1. Harvest Control Rule for Northeast Arctic Saithe
- should predation on herring be taken into account?
Sigbjørn Mehl and Ingolf Røttingen, IMR, Bergen, Norway
Per Sandberg, Directorate of Fisheries, Bergen, Norway
Background
Methods
Northeast Arctic Saithe is an important predator on Norwegian spring-spawning herring. In an ecosystem based management, this aspect should be taken into consideration when developing Harvest Control Rules (HCR) for both fish stocks. In this work, the HCR for herring (agreed upon in 2001) was kept fixed, and we asked; will the optimal HCR for Northeast Arctic saithe be affected when accounting for the predation on herring?
First, the diet of saithe was established through a stomach-sampling program and scaled to cover the entire saithe stock. Next, for the part of the diet consisting of herring (primarily 0 and 1 group) the potential future loss in the herring fishery was calculated. Finally, by assuming a proportional relationship between the size of the saithe stock and its consumption of herring, the relevance of predation for the HCR of saithe was explored.
The predation by saithe in 1998 generates the following potential loss (in tonnes) in the herring fishery during the period
The predation by saithe each year generates the following total loss (in tonnes) in the herring fishery
Long term values of SSB of saithe, annual catch value of saithe and annual predator costs (in mill. NOK), all as a function of fishing mortality of saithe
(Based on SSB and Yield per recruit figures, price of herring and saithe of 6 NOK pr kg each)
Results and conclusions
The Northeast Arctic saithe stock consumes a significant amount of young Norwegian spring-spawning herring and reduces the potentially yield in the herring fishery
When the price of saithe increases relative to the price of herring, the predator costs are of less importance for the total economic yield
The total economic yield will increase with increasing F on saithe (also above Fpa=0.35). However, this increase is marginal for Fs above 0.30, while the expected spawning stock biomass of saithe is reduced more severely
Considering the uncertainties in data and assessment, and the need for stability in quotas from year to year, one should consider carefully if such a marginal increase in expected yield is desirable