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CS 1: Barents/Norwegian Sea

Background

Norwegian spring-spawning herring and North-east Arctic cod migrate between the Norwegian Sea and the Barents Sea. The other main species in the Barents/Norwegian Sea ecosystem, the capelin, spends its entire life within the Barents Sea. Any consideration of cod and herring dynamics in this area needs to consider the dynamics of capelin as the three species are closely linked. It is therefore necessary to consider spatial effects in order to adequately model the stock dynamics of these species. In particular fishing effort on the herring is located only in the Norwegian Sea, and predation and cannibalism is partly controlled by spatial and temporal overlap of the stocks. The fisheries for capelin and herring are single-species fisheries with very low by-catches of other species. There is significant by-catch of haddock, saithe and other demersal fish in the cod fisheries. However, the catch of cod is much higher than of other demersal fish species, so for the sake of studying cod recovery, it is not necessary to consider mixed fisheries.

Norwegian spring-spawning herring and Barents Sea capelin have both been through strong declines from which they have recovered (ICES 2004b). Norwegian spring-spawning herring collapsed around 1970, and it took about 20 years for the stock to recover. The Barents Sea capelin stock has fluctuated strongly in the last 20 years, and has recently (2003) collapsed for the third time during that period. The herring collapse is thought to be mainly due to over fishing, while an important reason for the capelin collapses seems to be herring-capelin interactions (Hamre 1994, Gjøsæter and Bogstad 1998). Strong year classes of herring growing up in the Barents Sea, and predation by young herring (mainly ages 1 and 2) on capelin larvae may seriously impact capelin recruitment and cause the capelin stock to collapse (Gjøsæter and Bogstad 1998). Strong year classes of herring are relatively infrequent, so capelin is able to recover in periods when the abundance of young herring is low. Cod has not collapsed despite being subject to high fishing pressure over long periods of time. Cod was depleted and probably close to collapsing in the late 1980s, when the fishing pressure was high, and the stock size was over-estimated. At the same time, cod individual growth and fecundity was low partly due to the collapse of the capelin stock. However, strong regulations of the fishery (F going from 0.98 in 1988 to 0.27 in 1990, ICES 2004a), combined with favourable environmental conditions allowed the stock to recover quickly.

Capelin is the main prey item for cod, and influences the cod growth and maturity rate. When capelin abundance is low, cod switches to other food items, including own juveniles. During the first capelin collapse, the abundance of alternative fish prey was low. Cod then switched to krill and amphipods as prey, and individual growth and condition of cod was strongly reduced. During the next two capelin collapses, the abundance of alternative fish prey was much higher. Cod then switched to other fish as well as krill/amphipods as prey, and the impact on individual growth of cod was much lower than during the first capelin collapse (Bogstad and Mehl 1997, ICES 2004a).
Recruitment of cod and herring is quite variable, and environmental conditions are believed to have a strong impact on the recruitment variability. A number of studies of the fecundity and the population dynamics of cod and herring in the first 6 months of their life have been carried out in recent years.

Hypotheses

The following hypotheses will be investigated for the cod-capelin-herring system in the Barents Sea:

  1. Population dynamics, e.g. stock level, reproductive potential, recruitment success, early life-stage dynamics, of the key species vary according to fishery strategy and environmental conditions.
  2. Modelling the impact of environmental factors and predation/cannibalism on fish stocks dynamics can lead to better evaluation of fishery recovery strategies
  3. Using the approach outlined in this document it is possible to compare the likelihood of success, and associated uncertainty, of various recovery strategies under different environmental conditions.

References

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