Eastern Baltic Cod Research Articles

Implications of stock recovery for a neighbouring management unit: experience from the Baltic cod

Cod in the Baltic Sea is assessed and managed as two separate stocks, i.e. eastern and western Baltic cod. The eastern Baltic cod has recently started to recover after several decades of severe depletion. In the present study, we suggest that the recovery of the eastern Baltic cod population has also substantially increased cod abundance in a specific area of the adjacent western Baltic management unit. This is investigated through long time-series of spatially resolved stock assessment data supplemented by genetic analyses of origin of the cod currently found in the transition area between the two populations. Due to immigrating cod from the east, there are currently large spatial differences in cod abundance and mean weight in the western Baltic management unit that raise new management concerns. First, the high abundance of cod of eastern origin found in the western Baltic management unit can mask the relatively poor state of the western Baltic cod population. Second, the current fishing mortality estimates for the entire western Baltic management unit, used as basis for fisheries management, are difficult to interpret as these are highly influenced by mixing of biological populations and the spatial distribution of fisheries.

Stock size matters more than vessel size: The fuel efficiency of Swedish demersal trawl fisheries 2002–2010

Fisheries management determines how much of each stock can be landed when, where and how fishing is permitted. It has been identified to strongly influence the environmental performance of the fishing industry, including fuel use. As fuel data for fisheries is scarce, especially on a detailed level, the aim of this study was to develop an approach for utilizing fleet-wide fuel data to estimate the fuel use of individual fisheries and mapping how fuel efficiency in Swedish fisheries is influenced by management. Swedish demersal trawl fisheries were studied between 2002 and 2010. Results show that the overall fuel efficiency has improved and interesting patterns between different fisheries and vessel sizes emerged. The difference in fuel efficiency per kilo landing between large and small trawlers was generally small, unless catch capacity was lowered e.g. by selective grids. Stock rebuilding was shown to be highly important for fuel efficiency, as fuel use was inversely correlated to the biomass of eastern Baltic cod. However, rebuilding can also lead to trade-offs e.g. in the case of selective trawling, where protection of depleted stocks comes at the cost of higher fuel intensity per landing. Finally, tax exemption of fuel use in fisheries was shown to maintain inefficient fisheries. These results could be used to reduce overall environmental impacts of fishing further by incorporating fuel use as an additional aspect into the fisheries management system.

Baltic cod (Gadus morhua callarias) recovery potential under different environment and fishery scenarios

The Atlantic cod (Gadus morhua) is one of the most significant fishery targets in northern seas. The cod occurs also in the brackish Baltic Sea, where besides fishing, several abiotic and biotic factors have a strong effect on their population dynamics. All cod stocks have diminished simultaneously due to heavy exploitation. Maintaining a sustainable fishery requires sufficient understanding of the recovery process and adequately tailored management plans. In marine habitats, temporal environmental fluctuations are often positively autocorrelated. Yet the temporal autocorrelation is often disregarded in models estimating fishery stock dynamics. For the Eastern Baltic cod (Gadus morhua callarias), the main environmental factors are water salinity and oxygen concentrations, which determine species distribution and reproductive success. The Baltic Sea's oceanographic characteristics make the ecosystem sensitive and climate change is expected to convert the type and magnitude of environmental fluctuations. This could potentially lead to inaccurate stock estimation and requires reassessment for any management strategy. We analyzed the recovery potential of the Eastern Baltic cod (Gadus morhua callarias) under different fishery and environmental scenarios. Our results illustrate how the unpredictability of stock size, catch, and recovery increase with increasing environmental autocorrelation. Under current environmental conditions and threshold methods fishing is at sustainable level. However, under more radical environmental fluctuations, like global warming, the estimation of cod population recovery becomes more difficult. Moreover, the precautionary fishing methods are important for population recovery by ensuring more sustainable fishing and preventing stock collapses.

Biological ensemble modeling to evaluate potential futures of living marine resources

Natural resource management requires approaches to understand and handle sources of uncertainty in future responses of complex systems to human activities. Here we present one such approach, the "biological ensemble modeling approach," using the Eastern Baltic cod (Gadus morhua callarias) as an example. The core of the approach is to expose an ensemble of models with different ecological assumptions to climate forcing, using multiple realizations of each climate scenario. We simulated the long-term response of cod to future fishing and climate change in seven ecological models ranging from single-species to food web models. These models were analyzed using the "biological ensemble modeling approach" by which we (1) identified a key ecological mechanism explaining the differences in simulated cod responses between models, (2) disentangled the uncertainty caused by differences in ecological model assumptions from the statistical uncertainty of future climate, and (3) identified results common for the whole model ensemble. Species interactions greatly influenced the simulated response of cod to fishing and climate, as well as the degree to which the statistical uncertainty of climate trajectories carried through to uncertainty of cod responses. Models ignoring the feedback from prey on cod showed large interannual fluctuations in cod dynamics and were more sensitive to the underlying uncertainty of climate forcing than models accounting for such stabilizing predator-prey feedbacks. Yet in all models, intense fishing prevented recovery, and climate change further decreased the cod population. Our study demonstrates how the biological ensemble modeling approach makes it possible to evaluate the relative importance of different sources of uncertainty in future species responses, as well as to seek scientific conclusions and sustainable management solutions robust to uncertainty of food web processes in the face of climate change.

The abundance and patterns of the egg and larval spatial distribution of Eastern Baltic cod Gadus morhua callarias (Gadidae) in 1931–1996

According to ichthyoplankton surveys of AtlantNIRO and published sources, the data on abundance of eggs and larvae of eastern Baltic cod Gadus morhua callarias at different spawning grounds of the Baltic Sea in 1931–1938, 1949–1956, and 1991–1996 are considered. We show that a similar type of spatial distribution of eggs and larvae of cod with localization of the greatest amount of its early ontogenetic stages in the Bornholm Basin in the southwest of the sea was observed in the 1930s and 1990s. The extension of the area of distribution was accompanied by an increase in the numbers of eggs and larvae of cod in all areas of the sea, and its decline was followed by a decrease in numbers, in the first turn, at northeastern spawning grounds in the Gotland Basin. The period of high numbers of eggs in 1949–1956 coincided with the century maximum of salinity of the near-bottom water layer.

Sexual maturation of cod (Gadus morhua L.) in the southern Baltic (1990-2006)

Summary Baltic cod, like other species, is susceptible to inter-annual fluctuations in sexual maturation, depending on the length, age, sex, extent of the habitat area, and stock abundance of the cod population. Maturity is one of the biological indicators used to detect changes in a stock that can be caused by fishing. To address these issues specifically for the eastern Baltic cod stock, long-term data (1990–2006) from Polish research vessels in the southern Baltic were examined. To date, the ICES has used the same maturity ogives over extended periods and assumed invariant sex ratios for the assessment of eastern Baltic cod. The combined maturity ogives calculated in the present study were markedly lower, particularly for age groups 2–4 (5), in all periods, than those used in the ICES assessment. Moreover, the proportion of females increased with length and age, suggesting that annual verification of the sex ratio is needed. The present study also revealed that the total length (L50%) and the age (mean age-at-maturity; MAM50%) at which 50% achieved first sexual maturity were higher for females than for males in the study period. The long-term mean L50% and MAM50% for females were 43.9 cm and 4.3 years, respectively, and for males 34.8 cm and 3.4 years. There was also a spatial difference between calculated maturity ogives, with slightly lower L50% (range: 1.4–8.6 cm) in the Gdansk Basin than in the Bornholm Basin. The increasing trend in fishing mortality observed in 1993–2004 (ICES data) did not translate into a temporal trend in calculated maturity ogives. However, changes in L50% and MAM50% reflected recruitment variations (ICES data). The significance of these findings is discussed in the context of the environment and recruits abundance.

BIOECONOMIC MODEL OF EASTERN BALTIC COD UNDER THE INFLUENCE OF NUTRIENT ENRICHMENT

Abstract The objective of this paper is to study the economic management of Eastern Baltic cod (Gadus morhua) under the influence of nutrient enrichment. Average nitrogen concentration in the spawning areas during the spawning season of cod stock is chosen to be an indicator of nutrient enrichment. The optimal cod stock is defined using a dynamic bioeconomic model for the cod fisheries. The results show that the current stock level is about half of the estimated optimal stock level and that the current total allowable catch (TAC) is about one‐fourth of the optimal equilibrium yield. The results also indicate that the benefit from a reduction in nitrogen very much depends on the harvest policies. If the TAC is set equal to the optimal equilibrium yield, the benefit of a nitrogen reduction from the 2009 level to the optimal nitrogen level would be about 604 million DKK over a 10‐year time horizon, given a discount rate of 4% per year. However, if a recovery management plan is chosen, the benefit would only be about 49 million DKK over a 10‐year time horizon.

Spatial management of marine resources can enhance the recovery of predators and avoid local depletion of forage fish

AbstractThe eastern Baltic cod stock has recently started to recover, after two decades of severe depletion, however with unexpected side effects. The stock has not re‐occupied its former wide distribution range, but remains concentrated in a limited area in the southern Baltic Sea. The biomass of forage fish, i.e., sprat and herring, is historic low in this area, which in combination with increasing cod stock results in locally high predation mortality of forage fish and cannibalism of cod. In line with low prey availability, body weight and nutritional condition of cod drastically declined. In the southern Baltic Sea, cod competes with pelagic fisheries for the limited resources of sprat and herring, while the largest biomass of these species is currently found outside the distribution range of cod. Accounting for spatial overlap between species is crucial in developing ecosystem based fisheries management to enhance the recovery of predator stocks.

Recovery of the Eastern Baltic Cod Fishery: Perspectives Revealed through Bioeconomic Modelling

The paper describes the situation of Polish cod fishery in the Baltic Sea and its perspectives in the future. The paper sets up a general model and exemplifies it by simple functional forms taking its offset in the problem of overcapacity in Polish fleet causing lack of profitability of the sector. The viable control approach is used in order to determine dynamic compatibility with defined constraints when making an intertemporal decision regarding the regulations affecting the stock of interest and the economy of the sector. The paper illustrates how the dynamic optimisation methodology can be used to determine the Optimal Economic Intertemporal Path and reveals potential benefits of following it.

Size matters: Short term loss and long term gain in a size-selective fishery

The maximum sustainable yield concept (MSY) and the ecosystem approach to fisheries management (EAFM) have been recently adopted by the European Commission with the objective to achieve, over the long term, the highest possible sustainable yield from a given exploited stock. In this context, a fishing mortality MSY reference level (i.e., FMSY) should be defined, taking into account recruitment, growth and natural mortality under current or recent ecosystem conditions. Thus, FMSY is used as a generic term for a robust estimate of a fishing mortality level that is associated with high sustainable yield in the long term, assuming the current harvesting regime in terms of size selectivity. In this study, using the Eastern Baltic cod as an example, we challenge this rather simplified view showing that by using a different harvest selectivity and thus changing the size range of harvested cod, it is possible to largely increase the yield and revenue from the fishery compared to the fishing mortality stipulated in the management plan (i.e., FMSY), while assuring sustainable high yield in the long term. Thus, implementing the MSY concept in terms of fishing mortality but neglecting selective harvesting effects will not achieve high long term sustainable yield for Eastern Baltic cod. The combination of size selective harvesting and economic reasoning may offer an important tool for the management of marine resources by potentially providing a common currency for the different stakeholders and offer guidance to achieve long term sustainability and human well-being. This would represent the natural step forward in the implementation of EAFM and MSY concepts.

Egg and early larval stages of Baltic cod, Gadus morhua, are robust to high levels of ocean acidification

The accumulation of carbon dioxide in the atmosphere will lower the pH in ocean waters, a process termed ocean acidification (OA). Despite its potentially detrimental effects on calcifying organisms, experimental studies on the possible impacts on fish remain scarce. While adults will most likely remain relatively unaffected by changes in seawater pH, early life-history stages are potentially more sensitive, due to the lack of gills with specialized ion-regulatory mechanisms. We tested the effects of OA on growth and development of embryos and larvae of eastern Baltic cod, the commercially most important fish stock in the Baltic Sea. Cod were reared from newly fertilized eggs to early non-feeding larvae in 5 different experiments looking at a range of response variables to OA, as well as the combined effect of CO2 and temperature. No effect on hatching, survival, development, and otolith size was found at any stage in the development of Baltic cod. Field data show that in the Bornholm Basin, the main spawning site of eastern Baltic cod, in situ levels of pCO2 are already at levels of 1,100 μatm with a pH of 7.2, mainly due to high eutrophication supporting microbial activity and permanent stratification with little water exchange. Our data show that the eggs and early larval stages of Baltic cod seem to be robust to even high levels of OA (3,200 μatm), indicating an adaptational response to CO2.

Stable Isotope Evidence for Late Medieval (14th–15th C) Origins of the Eastern Baltic Cod (Gadus morhua) Fishery

Although recent historical ecology studies have extended quantitative knowledge of eastern Baltic cod (Gadus morhua) exploitation back as far as the 16th century, the historical origin of the modern fishery remains obscure. Widespread archaeological evidence for cod consumption around the eastern Baltic littoral emerges around the 13th century, three centuries before systematic documentation, but it is not clear whether this represents (1) development of a substantial eastern Baltic cod fishery, or (2) large-scale importation of preserved cod from elsewhere. To distinguish between these hypotheses we use stable carbon and nitrogen isotope analysis to determine likely catch regions of 74 cod vertebrae and cleithra from 19 Baltic archaeological sites dated from the 8th to the 16th centuries. δ13C and δ15N signatures for six possible catch regions were established using a larger sample of archaeological cod cranial bones (n = 249). The data strongly support the second hypothesis, revealing widespread importation of cod during the 13th to 14th centuries, most of it probably from Arctic Norway. By the 15th century, however, eastern Baltic cod dominate within our sample, indicating the development of a substantial late medieval fishery. Potential human impact on cod stocks in the eastern Baltic must thus be taken into account for at least the last 600 years.

Why is the Eastern Baltic cod recovering?

The Eastern Baltic cod stock was until recently below safe biological limits and suffered from high fishing pressure. In most recent years, fishing mortality substantially declined and spawner biomass more than tripled. Similar developments have not been observed for any other depleted cod stock in the North Atlantic during the last few decades. This paper investigates relative impacts of changes in different ecological and management-related drivers, which could have contributed to the rapid recovery of the Eastern Baltic cod. The results show that the success to reduce fishing mortality below management target in 2008 was due to a combination of increased recruitment and improved compliance with TAC. The reversal of the negative trend in biomass and rebuilding of the stock to the present level were largely driven by increased recruitment. Harvest control rules of the multi-annual management plan for setting TACs currently maintain the fishing mortality at a low level, which allows the stock to accumulate biomass and further accelerate its recovery. Relatively strong incoming year-classes and recently better control over removals distinguish the Eastern Baltic cod from other depleted European cod stocks, which have not shown similar positive trends in recent years. Sound management measures and compliance to those as well as favourable biological conditions are required for a successful stock recovery. ► Recovery of Eastern Baltic cod is outstanding in comparison with other cod stocks. ► Elimination of illegal fishery is a prerequisite for reaching management targets. ► Rapid stock recovery requires high reproductive success.

Reproductive potential of the eastern baltic cod Gadus morhua callarias L. population

Trends in interannual variation in maturation and spawning terms of various age cohorts in the Eastern Baltic cod population in 1997-2009 were studied. Specific features in the age structure of the mature population part that were established by the end of the first decade of the 21st century were clarified. The role of cod age cohorts in the current population reproduction was considered taking into account the data on cod recruitment and fecundity.

Assessment of production of eggs of eastern Baltic cod (Gadus morhua callarias L.) on the basis of long-term ichthyoplankton data

Proceeding from long-term data on the numbers of eggs of cod in ichthyoplankton, the total annual production of cod eggs at four main spawning grounds of the Baltic Sea was calculated. It was shown that the long-term fluctuations of cod egg production were positively related to the dynamics of the volume of waters coming to the Baltic Sea in years of the North Sea advections. It is suggested that this dependence was determined by a set of adaptations providing the extension of cod reproduction upon the improvement of the environment.

The beauty of simplicity in science: Baltic cod stock improves rapidly in a ‘cod hostile’ ecosystem state

The Baltic Sea ecosystem is hypothesized to have undergone a regime shift during the last 3 decades, altering its functioning and the composition of its zooplankton and fish communities. The new stable state has been considered as 'cod hostile' due to reduced spawning success in cod, as well as increased predation on and declining food sources for cod larvae. Nonetheless, the eastern Baltic cod stock has recently recovered after more than 2 decades of low biomass and productivity. The recovery was mainly driven by a sudden reduction in fishing mortality and occurred in the absence of any exceptionally large year classes. The recovery of the cod stock during a 'cod-hostile' ecological regime indicates that fisheries are the main regulator of cod population dynamics in the Baltic Sea.

Robustness of egg production methods as a fishery independent alternative to assess the Eastern Baltic cod stock (Gadus morhua callarias L.)

At present, several cod stocks are outside safe biological limits and are managed under recovery plans. For these stocks Total Allowable Catches (TAC's) are generally low and quotas are accompanied by a broad variety of technical measures influencing the fishing patterns. Consequently, the input data to stock assessment models relying on catch statistics from the commercial fisheries is potentially biased and the perception of stock status may be incorrect. Egg production methods (EPM) provide a fishery independent alternative. Additionally, they provide better estimates of stock reproductive potential (SRP). Eastern Baltic cod (Gadus morhua callarias L.) has severely declined throughout the 2nd half of the 1980s and 1st half of the 1990s due to climate-driven adverse hydrographic conditions and high fishing intensity. Since 2007 the stock is managed under a long-term management plan and showed signs of recovery in most recent years. Since 1986, egg surveys have been carried out regularly in the Bornholm Basin, the most important spawning area of Eastern Baltic cod since mid-1980s. In the present paper the robustness of EPM towards simplification of spawning parameters and towards reduction of the number of egg surveys is tested applying three different methods requiring different numbers of egg surveys. We applied the annual egg production method (AEPM) requiring full egg survey coverage of the spawning season to estimate cod abundances in the Bornholm Basin. In addition, the daily fecundity reduction method (DFRM) and the daily egg production method (DEPM) were tested, the latter two methods requiring only single egg surveys, but require more complex reproduction input parameters. All three methods provided a comparable result, which was also expected as many spawning parameters were derived from the same underlying data sets. In a sensitivity analysis several input parameters were varied simultaneously up to 20% in both directions. EPM were especially sensitive towards changes in proportions of mature females at age, whereas changes in the various fecundity parameters and spawning fraction were less influential.EPM results followed the large scale spawning stock trends of the Baltic International Trawl Survey index, whereas the year to year variations of the index were not captured to well. EPM yielded spawning stock sizes in the same order of magnitude as provided by a spatially down-scaled multi-species stock assessment model.

Climate-driven long-term trends in Baltic Sea oxygen concentrations and the potential consequences for eastern Baltic cod (Gadus morhua)

AbstractHinrichsen, H-H., Huwer, B., Makarchouk, A., Petereit, C., Schaber, M., and Voss, R. 2011. Climate-driven long-term trends in Baltic Sea oxygen concentrations and the potential consequences for eastern Baltic cod (Gadus morhua). – ICES Journal of Marine Science, 68: 2019–2028. Variations in oxygen conditions in the Baltic are influenced by several mechanisms. Generally, the frequency and magnitude of major inflows have been identified as the most crucial process for the renewal of oxygen-depleted water masses in the Baltic Sea. Furthermore, enhanced degradation of suspended organic matter by bacteria over the past few decades has increased oxygen consumption. Finally, the effects of large-scale climate warming are causing long-term variations in oxygen content and saturation as an observed increase in temperature has led to a general decrease in oxygen solubility of water masses. Oxygen-dependent relationships based on field data and laboratory experiments were used to analyse the impact of the observed decrease in oxygen content on eastern Baltic cod (Gadus morhua) stock-specific processes (e.g. survival rates of eggs, settlement probability of juveniles, habitat utilization of spawning fish, age structure of successful spawners, food consumption rates of adult fish). The observed long-term decline in oxygen conditions in the Baltic Sea has had a seemingly generally negative impact on oxygen-related processes for the different life stages of eastern Baltic cod. Experimentally derived results of oxygen-driven processes were validated by field data.

Comparison of stock management with production, difference, and age-structured models using operating models

Management strategy evaluation (MSE) was used to test the assessment and management performance of three assessment methods in combination with harvest control rules. The assessment procedures considered were: the eXtended Survivors Analysis (XSA), the Schaefer production model, and the difference model. Four HCRs were considered: first, fishing mortality was set on the basis of the relationship between the current biomass and a reference biomass; second, fishing mortality was gradually reduced (or increased) until it reached a required target; and the third and the fourth HCRs were similar to the first and second but with imposed TAC constraints. The stock that was generated in the operating model (OM) resembled the eastern Baltic cod stock. For the XSA assessment, two options were used: XSA with default shrinkage of terminal fishing mortality to the average of the estimates, and XSA with low shrinkage. The simulations showed that for stock assessment, the XSA models performed much better than the difference and Schaefer models. However, for the data tested, the difference and Schaefer models performed somewhat better in terms of management performance than the XSA models, especially the XSA model with default shrinkage.

Biomanipulation: a tool in marine ecosystem management and restoration?

Widespread losses of production and conservation values make large-scale ecosystem restoration increasingly urgent. Ecological restoration by means of biomanipulation, i.e., by fishing out planktivores to reduce the predation pressure on herbivorous zooplankton, has proved to be an effective tool in restoring degraded lakes and coastal ecosystems. Whether biomanipulation may prove a useful restoration method in open and structurally complex marine ecosystems is, however, still unknown. To promote a recovery of the collapsed stock of Eastern Baltic cod (Gadus morhua), large-scale biomanipulation of sprat (Sprattus sprattus), the main planktivore in the Baltic Sea, has been suggested as a possible management approach. We study the effect of biomanipulation on sprat using a statistical food-web model, which integrates internal interactions between the main fish species of the Central Baltic Sea, with external forcing through commercial fishing, zooplankton, and climate. By running multiple, stochastic simulations of reductions in sprat spawning stock biomass (SSB) only minor increases in cod SSB were detected, none of which brought the cod significantly above ecologically safe levels. On the contrary, reductions in cod fishing mortality and/or improved climatic conditions would promote a significant recovery of the stock. By this we demonstrate that an ecosystem-scale biomanipulation, with the main focus of reinstating the dominance of cod in the Baltic Sea may likely be ecologically ineffective, operationally difficult, and costly. We argue that reducing exploitation pressure on Eastern Baltic cod to ecologically sound levels is a far more appealing management strategy in promoting a long-term recovery and a sustainable fishery of the stock.