Food consumption by clupeids in the Central Baltic: evidence for top-down control?

Considerable changes have taken place in the pelagic ecosystem of the Central Baltic Sea during the last decade. Owing to a combination of high fishing pressure and unfavourable hydrographic conditions, the cod (Gadus morhua) stock as the top predator in the system was reduced from a high level in the early 1980s to its lowest size on record in the early 1990s. The preferred prey species sprat (Sprattus sprattus) showed a significant increase in population size since the late 1980s to the highest level on record in recent years, while the herring (Clupea harengus), another important planktivore in the system, did not show such a response. We investigate whether fluctuations in clupeid stock size cascade down to the trophic level of mesozooplankton, based on stomach content data and daily ration estimates in combination with stock sizes estimated from Multispecies Virtual Population Analysis. Estimates of daily consumption by the populations of the two species for 1978–1990 were compared with standing stocks of mesozooplankton species. No evidence was found for food limitation as might be expected if clupeids exert a strong top-down control on mesozooplankton. Also no influence on interannual variability of mesozooplankton abundance was detected. However, predation did contribute to the seasonal development of two copepod species (Pseudocalanus elongatus and Temora longicornis)

Baltic cod reproduction in the Gotland Basin: annual variability and possible causes

Baltic cod spawning takes place in the deep basins and reproduction success is mainly related to environmental conditions (salinity and oxygen regimes, i.e. the 'reproduction volume'). Due to the Baltic Sea heterogeneity, cod reproduction success in the Southem and Centrat Baltic spawning grounds can differ significantly. Recent oceanographic changes i.e. decrease of water exchange and stagnation, as weil as a strong reduction of spawning stock caused the diminishing of the reproduction potential of the Gotland spawning grounds. The Gotland spawning grounds belong to four main cod spawning sites in the Baltic and historical analyses revealed that abundant generations of Baltic cod were produced when successful cod reproduction took place also in the Gotland Basin. Analyses of revised reproduction volume estimates for the Gotland Basin taking into account the spatial structure of hydrology in the basin during stagnation and aeration periods reveals high seasonal and inter-annual variability. To describe changes of abundance and distribution of the spawning stock and the recruits in relation to hydrographic conditions, results from trawl surveys carried out in 1975-1998 in the Gotland Deep are analyzed. In this analysis, the reproduction volume is used as a proxy for the environmental conditions

Spatial and temporal variability in the stomach content of herring and sprat in the Bornholm Basin

Herring and sprat stomachs and zooplankton samples were collected on 8 sites in the Bornholm Basin in May/June 1999 at different times of the day. Sprat showed a distinct increase in stomach content from early morning to the afternoon, whereas the stomach content of herring remained rather constant. Both herring and sprat showed a clear dominance of mesozooplankton in food composition with only small contributions of makrozooplankton and ichthyo-plankton. Both clupeid species showed similar preferences in selection of food items with a dominance of the copepods P. elongatus and T. longicornis. Compared to herring, other copepod species, especially Acartia spp. contributed to a larger extent to the food of sprat. Cladocerans played a less important role as food for both species. Spatial differences in stomach content have been revealed between the sampling sites. These resulted from differences in the distribution of food organisms in the water column as well as a different vertical distribution of the predators at different times of day. The share of juvenile copepods in herring stomachs was bigger than in sprat stomachs. Generally, selective feeding seemed to be stronger developed in sprat than in herring

Long-term trends in abundance of cladocerans in the Central Baltic: the influence of hydrography and predation by fish

Long-term dynamics (1960–1997) of the cladoceran species Bosmina coregoni maritima, Evadne nordmanni and Podon spp. are described for the Gdansk Deep and the Gotland Basin (Central Baltic Sea). By using correlation analyses on seasonal time-series, the influence of temperature and salinity on the abundance of cladoceran species was investigated. A clear affinity to higher temperature was found for B. coregoni maritima in summer as well as for E. nordmanni and Podon spp. in spring. In addition to temperature, association tests with salinity revealed besides species-specific preferences, regional and temporal differences. Contrary to B. coregoni maritima, both other species were positively associated to salinity in summer and autumn in the Gdansk Deep. In the Gotland Basin only E. nordmanni was positively correlated to salinity in autumn. Differences in the response to hydrographic variables are possibly stage specific, i.e. between resting eggs and adults, or due to a different adaptation to the abiotic environment

β-Cyclocitral, a Grazer Defence Signal Unique to the Cyanobacterium Microcystis

β-Cyclocitral is often present in eutrophic waters and is a well known source of airborne and drinking water malodor, but its production and functional ecology are unresolved. This volatile organic compound (VOC) is derived from the catalytic breakdown of β-carotene, and evidence indicates that it is produced by the activation of a specific carotene oxygenase by all species of the bloom-forming cyanobacterium Microcystis. Previous work has shown that β-cyclocitral affects grazer behavior, but the nature of this interaction and its influence on predator-prey dynamics was unresolved. The present study combined analytical and behavioral studies to evaluate this interaction by using Microcystis NRC-1 and Daphnia magna. Results showed that β-cyclocitral was undetectable in live Microcystis cells, or present only at extremely low concentrations (2.6 amol /cell). In contrast, cell rupture activated a rapid carotene oxygenase reaction, which produced high amounts (77 ± 5.5 amol β-cyclocitral/cell), corresponding to a calculated maximum intracellular concentration of 2.2mM. The behavioral response of Daphnia magna to β-cyclocitral was evaluated in a bbe© Daphnia toximeter, where β-cyclocitral treatments induced a marked increase in swimming velocity. Acclimation took place within a few minutes, when Daphnia returned to normal swimming velocity while still exposed to β-cyclocitral. The minimum VOC concentration (odor threshold) that elicited a significant grazer response was 750nM β-cyclocitral, some 2,900 times lower than the per capita yield of a growing Microcystis cell after activation. Under natural conditions, initial grazer-related or other mode of cell rupture would lead to the development of a robust β-cyclocitral microzone around Microcystis colonies, thus acting as both a powerful repellent and signal of poor quality food to grazer

Egg cannibalism in Baltic sprat (Sprattus sprattus L.)

Throughout the last 2 decades a shift from a cod- to a sprat-dominated system occurred in the upper trophic levels of the Central Baltic Sea. This was caused by a decline in the cod stock, due to recruitment failure and high fishing intensity, resulting in a decrease in predation pressure on sprat. Concurrently with the lowest cod stock size on record, sprat reached biomass values of above 2 × 106 t in 1992, being relatively stable afterwards. Besides predation mortality through cod and in recent years also an increasing fishing pressure, cannibalism on eggs may be a compensatory process limiting the reproductive success of sprat and hence contributing to the population regulation in the Central Baltic. Based on sprat stomach sampling on 21 cruises between March 1988 and July 1996 cannibalism on sprat eggs was investigated in the Bornholm Basin, one of the main spawning areas of Central Baltic sprat. Using a model of gastric evacuation to estimate daily food intake rates and a Virtual Population/Extended Survivor Analysis for computing predator population sizes, egg cannibalism rates were estimated. These were compared to egg abundance data from ichthyoplankton surveys and to preliminary estimates of seasonal egg productions. The study revealed significant interannual differences in the intensity of sprat egg cannibalism with considerable predation in 1990 to 1992 (>15% of the egg abundance during peak spawning and >60% of the seasonal production) and a reduction in most recent years (<16% of the corresponding abundance and production). As a possible reason for these differences a combination of changes in the vertical overlap of predator/prey and variability in the food environment were identified. Shortcomings of the applied methods and the possible impact of cannibalism on the reproductive success and population development of sprat in the Central Baltic Sea are discussed