ANSWER 1

Yes absolutely, although it is not a straighforward answer that warmer water results in X% smaller or Y% larger fish. There is a commonly observed effect, called the “temperature-size rule” that says that warmer temperatures result in faster growth, but ultimately lead to smaller adult sizes. This seems rather counter intutative at first, as one might expect faster growth to lead to larger sizes – but this is not the case, individuals just get to a given size (e.g., 30cm) quicker, but then plateau in the body length (sometimes called the ‘asymptotic length’) at smaller sizes (e.g., at 35cm instead of 40cm).

The size of the fish being caught depends on how much growing the fish indidivual has left. If you are only targetting the largest individuals within a population, then yes warming waters will likely result in smaller body sizes. If you are targetting the indivduals that have just matured but still have some growing left to do, then warmer water might result in more of these sized individuals (since they quickly grew to this size).

Let’s say you have a species that grows to 40cm, in warmer waters that might be reduced to 35cm. However if we look at the number of small, medium and large sized fish in the cool population (20x10cm, 10x25cm, and 5x40cm) versus the warm population (15x10cm, 15x25cm, and 2x35cm). The number of the biggest individuals has gone down (no more 40cm ones), however the number of medium fish (25cm) has increased since the higher growth rate means more of the small fish have grown to the medium sized fish.