unity – How can I manipulate a layer with click and drag functionality?

This works well on clicking layers but not click and dragging layers.

public class FishClicker : MonoBehaviour {

public LayerMask whatIsFish;

public float clickRadius = 0.1f;

void Update() {

    if (Input.GetMouseButtonDown(0)) {
        var mousePos = Camera.main.ScreenToWorldPoint(Input.mousePosition);
        var hit = Physics2D.OverlapCircle(mousePos, clickRadius, whatIsFish);

        if (hit != null) {
            var fish = hit.GetComponentInParent<Fish>();

            if (fish != null) { fish.DoClickThing(); }
        }
    }
}

}