> How did fish manage during the climatic optimum [1] some 8500 to 5500 years ago when it was between 1.5°C and 4°C warmer than it is now

From your wikipedia link:

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A study in 2020 estimated that the average global temperature during the warmest 200 year period of the HCO, around 6,500 years ago, was around 0.7 °C warmer than the mean for nineteenth century AD.

The HCO consisted of increases of up to 4 °C near the North Pole (in one study, winter warming of 3 to 9 °C and summer of 2 to 6 °C in northern central Siberia).[8] Northwestern Europe experienced warming, but there was cooling in Southern Europe.[9] The average temperature change appears to have declined rapidly with latitude and so essentially no change in mean temperature is reported at low and middle latitudes.

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So, the effect then was localized to the waters where there's more oxygen anyway, and having temperature going in different directions in different parts of Europe and at a much slower rate than current changes, would make it easier for the fish to migrate? Unclear how water temperatures behaved at different depths though.