Eels at the Edge: Science, Status, and Conservation Concerns

<em>Abstract.</em>—A lack of knowledge on the transition from the resident to the migratory phase has led to a series of studies on the silvering process. Silvering marks the end of the sedentary growth phase and the beginning of the migratory phase. A sixstage classification was developed to describe the physiological and morphological events that occur during this metamorphosis and the subsequent migration. Stages corresponded to a growth phase (I to FII), a premigrant stage for females (FIII), and migrating stages for both sexes (FIV, FV, and MII). Here, the objective was to develop a “silver index” using only external measurements to assess the degree of metamorphosis of eels, based on the same data set that was used in the former study. It consisted of a large number of both resident and migratory eels that were sampled at different times of the year with different types of fishing gear and at several locations representing various types of habitats. Discriminant Analysis was applied on external measurements only: (body length, body weight, pectoral fin length, and eye diameters). Total percentage of correct reclassification into the six silvering stages was 82%. The silver index (classification functions) was able to identify 91% of the migratory eels. This method, associated with proper sampling, could be utilized for the quantification of potential spawners given that they all reach their spawning grounds in the Sargasso Sea.

Eels at the Edge: Science, Status, and Conservation Concerns

<em>Abstract.—</em>The American eel <em>Anguilla rostrata </em>is declining in the St. Lawrence River watershed, where sex ratio is highly unbalanced in favor of females. Since the American eel is a panmictic species, this demographic dominance is implicated in reproductive potential of the species. The major objective of this study was to evaluate the reproductive strategies of five subpopulations of female eels. It was assumed that fecundity varies among subpopulations according to their migration distance because of the tradeoff between energy allocated to gonads and to somatic tissues. Thirty female silver eels were collected from each of five locations in the St. Lawrence watershed 2,850–4,300 km from the spawning area. Among subpopulations, mean length ranged from 67.9 to 104.3 cm, weight from 595 to 2,366 g, fecundity from 6.5 to 14.5 million oocytes, age from 20 to 23 years, gonadosomatic index from 2.9 to 4.1%, and somatic lipid content from 17.5 to 21.7%. Because of panmixia, no genetic influence on intersite variability is expected. Environmental differences in growth habitats and individual fitness might determine acquisition and allocation of resources, as well as subsequent variability in traits that would affect reproduction. In contrast to previous hypotheses, variations in such traits were attributed to eel size rather than migration distance. The number of oocytes per silver eel was positively correlated with length rather than negatively correlated with migration distance. In the St. Lawrence watershed, large eels are highly fecund regardless of their distance from the spawning ground.

Eels at the Edge: Science, Status, and Conservation Concerns

<em>Abstract.</em>—The New Zealand eel fishery comprises two species, the shortfin eel <em>Anguilla australis </em>and the New Zealand longfin eel <em>A. dieffenbachii</em>. A third species, the speckled longfin eel <em>A. reinhardtii</em>, is present in small numbers in some areas. Major fisheries in New Zealand are managed under the Quota Management System. Individual transferable quotas are set as a proportion of an annual total allowable commercial catch. The Quota Management System was introduced into the South Island eel fishery on 1 October 2000 and the North Island fishery on 1 October 2004. Freshwater eels have particular significance for customary Maori. Management policies allow for customary take and the granting of commercial access rights on introduction into the Quota Management System. Eel catches have remained relatively constant since the early 1970s. The average annual catch from 1989–1990 to 2001–2002 (fishing year) was 1,313 mt. Catch per unit effort remained constant from 1983 to 1989 and reduced from 1990 to 1999. Statistically significant declines in catch per unit effort for New Zealand longfin eel were found in some areas over the latter period. For management, an annual stock-assessment process provides an update on stock status.

Eels at the Edge: Science, Status, and Conservation Concerns

<em>Abstract.—</em>Yellow-phase American eels <em>Anguilla rostrata </em>migrate upstream extensively in Atlantic coastal river systems. However, few studies have focused on movements of large yellow American eels near dams in upper watersheds of Atlantic coastal rivers. We examined relationships between environmental variables (stream flow, water temperature, and lunar phase) and movements of radio-tagged yellow American eels (518–810 mm TL) near Millville hydroelectric dam in the lower Shenandoah River drainage of the upper watershed of the Potomac River system, West Virginia. Movements of yellow American eels differed seasonally. Water temperature and stream flow were associated with upstream migration during spring. Downstream movements during fall coincided with decreasing water temperatures and darker nights near the new moon. Eels overwintered in thermal refuge areas near tributary mouths. Localized irregular upstream and downstream movements during summer occurred near dusk and dawn and possibly reflected crepuscular foraging. Our study in the Potomac River drainage suggests the need for upstream eel passage at hydroelectric facilities when spring water temperatures exceed 15°C.

Eels at the Edge: Science, Status, and Conservation Concerns

Abstract.—The Japanese eel, Anguilla japonica, is an important food fish in East Asia, and catches of glass eels and of eels in freshwater appear to have declined dramatically in recent decades, causing increasing concern for the health of wild stocks. During that time, research efforts to understand its biology have progressed considerably. The spawning area was successfully outlined to the west of the Mariana Islands in 1991, and other research suggests that their recruitment success may be related to El Niño events, which appear to affect the transfer of leptocephali from the north equatorial current into the Kuroshio Current. Otolith microstructure and microchemistry studies have revealed various aspects of their early life history that relate to their oceanic larval migration. The discovery of sea eels that live in marine habitats without entering freshwater may change the common understanding of freshwater eel ecology and affect management plans. Most genetic studies suggest that the Japanese eel is composed of a single panmictic population throughout East Asia. Therefore, international management is needed among the countries of China, Taiwan, Korea, and Japan, where glass eels recruit from a common stock and are used extensively for aquaculture.

Eels at the Edge: Science, Status, and Conservation Concerns

<em>Abstract.</em>—In Taiwan, there has been a shortage of local Japanese eel <em>Anguilla japonica </em>elvers for culture, so culturists have imported American eel <em>Anguilla rostrata </em>(Le Sueur) elvers from North America to meet their needs. From 1999 to 2001, six exotic adult American eels were found in the estuary of the Kaoping River of Taiwan that had escaped from aquaculture ponds as young eels and stayed in the river until silvering. This study compares growth performance and migratory behavior, using otolith strontium (Sr)/calcium (Ca) ratios of those six American eels with cohabitating Japanese eels and American eels in North America. Regardless of sex, mean age at maturity of the exotic American eels was greater and mean annual growth rate was less than that of Japanese eels in Taiwan and similar to that of American eels in the southern United States. Sr/Ca ratios at the otolith edge of the six exotic American eels, which recorded their salinity history, increased significantly. Furthermore, four of the six exotic American eels spent more than one year in the high-salinity estuary. Their extended residence in the estuary may be due to a delayed spawning migration resulting from a failure to orientate and migrate properly to their native spawning site.

Eels at the Edge: Science, Status, and Conservation Concerns

<em>Abstract.</em>—Forty thousand American eel <em>Anguilla rostrata </em>elvers were released in a 400-ha lake in an eel-free watershed in eastern Québec in 1999. Subsequent sampling enabled the measurement of poststocking movements, growth, and sex ratio. Populations showed limited movements, and occupancy extended less than 3 km into inflowing tributaries by 2003. Annual growth increments in the lake (118 mm/ year) were the highest reported for the species, but growth increments in rivers (40 mm/year) were typical of those found elsewhere. Four of seven eels whose sex could be determined were females, in contrast to other sites in the St. Lawrence watershed where females are more than 99% of the population. American eel translocation to growth areas that have been blocked by artificial barriers may be a useful means to increase production of silver eels.

Eels at the Edge: Science, Status, and Conservation Concerns

<em>Abstract.—</em>Polymorphic microsatellite loci as genetic markers were used to reject the null hypothesis of panmixia for the Japanese eel, <em>Anguilla japonica</em>. Observed heterozygosity showed slight heterozygote deficiencies over all loci. One of the eight loci (MS-4) in one sample showed departure from Hardy-Weinberg equilibrium. Unbiased Nei’s genetic distances ranged from approximately 0.058 to 0.134. A slight genetic differentiation was determined by <EM>F</EM>_ST and <EM>R</EM>_ST statistics when adjusted with Bonferroni correction. Although isolation by distance is often observed in marine species, its use as a null hypothesis seems questionable. Although the freshwater eel is categorized as a catadromous fish, the value of genetic diversity obtained fell within that of marine fishes. A higher correlation (<EM>P </EM>< 0.001) resulting from AMOVA supports the separation of Japanese eels into two management units: a low-latitude group (Shantou, Tanshui, and Fangliao) and a high-latitude group (Daecheon-myon, Yalu River, Hangzhou, and Mikawa Bay). Such a population subdivision will be useful for further applications of fisheries conservation and management in the northwestern Pacific Ocean.

Eels at the Edge: Science, Status, and Conservation Concerns

<em>Abstract.—</em>The two main species of freshwater eels in New Zealand, the shortfin <em>Anguilla australis </em>and the endemic longfinned eel <em>A. dieffenbachii</em>, are extensively commercially exploited and also support important customary fisheries. Since there are no commercial glass eel fisheries in New Zealand, other indices must be used to indicate changes in recruitment over time. While there is some anecdotal evidence of reductions in glass eel recruitment, there is evidence of poorly represented cohorts of longfins within some populations, and modeling of these data indicate a substantial reduction in recruitment over the past two decades. Growth of both species is typically slow at 2–3 cm per year, meaning that both species are susceptible to commercial capture for many years until spawning escapement. Extensive commercial fishing has resulted in more substantial changes in length-frequency distributions of longfins than in shortfins; likewise, regional reductions in catch per unit effort are more significant for longfins. Theoretical models of silver eel escapement indicate that longfin females are especially susceptible to overexploitation. Shortfins would have been more impacted than longfins by loss of wetlands, but the impact of hydro stations on upstream access for juvenile eels and downstream access for silver eels would have been more severe for longfins. Overall, there is no clear evidence that the status of shortfin eel stocks has been seriously compromised by the extensive commercial eel fishery, but there is increasing evidence that longfins are unable to sustain present levels of exploitation.