Description: 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.

Description: Long-term macrobenthos data from Kiel Bight in the Western Baltic collected between 1968 and 2000 have been correlated with the winter NAO index (North Atlantic Oscillation Index) and other environmental data such as temperature, salinity and oxygen content in the bottom water in order to detect systematic patterns related to so far unexplained abiotic signals in the dynamics of zoobenthic species assemblages. The benthos data come from a cluster of five stations (Süderfahrt/ Millionenviertel) in Kiel Bay. Our investigations concentrated on the macrobenthic dynamics with a focus on the number of species m− 2 (species richness). Using logarithms and the time series analysis approach of Box/Jenkins (ARIMA modelling, transfer function modelling) it was shown that species richness was strongly influenced by the winter NAO (adjusted for a linear time trend within the 1968-2000 period) and salinity (with a shift/lag of four years). Bootstrapping experiments (i.e. sampling from the error process) and analysis of prediction power (by means of the one- or more-years leaving-out method) showed that the parameter estimates behaved in a stable way, leading to a relatively robust model.

Description: The synchrony of pelagic fish population dynamics with climate variability may impose significant alterations in their distribution and biomass, as well as catch composition, with potential effects on ecosystems and fisheries. This work examines the effect of the Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation (NAO) signals across the Mediterranean Sea sub-regions (western, central and eastern), with respect to small (European sardine Sardina pilchardus, European anchovy Engraulis encrasicolus, round sardinella Sardinella aurita and European sprat Sprattus sprattus) and medium (Atlantic mackerel Scomber scombrus, Atlantic chub mackerel Scomber japonicus, Atlantic horse mackerel Trachurus trachurus, Mediterranean horse mackerel Trachurus mediterraneus) pelagic fishes using various catch ratios and the mean temperature of the pelagic catch (MTpC) method for the period 1970–2014. The time until the pelagic fish communities react to the signals of the AMO and NAO, as revealed by the MTpC and catch ratios, varied among the Mediterranean sub-regions. The pelagic fishes of the central and eastern Mediterranean are those that responded most strongly to AMO variability, whereas those of the central and western Mediterranean also responded to the NAO. The effect of the NAO on pelagic fishes of the eastern Mediterranean was not significant.

Description: Northeast Atlantic marine ecosystems such as the Bay of Biscay, Celtic Sea, English Channel, Subpolar Gyre region, Icelandic waters and North Sea as well as the Mediterranean Sea show concomitant ‘regime shift’-like changes around the mid-1990s, which involved all biota of the pelagial: phytoplankton, zooplankton, pelagic fish assemblages, demersal fish assemblages and top predators. These shifts were caused by complex ocean-atmosphere interactions initiating large-scale changes in the strength and direction of the current systems, that move water masses around the North Atlantic, and involved the North Atlantic Oscillation (NAO), the Atlantic Meridional Overturning Circulation (AMOC), and the subpolar gyre (SPG). The contractions and expansions of the SPG and fluctuations of the Atlantic Multidecadal Oscillation (AMO) play a key role in these complex processes. Small pelagic fish population trends were the sentinels of these changes in the mid-1990s in the ecosystems under investigation.

Description: Atlantic cod (Gadus morhua) is an important recreational and commercial fisheries target species in the Northern hemisphere. Release rates are high in the recreational fishery due to regulatory and voluntary catch-and-release practice. Although post-release mortality of cod is relatively low, there is potential for further reductions. The most effective way to reduce post-release mortality is to minimize the catch of sublegal fish or non-target species and to reduce hooking injuries by using more selective fishing methods. This study investigated the influence of the lure/bait type on: (1) size of fish, (2) catch and harvest, (3) proportion of bycatch, (4) hooking location, and (5) injury (bleeding) in the western Baltic Sea recreational cod fishery. Data were collected via random onboard sampling of 35 charter vessel angling trips (778 anglers) and during two supplementary studies in the western Baltic Sea. Overall, the median total length was significantly higher for cod caught on artificial lures (39 cm) than for cod caught on natural bait (28 cm), leading to a 43% higher proportion of sublegal (〈38 cm) cod for bait than for lure. Median catch-per-unit-efforts (number of captured cod per angling hour) did not differ significantly between lure and bait angling (both: 0.49 cod per hour), whereas the median harvest-per-unit-effort (number of captured cod ≥ minimum landing size (38 cm) per angling hour) was significantly higher for lure (0.24 cod ≥38 cm per hour) than for bait angling (0.06 cod ≥38 cm per hour). The incidence of deep hooking and severe bleeding was significantly higher for bait angling. Furthermore, bait angling significantly increased bycatch of other species dominated by whiting (Merlangius merlangus) and European flounder (Platichthys flesus). Cod anglers can reduce the catch of sublegal cod and non-target species and minimize hooking injuries of released fish by using lures instead of bait in the western Baltic Sea. Thus, voluntary terminal gear recommendations may be an effective tool for anglers and managers to increase selectivity in recreational cod fisheries.

Description: Atlantic herring (Clupea harengus) has a complex population structure and displays a variety of reproductive strategies. Differences in reproductive strategies among herring populations are linked to their time of spawning, as well as to their reproductive investment which can be an indicator for migratory vs. stationary behavior. These differences are reflected in the number of oocytes (fecundity) and the size of the oocytes prior spawning. We studied potential mixing of herring with different reproductive strategies during the spring spawning season on a coastal spawning ground. It has been hypothesized that both spring and autumn spawning herring co-occur on this specific spawning ground. Therefore, we investigated the reproductive traits oocyte size, fecundity, fertilization success as well as length of the hatching larvae during the spring spawning season from February to April. We used a set of 11 single nucleotide polymorphism markers (SNPs), which are associated with spawning season, to genetically identify autumn and spring spawning herring. Reproductive traits were investigated separately within these genetically distinct spawning types. Furthermore, we used multivariate analyses to identify groups with potentially different reproductive strategies within the genetic spring spawners. Our results indicate that mixing between ripe spring and autumn spawners occurs on the spawning ground during spring, with ripe autumn spawners being generally smaller but having larger oocytes than spring spawners. Within spring spawners, we found large variability in reproductive traits. A following multivariate cluster analysis indicated two groups with different reproductive investment. Comparisons with other herring populations along the Norwegian coastline suggest that the high variability can be explained by the co-occurrence of groups with different reproductive investments potentially resulting from stationary or migratory behavior. Fertilization success and the length of the hatching larvae decreased with progression of the spawning season, with strong inter-individual variation, supporting our findings. Incorporating such complex population dynamics into management strategies of this species will be essential to build its future population resilience.

Description: This study presents a trilateral test array of new opto-acoustic Underwater Fish Observatories (UFOs) that were operated and tested in Kiel Bight as part of the “UFOTriNet” project. While hydroacoustic and optical techniques have so far been used individually to observe and monitor fish stocks, we present a coupled hybrid system consisting of an optical device intended to scan the near-field as a subsample of a spatially larger medium-to-far-field, scanned by an acoustical device. The optical device consists of two residual light amplifying camera modules able to detect and classify various marine species at a high resolution in the range of at max 4 meters in the study area. To compensate for this spatial limitation, the acoustical component consists of a 2D imaging sonar with a maximum range of 50 m, albeit with a lower resolution. Species affiliation, morphometric characteristics of fish and other marine organisms were stereo-optically detected and classified in the nearfield, blended with acoustical activity in medium to far range, and projected onto the entire insonified area using a hybrid algorithm. Through the synchronous acquisition of multiparametric abiotic and biotic data, UFO allows an automatic, continuous, and non-invasive long-term monitoring of various fish and other marine species and their habitats at regional hotspots. An 86-day multiparametric sample revealing an abrupt shift from a clupeid fish to a gelatinous plankton dominated regime in summer/autumn 2021 in Kiel Fjord is used to demonstrate the potential of UFO for various applications.