ALBERT

Description: The eastern Baltic (EB) cod (Gadus morhua) stock was depleted and overexploited for decades until the mid-2000s, when fishing mortality rapidly declined and biomass started to increase, as shown by stock assessments. These positive developments were partly assigned to effective management measures, and the EB cod was considered one of the most successful stock recoveries in recent times. In contrast to this optimistic view, the analytical stock assessment failed in 2014, leaving the present stock status unclear. Deteriorated quality of some basic input data for stock assessment in combination with changes in environmental and ecological conditions has led to an unusual situation for cod in the Baltic Sea, which poses new challenges for stock assessment and management advice. A number of adverse developments such as low nutritional condition and disappearance of larger individuals indicate that the stock is in distress. In this study, we (i) summarize the knowledge of recent changes in cod biology and ecosystem conditions, (ii) describe the subsequent challenges for stock assessment, and (iii) highlight the key questions where answers are urgently needed to understand the present stock status and provide scientifically solid support for cod management in the Baltic Sea.

Description: Oxygen minimum zones are increasing, yet the effects of these zones on the genetic composition of marine fish stocks has been neglected. We assessed the combined effects of stock size and structure, and the prevailing oxygen situation, on Eastern Baltic cod (ICES SD25) genetic diversity. For this purpose, we used an integrative long-term otolith sample and data series (1995-2013) to (1) calculate the approximate number of females with surviving eggs in a given year, i.e., contributing to reproduction (n F), and (2) the annually resolved cohort mean allelic richness as proxy of genetic diversity, based on 12 microsatellite markers. Cohort mean allelic richness showed strong year-to-year fluctuations though no permanent decline. Importantly, it was highly correlated with n F, but with an unexpected 1.5 year time lag that may be an artefact of Eastern Baltic cod ageing problems. Our findings indicated that environmental pressure can effect rapid alterations in exploited fish stock genetic composition, and pointed to the importance of large females for Eastern Baltic cod reproduction during stagnation periods. Considering the importance of standing genetic variation for the evolutionary potential of populations, this is relevant for projections of the future state of cod stocks under global change.

Description: In order to study short- and medium-term changes in the composition of species assemblages in tropical mangrove creeks, weekly day and night sampling was conducted between february and april 1999 (rainy season). For this purpose the mouth of a tidal mangrove creek was closed off at high tide with tapagem nets, and fish and prawns leaving with the ebbing tide were caught. Each tapagem yield was subdivided into consecutive catches. Catch yields ranged between 0.3 kg and 10.3 kg corresponding to minimum and maximum high water level. Mean catch weight during neap tide was 1,9 ± 1,5 kg and 5,7 ± 2,4 kg for spring tides. In total, 41 fish species (26 families) and four shrimp species (2 families) were caught. Important species were Cathorops sp. (Ariidae), Colomesus psittacus (Tetraodontidae), Selenaspis herzbergii (Ariidae), Pseudauchenipterus nodosus (Pseudauchenipteridae), the shrimp Penaeus subtilis (Penaeidae), Anchovia clupeioides (Engraulidae), Mugil spp. (Mugilidae) and Anableps anableps (Anablepidae), accounting for 34, 18, 10, 6, 5, 5, 4 and 4 % of total catch weigth of 66,3 kg. Mean fish biomass was 2,2 ± 1, 1 g/m3 or 1,9 ± 1,3 g/m2. Mean shrimp density was 0,06 ± 0,05 individuals/m3. 87 ± 6 % of the fishes caught were juveniles. Shrimp were mainly juveniles or subadults, indicating that tidal mangrove creek are an important nursery area for these taxa. Shannon-Weaver-diversity H' was low (0,9 ± 0, 1) which is typical for the rainy season period. Eveness J' was lowest at spring tide-night when more rare species entered the creeks and immigration of dominant species was even more severe. This was supported by the species-family ratio indicating highest niche diversity in fish families at spring tide-night. During ebb tide firstly species of the water surface and free water column attempted to leave the tidal mangrove creeks, followed by benthic orientated species as water continued to drain off. Younger fishes left earlier than older ones, whereas young P. subtilis left later than older ones. Feeding was the main reason for entering the mangrove tidal creeks. Some species were present with reproductively active specimens. Male Cathorops sp. were only caught during spring tide, especially at night. Although salinity dropped from 30 to 10 within a month, only abundances of three species (Anchoviella lepidentostole, Chaetodipterus faber und Cynoscion acoupa) increased significantly. Some indicators for reduced predation and realised predator avoidance in the mangrove creeks were found. Fourteen species correlated positive with the maximum water level per tide. This relationship was best described by a power function. According to this, catch yields were most clearly determined by the changes between spring and neap tide. Although less pronounced, constant changes between day and night were noticeable. The composition of the species assemblage in the mangrove tidal creeks, changing with every tide, was significantly determined by the combination of the factors "tide" and "time of day". According to this, four distinct compositions could be distingiushed: spring tide-night, spring tide-day, neap tide-night and neap tide-day. lt seems that maximum water level of a tide, being influenced by wind direction and strength, ultimately determines the actual composition of the species assemblage found in a catch. Neither water temperature nor salinity influenced the composition of the species assemblage as a whole. Considering tide and day/night cycle is recommended for future planning of sampling highly mobile taxa in macrotidal environments.

Description: Genetic data have great potential for improving fisheries management by identifying the fundamental management units—that is, the biological populations—and their mixing. However, so far, the number of practical cases of marine fisheries management using genetics has been limited. Here, we used Atlantic cod in the Baltic Sea to demonstrate the applicability of genetics to a complex management scenario involving mixing of two genetically divergent populations. Specifically, we addressed several assumptions used in the current assessment of the two populations. Through analysis of 483 single nucleotide polymorphisms (SNPs) distributed across the Atlantic cod genome, we confirmed that a model of mechanical mixing, rather than hybridization and introgression, best explained the pattern of genetic differentiation. Thus, the fishery is best monitored as a mixed-stock fishery. Next, we developed a targeted panel of 39 SNPs with high statistical power for identifying population of origin and analyzed more than 2,000 tissue samples collected between 2011 and 2015 as well as 260 otoliths collected in 2003/2004. These data provided high spatial resolution and allowed us to investigate geographical trends in mixing, to compare patterns for different life stages and to investigate temporal trends in mixing. We found similar geographical trends for the two time points represented by tissue and otolith samples and that a recently implemented geographical management separation of the two populations provided a relatively close match to their distributions. In contrast to the current assumption, we found that patterns of mixing differed between juveniles and adults, a signal likely linked to the different reproductive dynamics of the two populations. Collectively, our data confirm that genetics is an operational tool for complex fisheries management applications. We recommend focussing on developing population assessment models and fisheries management frameworks to capitalize fully on the additional information offered by genetically assisted fisheries monitoring.

Description: Atlantic cod (Gadus morhua) is a species of great ecological and economical importance in the Baltic Sea. Here, two genetically differentiated stocks, the western and the eastern Baltic cod, display substantial mechanical mixing, hampering our understanding of cod ecology and impeding stock assessments and management. Based on whole-genome re-sequencing data from reference samples obtained from the study area, we designed two different panels of Single Nucleotide Polymorphisms markers (SNPs), which take into account the exceptional genome architecture of cod. A minimum panel of 20 diagnostic SNPs and an extended panel (20 diagnostic and 18 biologically informative SNPs, 38 in total) were developed and validated to distinguish unambiguously between the western and the eastern Baltic cod stocks and to enable studies of local adaptation to the specific environment in the Baltic Sea, respectively. We tested both panels on cod sampled from the southern Baltic Sea (n = 603) caught in 2015 and 2016. Genotyping results showed that catches from the mixing zone in the Arkona Sea, were composed of similar proportions of individuals of the western and the eastern stock. Catches from adjacent areas to the east, the Bornholm Basin and Gdańsk Deep, were exclusively composed of eastern Baltic cod, whereas catches from adjacent western areas (Belt Sea and Öresund) were composed of western Baltic cod. Interestingly, the two Baltic cod stocks showed strong genetic differences at loci associated with life-history trait candidate genes, highlighting the species’ potential for ecological adaptation even at small geographical scales. The minimum and the extended panel of SNP markers presented in this study provide powerful tools for future applications in research and fisheries management to further illuminate the mixing dynamics of cod in the Baltic Sea and to better understand Baltic cod ecology.

Description: Coexistence of fish populations (= stocks) of the same species is a common phenomenon. In the Baltic Sea, two genetically divergent stocks of Atlantic cod (Gadus morhua), Western Baltic cod (WBC) and Eastern Baltic cod (EBC), coexist in the Arkona Sea. Although the relative proportions of WBC and EBC in this area are considered in the current stock assessments, the mixing dynamics and ecological mechanisms underlying coexistence are not well understood. In this study, a genetically validated otolith shape analysis was used to develop the most comprehensive time series of annual stock mixing data (1977–2019) for WBC and EBC. Spatio-temporal mixing analysis confirmed that the two stocks coexist in the Arkona Sea, albeit with fluctuating mixing proportions over the 43-year observation period. Depth-stratified analysis revealed a strong correlation between capture depth and stock mixing patterns, with high proportions of WBC in shallower waters (48–61% in 〈20m) and increasing proportions of EBC in deeper waters (50–86% in 40-70m). Consistent depth-specific mixing patterns indicate stable differences in depth distribution and habitat use of WBC and EBC that may thus underlie the long-term coexistence of the two stocks in the Arkona Sea. These differences were also reflected in significantly different proportions of WBC and EBC in fisheries applying passive gears in shallower waters (more WBC) and active gears in deeper waters (more EBC). This highlights the potential for fishing gear-specific exploitation of different stocks, and calls for stronger consideration of capture depth and gear type in stock assessments. This novel evidence provides the basis for improved approaches to research, monitoring and management of Baltic cod stocks.

Description: During the 2010s, Atlantic cod Gadus morhua L. in the eastern Baltic Sea experienced increasing infection loads of the parasitic nematode Contracaecum osculatum (Rudolphi) in their livers. Starting in 2021, a mandatory part of the routine sampling protocol on Baltic monitoring surveys is to assign a liver category to individual cod livers, based on the number of nematodes visible on the liver surface, to follow spatiotemporal changes in nematode infection loads. The validity of the liver category method has never been evaluated. Based on data from 642 cod livers, the method was verified and found to be a good predictor of the total number of nematodes. Moreover, the probability of cod being in a critical condition increased with the parasite load. In addition to their direct applicability to Baltic cod, the present findings may inspire others working with disease in fish stocks to include parasite monitoring.

Description: Range expansions can lead to increased contact of divergent populations, thus increasing the potential of hybridization events. Whether viable hybrids are produced will most likely depend on the level of genomic divergence and associated genomic incompatibilities between the different entities as well as environmental conditions. By taking advantage of historical Baltic cod (Gadus morhua) otolith samples combined with genotyping and whole genome sequencing, we here investigate the genetic impact of the increased spawning stock biomass of the eastern Baltic cod stock in the mid 1980s. The eastern Baltic cod is genetically highly differentiated from the adjacent western Baltic cod and locally adapted to the brackish environmental conditions in the deeper Eastern basins of the Baltic Sea unsuitable for its marine counterparts. Our genotyping results show an increased proportion of eastern Baltic cod in western Baltic areas (Mecklenburg Bay and Arkona Basin)—indicative of a range expansion westwards—during the peak population abundance in the 1980s. Additionally, we detect high frequencies of potential hybrids (including F1, F2 and backcrosses), verified by whole genome sequencing data for a subset of individuals. Analysis of mitochondrial genomes further indicates directional gene flow from eastern Baltic cod males to western Baltic cod females. Our findings unravel that increased overlap in distribution can promote hybridization between highly divergent populations and that the hybrids can be viable and survive under specific and favourable environmental conditions. However, the observed hybridization had seemingly no long-lasting impact on the continuous separation and genetic differentiation between the unique Baltic cod stocks.

Description: Understanding individual growth in commercially exploited fish populations is key to successful stock assessment and informed ecosystem-based fisheries management. Traditionally, growth rates in marine fish are estimated using otolith age-readings in combination with age-length relationships from field samples, or tag-recapture field experiments. However, for some species, otolith-based approaches have been proven unreliable and tag-recapture experiments suffer from high working effort and costs as well as low recapture rates. An important alternative approach for estimating fish growth is represented by bioenergetic modelling which in addition to pure growth estimation can provide valuable insights into the processes leading to temporal growth changes resulting from environmental and related behavioural changes. We here developed an individual-based bioenergetic model for Western Baltic cod (Gadus morhua), traditionally a commercially important fish species that however collapsed recently and likely suffers from climate change effects. Western Baltic cod is an ideal case study for bioenergetic modelling because of recently gained in-situ process knowledge on spatial distribution and feeding behaviour based on highly resolved data on stomachs and fish distribution. Additionally, physiological processes such as gastric evacuation, consumption, net-conversion efficiency and metabolic rates have been well studied for cod in laboratory experiments. Our model reliably reproduced seasonal growth patterns observed in the field. Importantly, our bioenergetic modelling approach implementing depth-use patterns and food intake allowed us to explain the potentially detrimental effect summer heat periods have on the growth of Western Baltic cod that likely will increasingly occur in the future. Hence, our model simulations highlighted a potential mechanism on how warming due to climate change affects the growth of a key species that may apply for similar environments elsewhere.