Recruitment of Baltic cod and sprat stocks: identification of critical life stages and incorporation of enviromental variability into stock-recruitment relationships

The recruitment processes of Baltic cod and sprat were analysed and critical periods were identified by addressing the major impact factors on individual early life history stages separately and relating observed abundance data between successive stages. For cod, recruitment appeared to be dependent on egg survival, with low oxygen concentration in dwelling depths and predation by clupeids as the major causes for egg mortality. Surviving egg production and larval abundance were weakly correlated, whereas larval abundance was significantly related to year class strength. This indicated that the period between the late egg and the early larval stage is critical for cod recruitment. A potential variable identified to affect this life stage was prey availability for larvae. For sprat, early and late egg stage production as well as late egg stage production and larval abundance were significantly related. However, year class strength was largely independent of larval abundance. Thus, the period between the late larval and early juvenile stage appeared to be critical for sprat recruitment. Potential variables identified to affect this life stage were ambient temperature and wind stress. Environmental factors showing statistically significant covariance with the survival of one of these critical life stages were incorporated into stock-recruitment models for individual spawning areas separately and for the Central Baltic combined

The vertical distribution of ichthyoplankton in relation to the hydrographic conditions in the Eastern Baltic

The vertical distribution of eggs and larvae of cod (Gadus marhua cal/arias), sprat (Sprattus sprattus balticus) and four-bearded rockling (Onas cimbrius) has been investigated in the Gdansk Deep and the Gotland Basin in May - June and July - August 1996 and 1997. Sprat eggs occurred in the wide range of depth, but cod eggs were restricted to a tiny water layer. Mean depth of cod eggs was 95 - 87 m in the Gdansk Deep and 112 -95 m in the Gotland Basin. More cod eggs than expected were found at low salinity in the period of most intensive spawning: up to 5% at 9 psu and 22 % at 10 psu in July 1996 and 8% at 10 psu in July 1997. Mean depths of sprat eggs diminished from 81 m in spring to 58 m in summer in the Gdansk Deep and from 94 m to 65 m in the Gotland Basin. At the same time sizes of sprat eggs also decreased Most of the eggs were found at the depth of the pycnocline. No significant differences in mean depths of eggs of sprat at different stages of development were discovered. Reduction in mean diameters of sprat eggs with depth and density was observed. In contrast, sizes of cod eggs showed no clear tendencies to change either from spring to summer or with changes in water density

Egg mortality: predation and hydrography in the central Baltic

Cod and sprat are the dominant fish species in the Baltic pelagic ecosystem, both of great economic importance and ecologically strongly interlinked. Management of both species is challenged by highly variable recruitment success. Recent studies have identified predation and hydrographic conditions during the egg phase to be of critical importance. Two years of extensive field investigations in the Bornholm Basin, central Baltic Sea, were undertaken. In 2002, a typical stagnation situation characterized by low salinity and poor oxygen conditions was investigated, and in early 2003, a major inflow of North Sea water completely changed the hydrographic conditions by increasing salinity and oxygen content, thereby altering ecological conditions. The goal was to quantify egg mortality caused by predation and hydrography, and to compare these estimates with independent estimates based on cohort analysis. Results indicated high intra-annual variability in egg mortality. Cod and sprat egg mortality responded differently to the major Baltic inflow: mortality related to hydrographic conditions increased for sprat and decreased for cod. On the other hand, predation mortality during peak spawning decreased for sprat and increased for cod

Baltic cod recruitment – the impact of climate variability on key processes

Large-scale climatic conditions prevailing over the central Baltic Sea resulted in declining salinity and oxygen concentrations in spawning areas of the eastern Baltic cod stock. These changes in hydrography reduced the reproductive success and, combined with high fishing pressure, caused a decline of the stock to the lowest level on record in the early 1990s. The present study aims at disentangling the interactions between reproductive effort and hydrographic forcing leading to variable recruitment. Based on identified key processes, stock dynamics is explained using updated environmental and life stage-specific abundance and production time-series. Declining salinities and oxygen concentrations caused high egg mortalities and indirectly increased egg predation by clupeid fish. Low recruitment, despite enhanced hydrographic conditions for egg survival in the mid-1990s, was due to food limitation for larvae, caused by the decline in the abundance of the copepod Pseudocalanus sp. The case of the eastern Baltic cod stock exemplifies the multitude effects climatic variability may have on a fish stock and underscores the importance of knowledge of these processes for understanding stock dynamics

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

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

Spawning of bluefin tuna in the black sea: historical evidence, environmental constraints and population plasticity

<div><p>The lucrative and highly migratory Atlantic bluefin tuna, <em>Thunnus thynnus</em> (Linnaeus 1758<em>;</em> Scombridae), used to be distributed widely throughout the north Atlantic Ocean, Mediterranean Sea and Black Sea. Its migrations have supported sustainable fisheries and impacted local cultures since antiquity, but its biogeographic range has contracted since the 1950s. Most recently, the species disappeared from the Black Sea in the late 1980s and has not yet recovered. Reasons for the Black Sea disappearance, and the species-wide range contraction, are unclear. However bluefin tuna formerly foraged and possibly spawned in the Black Sea. Loss of a locally-reproducing population would represent a decline in population richness, and an increase in species vulnerability to perturbations such as exploitation and environmental change. Here we identify the main genetic and phenotypic adaptations that the population must have (had) in order to reproduce successfully in the specific hydrographic (estuarine) conditions of the Black Sea. By comparing hydrographic conditions in spawning areas of the three species of bluefin tunas, and applying a mechanistic model of egg buoyancy and sinking rate, we show that reproduction in the Black Sea must have required specific adaptations of egg buoyancy, fertilisation and development for reproductive success. Such adaptations by local populations of marine fish species spawning in estuarine areas are common as is evident from a meta-analysis of egg buoyancy data from 16 species of fish. We conclude that these adaptations would have been necessary for successful local reproduction by bluefin tuna in the Black Sea, and that a locally-adapted reproducing population may have disappeared. Recovery of bluefin tuna in the Black Sea, either for spawning or foraging, will occur fastest if any remaining locally adapted individuals are allowed to survive, and by conservation and recovery of depleted Mediterranean populations which could through time re-establish local Black Sea spawning and foraging.</p> </div

The Time Course of the Influence of Valence and Arousal on the Implicit Processing of Affective Pictures

In the current study, we investigated the time course of the implicit processing of affective pictures with an orthogonal design of valence (negative vs. positive) by arousal (low vs. high). Previous studies with explicit tasks suggested that valence mainly modulates early event-related potential (ERP) components, whereas arousal mainly modulates late components. However, in this study with an implicit task, we observed significant interactions between valence and arousal at both early and late stages over both parietal and frontal sites, which were reflected by three different ERP components: P2a (100–200 ms), N2 (200–300 ms), and P3 (300–400 ms). Furthermore, there was also a significant main effect of arousal on P2b (200–300 ms) over parieto-occipital sites. Our results suggest that valence and arousal effects on implicit affective processing are more complicated than previous ERP studies with explicit tasks have revealed

Interannual and seasonal changes of the mortality rates of sprat eggs in the Gotland Basin (Eastern Baltic Sea).

No abstracts are to be cited without prior reference to the author.Instantaneous mortality coefficients (per day), mortality rates and total mortality coefficients of eggs of sprat (Sprattus sprattus balticus Schneider) have been calculated for the number of years from 1973 to 2004 using the databases of Latvian Fish Resources Agency (LATFRA) on ichthyoplankton and hydrology. The mortality rates increased at higher water temperature in the mean depth of eggs in June. Higher mortality rates were observed between 1st and 2nd stages of development rather than between 2nd and 3rd ones. Highest mortality rates and total mortality coefficients were registered in June. No big differences could be found in the value of mortality rate between the southern and central parts of the Gotland Basin, but total mortality coefficient during the incubation period of eggs was greater in the central part of the basin. Mean weighted values of total mortality coefficients were 93% in the central part of the basin and 88% in the southern part of it. Extremely high mortality of eggs has been observed in the samples collected in the water surface layer. No influences of the age structure of spawning stock and of the mean weight at age on the mortality rates of sprat eggs were discovered. Success of spawning could be linked to the total survival coefficient in the southern part of the basin in July – August

Interannual changes in the time of spawning of sprat in the Gotland Basin (Eastern Baltic Sea) in relation to the hydrographical conditions and the structure of spawning stock

No abstracts are to be cited without prior reference to the author.Time of peak spawning of sprat (Sprattus sprattus balticus Schneider) in the central and southern parts of the Gotland Basin (Eastern Baltic Sea) has been calculated for the period of years from 1973 to 2004 using the database of Latvian Fish Resources Agency (LATFRA) on ichthyoplankton. Sprat has very extended spawning season, but maximum abundance of sprat eggs in this region was observed mainly in the middle of June. Time of spawning altered from year to year, but only unclear trends could be traced. Time of peak spawning shifted slightly to later time in the 1980-s, but returned to the earlier period in the 1990-s. Some significant correlations were found between the time of peak spawning and the temperature of water in the layer where sprat spent previous December and January, as well as temperature of water in the mean depth of eggs. Colder temperatures led to spawning in a later time. Only a little influence of the age structure of the spawning stock of sprat on the time of spawning was found