Gulls dont eat herring but they are still connected by the food web. how might gull populations be affected?

Since the gulls act both as prey and predator, if it is not eating herring it can be fed upon by higher bird and hence contribute to the ecosystem.

Explanation:

Gulls are specific breed of birds which are considered intelligent among all the species of gulls. It acts both as a prey and predator. Ecologically these bird are very important as they keep a check on both their prey  and predators. Even if they do not feed on herring , they are an essential part of food ecosystem as large gull birds or other bird species prey on gulls. They become the food for higher birds.