Environmental Biology of Fishes 66: 143–154, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.
Diel and seasonal movements of radio-tagged freshwater eels, Anguilla spp., in two New Zealand streams Donald J. Jellyman & Julian R.E. Sykes National Institute of Water and Atmospheric Research Ltd. P.O. Box 8602, Christchurch, New Zealand (e-mail:
[email protected]) Received 5 December 2001
Accepted 20 June 2002
Key words: telemetry, habitats, home range, crepuscular Synopsis We studied diel and seasonal movements of 21 radio-tagged shortfinned, Anguilla australis Gray, and longfinned, A. dieffenbachii, eels in two small New Zealand streams. Movements of eels commenced at dusk, with a higher proportion of shortfinned eels moving per night than longfinned eels, and also moving greater distances. Both species often showed extensive movements immediately after tagging, but thereafter movements were limited. In the smaller stream, home ranges averaged 30 and 10 m for shortfinned and longfinned eels, respectively, but not all eels were active on every night. There were no seasonal differences in mean distances moved. In both streams, eel movement was almost exclusively bankside, and seldom cross-channel; eels also showed considerable fidelity to a particular bank. Shortfinned eels were most commonly found in runs, and longfinned eels in riffles.
Introduction Anguillid eels have extensive marine migrations, but their freshwater migrations are more limited, both spatially and temporally. Temperate freshwater eels have a winter-spring recruitment (Sloane 1984a, Jellyman et al. 1999), a summer upstream migration of juveniles (Jellyman 1977), and a summer–autumn downstream migration of maturing adults (Burnet 1969a, Todd 1981, Sloane 1984b). Compared with such extensive juvenile and adult migrations, the intervening period of freshwater residence, the yellow eel phase, is one of comparatively little movement (e.g., Anguilla rostrata Gunning & Shoop 1962, Helfman et al. 1983, Bozeman et al. 1985, Oliveira 1997; A. australis Burnet 1969b, Beumer 1979, Chisnall & Kalish 1993, and A. dieffenbachii Burnet 1969b, Chisnall & Kalish 1993). Freshwater eels sustain important commercial and customary fisheries in New Zealand (McDowall 1990, Jellyman 1993). The endemic longfinned eel, A. dieffenbachii, is generally slow-growing
(Burnet 1969b, Jellyman 1997), and age at migration normally ranges from 25–60 years (Todd 1980), but may exceed 90 years (Jellyman 1995). Recent research has raised concern about the sustainability of present levels of exploitation (Hoyle & Jellyman 2002). Consequently, research is underway to develop predictive models to simulate varying levels of exploitation. An important component of such models is the vulnerability of both species of eel to capture by fyke nets, the most common form of commercial capture in New Zealand (Jellyman 1993). As part of this study, information was required on both the short term (diel and weekly) and long-term (seasonal) movements of a wide size range of both species of eel. Therefore the objectives of the present study were to determine for both species (1) whether all eels were active on all nights during summer; (2) was activity primarily crepuscular (dusk/dawn); (3) did eels show restricted movements (have a home range); (4) were there seasonal differences in activity; (5) did eels use different habitats during day and night, and would these differ between the species?
144 Methods Sampling locations The diel movement study took place in the Cust River, a channelised tributary of the Kaiapoi River, which in turn enters the lower Waimakariri River, South Island, New Zealand (Figure 1). The river sinuousity is confined by manmade banks, and consequently the channel is rather uniform and shallow in cross-section, with few deep pools. Aquatic plant growth is confined to extensive marginal growth of cress, Rorippa nasturtiumaquaticum. Flow data came from a recorder 0.7 km above the study reach, while water temperatures were recorded by a temperature logger located in the centre of the study reach (Table 1). Habitat composition (extent of run, pool, riffle, and backwater) was calculated by direct measurement of each habitat type. In addition, the longitudinal distributions of the various habitat types were measured, so that subsequent locations of individual eels could be assigned to particular habitats. Most of the study of seasonal movement was carried out in the South Branch (Otukaikino), a springfed tributary of the lower Waimakariri River. This site was in the upper section of the reach used by Burnet (1968) to study eel-trout interactions, 9 km upstream
of the confluence with the Waimakariri River. Flow and temperature measurements (Table 1) came from a recorder placed in the centre of the study section. Being spring-fed, flow is relatively constant, with main variation due to periodic upstream abstraction for irrigation. The stream has an abundant growth of macrophytes, mainly Elodea canadensis, Veronica anagallis, and Ranunculus tricophyllus, although ingress of sediment has resulted in a considerably lower fish density than during Burnet’s study (D.J.J., pers. obs.).
Tagging and tracking procedure Radio tags were of two types. Those used for the diel (short term) study were smaller (45 × 12 mm, 200 mm antenna, 7.5 g), did not incorporate a motion sensor, and had a life of 3 months. Tags used in the seasonal (longer term) study were 43 × 18 mm with a 220 mm antenna, weighed 16 g, and were designed to transmit for 9 months at 30 pulses per minute (ppm). Each of the 10 tags used incorporated a motion sensor which reduced transmission frequency to 15 ppm after a 5 s period of inactivity – this enabled actual activity to be noted and also extended the life of the batteries. All tags were manufactured by SIRTRACK Ltd (Havelock North, New Zealand), and transmission
Figure 1. Map showing the study reaches in the Cust River and South Branch, South Island, New Zealand. Dotted lines indicate the extent of the study reaches.
145 Table 1. Summary of the size, flow, water temperature and habitat types of the South Branch and Cust River study sites.
Length of study site (km) Maximum depth (m) Mean width (m) Flow (m3 s−1 ) Mean (SE) Range Temperature (◦ C) Mean (SE) Range Habitat (%) Run Pool Riffle ∗
South Branch
Cust River
1.2 0.9 12
1.0 0.55 9
0.67 (0.01)∗ 0.18–1.12
0.42 (0.01)∗∗ 0.27–0.82
12.3 (0.03)∗ 6.1–20.5
13.9 (0.06)∗∗ 5.3–22.3
95 3 2
64 24 12
May 2000–June 2001, ∗∗ Jan–June 2001.
frequencies ranged from 160.300–160.900 MHz. We used an Advanced Telemetry Systems R2100 receiver; initially the aerial used was a hand-held 3-element Yagi, which was sometimes used in conjunction with a variable attenuator (10–60 dB) to determine more accurate locations of eels. However, a 3-element aerial manufactured by NIWA Instrument Systems, had a more defined null-point and a greater range, and was the aerial used most often during the study. With experience, it was usually possible to locate an eel to within ±0.5 m of its actual location. Feeding (yellow) eels were obtained by electrofishing with a portable backpack machine (EFM300, NIWA, Christchurch, New Zealand). As we wanted eels of sufficient size so that tag weight did not exceed 2% of body weight (e.g., Moser et al. 1990, Thorstad et al. 2000), the minimum eel sizes targeted were 800 g in the South Branch, and 375 g in the Cust River. Captured eels were anaesthetised with 2-phenoxyethanol. Eels were individually measured (TL, mm) and weighed (g), and tags surgically implanted within the body cavity using the technique of Brown & MacKay (1995). Incisions were initially closed with surgical staples, but problems with the soft musculature of longfinned eels meant that staples were abandoned in favour of synthetic sutures. Tagged eels were then placed in live-boxes within the stream to recover for 0.5 h before release. The unique frequency of each tag was used to differentiate between individual eels (Table 2). Only two shortfinned eels (hereafter called shortfins) >800 g were captured and tagged in the South
Table 2. Sizes and identification number (=radio frequency, MHz) of eels radio-tagged in the South Branch and the Cust River, New Zealand. Species
Length (mm)
Weight (g)
Identification number
Duration of tracking (days)
South Branch S 720 S 748 L 657 L 705 L 726 L 812 L 881 L 912 L 1052
948 1064 922 863 924 1851 2038 1979 4010
459 300 399 315 478 440 360 338 419
29∗ 387 387 210∗ 2931 562 387 387 387
Cust River S S S S S S L L L L L L
254 283 325 315 377 419 241 400 430 500 1610 1365
698 840 618 757 719 800 639 816 678 778 735 656
43∗ 88 88 88 88 88 88 88 88 46∗ 283 31∗
466 479 496 534 549 563 422 526 551 571 762 780
S = shortfin (A. australis); L = longfin (A. dieffenbachii), and length of successful tracking periods. *Tag expulsion suspected. 1 Contact lost due to transmission failure or migration out of study reach. 2 Eel swam 1200 m upstream into swamp and where it could not be tracked. 3 Eel swam upstream 2025 m before contact lost.
Branch, compared with nine longfinned eels (longfins) (Table 2). Equal numbers of both species were tagged in the Cust River, although only two of the six shortfins and five of the six longfins exceeded the desired threshold weight of 375 g. Prior to eel tagging, reaches of both streams (1.5 km of the South Branch, 2.0 km of the Cust River) were measured by hip chain; marker pegs that had the distance upstream from a designated start point written on them, were then hammered into the bank at 50 m intervals. Tracking was principally done from the riverbank, although the presence of dense bankside gorse, Ulex europaeus, and willow, Salix spp. in the South Branch meant that it was necessary to enter the water for some sections – when this was necessary,
146 tracking was always done in an upstream direction to minimise possible disturbance to eels. Once the position of an eel had been established, a plastic stake bearing the tag frequency was placed at the stream edge or bank directly opposite the eel. The location (metres upstream) was then determined by measuring or pacing to the nearest marker peg. Tracking at night required the use of a battery-powered headlamp, but this was switched off within approximately 10 m of each eel, again to avoid disturbance. Tracking in the South Branch commenced on 5 April 2000 and concluded on 27 April 2001, while tracking in the Cust River was between 29 January and 27 April 2001. To monitor diel activity, eels in the Cust River were tracked at hourly intervals for 3 successive nights over 3 periods i.e. 27 February–2 March, 13–16 March, 25–28 March. During the first period, tracking was continuous for 65 h, (3 nights, plus 2 intervening days), but thereafter tracking was confined to duskdawn (1800–0700 h). The diel activity data from the Cust River were used to provide estimates of home range, being averages of maximum distance travelled per night, for all eels. Home range ‘the area over which an animal normally travels’ (Gerking 1953), was estimated by averaging the sum of nightly distance travelled, (irrespective of whether upstream or downstream), by individual eels. To observe any influence of changes in water level and temperature on distance moved, the mean distance moved per night for either species was regressed against mean water level or water temperature of that day (day 0); the possibility of a lagged response to these variables was also tested by using the level or temperature for both of the preceding two days (days 1 and 2). The Cust River data were also used to determine any changes in day and night (2000–0600 h) habitat usage. For this, all habitat observations (for shortfins, 356 day and 625 night observations; for longfins, 276 day and 447 night observations) were compared (χ 2 , 2-tailed test). In addition, the extent (m2 ) of stream habitats (run, pool, riffle) were compared with habitats used by both species (χ 2 , 1-tailed test), to see whether habitat use was arbitrary or not. Monitoring of longer term movements took place at approximately biweekly intervals in the South Branch, except during winter (July–September) when monthly visits were made. In the Cust River, seasonal (summer) movement was recorded by visits at approximately 10-day intervals.
Results Duration of tracking Not all eels could be tracked over the entire period of observation (Table 2). Expulsion of tags was suspected for several eels where no movement had been recorded for five successive periods, but subsequent searches failed to find either tags or dead eels. Diel movement Eels in the Cust River showed differing patterns of diel activity. Figure 2 illustrates the range in activity observed: eel # 840 (shortfin, length 479 mm) showed a reasonably consistent pattern of upstream movement after dusk each night, before returning downstream at dawn (but at dawn of the second day, not returning to the exact position occupied during the first day). Eel # 618 (shortfin, 496 mm) tended to move initially downstream, and was active over a shorter period on all three nights than eel # 840. In contrast, eel # 678 (longfin, 551 mm) did not move on the first night, showed a single upstream movement on the second night, and more extensive movement prior to dawn on the third night. The mean diel activity recorded for both species over 9 nights and 3 days (Figure 3) showed an initial high peak of activity immediately after dusk. For shortfins, there was a tendency for reducing activity throughout the night with abrupt cessation at dawn; no daytime activity was recorded. Likewise, activity of longfins commenced at dusk, but average movement for this initial activity of longfins exceeded that of shortfins, but thereafter there was a marked reduction in activity, until a secondary peak prior to dawn. An interesting feature of eel movements was the fidelity for movement along either bank, with movements from one side to the other uncommon. During the diel observations, several eels did move from one bank to the other, usually returning to their original bank by dawn, but such cross-channel movements constituted only 1.2% of all diel movements for shortfins (n = 840) and 0.3% for longfins (n = 591). A comparison of total hourly activity (the proportion of all hours when some of movement was recorded), showed that a much higher proportion of shortfins (34.4%) were active than longfins (12.7%). Not only did shortfins move more frequently than longfins, but
147
Figure 2. Patterns of diel activity of eels, Anguilla spp., 27 February–2 March 2001, in the Cust River, New Zealand. (a), eel # 840 (A. australis, 479 mm); (b), eel # 618 (A. australis, 496 mm); (c), eel # 678 (A. dieffenbachii, 551 mm). Horizontal bars show times of daylight (non-hatched) and dark (hatched). Positive values represent upstream movement, negative values represent downstream movement.
when they did move they tended to move longer distances. Thus, the mean distance travelled per movement was 10.7 m for shortfins and 6.0 m for longfins. The greatest total distance travelled over the nine nights of observation was 686 m for a shortfin, and 126 m for a longfin. The maximum distance travelled during any single hour was 126 m by a shortfin, compared with 115 m for a longfin. Not all eels were active on every night (Table 3). For example, eel # 757 showed some movement during the first and third periods of observation, but no movement during the second period. Only one eel (# 800) showed some movement on all 9 nights, with the least movement being for eel # 698 (movement on only 3 nights). On average, 42% and 40% of shortfins and longfins, respectively, showed some movement over all 9 nights, although 22% of shortfins but
only 4% of longfins moved five or more times per night. There were no significant relationships (p < 0.05) for either species between the mean distance moved per night and mean daily water level or water temperature for that day or either of the preceding two days. As cross-channel movements were rare, home range was expressed in linear metres moved, rather than square metres. The average home range (over 9 nights), for shortfins (29.9 m, range 1–126 m, SE 5.2, n = 36) was significantly greater than that for longfins (9.6 m, range 2–115 m, SE 5.9, n = 19); t-test, p < 0.01. Seasonal movement Movements of the five South Branch eels that retained tags for the entire period (or almost the entire period,
148
Figure 3. Mean diel activity (±SE) for (a), A. australis (N = 7) and (b), A. dieffenbachii (N = 3) in the Cust River, New Zealand. Table 3. Number of hours per night (maximum of 11 h) when some movement of eels was recorded in the Cust River, New Zealand. A. australis Eel No. Period
1
Period
2
Period
3
Night Night Night Night Night Night Night Night Night
1 2 3 1 2 3 1 2 3
A. dieffenbachii
698
757
840
618
719
800
816
639
678
7 4 0 0 0 0 1 0 0
1 3 1 0 0 0 4 2 3
7 6 9 0 7 1 4 0 9
8 4 7 5 0 4 4 11 7
0 0 0 2 6 1 6 0 2
9 5 9 4 5 1 10 6 8
3 1 1 1 6 2 3 0 4
1 3 1 0 0 0 0 0 1
0 2 7 2 0 0 4 3 1
eel # 478), Figure 4, show that for four fish, the most extensive movement recorded was almost immediately after tagging, and was usually upstream. Only one of these fish, # 478, then returned to the position it occupied pre-tagging, and thereafter moved a maximum longitudinal distance of 145 m. For the three eels that remained upstream, the maximum distance they subsequently moved was 25 m, 42 m, and 123 m, respectively. Of the three eels that had shorter
movement records, two eels initially went downstream, although one (# 315) then returned to within 8 m of its tagging position before gradually moving downstream again. For estimates of average seasonal distances moved for longfins, the first 12 days of record were ignored, as these often coincided with extensive movement, probably associated with the tagging process. With the exception of two eels that showed no movement
149
Figure 4. Movements of eels, Anguilla spp., in the South Branch, New Zealand. (a), eel # 300 (A. australis, 748 mm); (b), eel # 399 (A. dieffenbachii, 657 mm); (c), eel # 478 (A. dieffenbachii, 726 mm); (d), eel # 360 (A. dieffenbachii, 881 mm); (e), eel # 419 (A. dieffenbachii, 1052 mm). Positive values represent upstream movement, negative values represent downstream movement.
during winter (Table 4), the remaining five eels showed some movement throughout all seasons. Of these five eels, three showed maximum movement during summer, two during winter, and one during autumn, but the mean seasonal distances moved were not significantly different (ANOVA, p > 0.05). Movement data during summer were available for Cust River eels. Eels frequently showed extensive movement immediately after they were tagged (Figure 5a,e), but thereafter movement was generally limited. Several eels showed short term relocation to a different site(s), generally upstream
(e.g., eels # 618, 639), but sometimes downstream (eel # 757). Habitats used Eels were almost invariably located at the river banks; from all the records of positions of individual eels in the South Branch (n = 256, both species combined), only 4 (1.2%) were recorded from the ‘middle’ of the stream, with 38 and 214 from the left and right banks, respectively. Eels also showed considerable fidelity
150 for a particular bank; three eels were always recorded along the same bank, and the overall average for ‘same bank residence’ among individual eels was 85% (range 61–100%, SE 6). For the Cust River, of the total number of positions recorded (shortfins, n = 1005; longfins, n = 721), none was away from the banks. Both species showed considerable fidelity for residence at the same Table 4. Mean distance moved (md−1 ) per season for longfinned eels (A. dieffenbachii) eels in the South Branch, New Zealand. The mean distances (and SE) are for those eels with records for all seasons. Season Eel No.
Autumn
Winter
Spring
Summer
399 315 478 360 338 419 Mean (SE)
0.29 4.00 0.43 0.92 4.84 0.43 1.82 (0.83)
0.51 0.35 7.54 0 0 3.86 2.04 (1.25)
0.18 0.34 1.33 1.16 0.09 0.69 0.63 (0.21)
0.07 — 2.11 1.82 0.03 4.16 1.64 (0.76)
— = no data.
bank, with the six shortfins averaging 95% fidelity (range 84–100, SE 2) and the five longfins 98% fidelity (range 89–100, SE 2). As indicated previously, crosschannel movements were uncommon. Riparian grass and cress provided most daytime cover in the South Branch (63%, 66 observations), compared with 88% in the Cust River (40 observations); debris clusters accounted for an additional 10% of cover used in both waterways. Comparisons of the counts of either species from various habitat types used in the Cust River (Table 5) showed differences both between the species and within each species (diel differences). Thus while runs were the most common habitat used by shortfins during both day and night, a greater proportion of shortfins were found in pools at night. These differences were significant (χ 2 , p < 0.01), as were comparisons of both day and night habitat used with habitat available (using data from Table 1). A much higher proportion of longfins (64%) was found in riffles during the night than during the day (42%), and these diel habitat uses were also significantly different (χ 2 , p < 0.01), as were
Figure 5. Movements of eels, Anguilla spp., in the Cust River, New Zealand. (a), eel # 840 (A. australis, 479 mm); (b), eel # 618 (A. australis, 496 mm); (c), eel # 757 (A. australis, 534 mm); (d), eel # 639 (A. dieffenbachii, 422 mm); (e), eel # 816 (A. dieffenbachii, 526 mm); (f), eel # 678 (A. dieffenbachii, 551 mm). Positive values represent upstream movement, negative values represent downstream movement.
151 Table 5. Number of times both species of eel were recorded using various habitats in the Cust River, New Zealand, during day and night. Habitat
A. australis
A. dieffenbachii
Day
Night
Day
Night
Run Pool Riffle n
191 113 52 356
323 284 18 625
86 73 117 276
51 111 285 447
n = number of observations.
differences between day or night habitat used and habitat available. Interspecific comparisons of both day and night habitats used were also significantly different (χ 2 , p < 0.01). Discussion Expulsion of surgically implanted tags occurs in a range of fish species (Moore et al. 1990), usually through a rupture of the incision, but also through tag encapsulation and expulsion through the intestine (Baras & Westerloppe 1999). In the present study, tag loss was suspected but not proven. The suturing method used has been shown to have a similar healing rate to other closing procedures, but a lower survival rate (Baras & Jeandrain 1998). The tagging process often seemed to produce some aberrant behaviour, as evidenced by the initial extensive movements that several eels made. Similarly, attachment of external sonic tags to eels in Lake Ellesmere was suspected of causing shortterm, extensive movement (Jellyman et al. 1996). Other radio-tracking studies have also recorded extensive post-tagging movement by some eels (e.g., LaBar & Facey 1983, Baras et al. 1998). Diel periodicity in the activity of both shortfins and longfins was clearly evident in the present study. The onset of activity at dusk has been related to feeding in both small (Sagar & Glova 1998, Glova & Jellyman 2000) and large eels (Ryan 1984). Similar nocturnal activity rhythms have been demonstrated for other Anguilla species, both in the laboratory (Edel 1976, Van Veen et al. 1976), and in the field (Bohun & Winn 1966, Westin & Nyman 1979, Helfman et al. 1983, Sorensen et al. 1986, Baras et al. 1998). Eels are negatively phototrophic (Van Veen et al. 1976), but their highly developed olfactory and sensory systems (Tesch 1977), enable them to forage under conditions
of low light intensity. During the day, eels are largely sedentary, inhabiting areas that provide some shade; the density of large A. dieffenbachii has been directly related to the amount of suitable cover available (Burnet 1952). Pronounced crepuscular (dusk-dawn) activity was observed in longfins in the present study, but was less apparent in shortfins. Such activity has been observed in juvenile shortfins in the wild (Jellyman & Ryan 1983), but not from experiments using small eels (
