Rethinking fish stock management: new research challenges the "fewer fish, faster growth" paradigm

"Reducing stock levels to boost growth – assuming that less competition will automatically increase growth rates – carries significant risks. Our research suggests that managing fish stocks at higher levels could be a safer and more sustainable approach”, says Massimiliano Cardinale, lead author of the study and a researcher at the department of Aquatic Resources at SLU.

Building on decades of observations on Northeast Atlantic fish stocks, the study explores how population density (the number of fish in a stock) affects fish growth in exploited fish stocks.

Key findings:

• Positive effect of dense stocks: mean weight at age of many fish stocks is often stable or even increased despite high population density in several stocks. This challenges the simple idea that fish always grow more slowly in large stocks because of other competing factors.
• Weak density dependence effect on weight: age-specific growth rates are only limitedly explained by density dependence – meaning stock size does not impact weight in wild fish stocks as much as expected by theory.
• Risk of misinterpretation: year-to-year variations in growth are highly correlated. This can bias detection and interpretation of density-dependence and overlook the impact of other competing ecological processes which also influence growth.
• Management implications: deliberately reducing stock levels to boost growth, by assuming that released competition will promptly result in an increase in growth, carries substantial risks and it should be carefully considered and weighted against the benefits of managing larger stocks.

The study Occurrence and Strength of Instantaneous and Intracohort Density-Dependence in Northeast Atlantic Fish Stocks has been published in Ecology and Evolution.