Broad-Scale Climate Influences on Spring-Spawning Herring (Clupea harengus, L.) Recruitment in the Western Baltic Sea

Climate forcing in complex ecosystems can have profound implications for ecosystem sustainability and may thus challenge a precautionary ecosystem management. Climatic influences documented to affect various ecological functions on a global scale, may themselves be observed on quantitative or qualitative scales including regime shifts in complex marine ecosystems. This study investigates the potential climatic impact on the reproduction success of spring-spawning herring (Clupea harengus) in the Western Baltic Sea (WBSS herring). To test for climate effects on reproduction success, the regionally determined and scientifically well-documented spawning grounds of WBSS herring represent an ideal model system. Climate effects on herring reproduction were investigated using two global indices of atmospheric variability and sea surface temperature, represented by the North Atlantic Oscillation (NAO) and the Atlantic Multi-decadal Oscillation (AMO), respectively, and the Baltic Sea Index (BSI) which is a regional-scale atmospheric index for the Baltic Sea. Moreover, we combined a traditional approach with modern time series analysis based on a recruitment model connecting parental population components with reproduction success. Generalized transfer functions (ARIMAX models) allowed evaluating the dynamic nature of exogenous climate processes interacting with the endogenous recruitment process. Using different model selection criteria our results reveal that in contrast to NAO and AMO, the BSI shows a significant positive but delayed signal on the annual dynamics of herring recruitment. The westward influence of the Siberian high is considered strongly suppressing the influence of the NAO in this area leading to a higher explanatory power of the BSI reflecting the atmospheric pressure regime on a North-South transect between Oslo, Norway and Szczecin, Poland. We suggest incorporating climate-induced effects into stock and risk assessments and management strategies as part of the EU ecosystem approach to support sustainable herring fisheries in the Western Baltic Sea

Predation on Atlantic herring (Clupea harengus ) eggs by the resident predator community in coastal transitional waters

Atlantic herring (Clupea harengus) migrates from offshore to coastal areas to spawn and their eggs and larvae may substantially increase prey resources for resident predators. We combined an in situ predator exclusion experiment using eggs naturally spawned on submerged aquatic vegetation and field observations of predator abundance to estimate the magnitude of predation mortality of herring eggs. During our predator exclusion experiment, performed in an important spawning ground in the southwest Baltic Sea, 20% of the herring eggs were consumed resulting in an extrapolated predation of 42% of all eggs between spawning and hatch. Abundance and stomach content analyses indicated that one predator (threespine stickleback, Gasterosteus aculeatus) was responsible for the majority of the predation impact. Predation mortality estimates from this in situ study were more than 10-fold higher than those of an empirical egg predation model for the same predator in the same region. Our findings highlight the potential of resident predators to regulate the survival of early life stages of ocean-going fishes that rely on the nursery functions of inshore transitional waters

Food-limited growth of larval Atlantic herring Clupea harengus recurrently observed in a coastal nursery area

Food-limited growth of larval fish, defined as growth rates lower than observed in other habitats or from laboratory experiments at a given temperature, is rarely reported in field studies. This would imply that either larval fishes are living in an environment characterized by plenty of food, that nutritional condition selective mortality (i.e., eliminating the weak) is very strong, or this impression is caused by misinterpretation of data concerning e.g., poor taxonomical resolution of potential prey items, i.e., total potential prey abundance is high, but positively selected food is actually scarce. We analyzed RNA:DNA derived growth rates of herring larvae (Clupea harengus L.) and taxonomically differentiated prey field data of six consecutive spring seasons from the Kiel Canal, an artificial waterway in northern Germany, in order to test if food-limited growth in larval fish can occur recurrently in coastal habitats. In all years analyzed, larval growth rates decreased simultaneously with prey abundance at the end of each larval season. Furthermore, larval growth rates were observed to be lower than mean growth rates observed in another herring larvae nursery area at temperatures above 15 °C. Asymptotic relationships between prey abundance and larval growth rates were observed, further supporting the hypothesis of food-limitation. As larval growth was best explained by the abundance of the numerically dominant calanoid copepod Eurytemora affinis, the paramount importance of the dominant prey item is highlighted. We conclude that food limitation can be a severe and re-occurring issue for larval fish in coastal habitats, and that certain prey items play a crucial role in determining larval growth rates, and therefore potentially recruitment

Linking individual physiological indicators to the productivity of fish populations: a case study of Atlantic herring

Physiological measures can help to identify environmental thresholds that constrain organismal-level performance. Relating these thresholds, in a cause-and-effect manner, to long-term changes in the vital rates (e.g. growth, survival, reproduction) of wild populations has the potential to generate robust science advice needed to support conservation efforts. Here we investigate the hypothesis that the decreasing annual productivity (i.e. larval and juvenile abundances) of Western Baltic Spring-Spawning (WBSS) herring over the last decade is linked to warmer springs exceeding the physiological optimum of early life stages. First, we used laboratory experiments to identify the optimal and arrhythmia-inducing temperatures for cardiac function in herring larvae (approx. 16 °C and 21 °C, respectively), which were not significantly influenced by rearing temperature (7, 11 or 15 °C). These laboratory results matched well the decreased growth rates determined in the wild for larvae at temperatures beyond 17 °C. Second, we calculated a thermal threshold index based on the number of days above the optimal 16 °C threshold during the herring spawning time (March-June), which significantly increased from 1992 to 2017 for a major spawning ground of WBSS herring. Over the same time period, the thermal threshold index was significantly correlated to decreased annual productivity of WBSS herring. This finding suggests that warming is at least partially responsible for the steady decline in annual productivity of this population over the past decade. This study adds to the growing body of evidence that physiological measurements can be used as indicators of population resilience, and that the knowledge gained from laboratory experiments can be translated into advice for effective single-species (and eventually ecosystem-based) conservation and management

Reduced Reproductive Success of Western Baltic Herring (Clupea harengus) as a Response to Warming Winters

Shallow estuaries, bays, and lagoons are generally considered hot spots of ocean productivity that often adjust rapidly to seasonal variations in atmospheric temperatures. During spring when biological reproductive processes begin in the temperate zones, regional climate variability can be immense and uncovering a non-linear biological response, such as fish recruitment to changing temperature regimes might be challenging. Using herring as a paradigm for a response of coastal spring productivity to regional climate drivers, we demonstrated how the annual timing of spawning periods can significantly affect the reproductive success of spring-spawning herring (Clupea harengus) in the western Baltic Sea. An investigation of spawning phenology in consecutive years indicated a temperature threshold range of 3.5–4.5°C triggering initial spawning in the coastal zone. Based on this finding, we analyzed the timing of larval hatching peaks, larval survival and recruitment to the adult population relative to multi-decadal time-series of seasonal sea-surface temperatures. The results revealed that the late seasonal onset of cold periods the corresponding elongation of the period where larvae hatch from the eggs and early larval hatching peaks significantly reduced larval production in a coastal nursery area and finally lead to a reduced abundance of juveniles in the entire distribution area. Using a combination of field research and time series analysis, we presented precedence for shifting regional winter regimes providing a present-day stressor to reproductive capacity of a central component of the coastal food web

Elemental Inventory in Fish Otoliths Reflects Natal Origin of Atlantic Herring (Clupea harengus) From Baltic Sea Juvenile Areas

Despite centuries of human exploitation and research on Atlantic herring (Clupea harengus) in Europe, there is still much uncertainty on where major nursery areas are located. However, understanding the quantitative contribution of particular coastal systems to adult fish populations is of utmost importance to secure sustainable fish resources. Routinely, marker elements indicating certain hydrological conditions, which are incorporated into calcified structures, the so-called otoliths, are used to trace the origin of fish. However, as in the Baltic Sea, small and large scale salinity gradients potentially masking specific salinity signals. Based on the entire elemental inventory of the otolith core region, indicating the chemical signature of the spawning area, we developed a unique elemental fingerprinting index (EFI), allowing comparisons of multi-elemental chemical signatures from within and between herring juvenile areas. Our results show significantly distinct chemical “fingerprints” on the scale of particular bays and estuaries, which were not detectable with the usual marker elements. We further demonstrate that heavy metals levels drive the potential to distinguish natal origin of herring. These findings provide an essential baseline for further studies on the impact of small scale productivity for exploited fish resources and central components of marine food webs

HERRING : An analysis of spawning ground management, ecological conditions and human impacts in Greifswald Bay, Vistula Lagoon and Hanö Bight.

This book compiles the findings of the HERRING project which was conducted from 2012 until 2015 and part-financed by the EU South Baltic Programme. The main objective of the HERRING project is to improve the consideration of including herring spawning grounds in coastal management. Herring as a resource recourse would be part of the economic development of coastal areas, and HERRING strongly emphasizes the importance of foster an integrated coastal management in the South Baltic Sea. Three case study areas in Germany, Poland and Sweden serve as the basis of the approach, which can be roughly distinguished in two parts. The analysis of the ecological parameters and conditions as well as the impacts of present and future human activities, spatial uses and natural changes The analysis and compilation of the multi-level institutions and manage- ment instruments that govern the use and protection of coastal herring spawning grounds. The management of coastal spawning areas can function as an example to show the huge diversity of interest, demands and actors that need to be considered for the sustainable use of resources and ecosystems.https://commons.wmu.se/mer_book/1002/thumbnail.jp

Caught in the middle: bottom‑up and top‑down processes impacting recruitment in a small pelagic fsh

Understanding the drivers behind fluctuations in fish populations remains a key objective in fishery science. Our predictive capacity to explain these fluctuations is still relatively low, due to the amalgam of interacting bottom-up and top-down factors, which vary across time and space among and within populations. Gaining a mechanistic understanding of these recruitment drivers requires a holistic approach, combining field, experimental and modelling efforts. Here, we use the Western Baltic Spring-Spawning (WBSS) herring (Clupea harengus) to exemplify the power of this holistic approach and the high complexity of the recruitment drivers (and their interactions). Since the early 2000s, low recruitment levels have promoted intense research on this stock. Our literature synthesis suggests that the major drivers are habitat compression of the spawning beds (due to eutrophication and coastal modification mainly) and warming, which indirectly leads to changes in spawning phenology, prey abundance and predation pressure. Other factors include increased intensity of extreme climate events and new predators in the system. Four main knowledge gaps were identified related to life-cycle migration and habitat use, population structure and demographics, life-stage specific impact of multi-stressors, and predator–prey interactions. Specific research topics within these areas are proposed, as well as the priority to support a sustainable management of the stock. Given that the Baltic Sea is severely impacted by warming, eutrophication and altered precipitation, WBSS herring could be a harbinger of potential effects of changing environmental drivers to the recruitment of small pelagic fishes in other coastal areas in the world.publishedVersio

Mixed-stock analysis of Atlantic herring (Clupea harengus): a tool for identifying management units and complex migration dynamics

We developed and validated a mixed-stock analysis (MSA) method with 59 single-nucleotide polymorphisms selected from genome-wide data to assign individuals to populations in mixed-stock samples of Atlantic herring from the North and Baltic seas. We analysed 3734 herring from spawning locations and scientific catches of mixed feeding stocks to demonstrate a "one-fits-all" tool with unprecedented accuracy for monitoring spatio-temporal dynamics throughout a large geographical range with complex stock mixing. We re-analysed time-series data (2002-2021) and compared inferences about stock composition with estimates from morphological data. We show that contributions from the western Baltic spring-spawning stock complex, which is under management concern, have likely been overestimated. We also show that a genetically distinctive population of western Baltic autumn spawners, ascribed low fisheries importance, contributes non-negligible and potentially temporally increasing proportions to mixed-stock aggregations, calling for a re-evaluation of stock definitions. MSA data can be implemented in stock assessment and in a variety of applications, including marine ecosystem description, impact assessment of specific fleets, and stock-rebuilding plans