Egg cannibalism in Baltic sprat Sprattus sprattus

Abstract

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 intens~ty, 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 X 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 Bornholrn 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 PopulationIExtended 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 m the Central Baltic Sea are discussed.

Developing Baltic cod recruitment models. II. Incorporation of environmental variability and species interaction

“…In inflow situations, when oxygenated water occurred over the whole water column, the maximum average depth of capture encountered in 1994, a year when anoxic conditions did not exist in the basin, was utilised as dwelling depths of clupeids. Applying a linear relationship between the ratio of hourly consumption and egg abundance with the vertical predator-prey overlap index (n = 8; sprat: r 2 = 0.77, herring: r 2 = 0.42) enabled the prediction of consumption rates by predator populations (Köster and Möllmann 2000b) for the main spawning periods in May 1976May -1992. For the period from 1993 on, characterised by late cod spawning activity , the mean observed consumption relative to cod egg availability was utilised.…”

Impact of Climate Change on Fish Population Dynamics in the Baltic Sea: A Dynamical Downscaling Investigation

“…The flatness of the curves suggests a wide range of spawner biomass over which recruitment may vary independently of spawner biomass. The flatness indicates that density-dependent processes such as egg cannibalism (Köster and Möllmann 2000) are not strong enough to cause declines in recruitment substantially below maximum recruitment.…”

The influence of different salinity conditions on egg buoyancy and development and yolk sac larval survival and morphometric traits of Baltic Sea sprat (<i>Sprattus sprattus balticus</i> Schneider)

“…Laurence and Howell (1981) stated that "it is hard to attach any significance in an ecological sense to differences spanning only 0.6 days", which they found as a statistically significant result of accelerated egg development at increased salinity (range 28 to 38) in yellowtail flounder (Limanda ferruginea). However, for the situation of sprat in the Central Baltic Sea, reduced egg development time could be beneficial for survival due to a high predation pressure on eggs by clupeids (Köster and Möllmann, 2000). Sprat eggs in this study were also incubated in salinity conditions exceeding the natural range experienced in the central Baltic Sea (30 and 35), but development time remained the same in all experiments.…”

“…Köster and Möllmann, 2000) conditions experienced by early life stages (ELS). Sprat in the Baltic Sea inhabits waters with the lowest salinities and temperatures experienced by this species (Muus and Nielsen, 1999) and displays adaptations to the environment (Kändler, 1941;Parmanne et al, 1994).…”