Final answer:
A 8.0 kg fish that is neutrally buoyant at the surface of a lake will remain at the surface because the upward buoyant force exactly balances its weight. Neutrally buoyant objects do not sink or float but stay suspended in the fluid at their current depth. Adjustments to the fish's swim bladder could potentially cause it to sink or float.
Step-by-step explanation:
The subject in question relates to physics, specifically the concept of buoyancy, which is a force experienced by objects when they are in a fluid. According to Archimedes' Principle, an object will experience a buoyant force equal to the weight of the fluid it displaces. Since the 8.0 kg freshwater fish is neutrally buoyant at the surface of the lake, it means the buoyant force exactly balances its weight, and thus it will neither sink nor float but remain at the surface. The fish achieves this by having a density that is equal to the water around it. Should the fish change its density by, for example, adjusting its swim bladder, it could either sink or float.
If a fish like the 85.0-kg grouper mentioned in the reference has a body density slightly less than that of the water, it will tend to float. To stay submerged, this fish must exert a force downwards to counteract the upward buoyant force. In the case of the grouper, the required force would be just enough to overcome the slight difference in densities, indicating that it would need to swim downwards or adjust its swim bladder to stay beneath the surface of the water.