Report on improved predictive models of growth, production and stock quality

SEAwise research

This study focused on several different measures typically used to assess the growth, condition, and maturity of individuals within fish stocks, examining them against ecological factors in order to better understand how they may respond in real life. The measures used were weight and length, how healthy and nourished fish were, the length at which 50% of fish in a population reached sexual maturity, and the increments between the accumulation of calcium carbonate on the ear bones which indicate age (similar in a way to how you can count the growth rings of trees).

The amount and level of detail of the underlying data used in this study varied across the SEAwise Case Study regions. This means that a variety of different methods needed to be used, and as such the results are not directly comparable across the cases. Nevertheless, here’s a snapshot of key findings from each region:

• Baltic Sea: For herring in the Gulf of Riga, the main driver of annual growth in individuals appears to be how many copepods – a key source of food – are around. By contrast, factors such as summer temperatures during the main feeding period for herring did not have a significant effect. 
• Mediterranean Sea: When it came to size at first maturity, condition, and growth, warmer temperatures at the bottom of the sea were the key environmental driver for many species. But for female deep-water rose shrimp specifically, higher levels of salinity at the bottom of the sea mattered more. 
• North Sea: Growth was found to be density-dependent for some species, meaning that the number of fish in the stock affected the growth rate of individuals within it. Meanwhile, the most important environmental driver of growth tended to be surface temperature. For example, growth in adult sole increased as temperature did . However, for juvenile fish, temperature increases suppressed their growth; this could be linked to changes in food availability and/or competition within populations.
• Western Waters: As in the North Sea, growth was found to be density-dependent for many of stocks. The sizes of individual anchovy and sardine were found to have declined over time, whereas variations in size were less significant for benthic and demersal species. Focusing specifically on anchovy in the Bay of Biscay, temperature appears to have a more significant effect on adult growth than it does for juveniles, which are more affected by spawning stock biomass (the combined weight of all sexually mature females in a stock).