Project Tuna

Investigating the potential for an eel aquaculture industry

original article at ruraldelivery.net

Eel aquaculture has been the subject of debate in New Zealand for many years. Early attempts at culture failed through a combination of inappropriate scale and poor technology choices. More recently, concerns over the sustainability of harvesting glass eels have hindered development.

Māori are significant eel quota holders and eel (or tuna) are a highly valued customary, recreational and commercial species for Māori. There are currently a number of iwi and Māori organisations who are seeking to participate in eel aquaculture at a significant commercial scale for both customary and commercial benefit.

Although some technical hurdles remain to be overcome there is a general consensus that eel aquaculture offers a significant development opportunity for Māori. Based on current market prices, a 100 tonnes farm (requiring 1 million/ 200 kg glass eels) has the potential to yield over $4.5 million in export income. Some estimates suggest that the New Zealand eel farming industry could be as large as 4,000 tonnes. 

The most significant bottleneck to development of an eel aquaculture industry remains the need to ensure long-term sustainable access to glass eels for on-growing. Internationally, the decline of wild eel fisheries has resulted in over-harvesting of glass eels for farming; a situation that needs to be prevented here.

Hatchery culture of eels is difficult and is unlikely to ever be commercially viable. So, overcoming the hurdle of accessing wild glass eels stocks in a sustainable manner will be critical in order to establish and secure a highly valuable eel export industry in New Zealand.

A three-year long project looking at the opportunity for wild harvest of glass eels has been undertaken, to support the development of an eel aquaculture industry. Scientist Dr Anke Zernack was the project manager in the Rangitane North Island Fisheries Trust working with Terry Hape (Rangitane iwi), among others.

The project’s goal was to move from an extended phase of data gathering around glass eel recruitment to one of positive action towards sustainable commercialisation of eel aquaculture. This requires underpinning science to develop innovative tools and technologies for accessing and on-growing eel stocks that are not site-specific and give regulatory authorities and industry confidence in the industry’s sustainability.

Dr Zernack says the project set out to answer four questions: 

1.   What quantities of glass eels are available? 

While large rivers will ultimately be the most important for sustaining a New Zealand short-fin eel aquaculture industry, the project focused on smaller catchments in the Rangitāne North Island (RNI) rohe.

Working in a small catchment allowed effective testing of new methods, including; effective glass eel capture methods, implementation of repeatable juvenile eel monitoring approaches, and development of appropriate eel stock assessment models.

Glass eel fishing was specifically targeted at predicted peak glass eel migration events, i.e., high tide conditions, during new or full moon events, in the hours of darkness. The schedule for monitoring glass eels was based on approximately 32 fishing days per fisherman/catchment, between late July and early December.

2:   What are sustainable levels of harvest? 

This assessment focused on juvenile short-fin eels because the factors that influence the survival of older and larger eels (such as competition from other eels, limited suitable cover, predation) were considered to be largely natural and unrelated to glass eel harvesting in the study.

Eel population surveys were conducted annually (in 2014, 2015 and 2016) in the Whakataki and Castlepoint catchment in the summer/autumn months following glass eel fishing. To determine the abundance of juvenile short-fin eels in the study reaches, density estimates were adjusted to take into account smaller eels that were buried in the substrate during the day and inaccessible to electric fishing current.

The age compositions of juvenile short-fin eels present were also estimated to the total numbers present in both waters.

3:   Can we develop means of separating short-fin and long-fin glass eels for culture?

The short-fin eel is the species most amenable to eel aquaculture, but, while short-fins dominate most glass eel catches, short-fins and long-fins occur in mixed shoals at the glass eel stage.

To provide single-species glass eel seed stock to eel aquaculture and to protect longfin eel stocks, it is essential to physically separate the two species with a high degree of certainty. Given that both species in the wild appear to make specific choices about the type of stream they migrate, the study investigated a potential behavioural approach to separate the species using their responses to olfactory cues. Species separation trials were undertaken in years one and three of the project.

4:   Is the culture of New Zealand glass eels to market size economically viable?

To address the commercial viability and performance of the New Zealand glass-eel stock under intensive culture conditions, a small glass eel holding unit was developed at Oringi Business Park, near Dannevirke.

Initially the small recirculating aquaculture system consisted of 6 eel rearing tanks, designed to accommodate up to 100 kg of eels. It was upgraded prior to Year 2 trials to improve water quality and temperature control within the system and provide a more robust system that is easier and safer to operate.

Water quality parameters within the system (temperature, oxygen, pH and salinity) were recorded automatically via probes connected to a data logger to ensure system parameters remained within the normal operating limits for eel culture. In addition Ammonia, Nitrite, Nitrate and Chlorine levels were measured weekly using chemical test kits.