If the algae population decreases by 50%, the goby fish population is likely to decrease, as their primary food source is reduced. Therefore, the correct response would be that the goby fish population will likely decrease, not increase.
Neither of the provided responses seems completely accurate, as the correct answer should state that the goby fish population will likely decrease due to the reduced availability of food.
However, if the option were to say "It will decrease" or "It will decline in relation to the decrease in algae," that would be more aligned with ecological principles. The other option regarding carrying capacity could also be possible, depending on the initial status of the goby population in relation to its carrying capacity.