Let’s begin with a question that would require you to work through various different processes. You might begin this by breaking down the fact that they will face some problems that are universal to any organism, be it underwater or on land, and some that are unique to being underwater. Oxygenation and temperature, for example, are issues on land too. The temperature in certain seas, or in deeper water, might be much cooler – just as it would be in certain areas on land or at higher altitude. Meanwhile, oxygenation might be lower in certain pockets of water, whilst it could also be lower at altitude, again, on land.
Unlike land organisms, you might explain that fish are affected by pH changes. As an example, you might outline that at a very high pH (above 9) ammonium is converted to ammonia, which is fatal to fish. pH levels will also affect bacteria, which in turn would be necessary for the ecosystem in which the fish lives – they will likely clean the water, and alter nutrient levels. Additionally, fish are subject to pressure changes – something that would only affect a land organism if it were to reach an exceptionally high altitude. Of course, one would expect the fish to be adapted to the level at which it swims in the water and therefore have few problems with the pressure at lower altitudes of water. We might also highlight light levels being an issue at greater depths.
Lastly, we should highlight man’s effect on the oceans – plastic pollution, oil spills, and other detritus damaging habitats and creating issues for fish and other marine life.
With this question, you might not be expected to go into huge detail, but should show an ability to think ‘big picture’ and work with the tutor to discuss your points.