Bottom Temperature Effect on Growth of Multiple Demersal Fish Species in Flemish Cap, Northwest Atlantic

This study investigates the effects of warming water on growth in seven demersal fish species including Atlantic cod (<i>Gadus morhua</i>), American plaice (<i>Hippoglossoides platessoides</i>), Greenland halibut (<i>Reinhardtius hippoglossoides</i>), roughhead grenadier (<i>Macrourus berglax</i>) and three species of redfish (<i>Sebastes</i> spp.) in the Northwest Atlantic and compares the changes in growth across species. Length-at-age data were collected from EU bottom trawl surveys from 1993 to 2018, and bottom temperature data were obtained from the Copernicus Marine Service. Generalised additive mixed models (GAMMs) were used to describe the temperature effects on growth. The analysis was carried out separately for males and females. Both sexes of all species except American plaice showed significant temperature effects on growth. To obtain the growth parameters, von Bertalanffy growth functions (VBGFs) were fitted to the predictions from best-fit GAMMs for all species and both sexes under five different bottom temperature scenarios (3, 3.5, 4, 4.5 and 5 °C). The predictions from all best-fit GAMMs were broadly similar in form to the fitted von Bertalanffy growth functions (R² > 90%). Increased bottom temperature generally resulted in a decrease in the asymptotic length (<i>L_∞</i>) and an increase in the growth rate (<i>k</i>). The species with the most dramatic increase in <i>k</i> over the temperature range of 3 °C to 5 °C was Atlantic cod, for which <i>k</i> increased from 0.05 to 0.13 year^-1 in females and from 0.08 to 0.14 year^-1 in males. The maximum length (<i>L_max</i>), predicted by the VBGF at maximum age generally declined from 3 °C to 5 °C. The species with the most pronounced decline in <i>L_max</i> was beaked redfish (<i>S. mentella</i>). An increase in the proportion of smaller individuals could impact population productivity and result in lower biomass available to fisheries. Uneven changes in fish growth in the warming ocean could also have wider ecological implications and alter the trophic landscape.