Experienced pāua divers will be aware that some bays, which are now home to only a few sparsely distributed pāua, previously produced big pāua catches. No matter how much time passes or how well pāua populations along the rest of the coast are rebuilding, certain areas just never seem to recover. Why is this and what can be done about it?
The problem is that in these depleted areas there are not enough mature adults to support viable spawning. Pāua are broadcast spawners that need a critical mass of mature adults to breed successfully. Because pāua don’t move from one bay to the next and the larvae settle very close to where they were spawned, an area that is depleted becomes stuck in a never-ending cycle of poor spawning that prevents recovery.
The only way to break this cycle is to transfer more pāua into the area so that a viable spawning population can be established again. The pāua industry has developed several different techniques for enhancing pāua fisheries, which we will explore in a series of articles. In this first article we look at a technique known as ‘out-planting’. Out-planting involves selecting brood stock (parent pāua) from the area that will be out-planted, and using them to grow juvenile pāua in land-based hatcheries.
The juveniles are grown to a size that offers them the best protection from predators such as fish, starfish, and crayfish. In New Zealand we have typically outplanted pāua when they reach 10-15mm in length, which is why the technique is sometimes referred to as reseeding.
Although it’s hard to determine precisely, we estimate that over the first few months it is likely up to 50 percent of the out-planted juveniles are killed by predators. Four or five years later, when they reach harvestable size, only 10 to 15 percent of the original pāua will still be alive. Hatchery-reared pāua are particularly vulnerable to predation because they are not used to predators. Also, the smaller the juveniles are, the more predators can fit them in their mouths and eat them. Whatever the reasons, out-planting in New Zealand is not achieving good survival rates, so we’re looking into how other countries have overcome this problem. Western Australia is a good source of research about successful out-planting.
The Western Australians have a system of artificial reefs so scientists can readily survey the survivorship of juvenile pāua (abalone) and continue to monitor them as they grow to adulthood. The monitoring results show that when abalone are out-planted at 40mm, only 20 percent are eaten by predators over the first three months. This suggests that increasing the size of out-planted pāua should greatly enhance survival rates.
Another important result from the Australian programme is that once the juveniles adapt to their new surrounds their mortalities are greatly reduced. After the first three months, survival and growth in the outplanted population was the same as in the wild population. By harvest time, the Western Australians can expect 50 percent survival – a lot more successful than in New Zealand. The lesson from this research is that increasing the survival rates over the first three months from out-planting is the key to success. The challenge for us in New Zealand is to develop an economically-viable method of growing pāua to 40mm for out-planting.
This will require a two-step process, with the first step being to grow the juveniles in a land-based hatchery for six months until they are 10mm long. The second step is to transfer them to a marine farm where they can be grown for another year to reach 40mm. While New Zealand is proficient in land-based hatchery technology, we have no marine farms producing juvenile pāua. However, we can adapt technology from the experiences of countries like China, where the majority of abalone production is from marine farms. Although it will take more experimentation, we have no doubt that our investment in outplanting will be worthwhile. Increasing pāua biomass up in bays that are currently not contributing to a fishery will produce more pāua to support commercial, customary, and recreational catches. With a larger biomass of mature adults, we can be confident that spawning will be successful and that pāua fisheries will continue to be sustainable.