Foxes have been present on Herring Island in high numbers for at least ten years. Considerable monitoring of fox numbers at the island has been undertaken by Clive Marks (DCNR, Keith Turnbull Research Institute) and the island is one of two sites being used for a pilot study of new fertility control techniques. The island is used as a natal site for which it is suitable by having high refuge values and low levels of nocturnal disturbance by humans. Individuals move across the river to and from the island on a regular basis for nocturnal foraging in the surrounding parkland and urban areas. As a result, foxes are present on the island in greater densities than the 2.8 ha of habitat could of itself support, and probably acts as a daytime refuge for foxes which range over a much larger area.

There are a number of major constraints to fox control on Herring Island. Foxes are abundant in the local area and the river is not a barrier to fox movement. Therefore control of foxes by fumigation, poison or other method is likely to be successful only in the short term. New foxes are likely to migrate to the island within a period of 10-30 days after control. Successful control of foxes on the island is therefore dependent upon preventing migration of foxes onto the island. The only way to effectively do this is by adequate fencing.

Fencing would need to encompass the entire perimeter of the island. It could be located at the foot of the levee bank, or located at or in the water's edge. Foxes are efficient climbers and effective fencing would require the following,

Fencing the island raises a number of concerns for wildlife management including reduced access to the island by native fauna particularly waterfowl access to the riparian fringe. Negative impacts on other amenity values of the island include: Fencing at the water's edge, either at the bank, or in the water, would have additional practical constraints. It would be highly prone to damage during periods of high flood, would collect water-borne detritus which may in the end provide 'ladders' over the fence, and may be surmounted by individuals during flood periods.

Due to all of the above constraints, and the absence of rare or threatened fauna within the island for which predation by foxes is a major threat, fencing is not recommended for the island.

In the long-term, the only possible practicable method of fox control on the island is fertility and genetic control measures. The efficacy of fertility control techniques currently being trialled is unknown, and will not be available for implementation in the short term.