143 research outputs found
Cruise Report R.V. Alkor AL 390 : 12.04. – 26.04.2012
Port calls: Rønne, Denmark, 19.04.2012 – 21.04.201
Cruise Report R.V. Alkor AL 415
11.05. – 24.05.2013
Port calls: Visby, Sweden 18.05.2013 – 19.05.201
Cruise Report: 12.05. – 25.05.2011 R.V. Alkor Cruise No.: AL 373
Port calls: Stettin, Poland, 15.05.2011 – 16.05.201
Cruise Report R.V. Alkor AL 392 [AL392]
13.05. – 29.05.2012
Port calls: Ystad, Sweden 20.05.2012 – 21.05.201
Small scale distribution patterns and vertical migration of North Sea erring larvae (Clupea harengus, Teleostei: Clupeidae) in relation to abiotic and biotic factors
The distribution of herring larvae in relation to environmental conditions and the occurrence of possible prey and predator organisms was studied during a 4-day period on a permanent station in the northern North Sea in September 1999. The vertical distribution of herring larvae was sampled in 20-m intervals by means of a multiple-closing net. To resolve the small-scale patchiness of herring larvae and planktonic prey and predator organisms, a towed in-situ video system was used, the Ichthyoplankton Recorder. A diel vertical migration of herring larvae was observed with different intensities depending on their body length. Small larvae (16 mm) showed the highest abundances in the upper water layers during the day and were concentrated in deeper water layers during the night. The presented results appear to be relevant for individual-based modelling of the fate of larval herring populations.
La distribución a pequeña escala y la migración vertical de larvas de arenque del Mar del Norte (Clupea harengus, Teleostei: Clupeidea) en relación con factores abióticos y bióticos. – Se han estudiado la distribución de las larvas de arenque en relación a las condiciones ambientales y la incidencia de posibles presas y organismos depredadores a lo largo de un periodo de 4 dÃas en una estación permanente situada en el Mar del Norte en septiembre de 1999. La distribución vertical de las larvas de arenque fue muestreada en intervalos de 20 m mediante una red de apertura múltiple (multiple opening/closing net). Para determinar a pequeña escala la heterogeneidad en la distribución de las larvas de arenque, asà como de las presas y depredadores planctónicos, se utilizó un sistema de vÃdeo in-situ (The Ichthyoplankton Recorder). Se observó una migración vertical nictimeral con diferentes intensidades dependiendo de la longitud corporal de las larvas. Las larvas pequeñas (16 mm) mostraban su mayor abundancia en las capas superiores de la columna de agua durante el dÃa y se concentraban en capas de agua más profundas durante la noche. Los resultados presentados en este trabajo pueden ser relevantes para la modelización (individual based modeling) del destino de las poblaciones de larvas del arenque
Abundance, horizontal and vertical distribution of the invasive ctenophore Mnemiopsis leidyi in the central Baltic Sea, November 2007
The distribution and abundance of the invasive ctenophore Mnemiopsis leidyi in the Bornholm Basin, an important spawning ground of several fish stocks, and in adjacent areas in the central Baltic Sea was studied in November 2007. The study showed that M. leidyi were relatively small (body length 18.6 ± 7.6 mm) and they were patchily distributed over a large part of the investigated area. Specimens were found on 68 and 59% of stations sampled with a Bongo net (n=39) and an Isaac-Kidd midwater trawl (n=51), respectively. Vertically, the highest densities of M. leidyi occurred at 40 to 60 m around the halocline.
Horizontally, the highest abundances were found north and west of Bornholm, but relatively high densities were also observed in the Slupsk Furrow. The mean abundance was 1.58 ± 2.12 ind. m-2, the peak abundance was 8.92 ind. m-2, and the average and peak population density were 0.03 ± 0.05 and 0.28 ind. m-3, respectively. The abundances are low compared to densities recently observed in other areas of the Baltic region (e. g. Limfjorden, Åland Sea) and the estimated predation impact on zooplankton by M. leidyi was negligible in November 2007. However, because of the ctenophore’s wide distribution in the central Baltic Sea, its ability for rapid population growth, and its potential influence on fish stocks by competing for food and by preying on fish eggs and newly hatched larvae, close monitoring of the future development of M. leidyi in the Baltic Sea is strongly recommended
Reproductive ecology of Baltic sprat and its application in stock assessment
Sprat is an important ecological key species in the Baltic Sea. Temporal and spatial variability in reproductive traits of Baltic sprat was investigated in relation to biotic and abiotic parameters. The seasonality of important spawning traits were analysed. Models of sex ratio and maturity at length were established. The ovarian dynamics in relation to postovulatory follicles (POF) were described with the major finding that the degeneration of POF equals the spawning interval. A temperature dependent model on Baltic sprat egg development was established. All obtained results were finally used to implement the Daily Egg Production Method (DEPM) to the Baltic sprat stock. The knowledge obtained by the present thesis will serve as basis to (i) enhance existing assessment methods and to test alternative indices for sprat stock reproductive potential, (ii) implement alternative assessment methods and (iii) further investigate the population dynamics and ecology of Baltic sprat
Temperature effects on vital rates of different life stages and implications for population growth of Baltic sprat
Baltic sprat (Sprattus sprattus balticus S.) is a key species in the pelagic ecosystem of the Baltic Sea. Most stocks of small pelagic species are characterized by natural, fishery-independent fluctuations, which make it difficult to predict stock development. Baltic sprat recruitment is highly variable, which can partly be related to climate-driven variability in hydrographic conditions. Results from experimental studies and field observations demonstrate that a number of important life history traits of sprat are affected by temperature, especially the survival and growth of early life stages. Projected climate-driven warming may impact important processes affecting various life stages of sprat, from survival and development during the egg and larval phases to the reproductive output of adults. This study presents a stage-based matrix model approach to simulate sprat population dynamics in relation to different climate change scenarios. Data obtained from experimental studies and field observations were used to estimate and incorporate stage-specific growth and survival rates into the model. Model-based estimates of population growth rate were affected most by changes in the transition probability of the feeding larval stage at all temperatures (+0, +2, +4, +6 °C). The maximum increase in population growth rate was expected when ambient temperature was elevated by 4 °C. Coupling our stage-based model and more complex, biophysical individual-based models may reveal the processes driving these expected climate-driven changes in Baltic Sea sprat population dynamics
Oxygen depletion in coastal seas and the effective spawning stock biomass of an exploited fish species
Environmental conditions may have previously underappreciated effects on the reproductive processes of commercially exploited fish populations, for example eastern Baltic cod, that are living at the physiological limits of their distribution. In the Baltic Sea, salinity affects neutral egg buoyancy, which is positively correlated with egg survival, as only water layers away from the oxygen consumption-dominated sea bottom contain sufficient oxygen. Egg buoyancy is positively correlated to female spawner age/size. From observations in the Baltic Sea, a field-based relationship between egg diameter and buoyancy (floating depth) could be established. Hence, based on the age structure of the spawning stock, we quantify the number of effective spawners, which are able to reproduce under ambient hydrographic conditions. For the time period 1993–2010, our results revealed large variations in the horizontal extent of spawning habitat (1000–20 000 km2) and oxygen-dependent egg survival (10–80%). The novel concept of an effective spawning stock biomass takes into account offspring that survive depending on the spawning stock age/size structure, if reproductive success is related to egg buoyancy and the extent of hypoxic areas. Effective spawning stock biomass reflected the role of environmental conditions for Baltic cod recruitment better than the spawning stock biomass alone, highlighting the importance of including environmental information in ecosystem-based management approaches
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