Co-aparición de las larvas de sardina europea (Sardina pilchardus), anchoa (Engraulis encrasicolus) y espadín (Sprattus sprattus) en hábitats del sur del Mar del Norte: abundancia, distribución y condición bioquímica

Spawning populations of European sardines (Sardina pilchardus) and anchovies (Engraulis encrasicolus) have become re-established in the southern North Sea after a ~30-year absence and now co-occur with sprat (Sprattus sprattus). Consequently, little is known concerning potential interactions among these three species in this region. Based upon parallel cruises conducted in June/July 2005, we compared the larval abundance, size-distributions and biochemical-based condition of these species among 1) nearshore (Wadden Sea) areas and offshore (German Bight) areas that were 2) vertically mixed, 3) frontal zones, or 4) stratified waters. In general terms, larval condition (RNA:DNA) was relatively high at all stations. Although fronts clearly acted to concentrate larvae, larval condition was not necessarily higher. For example 9% of sardines captured at the tidal mixing front were categorised as starving, while no starving larvae were sampled in the stratified water masses. Habitats of sardine and sprat larvae were similar, whereas anchovies were primarily restricted to nearshore areas. This is the first study examining the potential role of near- and offshore habitats as nursery areas and the extent to which resource (habitat) partitioning exists among the larvae of sprat and newly established anchovy and sardine in the North Sea.La puesta de la sardina (Sardina pilchardus) y de la anchoa (Engraulis encarsicolus) se han restablecido en el sur del mar del norte después de casi 30 años de ausencia, apareciendo ahora juntamente con el espadín (Spattus sprattus). consecuentemente, poco se conoce de las interacciones entre estas tres especies en esta región. se ha llevado a cabo dos campañas entre Junio y Julio 2005, comparando la abundancia larvaria, distribución de tallas y la relación arn:aDn de estas tres especies de clupeidos entre: 1) zonas costeras (Mar Wadden) y mar abierto (Bahía alemana), 2) mezcla vertical, 3) zonas frontales y 4) masas de aguas estratificadas. en términos generales, la condición larvaria (relación arn:aDn) fue relativamente alta en todas las estaciones. aunque las zonas frontales actúan claramente como concentradoras de larvas, la condición larvaria no fue necesariamente más alta allí. Por ejemplo, el 9% de las larvas de sardinas capturadas en la zona de mezcla de marea fue clasificada en estado de inanición, mientras que las larvas de sardinas en estado de no inanición fueron capturadas en zonas de masas de aguas estratificadas. el hábitat de las larvas de sardina y de las larvas de espadín fueron más similares entre sí, mientras que las larvas de anchoas fueron encontradas en zonas costeras. este estudio es el primero en investigar la importancia del hábitat de estas especies en zonas costeras y mar abierto como áreas de cría, así como determinar hasta que punto existe una partición en los recursos (hábitat) entre las larvas de espadín y las de las especies nuevamente establecidas, sardina y anchoa, en el sur del mar de norte

Respiration rates of the polyps of four jellyfish species: Potential thermal triggers and limits

AbstractThe bloom dynamics of metagenic jellyfish are regulated, to a large degree, by the asexual reproduction of benthic polyps. The ecophysiology of polyps is poorly studied compared to pelagic (ephyrae and medusae) life stages. We measured unfed (routine) respiration rates (RR) of the polyps of four scyphozoan species (Cyanea capillata, Aurelia aurita, Aurelia labiata and Aurelia limbata) acclimated to six temperatures between 7 and 20°C and one species (A. aurita) under hypoxic conditions. Strong increases (Q10~7 to 13) in RR occurred after subtle warming across specific test temperatures (e.g., 12 to 15°C for C. capillata, A. labiata, and A. aurita). In some species, RR at 20°C was lower than at 15 or 18°C suggesting that sub-optimally warm temperatures were approached. Polyps of A. aurita were unable to maintain RR below 11, 22 and 24% O2 saturation at 8.0, 15.5 and 19.0°C, respectively. Despite obvious differences in activity and habitat, rates of respiration in polyps, ephyrae and medusae of A. aurita at 15°C appear similar after taking into account differences in body size. A literature comparison of polyp respiration rates suggests a narrowing of thermal windows in individuals collected from higher latitudes. Common garden experiments are needed to thoroughly examine potential local adaptation

Comparing observed and modelled growth of larval herring (Clupea harengus): Testing individual-based model parameterisations

Experiments that directly test larval fish individual-based model (IBM) growth predictions are uncommon since it is difficult to simultaneously measure all relevant metabolic and behavioural attributes. We compared observed and modelled somatic growth of larval herring (Clupea harengus) in short-term (50 degree-day) laboratory trials conducted at 7 and 13°C in which larvae were either unfed or fed ad libitum on different prey sizes (~100 to 550 μm copepods, Acartia tonsa). The larval specific growth rate (SGR, % DW d-1) was generally overestimated by the model, especially for larvae foraging on large prey items. Model parameterisations were adjusted to explore the effect of 1) temporal variability in foraging of individuals, and 2) reduced assimilation efficiency due to rapid gut evacuation at high feeding rates. With these adjustments, the model described larval growth well across temperatures, prey sizes, and larval sizes. Although the experiments performed verified the growth model, variability in growth and foraging behaviour among larvae shows that it is necessary to measure both the physiology and feeding behaviour of the same individual. This is a challenge for experimentalists but will ultimately yield the most valuable data to adequately model environmental impacts on the survival and growth of marine fish early life stages. Comparación entre crecimiento observado y predicho de larvas de arenque (Clupea harengus): analizando parametrizaciones de modelos basados en individuos. – Los experimentos que analizan directamente las predicciones de crecimiento generadas por modelos basados en individuos (IBM) son poco comunes puesto que resulta difícil medir simultáneamente todos los atributos metabólicos y conductuales. En este estudio, comparamos el crecimiento somático observado y el estimado a partir de modelos de larvas de arenque (Clupea harengus) en experimentos de laboratorio a corto plazo (50 grados-día) a 7 y 13°en los que las larvas fueron mantenidas en condiciones de ayuno o recibieron alimentación ad libitum con diferentes tamaños de presa (copépodos, Acartia tonsa, de aproximadamente 100 a 500 μm). Las estimas de tasa específica de crecimiento (SGR, % de peso seco por día) fueron, en general, sobreestimadas por el modelo, especialmente para larvas que se alimentaron con presas grandes. estimas del modelo se ajustaron a dos escenarios para explorar el efecto de 1) variabilidad temporal en la alimentación de las larvas, y 2) disminución en la eficiencia de asimilación debida una rápida evacuación del tubo digestivo a tasas de alimentación altas. Con estos ajustes, el modelo describió bien el crecimiento larvario para temperaturas, tamaños de presa y edades de las larvas, indicando que las parametrizaciones metabólicas son robustas. Aunque los experimentos llevados a cabo con grupos de larvas verificaron los modelos de crecimiento, la variabilidad en el crecimiento y conducta de alimentación entre larvas sometidas a las mismas condiciones ambientales ponen de relieve la necesidad de que las medidas fisiológicas y de conducta vayan emparejadas y sean tomadas a nivel individual. Esto representa un reto para los experimentalistas, pero a largo plazo generará datos valiosos para los modeladores encargados de simular efectos ambientales sobre las tasas vitales de estadíos tempranos de desarrollo de peces marinos

Depth-dependent nutritional condition of sprat Sprattus sprattus larvae in the central Bornholm Basin, Baltic Sea

Bimodal depth distribution patterns observed for sprat Sprattus sprattus larvae in previous field studies conducted in the deep basins of the Baltic Sea have led researchers to hypothesise that larval sprat condition was depth-dependent. We examined this hypothesis by measuring morphological, biochemical and otolith-based proxies for nutritional condition in sprat larvae collected in discrete 5 m depth intervals from the surface to the bottom in the central Bornholm Basin. Similar to earlier studies, larval sprat were most abundant in 2 depth strata (0 to 10 and 65 to 75 m). Their nutritional condition in surface and deep waters was not uniformly expressed by the different indices. For example, sprat larvae from 0 to 10 m could not be distinguished from conspecifics caught at 65 to 75 m by a long-latency condition proxy (otolith-based growth rates). Similarly, a medium-latency proxy (RNA:DNA) did not suggest differences in condition between the depths. However, short-latency proxies (protein:standard length and DNA:dry weight) supported the depth-dependent condition hypothesis. The lack of correspondence and pitfalls associated with the use and interpretation of multiple condition indices (e.g. the influences of temperature and body size) are discussed and recommendations to strengthen these various metrics are provided

Survival probability of larval sprat in response to decadal changes in diel vertical migration behavior and prey abundance in the Baltic Sea

We employed a coupled three-dimensional biophysical model to explore long-term inter- and intra-annual variability in the survival of sprat larvae in the Bornholm Basin, a major sprat spawning area in the Baltic Sea. Model scenarios incorporated observed decadal changes in larval diel vertical distribution and climate-driven abiotic and biotic environmental factors including variability in the abundance of different, key prey species (calanoid copepods) as well as seasonal changes, long-term trends, and spatial differences in water temperature. Climate forcing affected Baltic sprat larval survival both directly (via changes in temperature) and indirectly (via changes in prey populations). By incorporating observed changes in larval diel vertical migration, decadal changes in modeled and observed survival of Baltic sprat agreed well. Higher larval survival (spawning stock biomass) was predicted in the 1990s compared to the 1980s. After changing their foraging strategy by shifting from mid-depth, low prey environment to near-surface waters, first-feeding larvae encountered much higher rates of prey encounter and almost optimal feeding conditions and had a much higher growth potential. Consequently, larvae were predicted to experience optimal conditions to ensure higher survival throughout the later larval and early juvenile stages. However, this behavioral shift also increased the susceptibility of larvae to unfavorable winddriven surface currents, contributing to the marked increase in interannual variability in recruitment observed during the past decade

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

Combining modeling with novel field observations yields new insights into wintertime food limitation of larval fish

Recruitment success of marine fishes is generally considered to be highly dependent on larval growth and survival. In temperate ecosystems, fish larvae are sensitive to food limitation during the low productivity seasons, particularly if water temperatures and concomitant larval metabolic costs increase due to climate change. We combined 7 years of in situ sampling of larval fish, novel observations on zooplankton via automated image analyses, and larval physiological modeling to explore feeding conditions of Atlantic herring larvae (Clupea harengus) in the North Sea. The observed plankton size-structure was close to the theoretical optimum for larval foraging, but not the biomass. Our results for autumn larvae supported Hjort's critical period hypothesis: small first-feeding larvae were predicted to have a high probability of starvation, whereas larvae > 13 mm were able to reach their maximal growth capacity. In winter, the majority of herring larvae of all tested sizes (5–27 cm) experienced food-limitation with over 35% probability of starvation. Sensitivity analysis suggested that young herring larvae improve their growth performance and probability of survival if feed not only on copepods and their life-stages but include other microplankters in their diet. Given projected warming of the North Sea, our model predicts that herring larvae would require 28% (35%) more prey biomass in autumn (winter) to sustain their growth and survival in the future. This finding together with the ongoing low recruitment of North Sea herring underscore the importance of future micro- and mesoplankton monitoring within a scope of wintertime larval fish surveys.publishedVersio

Predation on Atlantic herring (Clupea harengus ) eggs by the resident predator community in coastal transitional waters

Atlantic herring (Clupea harengus) migrates from offshore to coastal areas to spawn and their eggs and larvae may substantially increase prey resources for resident predators. We combined an in situ predator exclusion experiment using eggs naturally spawned on submerged aquatic vegetation and field observations of predator abundance to estimate the magnitude of predation mortality of herring eggs. During our predator exclusion experiment, performed in an important spawning ground in the southwest Baltic Sea, 20% of the herring eggs were consumed resulting in an extrapolated predation of 42% of all eggs between spawning and hatch. Abundance and stomach content analyses indicated that one predator (threespine stickleback, Gasterosteus aculeatus) was responsible for the majority of the predation impact. Predation mortality estimates from this in situ study were more than 10-fold higher than those of an empirical egg predation model for the same predator in the same region. Our findings highlight the potential of resident predators to regulate the survival of early life stages of ocean-going fishes that rely on the nursery functions of inshore transitional waters