Multispecies considerations for the central Baltic stocks: cod in Subdivisions 25–32, herring in Subdivisions 25–29 and 32, and sprat in Subdivisions 22–32

ICES  intends  to  provide  multispecies  advice  on  fisheries  for  some  ecosystems  (see  Section 1.2)  and  encourages  managers to  apply such  an  approach  to  the central  Baltic  Sea.  The  present  section  may  serve  as  a  starting  point  for  a  dialogue  between  ICES  and  managers  to  foster  the  development  of  a  multispecies  management  system  for  the  Baltic.  This text  uses  implicit  management  objectives  and  risk  tolerance that need  to  be  validated  by  managers.  If  managers  decide to adopt a multispecies management approach a transition period from the present management will be required. As this work is mostly illustrative, ICES focuses on the most obvious interactions between commercially exploited fish stocks in the area and does not attempt to provide a full foodweb model.

The  main  result  of  the  present  preliminary  quantitative  multispecies  analysis  for  the  central  Baltic  Sea  (Subdivisions 25–29  and 32  excl.  Gulf  of  Riga)  is  that,  compared  to  the  present  single-species  approach,  it  could  be  possible  to  increase the sum of the sustainable yields in tonnes of the three species combined; the growth of individual fish would be  improved  if  multispecies  interactions  were  taken  into  account  when  setting  target  Fs.  However,  cod  yields  will  remain about the same, whereas the probability  of  low  cod  spawning-stock biomass (SSB)  will  increase. Multispecies considerations  indicate  a  multitude  of  solutions,  all  being  biologically  sustainable.  The  societal  choice  between  these  must be based on social and economic considerations and informed by social and economic impact assessments.

These results are derived assuming that there is full spatial overlap for all three stocks. The geographical overlap of cod and  clupeid  stocks  is  currently  small,  with  cod  found  mainly  in  the  south  (Subdivision  25)  and  clupeids  in  the  north  (Subdivisions 28–29 and 32)