Stock collapse and its effect on species interactions: cod and herring
in the Norwegian-Barents Seas system as an example
Abstract
Both the Norwegian Spring Spawning herring (Clupea harengus) and the
Northeast Arctic cod (Gadus morhua) are examples of how the
overexploitation of marine fish populations was leading to a strong
reduction even so stock collapse, with a strong decline in the
associated fisheries, followed by a recovery. Cod and herring are both
part of the Barents Sea ecosystem, which experienced major warming
events in the early (1920-1940) and late 20th century. While the
collapse or near collapse of these stocks seems to be linked to
instability created by overfishing and climate, the difference of
population dynamics before and after is not fully understood. In
particular, it is unclear how the changes in population dynamics before
and after the collapses are associated with biotic interactions. The
combination of the availability of unique long-term time series for
herring and cod makes it a well-suited study system to investigate the
effects of collapse. We examine how species interactions may differently
affect the herring and cod population dynamic before and after a
collapse. Particularly we explore, using a GAM modelling approach, how
herring could affect cod and reciprocally. We found that the effect on
herring of cod biomass that was generally positive (i.e., covariation)
became negative after the collapse (i.e., predation or competition).
Likewise a change occurred for the cod, the juvenile herring biomass
that had no effect before the collapse had a negative one after. Our
results indicate that population collapses may lead to altered
inter-specific interactions as well as altered response to abiotic
environmental variations. While the stocks are at similar abundance
levels before and after the collapses the system is potentially
different in its functioning and may require different management
action.