In this scenario fleets of poachers are loose in the southern ocean thumbing their noses at conservation efforts
Naturally this is a rather seasonal issue. Few ships venture into Antarctic waters in the dark, cold winter months.
There are three levels of intervention.
1) Recording poaching in an attempt to gather evidence and identify ships so that they can be pursued by legal means.
2) Hinder poaching to prevent damage to the environment by illegal fishing
3) Board, arrest and tow.
The current approach is level one. This is carried out by OPVs at a cost of at least quarter a million dollars per sea day or P3K Orions at a cost of $84,000 per flight hour.
This site would recommend using LEO satellites to locate the ships. Using synthetic aperture radar an ocean going trawler is a relatively large target. This need not be a NZ satellite. Other nations sell time on LEO earth observation satellites. Some ships may even self report their position via the AIS system.
Having located the ships generally the lowest cost way to record them would be by using the Orion UAV platform. This would transmit video evidence immediately to base. It could also be used to relay warnings to cease operations on marine radio which would give the impression that a ship was in the vicinity. With the right amount of detective work a captain might learn a red notice was issued by Interpol for his arrest without ever knowing what had recorded the evidence or was relaying the message.
There are, of course, some issues that would need to be addressed. Most LEOs are equatorial. High latitudes are hard orbits for such satellites. Another issue is wing icing. The Orion aircraft would need de-icing added for Antarctic patrols and that would reduce range.
As an indicator of relative size the KV Svalbard alongside MV Praia de Santa Cruz a 2218T trawler
While its nice to pursue everything legally it doesn't do much to save the toothfish. Stopping fishing operations is the only way to do that. Unfortunately the current OPVs are simply too small to do that. At 2200T they are much smaller than larger trawlers and factory ships. That's where the Svalbard type EPVs come in. At 6300T plus ice breaking hulls they are built for mixing it in shoving matches. They also have high pressure fire fighting systems which can be mixed with Skunk to make life on board the target vessel even more unpleasant. If the seas are calm enough they can carry SH-2G helicopters to hinder fishing operations over a wider area. If necessary the ships can even tow captured vessels back to port.
Board and Arrest
Boarding in rough seas from a RHIB is only possible if the target vessel agrees to be boarded. Helicopters are another option but they too can be risky to launch and dangerous to maneuver over the pitching decks of fishing vessel. The only way to safely get Marines aboard might be using a Martin jetpack. The jetpack has a very small landing footprint. Boarding one Marine with a shotgun should be enough to convince the crew to give up. While a jetpack may cost twice as much as RHIB it is twice as hard to avoid. This is obviously an application that would require development by Martin Jetpack and perhaps investment by Defence Ventures.