ANSWER 1

Written response:

Fishes are primarily ‘ectotherms’ – meaning their body temperature depends on the surrounding water temperature and not internal heat production. As a result, ocean temperatures have a big effect on when and how fish reproduce. Many species rely on seasonal temperature fluctuations to trigger spawning. But if temperatures dramatically fluctuate from seasonal norms (e.g. during marine heatwave events), spawning may be delayed or brought forward, which can throw off the timing between hatching and when ocean conditions are suitable or food is available for young fish.

Early life stages like eggs and larvae are particularly sensitive to environmental conditions. Sudden or extreme temperature changes can reduce egg survival or even speed up development too much. Larvae that hatch too early may run out of yolk before they can find food. Even small yet rapid changes in temperatures, like 1–2 °C of change over a couple of days, can increase stress and reduce survival at these stages. Juveniles and adults usually cope better, but frequent fluctuations still take a toll.

In terms of limits to species adaptability, different species have preferred temperature ranges where individuals perform best and limits beyond which they simply can’t survive. In general, the maximum temperatures that occur at fishes equatorward (northern in the Southern Hemisphere) distribution limits provide a rough approximation of species upper thermal tolerance limits, while minimum temperatures that occur at poleward (southern in the Southern Hemisphere) distribution limits generally describe species lower thermal tolerances. Some fishes have the capacity to adapt to temperatures that are slightly outside this range through short-term acclimation or may genetically adapt over many generations. But not all species have the physiological flexibility for short-term acclimation and genetic adaptation takes a long time.

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