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Locomotor Activity and Concentration of Thyroid Hormones in Migratory and Sedentary Juvenile American Eels a

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b

Martin Castonguay , Jean-Denis Dutil , Céline Audet & Roberta Miller

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a

Ministère des Pêches et des Océans, Institut MauriceLamontagne , Casier Postal 1000, Mont-Joli, Québec, G5H 3Z4, Canada b

Institut National de la Recherche ScientiftqueOcéanologie , 310 des Ursulines, Rimouski, Québec, G5L 3A1, Canada c

Ministère des Pêches et des Océans, Institut MauriceLamontagne Published online: 09 Jan 2011.

To cite this article: Martin Castonguay , Jean-Denis Dutil , Céline Audet & Roberta Miller (1990) Locomotor Activity and Concentration of Thyroid Hormones in Migratory and Sedentary Juvenile American Eels, Transactions of the American Fisheries Society, 119:6, 946-956, DOI: 10.1577/1548-8659(1990)1192.3.CO;2 To link to this article: http:// dx.doi.org/10.1577/1548-8659(1990)1192.3.CO;2

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Transactions of the American Fisheries Society \ 19:946-956. 1990 Copyright by Ihc American Fisheries Society 1990

Locomotor Activity and Concentration of Thyroid Hormones in Migratory and Sedentary Juvenile American Eels MARTIN CASTONGUAY AND JEAN-DENIS DUTIL Ministere des Peches et des Oceans, Institut Maurice-Lamontagne Cosier Postal WOO. Mont-Joli. Qutbec, G5H 3Z4. Canada CfeLINE AUDET Institut National de la Recherche Scientifique—Oceanohgie 310 des Ursulines. Rimouski, Qutbec, G5L 3Alt Canada

ROBERTA MILLER Downloaded by [Fisheries and Oceans Canada] at 07:40 17 June 2015

Ministere des Peches et des Oceans. Institut Maurice-Lamontagne Abstract.—We compared spontaneous locomotor activity and thyroidal activity between juvenile yellow American eels Anguilla rostrata caught climbing waterfalls (migratory) and juvenile yellow American eels captured in an estuary (sedentary). Migratory fish were three times more active than sedentary fish during the first day of the experiment, both in natural light-dark conditions and in constant darkness. Migratory fish also had a mean concentration of plasma thyroxine (21.6 ng/mL) twice as high as that of sedentary fish (9.9 ng/mL). Mean concentrations of plasma triiodothyronine differed little between migratory fish (4.3 ng/mL) and sedentary fish (4.8 ng/mL). These results suggest that the thyroid gland is associated with migration of juvenile yellow American eels because no other life history transition that could involve the thyroid was occurring in the migratory individuals. Migrations of anadromous fishes from the ocean to home streams and spawning grounds are determined by well-documented homing mechanisms (e.g., Scholz et al. 1976). The oceanic migrations of planktonic anguillid eel larvae are probably not determined by such homing mechanisms (Williams and Koehn 1984; Avise et al. 1986). Anguillid leptocephali are carried by oceanic currents for periods that range from months to years before they metamorphose into unpigmented elvers (glass eels). On the north shore of the Gulf of St. Lawrence, glass eels and elvers of the American eel Anguilla rostrata migrate into estuaries of small streams within a short period between mid-June and mid-July (Dutil et al. 1989). They migrate through estuaries by use of selective tidal stream transport (McCleave and Kleckner 1982; McCleave and Wippelhauser 1987; Wippelhauser and McCleave 1987). However, subsequent upriver movements of juvenile yellow American eels are poorly understood, as are factors that control the establishment of individuals in particular areas of river systems. The behavior of juvenile eels bypassing waterfalls has been described for many anguillid species (Jellyman 1977; Liew 1982; Sloane 1984; Moriarty 1986; Legault 1988; Dutil et al. 1989). In northern latitudes, the climbing of waterfalls by American eels involves more than one year-class

(Michaud et al. 1988) and follows clear nocturnal and seasonal patterns (Dutil et al. 1989). These observations show that the upstream migration in fresh water is not restricted to the elver stage, but that some juvenile yellow eels, often considered to be sedentary, can also undertake a migration to colonize freshwater habitats located higher in river systems (see also Liew 1982). Juvenile American eels migrating upriver may differ from sedentary juveniles in terms of level of spontaneous locomotor activity and of rheotropic response. We first hypothesized that juveniles captured at waterfalls (hereafter called migratory) would exhibit a higher level of spontaneous locomotor activity than juveniles caught in an estuary (hereafter called sedentary). We measured spontaneous locomotor activity under two photoperiods in the laboratory to determine whether differences in locomotor activity between migratory and sedentary American eels would occur regardless of the light regime. Numerous workers have shown that a wide variety of physiological and environmental factors can influence thyroid activity. In migratory teleosts, the thyroid gland appears to be maximally active during anadromous and catadromous migration (Leatherland 1982). The relationship between thyroid activity and fish migration is equivocal, because migrations are usually associated with

946


JUVENILE AMERICAN EELS

transition between salt and fresh water, or sexual maturation, or both, all of which may involve the thyroid (see Bales 1979 and Leatherland 1982 for reviews of the roles of the thyroid gland). Relationships between thyroid activity and migratory behavior have been suggested or demonstrated for a number of fish: coho salmon Oncorhynchus kisutch (Grau et al. 1981), Atlantic salmon Sal mo salar (Youngson and Simpson 1984; Youngson et al. 1986), anadromous rainbow trout (steelhead) Oncorhynchus mykiss (Ewing et al. 1984; Birks et al. 1985), goby Rhinogobius brunneus (Iwata and Honma 1986), ayu Plecoglossus altivelis (Tsukamoto et al. 1988); see also reviews by Fontaine (1975) and Woodhead (1975). Histological observations of European eels Anguilla anguilla suggest that the thyroid gland is active during the elver stage (reviewed by Fontaine 1975). Stimulation of the thyroid, associated with metamorphosis from the yellow to the silver stage in the European eel, has also been observed histologically (Callamand and Fontaine 1942) and chemically (Leloup 1959). Our field study provided a rare opportunity to investigate levels of thyroid hormones during migration of juvenile American eels. This river migration occurs in the absence of either sexual maturation or larval metamorphosis. No transition between fresh and salt water is involved either; Michaud et al. (1988) showed that American eels in our study river do not cover the distance between the estuary and the waterfalls during their first year in fresh water. Though such factors as prey availability and social interactions escaped our control in the field, we hypothesized that migratory juvenile American eels would have a greater thyroid gland activity than sedentary juvenile American eels. Fishing Gears and Sampling Stations We examined locomotor activity of captured eels in the laboratory. Migratory American eels all came from the same station as previously described by Michaud et al. (1988) and Dutil et al. (1989), who studied the timing of American eel migrations in Petite Trinite River on the north shore of the Gulf of St. Lawrence (49°32'N, 67°14'W). Fish were captured in July between 2200 and 0000 hours as they climbed the wet bedrock on the edge of two waterfalls 4 km upstream from the mouth of the river (riverine migration is nocturnal at waterfalls). Petite Trinite River is 35 km long and drains a 200-km2 basin of boreal vegetation. Stream flow averages 3 mVs in August. Sedentary American eels of a similar size are

947

difficult to catch in estuaries, particularly the Petite Trinite River estuary. Therefore, sedentary fish were obtained from tidal portions of estuaries of the Trinite and Calumet rivers (14 km south and 7 km north of Petite Trinite River, respectively). Freshwater conditions occur at the estuarine stations most of the time: salt water (25%o) is encountered during not more than four high tides per month. August stream flows average 8 mVs in Trinite River and 2 mVs in Calumet River. These rivers have temperature regimes very similar to that of Petite Trinite River, recordings made in successive years indicate that temperatures average 18°C in late July with day-night fluctuations of 1-2°C (Dutil, unpublished). For locomotor activity experiments, we captured migratory fish with hand nets and sedentary fish with baited minnow traps. Both migratory and sedentary fish were kept for up to 5-7 d in holding cages anchored in Trinite Lake until the required number had been captured. The fish were then transferred to the laboratory, where they were given a 13-17-h acclimation period in the experimental tank before locomotor activity experiments began. To determine concentrations of thyroid hormones in American eels, we obtained migratory fish from the same location and with the same gear described above, and caught sedentary fish from the Calumet River in baited minnow traps and with an electrofishing apparatus. In order to standardize time of blood sampling, we took blood samples at night between 0000 and 0600 hours, that is, immediately after capture for migratory fish and a few hours after capture for sedentary fish. The animals were left undisturbed in holding tanks for the period between capture and blood sampling.

Methods Locomotor activity.—We recorded spontaneous locomotor activity with a video camera by filming a 45 x 32-cm counting area of a 120-cm-diameter annular tank (inner diameter, 50 cm) in which a gentle current (< 10 cm/s) was flowing. The counting area was delineated by white plastic taped to the tank bottom. A pebble substrate, which covered all but the counting area of the tank bottom, allowed the fish to hide when inactive. A movement was defined as a head crossing an edge of the counting area. A red light, which was constantly on, provided light for the camera at night. According to an absorbance study of their visual pigments (Beatty 1975), American eels do not see


948

CASTONGUAY ET AL.

wavelengths in the red end of the light spectrum. Temperatures were 20-22^ for all experiments. Experiments were run between 12 July and 1 August 1986 in a recirculation system with fresh water from Trinite River. The mean times of sunrise and sunset were 0440 and 2020 hours. Four experiments were conducted: sedentary fish in natural light-dark conditions (LD), migratory fish in natural light-dark conditions, sedentary fish in constant darkness (DD), and migratory fish in constant darkness. For each experiment, the activity of 12 American eels was filmed for 72 h. The experimental facility was visited every 6 h to change the videotape and measure water temperature. The annular tank was in a small building with large windows that were sealed for DD experiments. Every precaution was taken to minimize disturbance of the animals. All experiments started and ended at 0600 hours. We recorded the number of movements per half hour, with and against the current, for any fish passing through the counting area, as the index of spontaneous locomotor activity. Therefore, the results of a given experiment consisted of 144 activity measures with the current and 144 measures against the current. It was not possible to track the movement of individual fish on the films, and therefore we could not estimate the variability of activity of individual fish within any experiment. Mean total lengths (TL, mm ± SD) of the animals in each experiment were sedentary fish, LD: 274 ± 123; migratory fish, LD: 219 ± 45; sedentary fish, DD: 299 ± 57; and migratory fish, DD: 183 ± 47. Age determination of American eels from Petite Trinite River with otoliths is impossible because false annuli cannot be distinguished from annually produced ones (Michaud et al. 1988). Therefore, we could not compare age distributions between sedentary and migratory fish. Thyroid gland histology.— American eel heads were preserved for histology in Bouin's fixative for 2 d and were then transferred to 70% ethanol. A small piece of tissue containing the thyroid gland was dissected from the lower jaw, dehydrated, cleared, and impregnated with paraffin. Serial sections 7 Mm thick were stained with Schiff s reagent, hematoxylin, and picro-indigo-carmine. Thyroidal cell height was measured (precision of ±0.4 Mm) from center sections of follicles at 1,000 x magnification. Four cells, one at each end of two diagonals, were measured for each of 10 follicles, giving 40 cell measurements per fish. Thyroid hormones.—Blood sampling started at 0000 hours, upon return from the waterfalls, and

ended at 0600 hours. Fish were anesthetized with tricaine (MS-222). Blood was obtained by caudal puncture. Because many specimens were small (< 1 g), some blood samples had to be pooled in order to get a sufficient volume for analysis. When pooled, blood samples were grouped according to four size-classes (349 mm TL) to account for possible differences in thyroid hormone concentrations with size. For migratory fish, 18 of the 24 thyroxine (T4) determinations and 16 of the 22 triiodothyronine (T3) determinations were of pooled blood samples. For sedentary fish, only 4 of the 54 T4 determinations and 3 of the 38 T3 determinations were of pooled blood samples, because sedentary fish had a better condition factor than migratory fish (see Results section). Pooled blood samples contained blood from 2-8 fish, except one sample included blood from 14 fish. After centrifuging and decanting plasma samples, we froze them at -80°C. We determined plasma concentrations of T3 and T4 (expressed as ng/mL) with radioimmunoassay kits for human T3 and T4 (Kallestad Diagnostics numbers 846 [T3] and 838 [T4]). Samples from migratory and sedentary fish were assayed in random order. Assays were also done in duplicate when the volume of the sample made it possible (26 of the 60 samples for T3 assays and 62 of the 78 samples for T4 assays). Differences between duplicates, expressed as a percentage of the mean values, averaged 4.6% for T3 and 10.1% for T4. For measurements of T3, the assay kits required 100 ML of plasma. When less than 100 ML was available, plasma samples were diluted with T3free human serum in order to meet the critical volume of 100 ML. Concentrations of T3 in our samples were calculated based on the dilution factor. Serial dilutions of the plasmas of yellow and silver American eels showed a strong linearity (r 2 = 0.989 and 0.987, respectively) and paralleled the lines obtained from undiluted standards. Because of the low T4 concentrations in fish plasma, standards for the low end of the standard curve were prepared by diluting standards provided with the radioimmunoassay kits with T4free human serum. We consistently obtained adequate standard curves in this manner. Statistical analysis. —For each light regime, the difference in the level of locomotor activity between sedentary and migratory fish was tested for movements with and against the current separately. For a given light regime and type of movement, we calculated and compared linear regres-

949


JUVENILE AMERICAN EELS

sions of the log,-transformed number of movements per 30 min as a function of 30-min time periods for sedentary and migratory fish, using a separate slope model (GLM procedure of SAS version 5, SAS Institute 1985). Because we had pooled blood samples, we could not estimate variance of thyroid hormone concentrations among individual fish. We therefore conducted tests for differences in plasma concentrations of T3 and T4 between migratory and sedentary fish and among size-classes with the nonparametric Kruskal-Wallis test, because nonparametric tests are based on rank rather than on variance. When assays were duplicated, the mean value was used. We compared values of thyroidal cell height between migratory and sedentary fish with analyses of variance and a posteriori Student-NewmanKeuls' tests, after the values were loglo transformed to homogenize the variance (Scheffe-Box test, P > 0.05; Sokal and Rohlf 1981) and normalize the distribution. Results Locomotor Activity Migratory American eels were much more active than sedentary American eels, in both LD and DD (Table 1). By the end of the 3-d period, migratory fish had performed roughly twice as many movements as sedentary fish. Migratory fish performed 18% more movements in LD as in DD, but sedentary fish maintained a similar level of spontaneous locomotor activity in LD and DD. Movements generally occurred in the direction of the current (92.5% of all movements; Figures 1, 2). Movements against the current represented only 5% of the movements recorded in LD and 10% of those recorded in DD. Movements against the current were more frequent in sedentary fish (952 movements, 15% of total activity) than in migratory fish (536 movements, 4% of total activity). Numbers of movements with or against the current at the beginning of all experiments were always greater in migratory fish than in sedentary fish (tests on regression line intercepts, P < 0.02). Throughout the 3-d period in migratory fish, the level of activity gradually declined in both LD (slope = -0.010, P = 0.02 for movements with the current; slope = -0.008, P = 0.002 for movements against the current) and DD conditions (slope = -0.010, P - 0.003 with the current; slope = -0.013, P = 0.0001 against the current).

TABLE 1.—Total number of movements per day of American eels from sedentary and migratory groups held under natural light-dark conditions or in constant darkness. Natural pholoperiod

Day

Sedentary

Migratory

1 2 3

1,079 1,016 1,018

3,359 2,750 1,302

Constant darkness Sedentary Migratory 1,005 1,174 1,062

2,704 1,874 1,444

In contrast to migratory fish, sedentary fish in LD increased their movements with the current over time (slope = 0.016, P = 0.003), though the number of movements against the current remained unchanged (P = 0.74). In DD, sedentary fish exhibited an increase in the number of movements against the current (slope = 0.012, P = 0.0001). This increase in the number of movements against the current was accompanied by a reduction in the number of movements with the current (slope = -0.009, P = 0.005). Movements against the current had little effect on the overall difference in locomotor activity between migratory and sedentary American eels (Figures 1, 2). Differences in the level of activity between migratory and sedentary fish were greatest in the first day and gradually decreased in the second and third days of our experiments because the level of activity of migratory fish declined. In LD, migratory fish were 3.1 times more active in the first day and only 1.3 times more active in the third day than sedentary fish, which showed no decline (Table 1). In DD, migratory fish were 2.7 times more active in the first day and 1.4 times more active in the third day than sedentary fish, although sedentary fish made three times more movements against the current in the third day than in the first day (Table 1; Figure 2). In LD, both migratory and sedentary American eels exhibited a strong diel cycle in locomotor activity (Figure 1). The most active period occurred between 1500 and 0600 hours for migratory fish and between 1800 and 0300 hours for sedentary fish. Although migratory American eels were more active than sedentary ones in both day and night, the difference in the level of activity between these groups was greater by day (more than 7 times more movements between 0600 and 1800 hour compared with 2.5 times between 1800 and 0600 hours in the first day). Rhythmicity was less pronounced in DD, and the difference in level of activity between migratory and sedentary fish was more uniform over the 24-h period (Figure 2).

950

CASTONGUAY ET AL. 700650600

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FIGURE 1.—Number of movements of American eels with and against the current per 3-h period for the first and third days of a locomotor activity experiment under natural light-dark conditions. American eels were collected from sedentary and migratory groups.

Thyroidal Activity Concentrations of plasma T4 in migratory American eels were significantly greater than concentrations in sedentary American eels (KruskalWallis test, P < 0.0001; Figure 3). The mean plasma T4 concentration of migratory fish (21.6 ng/ mL, SD = 9.7, range = 9.8-56.8) was double that of sedentary fish (9.9 ng/mL, SD = 7.5, range = 1.4-^8.8). Plasma T4 concentrations did not differ significantly among size-classes for either sedentary or migratory American eels (Kruskal-Wallis test, P

> 0.1), although maximum values tended to increase with size in sedentary fish. A regression of plasma T4 concentrations on TL of unpooled sedentary fish was also nonsignificant (P > 0.4). No such regression could be calculated for migratory fish, because most T4 values were obtained from pooled blood samples. Plasma T3 concentrations of migratory American eels (mean = 4.3 ng/mL, SD = 2.3, range = 1.3-10.9) and sedentary American eels (mean = 4.8 ng/mL, SD = 2.3, range = 1.3-10.5) did not differ significantly (Kruskal-Wallis test, P > 0.4; Figure 4).

951

JUVENILE AMERICAN EELS MIGRATORY

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TIME OF DAY

FIGURE 2.—Number of movements of American eels with and against the current per 3-h period for the first and third days of a locomotor activity experiment under conditions of constant darkness. American eels were collected from sedentary and migratory groups.

In contrast with T4, T3 concentrations varied among size-classes of migratory fish (KruskalWallis test, P < 0.05) although not among sizeclasses of sedentary fish (Kruskal-Wallis test, P > 0.2). Small migratory American eels ( 0.08).

The regression of plasma T3 concentration on TL of sedentary fish (T3 = 0.0219TL - 0.867) was significant (P < 0.02, r2 = 0.20), indicating that a weak size effect may also have occurred in plasma T3 concentrations of sedentary fish. No regression was calculated for migratory fish because few blood samples were collected from individual fish. Migratory fish 150-249 mm long had a significantly larger mean thyroidal cell height than sedentary fish of similar size (analysis of variance, P < 0.001; Table 2), indicating a higher thyroidal activity in migratory fish. However, mean values of thyroidal cell height of migratory fish larger

than 249 mm were either not different or smaller than those of sedentary fish (Table 2).

952

CASTONGUAY ET AJL

that indicate that at least two age-classes migrate upstream simultaneously (Michaud et al. 1988). Most fish migrate upstream to the falls during their second year of freshwater residence, whereas others do not reach the falls until their third year (Dutil et al. 1989). This river system provided a rare opportunity to measure thyroid hormone concentrations in migrating fish that were sexually immature and were not preparing to make a transition from salt to fresh water or vice versa. Migratory American eels were much more active than sedentary American eels under condi23 73 «3 173 223 273 S23 S73 423 473 «3 §7.5 tions of both natural photoperiod and constant SEDENTARY darkness. Woodhead (1975) stated that migratory fish generally show an increase in random locoN - 54 8* motor activity. High levels of spontaneous locomotor activity is one behavioral manifestation of 40a migratory state. Because the movements of American eels are directional, one would expect 30 another behavioral component of this migratory state to be an increased positive rheotropism. The 2 number of movements against the current (posi10 tive rheotropism) was higher among migratory fish than among sedentary fish at the beginning of our experiments. Migratory fish exhibited a decline in 23 73 123 173 223 273 323 373 423 473 823 973 T CONCENTRATION ( ng.nUT > the number of movements against the current over FIGURE 3.—Percentage distributions of plasma thy- 3 d, and sedentary fish made more movements roxine (T4) concentration in migratory and sedentary against the current than migratory fish over the American eels (all size-classes pooled). N = sample size. 3-d period, particularly in DD. This decline in total activity of migratory fish in LD and DD may have been caused by a long holding period before Condition Factor locomotor activity measurements. However, miThe condition factor, denned as K = 100 • Wl gratory fish spent as long as sedentary fish in holdXL3 (W= weight in g, TL in cm), was significantly ing facilities, so one would expect differences in larger for sedentary fish (mean = 0.156, SD = locomotor activity between recently caught mi0.019, range = 0.103-0.196, N = 59) than for migratory and sedentary fish to be larger than we gratory fish (mean = 0.113, SD = 0.013, range = measured in our study. No explanation can be 0.080-0.136, N = 91; Kruskal-Wallis test, P < provided for the observed increase of movements with the current in LD and against the current in 0.0001). DD by sedentary fish. Discussion In our study, American eels remained active in Our study exemplifies a situation in which fish daytime in LD, whereas Dutil et al. (1989) obthat share a common gene pool (Williams and served no activity in daytime at the waterfalls and Koehn 1984; A vise et al. 1986) and inhabit sim- a sharp increase in climbing activity between 2100 ilar environments differ in migratory pattern. Some and 2300 hours. Elvers of various eel species exAmerican eels remain in the lower reaches of riv- hibit a diurnal activity pattern in fresh water (Jelers, whereas others migrate considerable distances lyman 1979; Sloane 1984; Sorensen and Bianchini up rivers. The migration pattern for American eels 1986). This may indicate that the upstream miin Petite Trinite River is complex and variable gration in Petite Trinite River also takes place in (Michaud et al. 1988; Dutil et al. 1989). Elvers do daytime and is momentarily interrupted by wanot reach the falls in the same year they enter the terfalls until darkness. However, one should be estuary (Michaud et al. 1988), even though the careful in relating activity in the laboratory to acfalls are only 4 km upstream. Fish at the falls tivity in the field, because most of the movements exhibit bimodal length and weight distributions recorded in the laboratory were directed downMIGRATORY

N • 22


60

ll

1

4

953

JUVENILE AMERICAN EELS 100 90MIQRATORY