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COSEWIC assessment and status report on the American Eel in Canada

Population size and trends

Studies on eel densities and sizes of components in Canada are limited. Types of available data vary among FEAs. Reported landings are available for all FEAs (see Limiting Factors and Threats section). All the data presented are compiled in Cairns et al. (unpubl. ms.). Spawning escapement in the American eel is considered to be the number of silver eels that survive fisheries, turbines, and other continental mortality factors to reach the open sea on their way to the spawning grounds.

FEA1 - Great Lakes and Western St.Lawrence(Ontario and western and central Quebec)

The number of eels in FEA1 (Figure 6) declined greatly from the mid-1980s through the 1990s (Castonguay et al. 1994a; Casselman et al. 1997; Casselman 2003). Commercial catches in Ontario declined sharply despite increased price per kg and fishing effort (Casselman 2003). Eel abundance has been severely reduced in Lake Champlain, the Ottawa and Richelieu river basins, the upper St. Lawrence River and Lake Ontario. Early declines have also been documented in three major tributaries on the US side of Lake Ontario: Oswego River (including Oneida Lake), Genesee River and Black River. Eel exclusion from these tributaries is due essentially to migration barriers.  Rising commercial catches in the Quebec portion of Lake St. Francis are in contrast to the overall trend FEA1. However, harvest in Lake St. Francis is quite small compared with that in other sections of the system (Casselman 2003) and might be associated with closure of the Ontario fisheries (J.M. Casselman, pers. obs.).

Figure 6.  Data sites and commercial fisheries within the ecological freshwater area FEA1, Great Lakes - Western St. Lawrence (records from Mandrak and Crossman 1992).

Figure 6.  Data sites and commercial fisheries within the ecological freshwater area FEA1, Great Lakes - Western St. Lawrence (records from Mandrak and Crossman 1992).

Indices of Juvenile Abundance

The longest-term data set on yellow eel recruitment in FEA1 is for upstream migrants ascending, during peak migration (31 days), the eel ladder at the Moses-Saunders dam (Castonguay et al. 1994a; Casselman et al. 1997; Casselman 2003). This ladder was first installed in 1974. Eels ascending the ladder had a mean age of about 6 in 1975 (Liew 1976), but mean age increased to about 12 in the 1990s (Casselman 2003). After a peak in 1982-1983, ladder counts dropped sharply and fell to record low levels in the late 1990s (Figure 7; Casselman 2003). The few eels that ascended the ladder in the 1990s were much larger and older (mean 493 ± 17 mm; 11.9 ± 1.1 years) than typical recruits before the decline (mean 363 mm ± 15 mm; 5.6 ± 0.1 years) (Casselman 2003). The number of juvenile eels climbing the eel ladder has declined from more than one million per year in the early 1980s to fewer than 4,000 annually (less than 60 eels per day) in most years since 1998 (J.M. Casselman, pers. obs.). The index showed upward movement in 2005, with a mean summer peak of 227 eels per day. Over the past 20 years (approximately one generation time for the St. Lawrence River-Lake Ontario component), recruitment at the eel ladder has fallen by three orders of magnitude (Casselman 2003) to approximately 0.2% of its levelin the early 1980s, and recruitment of young juveniles is now at minimal levels (J.M. Casselman, pers. obs.).

Figure 7.  Number of eels ascending the eel ladder per day at the Moses-Saunders dam in the upper St. Lawrence River at Cornwall, Ontario, during the 31 day summer peak migration period between 1974 and 2005 (from J.M. Casselman, OMNR). The insert shows mean daily summer and fall peak (31 days) counts between 1995 and 2005. In 2004, the fall peak (n = 274 per day) surpassed the mid-summer peak for the first time.

Figure 7.  Number of eels ascending the eel ladder per day at the Moses-Saunders dam in the upper St. Lawrence River at Cornwall, Ontario, during the 31 day summer peak migration period between 1974 and 2005 (from J.M. Casselman, OMNR). The insert shows mean daily summer and fall peak (31 days) counts between 1995 and 2005. In 2004, the fall peak (n = 274 per day) surpassed the mid-summer peak for the first time. However, these were not young eels but were larger individuals. Counts for 1996 are predictions from correlations between catch and eel ladder abundance

Young eels ascending climbing facilities on the west side of the Beauharnois dam were monitored in 1994-1995 and 1998-2005 (Bernard and Desrochers 2005).  Counts dropped from 24,721 in 1994 to 5,441 in 1998, and then rose to 51,694 in 2005.  The increasing trend in Beauharnois counts corresponds to the modest recent increases in the Moses-Saunders index of approximately 11,000 in 2004 and 2005 (J.M. Casselman, Queen’s University, pers. obs.).  The rising numbers at Beauharnois beginning in 1998 also correspond to a decrease in mean eel size, suggesting a declining mean age of eels that arrive at the dam.  Despite the recent increases, the number of eels counted through the west Beaharnois facility is still far short of the number needed to restore the annual passage that occurred at Moses-Saunders in the 1980s (ca. 1 million per year).  However, eels also move upstream from Beauharnois via the navigation locks, and the number using this route is unknown.

Upstream movements of yellow eels has been monitored at the Chambly dam on the Richelieu River since 1998 (Bernard and Desrochers 2005).  Numbers were high in the first two years the ladder operated, presumably because of pent-up demand by eels that had previously been unable to migrate.  Since 2000 there have been irregular increases, with 2,177 eels counted upstream in 2005.  Numbers of eels ascending the Chambly ladder are insufficient to restore historical abundance levels in upstream waters. The efficiency of this ladder is 60%.  The former commercial fishery in the Richelieu River (efficiency of 66%) harvested a mean of 34,000 kg (or approx. 23,000 individuals) of silver eels annually between 1920 and 1980.

Indices of Yellow Eel Abundance

Catch per unit effort (CPUE) in the trawl fishery for yellow eels in the Bay of Quinte (Lake Ontario), the commercial electrofishing index, and total reported landings for Ontario have declined in parallel with the Moses-Saunders recruitment index (Figures 8 and 7; Casselman 2003). The most recent electrofishing catches in eastern Lake Ontario are 0.5% of what they were in the late 1980s (J.M. Casselman, pers. obs.).

Figure 8.  Fisheries independent indices of abundance of yellow eels in FEA1 (from J.M. Casselman, OMNR): (A) bottom trawl CPUE from Bay of Quinte, Lake Ontario (1972-2004); (B) electrofishing CPUE from eastern Lake Ontario (1984-2004).

Figure 8.  Fisheries independent indices of abundance of yellow eels in FEA1 (from J.M. Casselman, OMNR): (A) bottom trawl CPUE from Bay of Quinte, Lake Ontario (1972-2004); (B) electrofishing CPUE from eastern Lake Ontario (1984-2004).

North AtlanticOscillation Index (NAOI)

The North Atlantic Oscillation Index (NAOI, National Center for Atmospheric Research 2005) describes the ratio of atmospheric pressures in the Azores and Iceland.  The NAOI is linked to a wide variety of biological processes in the North Atlantic Ocean (e.g. Jonsson and Jonsson 2004).  Eel recruitment in the past two decades to the upper St. Lawrence River and Lake Ontario is highly significantly negatively correlated with the NAOI when the appropriate age lags are taken into consideration (mean age of eels reaching the Moses-Saunders ladder is 9 years; ICES 2001, Casselman submitted; Figure 9). The standard procedure for examining the NAOI uses a fast Fourier transformed mean (Knights 2003). This provides an index of residuals, which can be compared with recruitment residuals. There is also a highly significant correlation between the NAOI and the lagged recruitment index of the European eel at Den Oever in the Netherlands (Knights 2003). Prior to the mid-1970s, lagged recruitment indices from both sites (and species) were relatively stable and were uncorrelated with the NAOI. After that period, strongly negative correlations appear (ICES 2001). However, short-term indices of early life stages or indices closer to the source of recruitment showed no apparent trends or correlations with the NAOI (Casselman, submitted).

Figure 9.  NAOI correlation with the recruitment index of the eel ladder with a 9 year lag (from J.M. Casselman, OMNR).

Figure 9.  NAOI correlation with the recruitment index of the eel ladder with a 9 year lag (from J.M. Casselman, OMNR).

Indices of Spawning Escapement

Quebec silver eel landings from the Richelieu River and the St. Lawrence estuary are considered under FEA1 since catches are composed principally of out-migrating females from lakes and rivers located upstream (Verreault et al. 2003; Verdon et al. 2003). Annual landings show dramatic declines since the 1980s (Figures 10 and 11). From 72.9 tonnes caught in 1981, the Richelieu River fishery collapsed in 1997 with a total catch of less than 5 tonnes and was closed in 1998 (Dumont et al. 1997; Verdon et al. 2003). In the Richelieu River, the decline has been partly related to the reconstruction of two old cribwork dams in the 1960s. No passage facilities for eels were provided, so upstream migration to Lake Champlain was impeded (Verdon et al. 2003). Between 1987 and 1997, average eel weight increased by 50%, which reflected an aging component that was not supplemented by recruits (Verdon et al. 2003). In the St. Lawrence estuary, total catch declined from 452 tonnes in 1980 to less than 82 tonnes in 2004 (Figure 10). Fishing effort in this area was relatively constant until 1996, and thereafter declined. Catch and effort have been recorded by two commercial fishermen in the estuary since 1985. CPUEs, calculated as eels harvested per m of net, decreased from 1985 to the late 1990s (Figure 10). CPUEs subsequently showed no consistent trend at the same time that effort decreased.

Figure 10.  Commercial indicators of silver eels within FEA1 (from Caron et al., submitted): (A) reported catch of silver eels in the Richelieu River (1975-1997); (B) reported catch of silver eels in the St. Lawrence estuary (1975-1984; 1986-2004); (C) CPUE and effort from two fisheries in the St. Lawrence estuary (1985-2004; from G. Verreault, MNRF, Secteur Faune Québec).

Figure 10.  Commercial indicators of silver eels within FEA1 (from Caron et al., submitted): (A) reported catch of silver eels in the Richelieu River (1975-1997); (B) reported catch of silver eels in the St. Lawrence estuary (1975-1984; 1986-2004); (C) CPUE and effort from two fisheries in the St. Lawrence estuary (1985-2004; from G. Verreault, MNRF, Secteur Faune Québec).

Figure 11.  Declared commercial catch of American eels in the St. Lawrence River (1920-2004) by sector in Quebec (from Caron et al., submitted).

Figure 11.  Declared commercial catch of American eels in the St. Lawrence River (1920-2004) by sector in Quebec (from Caron et al., submitted).

A fisheries-independent abundance index is provided by catches of eels at the Saint-Nicolas experimental trap fishery, near Quebec City, between 1971 and 2004 (Castonguay et al. 1994b; de Lafontaine et al., submitted). Most eels caught in the fall (1 September-1 November) are silver eels, although colour has not been systematically recorded. Fall catches showed intermittent high peaks in the 1970s (Figure 12). Fall catches were relatively stable between the late 1970s and the late 1990s. Fall catches in the late 1990s and early 2000s again showed intermittent peaks. September-October catches in a commercial trap (fixed gears), fished with relatively constant effort 1 km upstream from the experimental trap, declined substantially between the mid-1970s and the late 1990s (Figure 12). Mean weight of eels in the experimental trap has been increasing since measurements began in the mid-1990s (de Lafontaine et al., submitted), which suggests that mean age of eels taken in this gear is increasing.

Figure 12.  Total catches of eels recorded at Saint-Nicolas, near Quebec City (from de Lafontaine et al., submitted) (A) experimental trap fishery (1971-2004); (B) commercial fishery 1 km upstream (1977-1998). The beginning date of the commercial fishery varied among years but the fall catch was always entirely covered.

Figure 12.  Total catches of eels recorded at Saint-Nicolas, near Quebec City (from de Lafontaine et al., submitted) (A) experimental trap fishery (1971-2004); (B) commercial fishery 1 km upstream (1977-1998). The beginning date of the commercial fishery varied among years but the fall catch was always entirely covered.

Spawning escapement and fishing mortality on silver eels originating in FEA1 was estimated in 1996 and 1997 by means of mark-recapture experiments in the St. Lawrence estuary (Caron et al. 2003). Migrants upstream of Quebec City were estimated to number 488,000 in 1996 and 397,000 in 1997. Since exploitation rate by the commercial fishery in the estuary was estimated to be 19% in 1996 and 24% in 1997, total spawning escapement was estimated to be 396,000 in 1996 and 302,000 in 1997. Based on these data, Verreault and Dumont (2003) estimated silver eel departures from the upper St. Lawrence River – Lake Ontario from a model based on eel passage, commercial landings, percentage of migrating eels in the commercial catch, and turbine survival rate. Departures were estimated to be 525,000 in 1996 and 424,000 eels in 1997.