This blog post is provided by Carla Freitas, David Villegas‐Ríos, Even Moland and Esben Moland Olsen and tells the #StoryBehindThePaper for their article “Sea temperature effects on depth use and habitat selection in a marine fish community“, which was recently published in the Journal of Animal Ecology.

A cod rests between two rocks at the bottom of a southern Norwegian fjord. It is a sunny summer day; children jump and swim happily – enjoying surface water temperatures of 23°C. Just below them, eelgrass and algae beds are full of life, hiding invertebrates and small fish, a dream banquet for a hungry cod. But cod remain at depth for the time being, sheltering among boulders. It is too warm at the surface and it may need to wait several weeks before resuming diel vertical migrations to its favourite feeding habitats.

Cod habitat use and temperature

Using acoustic telemetry in Tvedestrand fjord, in southern Norway, Freitas et al. (2016) found that cod do not venture to shallow feeding areas, such as eelgrass and vegetated hard substrate, when surface temperatures rise above 16°C, remaining instead in deeper, colder waters, which seem to be less favourable in terms of food resources.

Temperature effects on a fish community

But, what about other fish species utilizing the same seascape? Sea surface temperature in this region ranges seasonally from 0 to over 20 °C, representing challenges and opportunities to the fish community which includes both cold‐, cool‐ and warm‐water affinity species. In this new study, Freitas et al (2021) acoustically tracked more than 100 individuals of Atlantic cod (Gadus morhua), pollack (Pollachius pollachius) and ballan wrasse (Labrus bergylta) and examined how coexisting species within a fish community adjusted their behaviour (vertical distribution in the water column and habitat selection) to cope with the thermal variation.

They found that cod used colder waters, compared with pollack and ballan wrasse. In contrast to cod, pollack and ballan wrasse occupied shallow areas when surface temperature increased in summer. During winter, when surface temperature dropped and the thermal stratification reversed (deeper waters were then warmer than the surface), pollack and ballan wrasse moved to deeper, relatively warmer areas, while cod occupied shallower, colder habitats.

Habitat selection

Though habitat selection was affected by temperature, species‐specific habitat selection was observed. When sea temperature was similar throughout depths and habitats, cod selected eelgrass and vegetated hard substrate, which probably provide suitable food resources, such as small fish and invertebrates. Pollack, in contrast, avoided eelgrass throughout the year and selected instead boulders and other hard substrates. These habitats may be more suitable for the hunting strategies of this piscivorous species. Ballan wrasse are known to feed on invertebrates and showed a preference for eelgrass, vegetated hard substrates and rocky walls with anemones. Interestingly, high sea temperature at the surface makes eelgrass suitable for ballan wrasse during summer but inaccessible for cod. The reverse is found in winter, when surface temperatures as low as 0 to 5°C make eelgrass accessible for cod but unsuitable for ballan wrasse.

Climate change implications

This study shows how cohabiting fish species respond to thermal heterogeneity, showing that temperature regulates the access to the different depths and habitats. Behavioural plasticity may be an important factor for coping with temperature variability and potentially for adaptation to climate change. However, the fitness cost of temporary deprivation from key feeding habitats are unknown. There are signs that cod in this region grow less during summer, contrary to what is observed in colder northern regions.

The autumn has arrived, and the hungry and slim cod now resumes diel vertical movements to shallow feeding habitats. Air temperature drop as winter approaches and eventually the fjord surface will freeze over. Pollack and ballan wrasse will move to deeper waters, while cod will thrive in shallow habitats and compensate for the reduced growth during summer – and hope for no further temperature extremes next summer. Projected sea surface temperature increases are expected to be detrimental for this cold-water species, while profitable for their warm-water affinity cohabitants.

Read the paper

Read the full paper here: Freitas, C, Villegas‐Ríos, D, Moland, E, Olsen, EM. Sea temperature effects on depth use and habitat selection in a marine fish community. J Anim Ecol. 2021; 00: 1– 14. https://doi.org/10.1111/1365‐2656.13497