Final answer:
Using a block design in this experiment for salmon helps control for species differences, allows for accurate comparison of treatments, and accounts for individual variability within each species.
Step-by-step explanation:
A valid reason to use a block design for this experiment is to control for the inherent differences between Pacific and Atlantic salmon. By grouping the fish into separate blocks based on their species, the biologist can ensure that each treatment group has an equal representation of both species. This eliminates the potential bias caused by species-specific factors that could affect the results.
Using a block design also allows for a more accurate comparison of the different treatments. By including both species in each treatment group, the biologist can observe how each treatment affects both Pacific and Atlantic salmon, providing a more comprehensive understanding of the effectiveness of the treatments.
In addition, using a block design helps to account for any variability among the fish within each species. By including multiple individuals from each species in each treatment group, the biologist can minimize the impact of individual differences and ensure that the results are more representative of the overall population.