Baltic Spring-spawning Herring Research Articles

Climate challenges for fish larvae: Interactive multi-stressor effects impair acclimation potential of Atlantic herring larvae

Fish early life stages are particularly vulnerable and heavily affected by changing environmental factors. The interactive effects of multiple climate change-related stressors on fish larvae remain, however, largely underexplored. As rising temperatures can increase the abundance and virulence of bacteria, we investigated the combination of a spring heat wave and bacterial exposure on the development of Atlantic herring larvae (Clupea harengus). Eggs and larvae of Western Baltic Spring-spawners were reared at a normal and high temperature ramp and exposed to Vibrio alginolyticus and V. anguillarum, respectively. Subsequently, mRNA and miRNA transcriptomes, microbiota composition, growth and survival were assessed. Both high temperature and V. alginolyticus exposure induced a major downregulation of gene expression likely impeding larval cell proliferation. In contrast, interactive effects of elevated temperature and V. alginolyticus resulted in minimal gene expression changes, indicating an impaired plastic response, which may cause cellular damage reducing survival in later larval stages. The heat wave alone or in combination with V. alginolyticus induced a notable shift in miRNA expression leading to the down- but also upregulation of predicted target genes. Moreover, both increased temperature and the Vibrio exposures significantly altered the larval microbiota composition, with warming reducing microbial richness and diversity. The outcomes of this study highlight the high sensitivity of herring early life stages towards multiple climate change-related stressors. Our results indicate that interactive effects of rapidly changing environmental factors may exceed the larval stress threshold impairing essential acclimation responses, which may contribute to the ongoing recruitment decline of Western Baltic Spring-Spawning herring.

Trends in a European coastal fishery with a special focus on small-scale fishers – Implications for fisheries policies and management

Worldwide, commercial small-scale fisheries are under increasing pressure. The study examines trends in a commercial Baltic Sea fishery with a special focus on the German small-scale fishery (both part-time and full-time fishery) over a period of 21 years (2000–2021) against the background of the declines in the two main target species of the fishery, namely western Baltic spring-spawning herring (Clupea harengus) and western Baltic cod (Gadus morhua). Landings, revenues, and the number of fishing businesses and vessels of the commercial fishery decreased over the past 21 years although the decline of the number of vessels was less pronounced for the small-scale fishery. The proportion of part-time fishers slightly increased locally. Fishers less dependent on cod and herring responded to declining revenues by diversifying their catches, and the proportion of revenues from flatfish, freshwater and diadromous fish species increased. The study was supplemented by a questionnaire sent to part-time fishers and excerpts from in-depth interviews with full-time fishers to add context to the study. The interviews indicated that family tradition, self-actualization, and nature experience but also a lack of alternatives due to the old age of fishers caused them to continue fishing despite poor economic prospects. A realistic assessment of the challenges and opportunities facing small-scale fisheries and the inclusion of social dimensions in fisheries policies are necessary, not least to develop incentives for small-scale fishers at the regional and local level to promote sustainable fisheries and facilitate fisher’s diversification.

A synthesis of a coastal northern pike (Esox lucius) fishery and its social-ecological environment in the southern Baltic Sea: Implications for the management of mixed commercial-recreational fisheries

We synthesize a large body of literature involving peer-reviewed work, grey literature and novel data analyses about the small-scale northern pike (Esox lucius) fishery in lagoon ecosystems in the southern Baltic Sea. Based on our comprehensive review that synthesizes ecological as well as social, economic and governance-related literature we derive implications for the management of mixed commercial-recreational fisheries in coastal areas. The interconnected shallow and biologically highly productive meso- to polytrophic lagoons (extension about 2000 km²) bordered by the peninsula of Fischland-Darß and the islands of Hiddensee, Rügen and Usedom in the southern Baltic Sea of Germany constitute an oligo- to mesohaline transitional habitat suitable for colonization by a range of freshwater fishes, including pike. In the Rügen area, pike successfully recruits in the mesohaline lagoons, but anadromous subpopulations and freshwater residents also exist in tributaries, forming a connected meta-population. The stock is co-exploited by a small-scale commercial fishery and a largely tourism-dominated recreational fishing sector that, depending on the angler type, values the pike for both consumption as well as for its trophy size. The recreational sector has risen in economic and social relevance since the German reunification in 1990 and today removes similar amounts of biomass than commercial fisheries. Pike is a prime target species of anglers, and recreational pike angling in the lagoons today generates a larger economic impact in terms of jobs created compared to the commercial pike fishing, where pike is typically one target among many freshwater fish. Stock assessments and stakeholder reports have revealed that the stock size and size of pike in the catch have been falling since 2010, fueling conflicts among fishers and anglers for space and fish. Reasons for the current decline of the pike stock involve multiple pressures operating jointly and possibly synergistically, such as local overharvest, loss of stock structure through past blocking of freshwater streams, eutrophication and macrophyte loss, predation mortality by natural predators, reduced availability of marine prey through declines of western Baltic spring-spawning herring (Clupea harengus), and poorly understood impacts of climate change. The assessment of current fishing mortality suggests a the stock stock is size/quality-overfished from the perspective of anglers, and fully exploited to slightly growth-overfished when judged against the reference point of maximum sustainable yield. The current biomass trend is negative. The current instantaneous fishing mortality rate, F, is estimated to range between 0.2 and 0.4 yr-1. Hence, fishing cannot be the sole culprit of the current stock decline because the current fishing mortality rates are too high for the underlying productivity, but not excessive. Because the current fishery no longer meets the expectations of recreational anglers, and angler numbers have recently declined, if the aim is to also suit anglers, reductions of fishing mortality would be useful in recovering the stock and fishing quality, coupled with restoration of access to flooded wetlands as spawning and nursery grounds, and control of other mortality sources of pike (e.g., cormorants). However, whether such actions indeed rebuild the fishery remains uncertain because of the potential for compensatory natural and fisheries mortality and other environmental changes affecting recruitment and abundance negatively. Policy makers may want to solve the allocation problem among commercial and recreational fisheries, install a robust monitoring system and a management framework that is inclusive of multiple perspectives and objectives and adaptive to novel productivity regimes and further structural changes in the mixed fishery. Further research on climate change impacts, food web changes, impacts of natural predators such as seals, cormorants or stickleback, and the behavioral and socio-economic aspects of commercial and recreational fisheries is warranted.

A productivity bottleneck in the Baltic herring (Clupea harengus membras): Early life-history processes and recruitment variability

Exogenous anomalies induced by contemporary climate change may severely impact dynamics of early life stages of fish. Here, we modelled how growth rate and abundance of postflexion larvae, and recruitment of Baltic spring-spawning herring (Clupea harengus membras) in the Pärnu Bay, Gulf of Riga (GoR) may respond to shifting climate variables. Higher larval growth rates were aligned with later seasonal emergence of yolk-sac larvae, while lower abundance of postflexion larvae occurred in years of earlier seasonal seawater warming. Cooler temperatures (<16 °C) in spring expanded the optimal thermal window for first-feeding herring larvae, attributable to the absence of early seasonal water temperature warming. Higher recruitment levels emerged in years of seasonally delayed warming and were associated with higher abundance of postflexion larvae. In recent decades, the trend towards earlier warming of the Baltic Sea in spring threatens to create a bottleneck to successful recruitment of herring. The existing paradigm that abundant Baltic herring year-classes occur only in the years following mild winters no longer stands as environmental conditions undergo rapid change. The relative contribution of Pärnu Bay larval nursery areas to recruitment has diminished as the suitable thermal window has been dramatically reduced in recent decades. Evolving thermal dynamics in the GoR have developed relatively recently and in future present a bottleneck for herring production.

Selection by higher-order effects of salinity and bacteria on early life-stages of Western Baltic spring-spawning herring.

Habitat stratification by abiotic and biotic factors initiates divergence of populations and leads to ecological speciation. In contrast to fully marine waters, the Baltic Sea is stratified by a salinity gradient that strongly affects fish physiology, distribution, diversity and virulence of important marine pathogens. Animals thus face the challenge to simultaneously adapt to the concurrent salinity and cope with the selection imposed by the changing pathogenic virulence. Western Baltic spring‐spawning herring (Clupea harengus) migrate to spawning grounds characterized by different salinities to which herring are supposedly adapted. We hypothesized that herring populations do not only have to cope with different salinity levels but that they are simultaneously exposed to higher‐order effects that accompany the shifts in salinity, that is induced pathogenicity of Vibrio bacteria in lower saline waters. To experimentally evaluate this, adults of two populations were caught in their spawning grounds and fully reciprocally crossed within and between populations. Larvae were reared at three salinity levels, representing the spawning ground salinity of each of the two populations, or Atlantic salinity conditions resembling the phylogenetic origin of Clupea harengus. In addition, larvae were exposed to a Vibrio spp. infection. Life‐history traits and gene expression analysis served as response variables. Herring seem adapted to Baltic Sea conditions and cope better with low saline waters. However, upon a bacterial infection, herring larvae suffer more when kept at lower salinities implying reduced resistance against Vibrio or higher Vibrio virulence. In the context of recent climate change with less saline marine waters in the Baltic Sea, such interactions may constitute key future stressors.

Separating two herring stocks from growth data: long-term changes in survey indices for Western Baltic Spring Spawning Herring (Clupea harengus ) after application of a stock separation function

Summary In the Baltic Sea, herring stocks are surveyed and managed according to a spatial allocation based on ICES (International Council for the Exploration of the Sea) subdivisions. In the western Baltic, the distribution areas of two stocks overlap: the Western Baltic Spring Spawning Herring (WBSSH) and the Central Baltic Herring (CBH). Survey results of length-at-age data indicate in Subdivision (SD) 24, which is a part of the WBSSH management area, that a considerable fraction of CBH is present and correspondingly erroneously allocated to WBSSH stock indices. Accordingly, a stock Separation Function (SF) based on growth parameters was established to identify the fraction of CBH in the WBSSH area. In the present study, the SF was applied to 8 years of data from the GERman Acoustic Survey (GERAS), which is conducted annually in autumn in ICES subdivisions 21-24. Results showed a distinct fraction of CBH in SD 24, and exclusion of the CBH greatly improved the quality of the GERAS index used in the assessment of the WBSSH stock.

On the potential benefits of marine spatial planning for herring spawning conditions—An example from the western Baltic Sea

Fisheries and marine spatial planning (MSP) still have a widely unsettled relationship. This paper reports on the potential benefits of MSP for the management of herring (Clupea harengus L.) stocks in the Greifswalder Bodden, a major spawning ground for western Baltic spring-spawning herring. The various pressures that have potential impacts on spawning conditions are identified based on a systematic literature review. Those anthropogenic activities that affect spawning conditions and could underlie MSP regulations are then analysed on the basis of the pressure maps to assess their importance for recruitment success in comparison to other pressures which are not subject to MSP by-laws, e.g. eutrophication. The results confirm that MSP could potentially improve the management of certain fish stocks and help to close existing gaps in European fisheries policy.

Abundance and distribution of larval herring, Clupea harengus (Actinopterygii: Clupeiformes: Clupeidae) in the Pomeranian Bay, Baltic Sea as an indicator of spawning sites

Herring, Clupea harengus Linnaeus, 1758, is one of the most important fish species in the Baltic Sea ecosystem. In the Polish part of the Baltic, it spawns along the entire Polish coast, predominantly in the Gdansk and Pomeranian bays, and in the Vistula Lagoon (Popiel 1955, Strzyzewska 1969, Fey 2001, Krasovskaya 2002). Baltic herring predominantly occurs in two forms depending on the spawning period. These are spring-spawning and fallspawning herring (Parmanne et al. 1994). Although spawning in the Pomeranian Bay occurs particularly in spring, some autumn spawning is noted in the area as well. For assessment purposes, a spatial stock separation based on ICES subdivisions (SD) is implemented. Five stocks are distinguished in the Baltic Sea (Anonymous 2012). Variable mixing of stocks is frequent due to migrations, for example between the spawning and feeding grounds. In the southern Baltic, two stocks are present: western Baltic spring-spawning herring (WBSSH; ICES Division IIIa and SDs 22–24), and central Baltic herring (CBH; ACTA ICHTHYOLOGICA ET PISCATORIA (2014) 44 (4): 309–317 DOI: 10.3750/AIP2014.44.4.05

Identifying the location and importance of spawning sites of Western Baltic herring using a particle backtracking model

Abstract The recruitment success of some herring stocks fluctuates strongly, and apparently, success is often already determined during the early life stages, i.e. before metamorphosis. In studying the survival of early life stages and its affecting factors, particularly those during the egg stage, it is crucial to examine the processes at the spawning sites, which often cannot be explored directly. A recent decline in the recruitment of Western Baltic spring-spawning herring (WBSSH) increases the urgency of filling the knowledge gap for this stock, especially because one bottleneck in the recruitment seems to occur before hatching. We examined the successful 2003–2009 spawning sites of WBSSH in the main spawning ground, the Greifswalder Bodden lagoon. Instead of using common techniques such as diving or underwater videography, which are usually unsuitable for mapping large areas, we applied a model approach. We tracked herring larvae at length 6–10 mm, recorded by larval surveys during March–June of the respective years, back to their hatching sites using a Lagrangian particle backtracking model. We compared the spawning areas identified by the model with the results of earlier field studies; however, we also analysed variations between years, larval length groups, and different applied growth models, which are needed to define hatch-dates. Although spawning sites could not be identified with high precision because of the strong diffusion in the area studied, results indicate that larvae up to 10 mm length are caught near their hatching sites. However, the location of successful spawning sites varied largely between years, with the main hatching sites situated in the Strelasund and the eastern entrance of the lagoon. This may reflect variations in spawning-site selection or quality. A better knowledge of the locations and relative importance of, and the processes occurring on, the different spawning sites will provide an important contribution to the sustainable management of this commercially valuable herring stock.

The effects of periodic marine inflow into the Baltic Sea on the migration patterns of Western Baltic spring-spawning herring

Abstract Western Baltic spring-spawning herring (WBSSH, Clupea harengus L.) perform seasonal migrations between feeding grounds in the Skagerrak and Kattegat and their spawning sites in the Western Baltic Sea. The Sound, connecting the Kattegat to the Western Baltic Sea, is an important aggregation and transition zone for this herring stock during its spawning migration. We analysed data from the German autumn acoustic surveys of the years 1993–2009. These data revealed at least two different distribution patterns of herring in autumn: herring generally aggregated in the Sound, but in some years the majority of herring were detected further south, being outside of the Sound by the time of the survey. We tested whether observed annual differences in the herring migration can be explained by either stock characteristics (age and size) or hydrographical variables (salinity and oxygen concentration). Our results suggest that rather than being related to stock characteristics, the distribution pattern of herring was related to environmental conditions, i.e. to marine inflow events into the Baltic Sea. Barotropic inflow events in late summer and early autumn seem to prevent deoxygenation in the Sound and thereby favour the prolonged aggregation of herring in the Sound.

Nutritional situation for larval Atlantic herring (Clupea harengus L.) in two nursery areas in the western Baltic Sea

Abstract The Greifswalder Bodden (GWB) is considered to be the most important spawning and nursery area for the western Baltic spring-spawning herring. However, the biotic and abiotic reasons for this are still unclear. Consequently, we investigated larval growth conditions in the GWB and in the Kiel Canal (KC), another nursery and spawning area of Baltic herring. We investigated prey quantity and quality [copepod abundance and essential fatty acid (EFA) concentration] as well as biochemically derived growth rates and fatty acid content of larval herring in spring 2011. A significant correlation between larval growth and larval EFA concentration could be observed in the GWB. The highest growth rates and EFA concentrations in the larval herring coincided with high food quality. Compensating effects of food quality on food quantity and vice versa could be observed in both the GWB and the KC. While larval growth rates in the KC were high early in the season, highest growth rates in the GWB were achieved late in the season. In conclusion, neither area was superior to the other, indicating similar growth conditions for larval herring within the region.

A Review on Herring, &lt;I&gt;Clupea Harengus&lt;/I&gt; (Actinopterygii: Clupeiformes: Clupeidae) Recruitment and Early Life Stage Ecology in the Western Baltic Sea

Herring, Clupea harengus L., is an important commercial fish species in the Baltic Sea region since medieval times. The western Baltic spring spawning herring (WBSS) is one of three major Baltic Sea herring stocks, supporting a significant over-regional fishery in the western Baltic Sea as well as in the Kattegat and Skagerrak area. One major component spawns in the vicinity of the German island of R&#252;gen and the associated major spawning ground, the Greifswalder Bodden (GWB), a shallow, semi-enclosed sub-system of estuarine character. Research of herring biology and ecology has an extensive history in the area probably due to the long fishery tradition and the importance of this particular herring stock for the local Baltic coast economy. In this review most of the scientific findings about WBSS in its spawning areas, mainly the GWB, is summarized. This study is based on critical review of over 120 publications and scientific sources from the past 100 years containing relevant information on possible consequences of multiple environmental and anthropogenic induced stressors of herring recruitment and productivity. Besides current peer reviewed literature a significant amount of grey literature was included, consisting primarily of papers and reports written in German language and representing the only historical data sources and published documentation of regional western Baltic herring ecology.

Discrimination of western Baltic spring-spawning and central Baltic herring (Clupea harengus L.) based on growth vs. natural tag information

Abstract Gröhsler, T., Oeberst, R., Schaber, M., Larson, N., and Kornilovs, G. 2013. Discrimination of western Baltic spring-spawning and central Baltic herring (Clupea harengus L.) based on growth vs. natural tag information. – ICES Journal of Marine Science, 70: 1108–1117. In the Baltic Sea, several stocks of herring (Clupea harengus L.) are surveyed and managed separately. For assessment purposes, a spatial stock separation based on ICES subdivisions is implemented. However, especially in the western Baltic, the distribution areas of two stocks, the western Baltic spring-spawning herring and the central Baltic herring, overlap. Results of regularly conducted surveys for assessment purposes indicated variable degrees of mixing of both stocks in the survey area, based on conspicuous differences in weights/lengths within certain age groups, especially in an area known for overlapping distribution of both stocks. At present, varying fractions of the central Baltic herring stock have not been taken into account during regular surveys conducted in the western Baltic, leading to possible undetected biases in assessment indices derived from these surveys. Additionally, methods otherwise applied for stock separation of Baltic herring so far are based on parameters that cannot readily be derived during regular surveys. In this paper, we present a simple and quick method to reliably allocate herring to either stock based on a separation function derived from survey-based length-at-age data, thus facilitating a more precise estimate of biomass and abundance indices from regular surveys and commercial fisheries.

Wind-induced variability in coastal larval retention areas: a case study on Western Baltic spring-spawning herring

The investigation of larval dispersal and retention, their variability and dependence on wind conditions, has become a major topic in fisheries research owing to potential effects on stock recruitment and stock structuring. The present study quantifies the wind induced variability of larval retention of herring in a highly productive coastal lagoon of the Western Baltic Sea. This lagoon, the Greifswalder Bodden, represents the main spawning area of Western Baltic Spring-Spawning Herring, a stock that has recently undergone a continuous decline in recruitment. The study tests whether this decline was related to changes in larval retention, more precisely to changes in wind conditions, the main forcing of the lagoon’s circulation. To answer this, a model approach was applied. Larvae were tracked as Lagrangian drifters under constant and variable wind conditions, examining the main drift patterns and reconstructing the incidents during the period of recruitment decline. For the latter, weekly cohorts of virtual larvae were released in the lagoon over the entire spawning period (April– June; >16 weeks). The fraction of retained larvae per cohort was related to observed larval abundances. On this basis, a new retention index was defined to evaluate the annual larval retention. The results presented cannot explain the observed recruitment decline but characterize the lagoon as an important larval retention area by virtue of unsteady wind conditions that prevent a steady outflow of larvae.

Finding the signal in the noise: objective data-selection criteria improve the assessment of western Baltic spring-spawning herring

Abstract Payne, M. R., Clausen, L. W., and Mosegaard, H. 2009. Finding the signal in the noise: objective data-selection criteria improve the assessment of western Baltic spring-spawning herring. – ICES Journal of Marine Science, 66: 1673–1680. In the art of fish-stock assessment, it is common practice to include all available data without properly testing their validity in terms of their signal-to-noise ratio. The western Baltic spring-spawning herring (Clupea harengus) stock has been historically difficult to assess in a reliable manner. The population is spread between the Skagerrak, Kattegat, the Danish islands, and the western Baltic, but the distribution depends on age and season. Although the distribution area is covered by five separate surveys, none covers the entire stock. Using all time-series data may cause high noise levels and could lead to a poor-quality assessment. We examine the temporal and spatial coverage of each survey in terms of current biological understanding of stock distribution and, employing the observed internal consistency between age classes within cohorts as additional criteria, select the most appropriate data subsets. Assessments based on the revised dataset show greatly improved quality in terms of both accuracy and precision. The results highlight the often-ignored principle that a judicious choice of input data, based on rational and justifiable selection criteria, can enhance the ultimate quality of a stock assessment.

When is year-class strength determined in western Baltic herring?

Abstract Oeberst, R., Klenz, B., Gröhsler, T., Dickey-Collas, M., Nash, R. D. M., and Zimmermann, C. 2009. When is year-class strength determined in western Baltic herring? – ICES Journal of Marine Science, 66: 1667–1672. Weekly surveys of larvae in the Strelasund and the Greifswalder Bodden were used to investigate when year-class strength is determined in western Baltic spring-spawning (WBSS) herring. An abundance metric of larvae reaching a length of 20 mm over the entire spawning season was constructed by accounting for increases in daily growth resulting from seasonal increases in temperature (5–20°C). The index was significantly correlated with the acoustic estimates of age-1 herring in the western Baltic Sea (r = 0.88) and with the estimates of year-class strength obtained from stock assessment (r = 0.65). Previous studies of herring elsewhere have reported that year-class strength is determined during the late larval stage, but we show that year-class strength can already be fixed at a larval length of only 20 mm. Although the index obtained may be used in stock assessment and predictions, the intriguing question remains, namely how can the signal of larval productivity from one single spawning component of WBSS herring reflect the year-class dynamics of the entire stock?

Description of growth by simple versus complex models for Baltic Sea spring spawning herring

The objective was to find a length–growth model to help differentiate between herring stocks (Clupea harengus l.) when their length–growth shows systematically different patterns. The most essential model restriction was that it should react robustly against variations in the underlying age range which varies not only over time but also between the different herring stocks. Because of the limited age range, significance tests as well as confidence intervals of the model parameters should allow a small sample restriction. Thus, parameter estimation should be of an analytical rather than asymptotic nature and the model should contain a minimum set of parameters. The article studies the comparative characteristics of a simple non-asymptotic two-parameter growth model (allometric length–growth function, abbreviated as ALG model) in contrast to higher parametric and more complex growth models (logistic and von-Bertalanffy growth functions, abbreviated as LGF and VBG models). An advantage of the ALG model is that it can be easily linearized and the growth coefficients can be directly derived as regression parameters. The intrinsic ALG model linearity makes it easy to test restrictions (normality, homoscedasticity and serial uncorrelation of the error term) and to formulate analytic confidence intervals. The ALG model features were exemplified and validated by a 1995 Baltic spring spawning herring (BSSH) data set that included a 12-year age range. The model performance was compared with that of the logistic and the von-Bertalanffy length–growth curves for different age ranges and by means of various parameter estimation techniques. In all cases the ALG model performed better and all ALG model restrictions (no autocorrelation, homoscedasticity, and normality of the error term) were fulfilled. Furthermore, all findings seemed to indicate a pseudo-asymptotic growth for BSSH. The proposed model was explicitly derived for of herring length-growth; the results thus should not be generalized interspecifically without additional proof.