Description: Estimates of the reproduetion volume of Baltic eod defined as the. total volume of water with salinity above 11 psu, oxygen coneentration above 2 mIlI and temperature above 1.5 °c are presented for the Bornholm Basin based on quasi-synoptie hydrographie surveys. The primary data set consisted of hydrographie observations recorded during 14 eruises in the period from 1991 to 1996 eovering a regular grid of 36 stations with a spacing of appr. 10 nm. This data set was supplemented by including two additional surveys from 1989 with 21 stations. A statistical method the objective analysis was applied to mup the horizontal distribution of the thickness of the spawning layer. Subsequently, the reproduetion volume was ealculated by basiri wide integration. Highly significant eorrelations (r ~ 0.89, p 〈 0.001) betwcen single point observations of the thickness of the spawning layer and the overall reproduetion volume in the Bornholm Basin was found for 4 stations loeated in the eentral part of the surveyed urea. Linear regression models for single stations and the basin wide reproduction volume were established representing different hydrographie scenarios from stagnation to inflow periods. The results are discussed with respect to a backward extension of the time series using historical single point observations.

Description: The volume of water with suitable oxygen and salinity conditions for survival and development of cod/eggs in the eastern Baltie Sea varies significantly within and among years. It has recently been shown that this volume of water (nreproduetive volumen), in addition to spawning stock biomass, is a major factor determining recruitment success in this eod population. However, it is unclear which oceanographie mechanisms are responsible for variability in reproductive volume, and how these processes interaet on a seasonal and interannual basis. In this study, we use time series observations to identify inter-relationships between hydrographie and biological processes (e. g., inflows of North Sea water, winter mixing processes, production and decomposition of organic matter) influencing reproductive volumes from 1952-1992. A frequent pattern of seasonal variability in 4 eastern Baltic spawning areas includes a spring maximum which decreases during the summer, followed by an inerease during the fall-winter months. However, this general pattern is violated in many years, and the magnitude of the seasonality is more pronounced, in some spawning areas than others. \Ve have begun to analyse these patterns and interpret them in the context of other fluctuating components of the eastern Baltic pelagic ecosystem. Preliminary results show that reproductive volumes deerease by an average of 9 - 39% between May and August, and that the decrease in the spawning area most important to long-term recruitment (Bornholm Basin) is temperature-dependen1. The seasonal decline in the Bomholm Basin also tends to be larger and more variable after 1964 than in the previous 12-year periode These patterns of reproductive volume variability may be important to eod reproduetive success beeause of a long and variable spawning period.