are estimated. The historical legislation on selectivity is reviewed and the expected selectivity for trawls is estimated. Management considerations concerning the mortality of escaping and discarded fish and wider management impacts have to be considered if improving selectivity. The review is ended by conclusions including reflections on needed research in the future.     

Mechanistic insights into the effects of climate change on larval cod

Understanding the biophysical mechanisms that shape variability in fisheries recruitment is critical for estimating the effects of climate change on fisheries. In this study, we used an Earth System Model (ESM) and a mechanistic individual‐based model (IBM) for larval fish to analyze how climate change may impact the growth and survival of larval cod in the North Atlantic. We focused our analysis on five regions that span the current geographical range of cod and are known to contain important spawning populations. Under the SRES A2 (high emissions) scenario, the ESM‐projected surface ocean temperatures are expected to increase by >1 °C for 3 of the 5 regions, and stratification is expected to increase at all sites between 1950–1999 and 2050–2099. This enhanced stratification is projected to decrease large (>5 μm ESD) phytoplankton productivity and mesozooplankton biomass at all 5 sites. Higher temperatures are projected to increase larval metabolic costs, which combined with decreased food resources will reduce larval weight, increase the probability of larvae dying from starvation and increase larval exposure to visual and invertebrate predators at most sites. If current concentrations of piscivore and invertebrate predators are maintained, larval survival is projected to decrease at all five sites by 2050–2099. In contrast to past observed responses to climate variability in which warm anomalies led to better recruitment in cold‐water stocks, our simulations indicated that reduced prey availability under climate change may cause a reduction in larval survival despite higher temperatures in these regions. In the lower prey environment projected under climate change, higher metabolic costs due to higher temperatures outweigh the advantages of higher growth potential, leading to negative effects on northern cod stocks. Our results provide an important first large‐scale assessment of the impacts of climate change on larval cod in the North Atlantic.     

Evidence of a hybrid-zone in Atlantic cod (Gadus morhua) in the Baltic and the Danish Belt Sea revealed by individual admixture analysis

The study of hybrid zones is central to our understanding of the genetic basis of reproductive isolation and speciation, yet very little is known about the extent and significance of hybrid zones in marine fishes. We examined the population structure of cod in the transition area between the North Sea and the Baltic Sea employing nine microsatellite loci. Genetic differentiation between the North Sea sample and the rest increased along a transect to the Baltic proper, with a large increase in level of differentiation occurring in the Western Baltic area. Our objective was to determine whether this pattern was caused purely by varying degrees of mechanical mixing of North Sea and Baltic Sea cod or by interbreeding and formation of a hybrid swarm. Simulation studies revealed that traditional Hardy-Weinberg analysis did not have sufficient power for detection of a Wahlund effect. However, using a model-based clustering method for individual admixture analysis, we were able to demonstrate the existence of intermediate genotypes in all samples from the transition area. Accordingly, our data were explained best by a model of a hybrid swarm flanked by pure nonadmixed populations in the North Sea and the Baltic Sea proper. Significant correlation of gene identities across loci (gametic phase disequilibrium) was found only in a sample from the Western Baltic, suggesting this area as the centre of the apparent hybrid zone. A hybrid zone for cod in the ecotone between the high-saline North Sea and the low-saline Baltic Sea is discussed in relation to its possible origin and maintenance, and in relation to a classical study of haemoglobin variation in cod from the Baltic Sea/Danish Belt Sea, suggesting mixing of two divergent populations without interbreeding.     

气候变化条件下全氟化合物对海岸带可持续发展的影响及管控策略

海岸带生态系统为人类提供了食物、水等丰富的物质以及文化、休闲等生态系统服务。但由于全氟化合物等污染物的排放和气候变化的影响,海岸带地区环境日益恶化、资源枯竭、极端气候灾害加剧。这些问题直接导致了海岸带水质下降、近海生物多样性减少、生物群落结构变化,海岸带生态系统可持续发展受到影响。全氟化合物在自然环境中难以降解,会沿食物链在生物体内累积和放大进而对人类健康造成风险。随着氟化工产业在我国的快速发展、新型全氟化合物的不断更替以及气候变化的加剧,全氟化合物对海岸带生态的影响面临着极大的不确定性。为了更好地服务海岸带生态系统的可持续发展,目前仍需要加快完善立法保障,在全国范围内对全氟化合物进行监测;将海洋与陆地作为一个有机整体,统筹陆地影响和海岸带管控,严格控制污染物的排放。     

A continental risk assessment of West Nile virus under climate change

Since first introduced to North America in 1999, West Nile virus (WNV) has spread rapidly across the continent, threatening wildlife populations and posing serious health risks to humans. While WNV incidence has been linked to environmental factors, particularly temperature and rainfall, little is known about how future climate change may affect the spread of the disease. Using available data on WNV infections in vectors and hosts collected from 2003–2011 and using a suite of 10 species distribution models, weighted according to their predictive performance, we modeled the incidence of WNV under current climate conditions at a continental scale. Models were found to accurately predict spatial patterns of WNV that were then used to examine how future climate may affect the spread of the disease. Predictions were accurate for cases of human WNV infection in the following year (2012), with areas reporting infections having significantly higher probability of presence as predicted by our models. Projected geographic distributions of WNV in North America under future climate for 2050 and 2080 show an expansion of suitable climate for the disease, driven by warmer temperatures and lower annual precipitation that will result in the exposure of new and naïve host populations to the virus with potentially serious consequences. Our risk assessment identifies current and future hotspots of West Nile virus where mitigation efforts should be focused and presents an important new approach for monitoring vector‐borne disease under climate change.     

Regional and global climate risks for reef corals: Incorporating species-specific vulnerability and exposure to climate hazards

Climate change is driving rapid and widespread erosion of the environmental conditions that formerly supported species persistence. Existing projections of climate change typically focus on forecasts of acute environmental anomalies and global extinction risks. The current projections also frequently consider all species within a broad taxonomic group together without differentiating species-specific patterns. Consequently, we still know little about the explicit dimensions of climate risk (i.e., species-specific vulnerability, exposure and hazard) that are vital for predicting future biodiversity responses (e.g., adaptation, migration) and developing management and conservation strategies. Here, we use reef corals as model organisms (n = 741 species) to project the extent of regional and global climate risks of marine organisms into the future. We characterise species-specific vulnerability based on the global geographic range and historical environmental conditions (1900–1994) of each coral species within their ranges, and quantify the projected exposure to climate hazard beyond the historical conditions as climate risk. We show that many coral species will experience a complete loss of pre-modern climate analogs at the regional scale and across their entire distributional ranges, and such exposure to hazardous conditions are predicted to pose substantial regional and global climate risks to reef corals. Although high-latitude regions may provide climate refugia for some tropical corals until the mid-21st century, they will not become a universal haven for all corals. Notably, high-latitude specialists and species with small geographic ranges remain particularly vulnerable as they tend to possess limited capacities to avoid climate risks (e.g., via adaptive and migratory responses). Predicted climate risks are amplified substantially under the SSP5-8.5 compared with the SSP1-2.6 scenario, highlighting the need for stringent emission controls. Our projections of both regional and global climate risks offer unique opportunities to facilitate climate action at spatial scales relevant to conservation and management.     

气候变化对“山水林田湖草”重大生态工程的影响

“山水林田湖草”重大生态保护与修复工程是中国对复杂生态环境治理的重要探索。鉴于目前大多数重大生态工程未系统地考虑气候变化对重大生态工程的影响问题,针对气候变化对高寒草地、北方林区以及风沙源区生态的影响,以实施了多年的三江源生态保护工程、三北防护林工程和京津风沙源治理工程为重点,分析了重大生态工程对全球气候变化的响应,解构了重大生态工程与气候的反馈关系和影响程度,指出了中国“山水林田湖草”重大生态工程实施过程中可能存在的问题,并给出了应对建议。结果表明：（1）气候变化对重大生态工程影响研究不足,尤其是涉及区域生态系统结构、功能、生物多样性与脆弱性等方面以及气候变化在工程实施效果贡献率的研究。（2）缺乏有效区分气候变化和工程实施效果的评估方法。目前能够定量识别气候变化对生态系统恢复的试验和方法比较少见,且缺乏对气候变化未来风险预估,导致制订的措施不能有效适应气候变化从而达到生态效益的最大化。所以在今后设计和实施“山水林田湖草”重大生态保护与修复工程时,要充分考虑自然规律、地理地带性差异和气候因素对生态系统的影响,并且加强建设生态综合监测和工程评价体系,根据未来不同的气候变化情景制订不同的措施,并依据气候动态预估来适当调整措施,使得生态工程能发挥出最大效益,以保证修复工程的系统性、区域性和整体性。     

Climate change and fishing: a century of shifting distribution in North Sea cod

Globally, spatial distributions of fish stocks are shifting but although the role of climate change in range shifts is increasingly appreciated, little remains known of the likely additional impact that high levels of fishing pressure might have on distribution. For North Sea cod, we show for the first time and in great spatial detail how the stock has shifted its distribution over the past 100 years. We digitized extensive historical fisheries data from paper charts in UK government archives and combined these with contemporary data to a time‐series spanning 1913–2012 (excluding both World Wars). New analysis of old data revealed that the current distribution pattern of cod – mostly in the deeper, northern‐ and north‐easternmost parts of the North Sea – is almost opposite to that during most of the Twentieth Century – mainly concentrated in the west, off England and Scotland. Statistical analysis revealed that the deepening, northward shift is likely attributable to warming; however, the eastward shift is best explained by fishing pressure, suggestive of significant depletion of the stock from its previous stronghold, off the coasts of England and Scotland. These spatial patterns were confirmed for the most recent 3½ decades by data from fisheries‐independent surveys, which go back to the 1970s. Our results demonstrate the fundamental importance of both climate change and fishing pressure for our understanding of changing distributions of commercially exploited fish.     

Ecological consequences of biological invasions: three invertebrate case studies in the north-eastern Baltic Sea

Population dynamics and ecological impacts of the cirriped Balanus improvisus, the polychaete Marenzelleria neglecta and the cladoceran Cercopagis pengoi were investigated in the north-eastern Baltic Sea. After an increase during the first decade of invasion, the density of M. neglecta and C. pengoi declined afterwards. The studied abiotic environmental variables did not explain the interannual variability in the seasonal cycles of M. neglecta and C. pengoi indicating that the species are at their initial phase of invasion. The population dynamics of B. improvisus was best described by water temperature. B. improvisus promoted the growth of the green alga Enteromorpha intestinalis. M. neglecta enhanced the content of sediment chlorophyll a and reduced growth and survival of the polychaete Hediste diversicolor and growth of the amphipod Monoporeia affinis. Concurrent with the invasion of C. pengoi the abundance of small-sized cladocerans declined, especially above the thermocline. C. pengoi had become an important food for nine-spined stickleback, bleak, herring and smelt.     

Spawning stock and recruitment in North Sea cod shaped by food and climate

In order to provide better fisheries management and conservation decisions, there is a need to discern the underlying relationship between the spawning stock and recruitment of marine fishes, a relationship which is influenced by the environmental conditions. Here, we demonstrate how the environmental conditions (temperature and the food availability for fish larvae) influence the stock–recruitment relationship and indeed what kind of stock–recruitment relationship we might see under different environmental conditions. Using unique zooplankton data from the Continuous Plankton Recorder, we find that food availability (i.e. zooplankton) in essence determines which model applies for the once large North Sea cod (Gadus morhua) stock. Further, we show that recruitment is strengthened during cold years and weakened during warm years. Our combined model explained 45 per cent of the total variance in cod recruitment, while the traditional Ricker and Beverton–Holt models only explained about 10 per cent. Specifically, our approach predicts that a full recovery of the North Sea cod stock might not be expected until the environment becomes more favourable.     

Variable choice affects estimations of vulnerability to climate change

For practical reasons, assessments of species' vulnerability to rising temperatures are often limited to measuring responses to a single ecological response variable, but this could result in an underestimation of vulnerability. Using the Cape Rockjumper Chaetops frenatus (‘Rockjumper’) we examined the thermal risk to nestling Rockjumpers for sublethal (i.e. reduced nestling mass gain) and lethal (i.e. increased nest predation) consequences of sustained hot weather under both current and predicted future climatic conditions (RCP 8.5). We used a direct approach to examine these risks, first as independent ecological responses and then as combined risk driven by both response variables (mass gain and predation risk). This study revealed that the inclusion of multiple climate-related responses affected the predicted vulnerability to climate change. Further, our analyses showed that increased vulnerability to climate change will vary within the Rockjumper's habitat. Our results demonstrate that the variability in predicted thermal risk depends on which response variable was used, with implications for how and where conservation practitioners direct their already limited resources.