The long and the short of it: looking after the needs of native eels

Don Jellyman presents a female longfin eel caught in a small stream in the Wairarapa. The eel weighs 4.5 kg and is 1.2 m long; we estimate that she is about 45 years old. (Photo: Eric Graynoth)

The cover index is a measure of instream and streamside cover. As the index rises, so does the number of large longfin and shortfin eels found in rivers and streams during the day.

Pulling out an aggregation of 24 longfin eels (26-47 cm long) from beneath a willow bush in the braided Aparima River. (Phoro: Eric Graynoth)

Eric Graynoth and his colleagues are studying the behaviour and habitat requirements of longfin and shortfin freshwater eels.

Throughout New Zealand, eels sustain important commercial, recreational, and Māori customary fisheries. They are also important in preserving natural predator-prey inter-relationships and supporting aquatic biodiversity. There are two principal species of freshwater eels in New Zealand. Shortfin eels (Anguilla australis) are most abundant in coastal rivers and lakes, while longfin eels (A. dieffenbachii) are more widely distributed in inland rivers and high-country lakes. Female eels grow larger than the males, and longfins are larger than shortfins: adult female shortfin eels average about 1.8 kg, while female longfins grow up to 15 kg and 1.5 m long.

In recent years, eel stocks have deteriorated due to land development, wetland drainage, and the construction of dams and weirs that block eel migrations. Large female eels are now scarce in heavily fished areas and are most abundant in national parks and streams that are too small to fish commercially. In many waters, large female eels have been replaced by high numbers of much smaller males, and this has important implications both for the fisheries and for the aquatic ecosystem and biodiversity. Māori fisheries have been severely affected by the scarcity as large eels were a major mahinga kai (food) for local iwi. Tuna (Māori for eel) are caught mainly in summer, but also in autumn during their heke (spawning migrations) to the sea.

The key focus of our research is determining the instream flows and aquatic habitats required to restore customary fisheries for these large eels. We are studying the eels' nocturnal and daytime behaviour and developing mathematical models to predict the flow regimes and other habitat features necessary to maintain adequate stocks and fisheries.

Daytime behaviour

Daytime surveys of eels tend to underestimate the value of shallow riffle-and-run habitat for large eels because eels more than 30 cm long are usually confined to deep pools and cover during the day. They hide in burrows in the river bed and banks or under woody debris and aquatic plants. Extraordinary aggregations – of up to 10 eels per square metre – can be found under uprooted willow trees and other debris in braided rivers where there is a shortage of other cover. The only eels we usually see during the day are large female longfins that are presumably too big to be eaten by black shags and other predators.

There is a very strong relationship between daytime densities of eels and the amount of instream and bank cover. To measure this we use a cover index that ranges from 0 where there is no suitable cover to 10 where the stream bed is completely covered with aquatic plants, woody debris, or other material. We've found that large eels are up to 1000 times more likely to be present during the day in locations with a cover grade of 10 than in locations where there is no cover (see graph at right). During the day, large eels are most abundant in deep, slow-flowing water, although they can be found in shallow water if there is sufficient cover. The composition of the stream bed generally has little influence on large eels, as they are too big to hide within the gravels and cobbles, although shortfin eels can burrow into mud.

Nocturnal behaviour

Our nocturnal surveys in various Canterbury and West Coast streams and rivers have shown that eels are widely distributed over most of the river bed at night, avoiding only the fastest runs and riffles. When dusk falls, eels leave their daytime shelters in pools and start foraging in the riffles and runs for small fish and aquatic invertebrates. They can move into quite fast water while feeding. For example, once when we were sampling at dusk, we were surprised to hear loud splashing coming from shallow riffles as eels left the pools and swam upstream to feed. They had their heads down in the riffles, feeding amongst coarse gravels and cobbles just below the main force of the current. Later in the night, the eels dispersed widely into open exposed positions in shallow runs and the margins of pools, where they would never be seen during the day. They show little preference for cover at night.

Flow requirements

The abundance and biomass of eels is influenced by a complex suite of biological and physical factors. Some of these factors, such as food supplies and instream resting and feeding habitats, are strongly related to flow regimes, the variations in the quantity and velocity of water moving through a stream or river. We have developed various mathematical models to predict how eels are likely to be affected by changes in flow regimes caused by land drainage, irrigation, and power-development schemes.

The models include depth, velocity, substrate, and eels' cover preference. A simulation we ran for a small braided river showed that the amount of daytime resting habitat rapidly rose as flows increased and peaked at about 1 cumec (cubic metre a second). By contrast, the amount of nocturnal feeding habitat rose much more slowly to a maximum at about 3 cumecs. We concluded that the amount of nocturnal feeding habitat in this river at low flows may be limiting the biomass of eels.

We are also using River Environment Classification (REC) and GIS-based models to predict the abundance and biomass of eels in different waters and to assess the value of national parks as reserves for eels. And we've developed bioenergetic models to predict how changes in temperatures and food supplies might influence eel growth rates and production. These bioenergetic models are data-intensive and have limited applicability, but they provide a theoretical background for our simpler, habitat-based models.

Teachers’ resource for NCEA AS: Biology 90161 (1.1), 90164 (1.4), 90457 (2.1), 90460 (2.4), 90461 (2.5), 90483 (2.9), 90713 (3.1), 90716 (3.4). See other curriculum connections at