One of the research projects undertaken by Erina Watene-Rawiri while at NIWA was the study of glass eels. This was done over 4 years. One of the reasons for the research was to compare volumes with historical data to see if there has been a decline in numbers. Another reason was to explore the possibility of producing eels commercially.
Back in 2002, we undertook a research project where we were monitoring the glass eel recruitment to the Waikato River.
I was working for NIWA, and we worked with the locals down at Port Waikato. Huakina Development Trust was the group that we engaged with. We fished for 5 months of the year over 4 years, and we were looking at when the glass eels were moving, what the volumes were and what the triggers may have been for migration.
There were a couple of objectives as to why we undertook the research. One of them was to compare historical data and to see if there actually was a decline happening, and the second reason we undertook it was to see if there was enough recruitment happening so that we were able to take a portion of the glass eels off and do aquaculture with them commercially.
We found out that the volume of glass eels coming up into the Waikato River had declined considerably since the earlier research that occurred in the 70s. So back in the 70s and even before that, there were records of glass eel runs being a metre wide, a metre deep, solid full of glass eels heading up the river for 3 or 4 days at a time. When we did our research, we got 10 minute little blips of maybe a few thousand eels happening at the top of a spring tide. So anecdotally and through our research, we found that the volumes of glass eels had declined.
While we were monitoring the recruitment of glass eels, we also took a small portion of 5 kilos, which is less than half a bucket, up to Bream Bay, which is our NIWA aquaculture facility, and we on-grew the glass eels to see how quickly we could get them to grow to a commercial size. In the wild, it can take 15 plus years for eels to become a commercial size. The large longfin eels can be 60 plus years. So we have six 1-tonne tanks up at Bream Bay, and we were feeding the glass eels artificial food, and some of them were commercial size within 2 years.
Because the recruitment of glass eels wasn’t significant enough, there was a bit of opposition, particularly from local iwi, to commercial big players coming in and taking seed stock from the wild and then using it for commercial on-growing of glass eels. There was a feeling from the locals that the glass eels should be populating our lakes and rivers first rather than generating a commercial return.
Following on from our on-growing project that we had up at Bream Bay, we brought the eels back to the Waikato River and released them. I think that’s a better way forward in terms of restoration and enhancement – we could take glass eels from the wild, on-grow them and then put them back into the wild later to support the wild population.
Dave Allen, NIWA, Taihoro Nukurangi
Glass eel still courtesy of the Vertebrate Zoology Division, YPM ICH.023326, Peabody Museum of Natural History, Yale University, New Haven, Connecticut, USA
NZ Eel Processing Co Ltd, Te Kauwhata
The Waikato Tainui College for Research and Development acknowledges the financial support given by the Waikato River Cleanup Trust Fund which is administered by the Waikato River Authority.
The Waikato River Cleanup Trust does not necessarily endorse or support the content of the publication in any way.