Quantifying environmental heterogeneity:habitat size necessary for successful development of cod Gadus morhua eggs in the Baltic Sea

Abstract

Spatial and temporal variability in environmental factors can exert major influences on survival and growth of livlng organisms. However, in many key areas of fisheries sclence (e.g. growth, survival and recruitment determination), environmental heterogeneity is usually ignored because of insufficient environmental or fisheries data or lack of evidence that such heterogene~ty impacts response variables. For the eastern Baltic Sea (ICES Subdivisions 25 to 321, we evaluated spatial and temporal differences in conditions affecting the sunrival of cod Gadus morhua L. eggs at survival on four distinct spawning sites within the assessment area. We intercalibrated ways of quantifying the volume of water ('reproductive volume') at each site where salinity, oxygen and temperature conditions permitted successful egg development. We have developed and compared a time series (1952 to 1996) of reproductive volumes among the areas to identlfy spatial differences. The results of 2 independent volume-estimation methods are comparable, indicating that highly significant differences exist among the sites, and that the westernmost spawning ground, Bornholm Basin, has on average the highest reproductive volume and the lowest variability among the 4 sites. These findings may be useful in evaluating how spatial and temporal variability in environmental conditions affect egg hatchng success and possibly recruitment in the Baltic stock.

Trophic cascades promote threshold-like shifts in pelagic marine ecosystems

“…Cod reproductive volume in the central Baltic Sea was calculated by using a contouring program that estimates the volume of water whose hydrographic conditions are considered suitable for the development of cod eggs; that is, with a salinity Ͼ11 psu and an oxygen concentration Ͼ2 ml⅐L Ϫ1 (29). These criteria are based on studies that have established the relation between Baltic cod egg survival and cod egg buoyancy, vertical distribution, and oxygen concentration (29).…”

“…29]. Higher salinity of the mid-deep waters enhances the buoyancy of cod eggs, preventing them from sinking into hypoxic water layers (29), but also favors the development of one of the key zooplankton prey for cod larvae; that is, the copepod Pseudocalanus spp. (30).…”

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

“…In the latter area, a water volume suitable for successful development of eggs, the so-called reproductive volume, was always available (Plikshs et al 1993;MacKenzie et al 2000). Other factors such as egg predation (Köster and Möllmann 2000a) have a regular and also more pronounced influence on the reproductive success of cod in this basin.…”

“…Estimates of the "reproductive volume," i.e., the volume of water suitable for successful egg development (Plikshs et al 1993), in the different subdivisions were obtained from MacKenzie et al (2000) for the peak spawning periods (Appendix Tables A1 and A2). However, for the Bornholm Basin, two modifications of this index were performed based on the suitability of the spawning volume for successful egg development.…”

“…As a result, several new time series are becoming available for variables relating to these processes. In particular, data series have been compiled for egg production (including sex-specific maturity ogives, Tomkiewicz et al 1997; individual fecundity, Kraus et al 2000), egg characteristics (buoyancy and viability; Nissling et al 1998), abiotic factors affecting egg survival (Plikshs et al 1993;Wieland et al 1994;MacKenzie et al 2000), estimates of egg and larval abundances in spawning areas (e.g., Plikshs et al 1993;Wieland et al 2000), hydrographic processes affecting larval and 0-group distribution (Voss et al 1999;Hinrichsen et al 2001), predation on early life history stages (Köster and Möllmann 2000a), and cannibalism (Sparholt 1996;Neuenfeldt and Köster 2000).…”