Deoxygenation impacts on Baltic Sea cod: Dramatic declines in ecosystem services of an iconic keystone predator

The intensified expansion of the Baltic Sea's hypoxic zone has been proposed as one reason for the current poor status of cod (Gadus morhua) in the Baltic Sea, with repercussions throughout the food web and on ecosystem services. We examined the links between increased hypoxic areas and the decline in maximum length of Baltic cod, a demographic proxy for services generation. We analysed the effect of different predictors on maximum length of Baltic cod during 1978-2014 using a generalized additive model. The extent of minimally suitable areas for cod (oxygen concentration >= 1 ml l(-1)) is the most important predictor of decreased cod maximum length. We also show, with simulations, the potential for Baltic cod to increase its maximum length if hypoxic areal extent is reduced to levels comparable to the beginning of the 1990s. We discuss our findings in relation to ecosystem services affected by the decrease of cod maximum length

Understanding the spatio-temporal dynamics of demersal fish species in the Baltic Sea

The Baltic Sea is one of the largest brackish areas in the world with mixed salt and fresh water. The particular hydrological characteristics of this sea, such as the temperature and salinity gradients from the Danish Straits to the northernmost part of the Gulf of Bothnia as well as the hypoxia of its deep waters, make it a unique ecosystem. During the last century the Baltic Sea has undergone major structural changes not only in its physical characteristics but also in the biological communities inhabiting it. Cod (Gadus morhua) and flounder (Platichthys flesus) are two key species of the Baltic Sea ecosystem. Cod in the Baltic has experienced huge changes in spatial distribution, variations in growth, mortality and abundance in the last 40 years, but still the environmental and the biological drivers that have caused these modifications are unclear. Much less is known for flounder even though it is predated by cod and could potentially compete with it for benthic resources. All these changes have caused and are causing problems to the fishing industry and the consequent management. The present situation of the Baltic Sea illustrates the importance of understanding on the processes causing spatial heterogeneity and changes in the fish population dynamics. Such knowledge is crucial in order to work out a better management of the resources. This essay has the aim of reviewing the state of the knowledge on the spatiotemporal dynamics of cod and flounder in the Baltic Sea and to point out the knowledge gaps that need to be filled

Understanding the spatiotemporal dynamics of demersal fish species in the Baltic Sea

Species are not homogeneously distributed and the interdependencies between trophic interactions, environmental factors and anthropogenic forcing determine how their distribution changes over time. Hence taking into account both the spatial and temporal components of the dynamics of potentially interacting species is essential in management and conservation. Cod and flounder are two key fish species of the Baltic Sea, both ecologically and commercially. However, their dynamics and interactions in the offshore demersal habitat have been largely neglected in the past. Therefore, the objective of this thesis was to obtain further knowledge on how their dynamics have changed in space and time in this region. This was achieved by investigating the long-term changes in the horizontal and vertical distribution of cod and flounder, and by quantifying their spatial overlap and potential interactions. The results show that large changes have occurred in the demersal fish community of the Baltic Sea during the past four decades. Cod collapsed and contracted to the south, while flounder increased both in abundance and extent of distribution in the central Baltic. A contraction in the vertical distribution have been shown for both species from the early 1990s, possibly due to a combination of expanded areas of hypoxia in deep waters and an increase in predation risk in shallow waters. These changes have increased the spatial overlap between life-stages and species, which may have amplified the interaction strength between cod and flounder. The changes in predator-prey and competitive interactions between cod and flounder are hypothesised to have contributed to the low abundances of flounder during the “cod outburst” in the early 1980s, and to the low condition and feeding level of juvenile cod in the last decades. These results are highly relevant both for fisheries management and marine spatial planning as they can be implemented in multispecies models or directly used to protect important areas and habitats. Moreover, the results could be used in stock assessments and management to take into account more realistically the dynamics of cod and flounder in the Baltic Sea

The Mediterranean and Black Sea STECF Stock Assessment Database

Since 2007, the Scientific Technical Economic Committee for Fisheries (STECF), for which Joint Research Centre (JRC) runs the Secretariat and all the data collection process, started collecting and organizing information on Mediterranean and Black Sea fisheries, and since 2009 performing standardized stock assessments on these fisheries during STECF expert working groups (EWGs). The stock assessment results have been documented in more than 30 reports STECF EWGs (https://stecf.jrc.ec.europa.eu/reports/medbs). Stock assessments performed during the STECF EWGs employ different approaches and tools, however, models implemented in the Fisheries Libraries in R (FLR, http://www.flr-project.org) were the most used. After almost 10 years of stock assessments in the Mediterranean and Black Sea, JRC extracted available stock assessment data from digital repositories of EWGs and compiled a STECF reference database. From each assessment contained in the STECF reports yearly time series of stock variables such as:Total Catch (total weight of all fish in the stock), Recruitment (total number of individuals entering yearly in the population), Spawning Stock Biomass (total weight of all sexually mature fish in the stock) and Fishing Mortality were extracted. The assessments data are made available through an online interactive dashboard under the STECF Data dissemination web page (https://stecf.jrc.ec.europa.eu/dd/medbs/ram) that allow readers to compare and contrast several stock assessments variables. Each stock assessment is linked via an URL to the original source of the stock assessment. According to EU CFP - Common Fisheries Policy (Reg. EU 1380/2013 and Reg. EU812/2015) all EU commercial fish stocks should be fished at a maximum sustainable yield (Fmsy). Biological reference points, Fref (Fmsy or a proxy as F 0.1 ) and were subsequently used to assess if the level of exploitation (F/Fmsy) is in line with the CFP objectives (F/Fmsy≤1) or not (F/Fmsy >1). The STECF database is the reference database for the computation of the Common Fisheries Policy monitoring indicators for the Mediterranean and Black Sea (https://stecf.jrc.ec.europa.eu/documents/43805/2092142/STECF+18-01+adhoc+-+CFP+Monitoring+2018.pdf) To ensure traceability and foster reproducible scientific research, all the data, code and references part of the compilation process are hosted on GitHub, a well-known version control software platform. The target audience of the dashboard ranges from governments, fisheries institutes, stakeholders, NGO’s and common citizens that want to check the status of marine fisheries resources evaluated. A copy of the Mediterranean and Black Sea STECF stock assessment results will be included, for the first time, in the next release of the RAM legacy database: a voluntary contributed worldwide stock assessments database, RAM legacy, (http://ramlegacy.org/). The RAM Legacy database includes fish stock assessments from all around the world’s oceans, and provides a unique source of information to make comparisons between fisheries and to perform global analysis of stock status.JRC.D.2-Water and Marine Resource

Changes in population depth distribution and oxygen stratification are involved in the current low condition of the eastern Baltic Sea cod (Gadus morhua)

During the past 20 years, hypoxic areas have expanded rapidly in the Baltic Sea, which has become one of the largest marine “dead zones” in the world. At the same time, the most important commercial fish population of the region, the eastern Baltic cod, has experienced a drastic reduction in mean body condition, but the processes behind the relation between deoxygenation and condition remain elusive. Here we use extensive long-term monitoring data on cod biology and distribution as well as on hydrological variations to investigate the processes that relate deoxygenation and cod condition during the autumn season. Our results show that the depth distribution of cod has increased during the past 4 decades at the same time of the expansion, and shallowing, of waters with oxygen concentrations detrimental to cod performance. This has resulted in a progressively increasing spatial overlap between the cod population and low-oxygenated waters after the mid-1990s. This spatial overlap and the actual oxygen concentration experienced by cod therein statistically explained a large proportion of the changes in cod condition over the years. These results complement previous analyses on fish otolith microchemistry that also revealed that since the mid-1990s, cod individuals with low condition were exposed to low-oxygen waters during their life. This study helps to shed light on the processes that have led to a decline of the eastern Baltic cod body condition, which can aid the management of this population currently in distress. Further studies should focus on understanding why the cod population has moved to deeper waters in autumn and on analyzing the overlap with low-oxygen waters in other seasons to quantify the potential effects of the variation

Skrubbskäddan stjäl östersjötorskens middag

På Östersjöns botten lever torsk och skrubbskädda, två fiskarter som är viktiga både för ekosystemet och för fisket. Under de senaste fyrtio åren har de båda arternas geografiska utbredning förändrats markant. Idag är torskbeståndet till stor del koncentrerat till i ett mindre område i södra Östersjön där det också finns mängder av skrubbskädda. Den ökade konkurrensen kan ha bidragit till att torsken blivit mycket magrare på senare tid

Long-term changes in spatial overlap between interacting cod and flounder in the Baltic Sea

The strength of interspecific competition and predator-prey interactions depends on the area of co-occurrence of the interacting species. Therefore, it is necessary to quantify the changes in the spatial overlap of trophically connected species to understand the outcomes of species interactions. In the Baltic Sea, the interplay between cod and flounder has previously been neglected. In this study, we use four decades of data on cod and flounder distributions covering the southern and central Baltic Sea to: (1) model and map the changes in the distributions of the two species using generalized additive models; (2) quantify the temporal changes in the potential competitive and predator-prey interactions between them using spatial overlap indices; (3) relate these changes in overlap to the known dynamics of the different cod and flounder populations in the Baltic Sea. Competition overlap has continuously increased for cod, from the beginning of the time-series. This is a possible cause of the observed decline in feeding levels and body condition of small and intermediate sized cod. Flounder overlap with large cod instead has decreased substantially, suggesting a predation release of flounder, potentially triggering its increase in abundance and distribution range observed in the last decades

A novel mutation in SACS gene in a family from southern Italy

A form of autosomal recessive spastic ataxia (ARSACS) has been described in the Charlevoix and Saguenay regions of Quebec. So far a frameshift and a nonsense mutation have been identified in the SACS gene. The authors report a new mutation (1859insC), leading to a frameshift with a premature termination of the gene product sacsin, in two sisters from consanguineous parents. The phenotype is similar to previously described patients with ARSACS

The effects of Contracaecum osculatum larvae on the growth of Atlantic cod (Gadus morhua)

Atlantic cod (Gadus morhua) from the Eastern Baltic stock have decreased in numbers and condition since the 1990′s. Among several causes, an increased prevalence and intensity of the nematode Contracaecum osculatum has been discussed. This increase has been attributed to a population increase of the parasites final host, the grey seal (Halichoerus grypus). Other studies have looked at the role of Contracaecum osculatum on cod growth and condition on recently caught cod, or done short term experimental studies in lab. This study instead investigated the importance of Contracaecum osculatum for cod growth in a sea pen based experiment, where cod were kept and fed in order to monitor growth. The results show that a higher density (number of nematodes per gram liver) decreases cod growth potential. If the number of nematodes exceeded 8 per gram liver cod did not grow in length, even when given generous amounts of food. Accounting for the lack of growth due to Contracaecum osculatum may improve stock assessments and increase the possibility to reach management targets

Is Diversity the Missing Link in Coastal Fisheries Management?

Fisheries management has historically focused on the population elasticity of target fish based primarily on demographic modeling, with the key assumptions of stability in environmental conditions and static trophic relationships. The predictive capacity of this fisheries framework is poor, especially in closed systems where the benthic diversity and boundary effects are important and the stock levels are low. Here, we present a probabilistic model that couples key fish populations with a complex suite of trophic, environmental, and geomorphological factors. Using 41 years of observations we model the changes in eastern Baltic cod (Gadus morhua), herring (Clupea harengus), and Baltic sprat (Sprattus sprattus balticus) for the Baltic Sea within a Bayesian network. The model predictions are spatially explicit and show the changes of the central Baltic Sea from cod- to sprat-dominated ecology over the 41 years. This also highlights how the years 2004 to 2014 deviate in terms of the typical cod–environment relationship, with environmental factors such as salinity being less influential on cod population abundance than in previous periods. The role of macrozoobenthos abundance, biotopic rugosity, and flatfish biomass showed an increased influence in predicting cod biomass in the last decade of the study. Fisheries management that is able to accommodate shifting ecological and environmental conditions relevant to biotopic information will be more effective and realistic. Non-stationary modelling for all of the homogeneous biotope regions, while acknowledging that each has a specific ecology relevant to understanding the fish population dynamics, is essential for fisheries science and sustainable management of fish stocks