The Kattegat cod stock is estimated to be at its lowest level since 1971. The biomass of reproducing fish has been reduced by as much as 95%. There are no signs of recovery and since the year 2000 this stock has been considered by ICES to be outside of safe biological limits. This is concluded by Francesca Vitale in her doctoral thesis “Reproductive aspects of Kattegatt cod: implications for stock assessment and management”.
The research by Vitale indicates that the situation for the Kattegatt cod could be even more alarming than previously believed. According to her research the female spawning biomass (FSB) of the Kattegatt cod may have been overestimated by as much as up to 35% for a period of more than 20 years. This is a consequence ofdue to using incorrect methodologies for calculating the proportion of mature individuals in the population, which has leading to an overrating of the amount of females that actually contribute to the reproductive potential.
The spawning stock biomass (SSB) of the Kattegatt cod is at a historical low, and far below its safe biological limit. The stock is compressed to a few age classes and the reproduction is mainly dependent on first time spawners. Assessments and management of fish populations rely on estimations of SSB which in turn are based on the proportion of mature fish within age classes in the population. But since SSB does not take into account the effect that the variation in length composition and fish condition has on the stock reproductive output, the reproductive potential gets overestimated when the stock is dominated by small individuals as is the case with the Kattegatt stock.
Vitale shows that a proper identification of mature individuals in the cod population is vital for a correct estimation of SSB and evaluating the status of the stock and thus establishing harvest levels. She points out that this overestimation of the stock reproductive potential may have led to the implementation of regulating measures far above the stock capacity, neglecting potential risks. Vitale suggests a revision of the Kattegatt cod stock assessment models and a re-evaluation of the reference points, based on increased stock-specific biological knowledge.