Doug A Dixon explains why the American eel is becoming a new consideration for US hydro power

US hydroelectric plant operators have been protecting and providing upstream and downstream passage for fish for years, but now protection of an entirely different sort of fish — the American eel — poses a new challenge.

The need for protection arises from a major decline in the American eel population. Historically disregarded in fisheries management efforts, US federal and state resource agencies are now developing management plans for harvest control, protection and stock restoration. Providing upstream and downstream passage for eels at dams and minimising turbine-induced mortality are main elements of the management effort of US hydro power operators. In accordance with the goals of promoting the development of scientifically sound policies, regulations and mitigative actions to comply with regulations, the Electric Power research Institute (EPRI) has initiated information-gathering and research efforts on the American eel to keep the dialogue and analysis efforts appropriately informed.

The concern over the health of the American eel stock began during the early 1990s. Two events contributed to this. Firstly, similar declines in the stocks of European and Pacific eels, which supported strong commercial fisheries in Europe and the Far East, resulted in the rapid development of a lucrative harvest of immature eels along the east coast of the US for overseas shipment. Overseas, immature eels were grown to market size at culturing facilities. The immature eels were sold to overseas buyers for up to US$500 a pound. Secondly, anecdotal data suggested a dramatic decline in American eel abundance had occurred in some regions and most notably in Canada. The Ontario Ministry of Natural Resources reported that the number of eels ascending an eel ladder on the Canadian side of the Moses-Saunders international hydro-electric project on the St Lawrence river during upstream migration dropped from 20,000 to 25,000 a day in the early 1980s, to about 100 a day in 1995.

Why the decline?

Several initial hypotheses were offered for the cause of the population decline. These included oceanic influences (changes in Gulf stream flow patterns), pollution, over-fishing, habitat alteration and loss, disease and impacts from hydroelectric operations. The hydroelectric focus results from partial blockage of historically available upstream freshwater habitat by dams (for immature eels), and from mortality incurred through turbine passage as silver eels migrate back to the sea. A lively debate, largely in the absence of scientific information among resource agencies, industry and environmental organisations has highlighted the need for different types of protection and passage.

In 1998, EPRI initiated a project to gather all relevant technical information on American eels. This effort focused on a review of life history and behaviour; population dynamics and stock status in North America; direct and indirect impacts of hydroelectric operations; existing protection and passage technology; and research needs for resolution of major impact and mitigation issues.

EPRI’s research effort was completed and published in early 1999. In addition to accomplishing a comprehensive review of recent technical literature on American eels, the report’s authors concluded the following:

•Analysis of time-series trends of eel abundance, compiled over a broad geographic range (Nova Scotia, Canada to Virginia, US), suggests a continent-wide decline in American eel abundance. The apparent decline, however, could also be an artifact of variation in recruitment, with stronger year classes occurring during the 1960s and 1970s than during other periods of time (some historical records suggest that there were indeed strong year classes during the late 1960s and 1970s. Limited data, however, precludes the testing of this latter hypothesis).

•The complexity of this species’ life history precludes the application of standard fisheries assessment and management approaches. Furthermore, basic population parameters such as annual survival, rate of changes in biological stages, factors that affect sex determination and recruitment are poorly quantified and understood.

•Potential indirect impacts to eels would most likely arise from changes in habitat availability altering density, growth and sex ratios, although no impacts of that nature were documented in the literature.

•Relative to the direct impact of impeding upstream migration, the complexity of the eel life history and natural patterns of distribution, and their ability to colonise heavily impacted basins, makes it difficult to quantify the effect of hydro development on habitat colonisation.

•Direct mortality due to turbine passage of downstream migrating eels has been estimated to range from 6% (Francis turbines) to 37% (Kaplan turbines).

•Significant advances in effective upstream passage for elvers and yellow eels have occurred. However, due in part to the behavioural variability of silver eels, substantial uncertainty exists regarding the effectiveness of traditional mitigation measures used for other migratory fish (eg salmon) for minimising downstream passage impacts (turbine mortality) on eels.

The study was unable to resolve the factors that may be responsible for the decline, particularly the effects of hydroelectric operations in relation to commercial harvest and other potential causes. The study, however, was able to focus subsequent analysis and debate on scientific information and on research to obtain that information where it is absent.

Developing a management plan

Subsequent to publication of the EPRI report, the Atlantic States Marine Fisheries Commission (ASMFC), issued a draft Fisheries Management Plan for American Eels. The draft plan, using much of the information provided in the EPRI report, takes a conservative approach to limiting sources of mortality (eg commercial harvest limits and turbine mortality) and enhancing recruitment and restoration of available habitat. One of the major objectives of the ASMFC plan is to institute a long term monitoring programme by the states to collect population information. This supports future assessment of population trends and determination of factors that affect the American eel population. The plan also addresses commercial harvest limits (almost all east coast states have banned the lucrative glass-eel fishery and curtailed the yellow-eel fishery), harvest reporting and export monitoring.

The draft ASMFC plan has broad implications for the eastern US hydro industry. Hydro projects on any streams, rivers and lakes throughout eastern North America represent a potential source of impact to the American eel. Habitat restoration may require provisions for upstream passage, and minimising turbine mortality may require turbine passage deterrents (such as screens and/or behavioural barriers) and downstream fishways. Preventing outmigrating or silver eels from passing through turbines and guiding them to downstream fishways remains a technical challenge. Information on the timing of outmigration, swimming behaviour and response to physical and behavioural deterrents is limited.

Future research and activities

EPRI, as a follow-up to its initial scoping study and in recognition of the importance of developing effective downstream eel fishways, has launched several additional research and information gathering initiatives, including the First International Symposium on Catadromous Eels, 21-22 August 2000 in St Louis, Missouri, US.

As part of this research, EPRI will also focus on collecting information on the effects of turbine passage of eels, specifically the individual component stresses (pressure, turbulence, turbine blade strikes) that contribute to turbine passage mortality. This additional information will provide data to the US Department of Energy’s efforts to develop Advanced Hydropower Turbine Systems (AHTS). AHTS are being developed to minimise injury and mortality of fish during turbine passage. Results of this review will be available in early 2001.

Life history of American and European Eels

The American eel (Anguilla rostrata) and European eel (Anguilla anguilla) are closely related. Each species is composed of a single population that spawns in ocean waters but has progency that grow to maturity in estuaries and freshwater streams, rivers and lakes (catadromous). The American and European eel spawn in an overlapping region of the mid Atlantic Ocean known as the Sargasso Sea. Newly hatched larvae drift with the Gulf Stream towards the North American and European coastal estuaries. During this drift period they transform to the ‘glass’ eel stage (unpigmented or transparent) followed by an elver stage (pigmented). Within a few months, the American eel transforms to the ‘yellow’ eel stage during which period it actively seeks colonisation of coastal, estuarine and freshwater habitat. The transformation to yellow stage by European eels is believed to take a considerably longer period, possibly one to two years. Following this colonisation, an extended life stage begins that may last from 5-30 years or more. During this period eels grow but do not mature sexually. Sexual maturation begins at the end of the yellow stage. This involves enlargement of fins and eye diameter, change of pigmentation from olive-yellow-bronze to olive-silver, and development of sex organs. Because of the pigmentation change this period is known as the ‘silver’ stage. Silver eels actively migrate from freshwater, estuaries and coastal areas back to the Sargasso Sea where they spawn and die, while their progeny begin their life cycle again.