Fishing impacts on age structure may conceal environmental drivers of body size in exploited fish populations

Using analysis of field survey size-at-age data, we examine responses of European plaice (Pleuronectes platessa) to spatial differences in environmental variables in the North Sea. Using available samples of plaice aged 1-7, northern and southern migrating groups of males and females grew differently. However, length-at-age growth patterns were not corroborated by complementary otolith-based estimates. Southern females and males were smaller than their northern counterparts until age 3. Southern males remained smaller up to age 7; by contrast southern and northern females reached similar size-at-age by year 4. Due to covariation, the influence of spatially variable environmental conditions was equivocal. However, temperature, depth, fishing pressure, phosphate levels, distance from shore, and conspecific density were all significant predictors of size for plaice aged 1-7. Our results suggest that fishing impacts on age structure limit the potential to examine the role of environmental variation on body size. For fish that rarely reach their full potential age and size, expected metabolic responses to warming may remain unexpressed, challenging predictions in a changing climate.

Automated summary

Using size-at-age data from field surveys, we examined how European plaice respond to spatial differences in environmental variables in the North Sea. We found that northern and southern migrating groups of males and females exhibited different growth patterns. However, these patterns were not supported by otolith-based estimates.