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

## TL;DR

This study shows that warming ocean temperatures are causing some fish species to grow faster but reach smaller maximum sizes, which could impact fisheries and ecosystems.

## Contribution

The study provides new insights into how warming affects growth rates and maximum sizes of seven demersal fish species using GAMMs and VBGFs.

## Key findings

- Atlantic cod showed the largest increase in growth rate with rising temperatures.
- Beaked redfish experienced the most significant decline in maximum size with warming.
- Higher temperatures generally reduced asymptotic lengths while increasing growth rates across most species.

## Abstract

This study examines temperature effects on the growth of seven demersal fish species in Flemish Cap. These species are Atlantic cod, American plaice, Greenland halibut, roughhead grenadier and three species of redfish. The length-at-age and bottom temperature data were analysed using generalised additive mixed models (GAMMs). The analysis was carried out separately for males and females. Both sexes of all species except American plaice showed significant temperature effects on growth. Best-fit GAMMs were used to predict the length at age under five temperature scenarios (3 °C, 3.5 °C, 4 °C, 4.5 °C and 5 °C). Von Bertalanffy growth functions were then fitted to the predictions to estimate asymptotic length and growth rate. As the temperature increased, most species showed an increase in growth rate but a decline in size at later life stages. Atlantic cod experienced the largest rise in growth rate, particularly in females. Beaked redfish showed the most drastic decline in size at maximum age. These findings suggest that warming oceans may lead to smaller fish, which could reduce the overall biomass available to fisheries. Differences in growth changes across species could also disrupt marine ecosystems and alter predator–prey relationships. Understanding these effects is important for fishery management and ecosystem conservation in the changing climate.

This study investigates the effects of warming water on growth in seven demersal fish species including Atlantic cod (Gadus morhua), American plaice (Hippoglossoides platessoides), Greenland halibut (Reinhardtius hippoglossoides), roughhead grenadier (Macrourus berglax) and three species of redfish (Sebastes 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 (L∞) and an increase in the growth rate (k). The species with the most dramatic increase in k over the temperature range of 3 °C to 5 °C was Atlantic cod, for which k 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 (Lmax), predicted by the VBGF at maximum age generally declined from 3 °C to 5 °C. The species with the most pronounced decline in Lmax was beaked redfish (S. mentella). 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.