The status of Eastern Baltic cod (EBC) Gadus morhua has remained poor despite low fishing mortality for over a decade, including a fishing ban since 2019. Although the decline in productivity can be explained by lower individual growth and survival rates, other aspects of life-history changes such as maturation patterns for EBC has so far not been sufficiently explored. According to current stock assessments, the median size at maturity (L₅₀) has halved from 40 to around 20 cm in total length since the 1990s, while the overall size distribution has become increasingly truncated. It has previously been suggested that changes in L₅₀ can be attributed to both fishing-induced evolution and phenotypic plasticity induced by growth rates. However, since L₅₀ is currently occurring around 20 cm, the maturation process must have been initiated at much smaller sizes, that is, long before the fish could be caught in the dominant trawl fishery at around 35 cm. In this study, we aimed to further investigate what drivers may have led to reduced productivity in EBC by determining variations in size at sexual maturity in longer time series than has been done before (1930s to 1980s) and include prey productivity and quality. We found that L₅₀ declined already in the 1930s and thereafter remained stable at around 40 cm up to the 1990s. On a centurial perspective, L₅₀ has been positively correlated to growth potential (L₉₅), length diversity, total stock biomass, total catch and yield per recruit, while Fulton's condition factor was not related to L₅₀. Our results suggest that the links between life-history parameters and external drivers are complex, but the present unprecedented early onset of maturity and hence decline in L₅₀ since the 1990s signals a decline in growth potential, which also has hampered the productivity of EBC.

Fiskeriförvaltningen har traditionellt haft en begränsad koppling till annan havsmiljöförvaltning. Den nuvarande situationen i Svealandskustens ekosystem är en bra illustration av varför vi behöver bli bättre på att integrera fiskeriförvaltning och fiskevård med arbetet för en bättre vattenkvalitet och friska bottenhabitat. För att underlätta en sådan integrering har fiskeaktörer och forskare gemensamt tagit fram en modell som beskriver Stockholms skärgårds ekosystem, med fisken i fokus.

Projektet BalticCAT har som målsättning att utveckla verktyg för kumulativa miljöbedömningar av Östersjöns ekosystem. Forskarna har fokuserat på vilka samverkande faktorer som lett fram till ett minskat torskbestånd, men även utvärderat näringsvävsmodeller som kompletterande metod.

Resultaten visade att syrebristen i Östersjöns djupa områden kan förklara förlusten av reproduktionsområden för torsk, men inte den minskande produktivitet och försämrade kondition som observerats under de senaste 25 åren. Resultaten visar att en kombination av övergödning och klimatförändringar troligen är viktiga för att förklara många av de förändringar vi ser i Östersjöns födovävar, vilket betyder att åtgärder mot dessa hot är av stor vikt för att upprätthålla biologisk mångfald och ett hållbart fiske.

Projektet har även utvecklat en näringsvävsmodell för hela Östersjön, som kan bidra till utvärdering av policy och förvaltningsåtgärder och till arbetet med bedömning av god miljöstatus för födovävar. Projektet har finansierats med medel från Naturvårdsverkets miljöforskningsanslag som finansierar forskning till stöd för Naturvårdsverkets och Havs-och vattenmyndighetens kunskapsbehov.

While the central Baltic herring (Clupea harengus) stock biomass is estimated to have been reduced by ∼80 per cent since the 1970 s, the local effects of this decline remain unassessed. Coastal commercial fishermen have witnessed a widespread depletion at the spawning grounds and generally low herring abundance in inshore areas. We carried out hydroacoustic surveys, supported by midwater trawling in August 2021, to test whether herring abundance and recruitment have changed over time in the inshore areas. We used assessed recruitment success, measured as young-of-the-year (YOY) abundance, as a proxy for herring abundance in several areas of the southeast Stockholm Archipelago and compared the results with previous investigations in this area (hydroacoustics 2000–2001 and midwater trawling 2002–2004 monitoring). We found that the composition of the fish community had undergone significant changes. While acoustic backscattering (s_A) representing the whole fish community were similar to previous estimates, species composition was significantly changed. YOY herring, previously dominant, had declined from 55% to 4% in numbers, whereas three-spined stickleback (Gasterosteus aculeatus) increased from no representation to 76%. The observed changes makes it imperative that further investigation concerning herring population structure, biomass, and recruitment are conducted.

Rapporten presenterar konceptuella modeller av ekosystemet i Stockholms skärgård, med fokus på fisk, som tagits fram i samarbete med fiskets intressenter och forskare. En modell har tagits fram tillsammans med fiskets intressenter, utifrån deras egna observationer och kunskap de plockat upp från forskningsstudier och andra undersökningar. En annan modell har tagits fram tillsammans med forskare med expertis om fisk, fiske och ekosystemet i Stockholms skärgård och angränsande kustområden och visar det vetenskapliga kunskapsläget. Slutligen presenteras en syntes av dessa båda modeller som ger en samlad bild av både intressenternas kunskap och forskningsläget. Denna modell belyser även likheter och skillnader mellan intressenternas förståelse och forskarnas bild byggd på traditionell vetenskap.

Syntesmodellen visar att det finns en stor samsyn mellan intressenter och forskare kring ekologiska samband i Stockholms skärgård och hur de påverkas av mänskliga aktiviteter. Det gäller exempelvis strömmingens viktiga roll i födoväven, att starka rovfiskbestånd kan bidra till att hålla tillbaka symptom på övergödning och att rovdjur som gråsäl och storskarv har en betydande påverkan på vissa fiskarter. Både observationer och forskning visar att strömmingen minskat kraftigt i området, vilket får stora konsekvenser för skärgårdens ekosystem och för fisket.

De konceptuella modellerna är en del i arbetet med att ta fram en ekosystembaserad regional förvaltningsplan för fisk och fiske i Stockholms skärgård. De framtagna modellerna bidrar med kunskap om vad som påverkar fiskbestånden i Stockholms skärgård och vilka lokala åtgärder som kan genomföras för att förbättra förutsättningarna för fisk och fiske. Det inkluderar fiskevårdsåtgärder för att återskapa fungerande reproduktionshabitat och vandringsvägar för fisk och åtgärder för att minska den negativa påverkan från fiskätande vilt. Modellerna visar också att det finns påverkansfaktorer som ligger utanför det regionala mandatet att påverka i dagsläget, framförallt fisket på strömming och andra fiskarter som förvaltas på Östersjöskala. Modellerna pekar också ut kunskapsbrister och vilken kunskapsinhämtning som behöver prioriteras för att möjliggöra en ekosystembaserad fiskeförvaltning i Stockholms skärgård.

The fish fauna of the Baltic Sea reflects its 9 KY history of Arctic and temperate conditions and is a mixture of species that have invaded from the Atlantic and the continental watersheds. In spite of the challenging environmental conditions, such as low salinity in the entire Baltic Sea and varying temperature conditions, limiting the possibilities for successful reproduction, the number of species is comparably high.

Except for the Baltic Ice Lake and certain stages of the cold Yoldia Sea and freshwater Ancylus Lake, the fish fauna of the Baltic Sea, as recorded by archaeological and historical notes, has to a large extent remained unchanged. Some freshwater and cold-water species such as Arctic char may have disappeared while others, such as fourhorned sculpin and eelpout, have adapted and persist as “ice age relicts.” There are few viable introductions of novel species; the round goby may be the most conspicuous invasive species. The extinction rate is still low; the loss of sturgeon and the common skate within the HELCOM (the Helsinki Commission) area is due to fishing, and, for the riverine sturgeon, due to damming.

Since the formation of the Baltic Sea, fishing has played an essential role in supplying coastal settlements and their hinterlands and in trading. The archaeological and historical records have indicated fishing conducted with varying intensity using different methods.

Herring fishing has been a significant economic driver from the Middle Ages onward. Recent archaeological findings indicate that organized fishing was established at the outer archipelagos along the present Swedish east coast on predominately herring and cod archipelagos for self-sufficiency shortly before or during the Viking Age, and later to engage in barter.

The fact that the cod abundance has sometimes been sufficient for letting cod fishing be the most important fishery in the northern Baltic Proper alongside the fishery on herring may indicate that the eastern cod stock had relatively high productivity even when the Baltic Sea was significantly less eutrophic than it has been since the mid-20th century. This preindustrial variability in cod abundance suggests that climatic changes leading to changes in inflows of oceanic water may have affected salinity levels in the Baltic Sea.

Fisheries show substantial variability, especially over the last century. Fish production may have increased due to nutrient enrichment of the Baltic Sea. Higher yields have also been obtained due to higher fishing intensity and technological changes. Fishing has, therefore, become a major driver in shaping fish stocks.

The eutrophication of the Baltic Sea, leading to higher primary productivity and increasing water temperature and reductions in ice cover, may have led to changes in ecosystem structure and productivity. It should be underscored that such changes may also be amplified by the increasing fishing pressure on the cornerstone species such as herring, leading to significant disruptions in the food web.

Explaining the recent decline of eastern Baltic cod (EBC) remains scientifically challenging. Brander proposes in a comment to Svedäng et al.that the observed trend in oxygen in SD 25 supports the idea that juvenile cod are balancing the physiological cost of living under mild hypoxiaby offsetting the risk of being eaten by diving seals and cormorants in shallower water with more oxygen. There are a number of objections tothis conjecture, besides the fact that supporting observations are missing. Hence, it is difficult to reconcile the long-term development of EBCunder varying oxygen conditions with the hypothesis that a small reduction in oxygen content can explain the current strong and uniform declinein growth observed in the entire southern Baltic Sea.

Hypoxia is presently seen as the principal driver behind the decline of the former dominating Eastern Baltic cod stock (EBC; Gadus morhua). It has been proposed that both worsening conditions for reproduction and lower individual growth, condition, and survival are linked to hypoxia. Here, we elucidate the ecological envelope of EBC in terms of salinity stratification, oxygen content, and benthic animal biomasses, and how it has affected EBC productivity over time. The spawning conditions started deteriorating in the Gotland Deep in the 1950s due to oxygen depletion. In contrast, in the Bornholm Basin, hydrographic conditions have remained unchanged over the last 60 years. Indeed, the current extent of both well-oxygenated areas and the frequency of hypoxia events do not differ substantially from periods with high EBC productivity in the 1970s–1980s. Furthermore, oxygenated and therefore potentially suitable feeding areas are abundant in all parts of the Baltic Sea, and our novel analysis provides no evidence of a reduction in benthic food sources for EBC over the last 30 years. We find that while reproduction failure is intricately linked to hydrographic dynamics, a relationship between the spread of hypoxia and the decline in EBC productivity during the last decades cannot be substantiated. 

Marine Protected Areas (MPAs) are an important tool for management and conservation and play an increasingly recognised role in societal and human well-being. However, the assessment of MPAs often lacks a simultaneous consideration of ecological and socio-economic outcomes, and this can lead to misconceptions on the effectiveness of MPAs. In this perspective, we present a transdisciplinary approach based on the Delphi method for mapping and evaluating Marine Protected Areas for their ability to protect biodiversity while providing Ecosystem Services (ES) and related human well-being benefits - i.e., the ecosystem outputs from which people benefit. We highlight the need to include the human dimensions of marine protection in such assessments, given that the effectiveness of MPAs over time is conditional on the social, cultural and institutional contexts in which MPAs evolve. Our approach supports Ecosystem-Based Management and highlights the importance of MPAs in achieving restoration, conservation, and sustainable development objectives in relation to EU Directives such as the Marine Strategy Framework Directive (MSFD), the Maritime Spatial Planning Directive (MSPD), and the Common Fisheries Policy (CFP).

Utfiskningen av sill/strömming kan få mycket allvarliga konsekvenser för kustekosystemen. Regeringen och ansvarig myndighet kan inte längre ducka för problemet, skriver fem forskare.