| GROWTH, DRIFT AND SURVIVAL PROBABILITY OF BALTIC COD EARLY LIFE STAGES |
| HUWER, B., Danish Institute for Fisheries Research, Dept. of Marine Fisheries, Charlottenlund Castle, Charlottenlund, Denmark, 2920, bhu@difres.dk
Hinrichsen, H-H., Leibniz-Institute of Marine Sciences IfM-Geomar, West Shore Campus, Duesternbrooker Weg 20, Kiel, Germany, 24105, hhinrichsen@ifm-geomar.de
Boettcher, U., Bundesforschungsanstalt fuer Fischerei, Institut fuer Ostseefischerei, Alter Hafen Sued 2, Rostock, Germany, 18069, uwe.boettcher@ior.bfa-fisch.de
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| This study aimed to investigate drift trajectories of Baltic cod juveniles as well as their growth and survival in relation to prey availability by utilizing an individual-based model incorporated into a 3-D hydrodynamic model, as the fate of larvae and juveniles is regarded to be of major importance for the recruitment success of Baltic cod. A main factor influencing the survival of larvae is growth which is in turn dependant on the availability of suitable prey. The main prey organisms of larval Baltic cod, the juvenile stages of Pseudocalanus elongatus, show large fluctuations in standing stocks associated to salinity changes. Salinity and oxygen levels in the Baltic are strongly influenced by the occurrence of inflow events from the North Sea. If the abundance of P. elongatus is high in the Bornholm Basin, the main spawning area of Baltic cod, it is favourable for larvae to be retained in the basin; if the abundance is low, it is more favourable for larvae to be drifted into shallower regions where other copepod species are available as suitable prey. Thus, retention or dispersion of larvae determines if they encounter suitable prey fields and have a chance to grow on to the juvenile stage.
Baltic cod juveniles were sampled in autumn in the period 1993-1996. Age and growth of the sampled fish were determined by means of otolith microstructure examination. Information on number (age) and width of otolith increments was coupled to a three-dimensional, hydrodynamic model of the Baltic Sea in order to determine the individual fish’s hatching/spawning location, drift trajectory and the temperatures and prey availabilities experienced on it’s drift path from the spawning ground to the location of catch.
The hydrographical conditions in the investigated years showed strong variations. The years 1993 and 1994 were characterized by inflows of saline, oxygen-rich water from the North-sea, likely to enhance conditions for feeding and growth of Baltic cod early life stages. 1995, on the other hand, was a stagnation year with no such inflow event and, accordingly, potentially adverse salinity and oxygen levels for Baltic cod feeding success and survival. 1996 was characterized by very cold water temperatures, possibly having a negative effect on larval growth and survival. Thus, results are expected to reveal inter-annual differences in growth, drift pattern and survival probability of Baltic cod early life stages that can possibly be related to the recruitment success of Baltic cod.
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