James D. McCleave

Species assemblages of leptocephali in the Subtropical Convergence Zone of the Sargasso Sea

Species assemblages of leptocephali are described in relation to density in the Subtropical Convergence Zone (STCZ) of the Sargasso Sea. Seven transects across fronts were made during four cruises in the late winter or early spring of 1983, 1985 and 1989. About 50 species from 13 families of eels were collected at 66 stations, but fewer than 10 species were abundant in all transects. Four oceanic species, two Anguilla species and Conger oceanicus appeared to be spawning in the STCZ. Leptocephali of most species whose adults inhabit the continental shelf were consistently large in size and were more abundant at or south of fronts and in the western transects. Leptocephali of the two most common oceanic species, Nemichthys scolopaceus and Serrivomer beanii, and the most common shelf species, Ariosoma baleaticum, were also more abundant in the western transects, but were abundant at some stations on both sides of fronts. Discontinuities in the assemblages of Anguilla and most shelf species occurred at the location of fronts that formed at the northernmost extent of southern Sargasso Sea surface water (defined as sigma-t < 25.6 kg m-j). These species were rare or absent in mixed convergence zone water (defined as sigma-t 25.2-25.6) north of the fronts. Cluster analysis and ordination of assemblages at 31 night stations reflected the greater species richness and abundance in the west and in the southern water mass. Patterns of assemblage structure within and among transects suggest that convergence of surface water toward fronts in the STCZ may concentrate leptocephali close to fronts and that frontal jets may transport leptocephali eastward.

Variation in Population and Life History Traits of the American Eel, Anguilla rostrata, in Four Rivers in Maine

We examined population traits of yellow American eels from nine sites with similar habitat characteristics in each of four rivers in Maine, U.S.A. Migrating silver eels were also collected to compare sex ratio, age and size at migration among the four rivers. Population density and biomass were not significantly different among rivers with mean ranges of 8.4–21.8 eels 100 m−2and 380–1485 g m−2. Pairwise comparisons of the slopes of weight–length relationships of log transformed data (pooled data: intercept = −6.007, slope = 3.094, r2= 0.99, and n = 3116) revealed no significant differences among rivers. Length–age relationships (pooled data: intercept = 87.826, slope = 23.444, r2= 0.76, and n = 2325) also showed no statistically significant pairwise differences in slopes among rivers. In all rivers, sexual differentiation was complete by 270 mm total length and age eleven. The sex ratios of migrating silver eels were not correlated with yellow eel sex ratios among the four rivers. Mean age at migration among the four rivers was significantly different for males only, with a range of 1.3 years. Both sexes had some significant differences in size at migration among rivers, but the biological importance of the differences is tenuous (male range: 15 mm, female range: 36 mm). The yellow and silver eel population traits from these four rivers showed little variation when riverine habitat was isolated. Variations in traits appeared to be greater when eels from non-riverine habitats may have been present.

Spawning ofConger oceanicus andConger triporiceps (Congridae) in the Sargasso Sea and subsequent distribution of leptocephali

SynopsisDistribution of leptocephali ofConger in the Western North Atlantic Ocean was studied using specimens from our collections, specimens from other collections, and various existing collection records. The presence of leptocephali ofConger oceanicus andConger triporiceps < 30 mm long over deep water in the southwestern Sargasso Sea in autumn and winter implies a protracted spawning period there. The subtropical convergence zone, meandering east-west across the Sargasso Sea, is probably the northern limit of spawning of both species. Spawning may also occur close to the Bahamas and Antilles.C. triporiceps may spawn also in the Caribbean Sea judging by the capture of small leptocephali in the western Caribbean and of the more southerly continental distribution of its juveniles. The claim of Johannes Schmidt in 1931 that the EuropeanC. conger spawns across the North Atlantic into the western Sargasso Sea is probably incorrect, because leptocephali ofConger are rare in the eastern Sargasso Sea and becauseC. triporiceps, with myomere numbers overlapping those ofC. conger, was recently described in the western North Atlantic. With increasing size, leptocephali ofC. oceanicus and a portion ofC. triporiceps spread westward and northward in the Florida Current and Gulf Stream, but larger leptocephali especially ofC. triporiceps are found also in the Caribbean and Gulf of Mexico. Spawning ofC. oceanicus in the Sargasso Sea indicates that adults cross the Florida Current-Gulf Stream, and successful leptocephali cross the current in the opposite direction to colonize juvenile habitat on the continental shelf, a migratory pattern similar to that of the American eelAnguilla rostrata (Anguillidae).

Rhythmic activity of migrating juvenile American eels Anguilla rostrata

Glass eels of the American eel Anguilla rostrata (Lesueur) utilize selective tidal stream transport, a series of semidiurnal vertical migrations in phase with the tide, to migrate up the Penobscot River estuary, Maine, U.S.A., in the spring. Glass eels, freshly collected during their migration, exhibited rhythmic activity with circatidal periodicities when tested in groups in the presence of a water current under constant conditions in the laboratory. Freshly collected groups of eels which had already completed their migration through tidal water did not exhibit circatidal activity rhythms under the same conditions. The activity rhythms of groups of eels freshly caught in tidal water appeared to be entrained by a 12.5 h cycle of water current reversals, but not by a 10.0 h cycle of current reversals. In other entrainment experiments water collected on flood and ebb tides was alternately added to the experimental tanks every 5.0 h. Under these conditions groups of eels freshly caught in tidal water did not display any rhythmic activity. Only 2 of 100 glass eels, freshly collected from tidal and non-tidal water, which were tested individually in static tanks, exhibited rhythmic activity. The activity rhythms would allow an eel to time its vertical migrations in the estuary despite varying environmental conditions.

LOCOMOTOR ACTIVITY RHYTHMS OF JUVENILE ATLANTIC SALMON (SALMO SALAR) IN VARIOUS LIGHT CONDITIONS

Atlantic salmon parr exposed to a 12 hr light-12 hr dark cycle (LD 12:12) for ten days were entrained to a 24.0 hr periodicity in locomotor activity. Thirty-five fish were light-active, 20 were dark-active and 32 were active primarily when lights were turned on or off. Fish maintained in constant conditions (75) were generally aperiodic. Five fish (of 30) in constant darkness (DD) showed evidence of 24.0 hr periodicity. Twelve fish exposed to a light signal of 1 hr duration recurring every 23 hr failed to become entrained. Fish in constant light (LL) showed more activity than fish in DD. The results suggest (1) that light-dark transitions are important in synchronizing locomotor activity rhythms and (2) that locomotor activity is not a good indicator of possible circadian oscillations in this species.

Spawning of American eel,Anguilla rostrata, relative to thermal fronts in the Sargasso Sea

SynopsisCollections of recently hatched American eel leptocehali including specimens less than 5.5 mm total length suggest the following hypotheses: 1) that thermal fronts separating the northern and southern surface water masses of the Sargasso Sea form the northern limit of American eel spawning, and 2) that some feature of the surface water mass in the southern Sargasso Sea serves as a cue for adult American eels to cease migrating and begin spawning activity.

A century of research on the larval distributions of the Atlantic eels: a re-examination of the data

The spawning areas of the Atlantic freshwater eels were discovered about a century ago by the Danish scientist Johannes Schmidt who after years of searching found newly hatched larvae of the European eel, Anguilla anguilla, and the American eel, Anguilla rostrata, in the southern Sargasso Sea. The discovery showed that anguillid eels migrate thousands of kilometers to offshore spawning areas for reproduction, and that their larvae, called leptocephali, are transported equally long distances by ocean currents to their continental recruitment areas. The spawning sites were found to be related to oceanographic conditions several decades later by German and American surveys from 1979 to 1989 and by a Danish survey in 2007 and a German survey in 2011. All these later surveys showed that spawning occurred within a restricted latitudinal range, between temperature fronts within the Subtropical Convergence Zone of the Sargasso Sea. New data and re‐examinations of Schmidts data confirmed his original conclusions about the two species having some overlap in spawning areas. Although there have been additional collections of leptocephali in various parts of the North Atlantic, and both otolith research and transport modelling studies have subsequently been carried out, there is still a range of unresolved questions about the routes of larval transport and durations of migration. This paper reviews the history and basic findings of surveys for anguillid leptocephali in the North Atlantic and analyses a new comprehensive database that includes 22612 A. anguilla and 9634 A. rostrata leptocephali, which provides a detailed view of the spatial and temporal distributions and size of the larvae across the Atlantic basin and in the Mediterranean Sea. The differences in distributions, maximum sizes, and growth rates of the two species of larvae are likely linked to the contrasting migration distances to their recruitment areas on each side of the basin. Anguilla rostrata leptocephali originate from a more western spawning area, grow faster, and metamorphose at smaller sizes of <70 mm than the larvae of A. anguilla, which mostly are spawned further east and can reach sizes of almost 90 mm. The larvae of A. rostrata spread west and northwest from the spawning area as they grow larger, with some being present in the western Caribbean and eastern Gulf of Mexico. Larvae of A. anguilla appear to be able to reach Europe by entering the Gulf Stream system or by being entrained into frontal countercurrents that transport them directly northeastward. The larval duration of A. anguilla is suggested to be quite variable, but gaps in sampling effort prevent firm conclusions. Although knowledge about larval behaviour is lacking, some influences of directional swimming are implicated by the temporal distributions of the largest larvae. Ocean–atmosphere changes have been hypothesized to affect the survival of the larvae and cause reduced recruitment, so even after about a century following the discovery of their spawning areas, mysteries still remain about the marine life histories of the Atlantic eels.

Simulation of the Impact of Dams and Fishing Weirs on Reproductive Potential of Silver-Phase American Eels in the Kennebec River Basin, Maine

Abstract I modeled the cumulative impact of hydroelectric projects with and without commercial fishing weirs and water-control dams on the production, survival to the sea, and potential fecundity of migrating female silver-phase American eels Anguilla rostrata in the Kennebec River basin, Maine. This river basin has 22 hydroelectric projects, 73 water-control dams, and 15 commercial fishing weir sites. The modeled area included an 8,324 km2 segment of the drainage area between Merrymeeting Bay and the upper limit of American eel distribution in the basin. One set of inputs (assumed or real values) concerned population structure (i.e., population density and sex ratio changes throughout the basin, female length-class distribution, and drainage area between dams). Another set concerned factors influencing survival and potential fecundity of migrating American eels (i.e., pathway sequences through projects, survival rate per project by length-class, and length–fecundity relationship). Under baseline conditio...

Olfactory clues play a critical role in the estuarine migration of silver-phase American eels

Estuarine migration of anosmic and control silver-phase American eels was examined during the fall spawning migration. Ultrasonic telemetry was used to track seventeen control and eight anosmic silver eels through 32 km of the Penobscot Estuary, Maine, U.S.A.. Twelve of seventeen control eels migrated out of the estuary in 97 h (approximately 4 d) on average. Only two of eight anosmic eels migrated out of the estuary. On average these two anosmic eels migrated out of the estuary within 180 h (approximately 7.5 d) of release and the other six had not left within 9 d. Most control eels progressed rapidly to the mouth of the estuary within a few days. Anosmic eels spent more time in the estuary and demonstrated different behavior from control eels due to their lack of olfaction. Some control eels moved with the appropriate tide, the ebb tide for transport out of the estuary, within one tidal cycle of being released into tidal freshwater. However, anosmic eels either did not move with the appropriate tide or took significantly longer to do so. Olfaction was probably used for orientation by control eels sensing chemical clues (organic and inorganic), which change throughout the tidal cycle. Increased migration times and errors in orientation were likely related to the inability of anosmic eels to use selective tidal stream transport for movement out of the estuary. Chemical clues seem to be one of the most important environmental clues used to guide estuarine migration of silver eels. However, a hierarchy of sensory mechanisms and environmental clues are most likely used for estuarine orientation.

Sexually Different Growth Histories of the American Eel in Four Rivers in Maine

Abstract Growth histories of yellow-phase American eels Anguilla rostrata collected in four rivers in Maine, were back-calculated from sagittal otolith increments. Our objectives were to first determine whether sexually dimorphic growth rates exist and then compare the growth histories of American eels from four rivers within a geographic region. For female eels, the maximum growth rate was 31.9 ± 1.7 mm/year at age 8, decreasing to 25.1 ± 2.9 mm/year at age 14. Males attained a maximum of 29.8 ± 1.6 mm/year at age 3, decreasing to a minimum of 17.9 ± 1.3 mm/year at age 11. Females grew faster than males after age 4 and had a slower reduction in growth rate with age. These faster growth rates among females were similar in all four rivers. The observed growth rates are not consistent with current life history hypotheses and may indicate an alternative life history strategy. Because female eels benefit from a larger size (i.e., size refuge, increased fecundity, and greater niche breadth), they would benefit...