The short answer: Copepods are a critical link in marine food webs, and oil exposure may affect their overwintering success and the timing of their emergence. If these effects are severe, there might not be enough copepods around in the spring for the fish to eat.
For a more detailed answer, check out this beautiful blog post by Dr. Elise Skottene about effects of oil exposure on copepods. Elise was a Guest Student in our lab in 2017. Her work uses transcriptional profiling to show that oil exposure may disrupt lipid metabolism in Calanus.
…and just this month, she published another paper showing that metabolic gene expression is altered by the presence of predators. This story is somewhat complicated because the response to predators depends on the nutritional status of the copepod. When the copepods are well-fed, they respond to predators by upregulating catabolic genes and developing faster. When the copepods are poorly fed, they seem less able to cope, and slow down their lipid metabolism even more in the presence of predators.