Finn Økland

Oceanic Spawning Migration of the European Eel (Anguilla anguilla)

Satellite tracking technology has allowed scientists to map part of the migration route of the European eel. European eels (Anguilla anguilla) undertake a ~5000-kilometer (km) spawning migration from Europe to the Sargasso Sea. The larvae are transported back to European waters by the Gulf Stream and North Atlantic Drift. However, details of the spawning migration remain unknown because tracking eels in the Atlantic Ocean has, so far, eluded study. Recent advances in satellite tracking enable investigation of migratory behavior of large ocean-dwelling animals. However, sizes of available tags have precluded tracking smaller animals like European eels. Here, we present information about the swimming direction, depth, and migratory behavior of European eels during spawning migration, based on a miniaturized pop-up satellite archival transmitter. Although the tagging experiment fell short of revealing the full migration to the Sargasso Sea, the data covered the first 1300 km and provided unique insights.

Survival of radio-tagged Atlantic salmon (Salmo salar L.) and trout (Salmo trutta L.) smolts passing a reservoir during seaward migration

High mortality-rates of seaward migrating salmonid smolts when passing reservoirs and lakes have earlier been found in the Danish River Gudena watershed. To reveal the causes of mortality of migrating smolts in Lake Tange, a 12 km long, shallow reservoir, 50 salmon smolts and 24 trout smolts were tagged with internal miniature radio-transmitters, and released in the river just upstream the reservoir on May 1, 1996. The salmon smolts were hatchery-reared, while the trout smolts were wild fish, caught in a smolt trap. The tagged smolts were tracked daily for 3 weeks, and when possible the cause of death was determined. During the 3-week period, 90% of the tagged smolts died. The main cause of death for both trout and salmon was predation from fish and birds. The most important predator was pike (Esox lucius L.), being responsible for 56% of the observed mortality. Avian predators were assumed to be responsible for 31% of the observed mortality. No trout smolts left the reservoir, but 5 salmon-smolts got out through the turbines. Others did traverse the reservoir, but were unable to enter the river downstream, and were later eaten. The present results suggest that mortalities for migrating smolts through Lake Tange are of such a magnitude, that stocking of juveniles in the river upstream is futile, and further, that the establishment of a natural population of salmon or sea-trout in river Gudena, upstream Tange, is unrealistic under present conditions.

Fjord migration and survival of wild and hatchery-reared Atlantic salmon and wild brown trout post-smolts

The behaviour of wild (n = 43, mean LT = 152 mm) and hatchery-reared (n = 71, mean LT = 198 mm) Atlantic salmon and wild anadromous brown trout (n = 34, mean LT = 171 mm) post-smolts with acoustic transmitters was compared in a Norwegian fjord system. There was no difference in survival between wild and hatchery reared salmon from release in the river mouth to passing receiver sites 9.5 km and 37.0 km from the release site. Mortality approached 65% during the first 37 km of the marine migration for both groups. There was no difference between wild and hatchery-reared salmon either in time from release to first recording at 9.5 km (mean 135 and 80 h), or in the rate of movement through the fjord (mean 0.53 and 0.56 bl s−1). Hatchery-reared salmon reached the 37 km site sooner after release than the wild salmon (mean 168 and 450 h), but rate of movement in terms of body lengths per second did not differ (mean 0.56 and 0.77 bl s−1). The brown trout remained a longer period in the inner part of the fjord system, with much slower rates of movement during the first 9.5 km (mean 0.06 bl s−1).

Remote monitoring of heart rate as a measure of recovery in angled Atlantic salmon, Salmo salar (L.)

The introduction of ‘Catch and Release’ fishery programs are now widely employed by fisheries managers in most Atlantic Provinces, primarily due to the recent decline of Atlantic salmon stocks on the east coast of Canada. However, there is still considerable debate among special interest groups and regulators as to the effectiveness of the technique. Heart rate telemetry has been utilized as a tool for the assessment of metabolic rate in wild fish by a number of investigators, and was employed in the present study in order to assess recovery following staged angling events in Atlantic salmon. Wild Atlantic salmon were successfully angled at 20 ±2 °C and 16.5 ±1 °C at Noel Pauls Brook, Newfoundland. In addition, hatchery reared Atlantic salmon were angled at the Ontario Ministry of Agriculture and Fisheries Research Station, Alma, Ontario, at a temperature of 8 ±1 °C. Survival rate for the angled salmon was 20% at 20 ± 2 °C; 100± at 16.5 ±1 °C; and 100% at 8 ± 1 °C. Mean resting heart rate for the fish angled at 16.5 °C and 20 ° C was approximately 1.6 and 1.8 times greater than that of fish angled at 8 °C. Heart rate, post angling, was found to increase 1.2 fold in the 8 °C group, 1.3 fold in the 16.5 °C group and approximately 1.15 fold in the 20 °C group. Time to recovery was assessed as a return to observed resting heart rate for each individual fish and was found to be similar for both the 8 °C and 16.5 °C angled groups (approximately 16 h). Although heart rate telemetry in fish is, perhaps, not an ideal measure of metabolic rate, the present study has demonstrated that remote monitoring of heart rate is a good indicator of post exercise physiological activity.

A wireless communication system for determining site fidelity of juvenile dusky groupers Epinephelus marginatus (Lowe, 1834) using coded acoustic transmitters

The value of marine reserves to protect threatened and vulnerable species, such as the dusky grouper (Epinephelus marginatus), is often uncertain because of a lack of knowledge about their ecology and habitat utilisation. To address this issue, a Wireless Communication System for location-based positioning of fish tagged with coded acoustic transmitters was installed to determine site fidelity of juvenile dusky groupers (mean total length = 30.4 cm, range: 20.5–40.0 cm; mean total weight = 470 g, range: 113–950 g) within a marine reserve (Island of Ustica, Sicily, Italy). The system was based on five wireless hydrophones distributed approximately 200 m apart along the coastline of the reserve, each monitoring acoustic signals from transmitters within a reception cell. The hydrophones retransmitted detected signals as a radio signal to a land-based receiver and data logger. Unique identifiers for each hydrophone permitted the identification of the detection cell containing the tagged fish. The wireless hydrophones had partly overlapping detection ranges, dividing the marine reserve into more than five different detection cells. The average detection range was 232 m, but coverage varied from site to site and with time, depending on many factors, such as ambient noise, wave action and signal strength of the transmitters. Despite some limitations, the Wireless Hydrophone System (WHS) combined with manual tracking provided a valuable tool for investigating site fidelity in the dusky grouper. Preliminary home ranges for the fish were estimated by the kernel method (probability level of 95%), based on a limited set of manual tracking data, to be a median 5312 m2 (range 1673–18626 m2).

Empirical observations of the spawning migration of European eels: The long and dangerous road to the Sargasso Sea

Fresh data on the timing and speed of the oceanic spawning migration of European eels suggest a new paradigm for spawning ecology. The spawning migration of the European eel (Anguilla anguilla L.) to the Sargasso Sea is one of the greatest animal migrations. However, the duration and route of the migration remain uncertain. Using fishery data from 20 rivers across Europe, we show that most eels begin their oceanic migration between August and December. We used electronic tagging techniques to map the oceanic migration from eels released from four regions in Europe. Of 707 eels tagged, we received 206 data sets. Many migrations ended soon after release because of predation events, but we were able to reconstruct in detail the migration routes of >80 eels. The route extended from western mainland Europe to the Azores region, more than 5000 km toward the Sargasso Sea. All eels exhibited diel vertical migrations, moving from deeper water during the day into shallower water at night. The range of migration speeds was 3 to 47 km day−1. Using data from larval surveys in the Sargasso Sea, we show that spawning likely begins in December and peaks in February. Synthesizing these results, we show that the timing of autumn escapement and the rate of migration are inconsistent with the century-long held assumption that eels spawn as a single reproductive cohort in the springtime following their escapement. Instead, we suggest that European eels adopt a mixed migratory strategy, with some individuals able to achieve a rapid migration, whereas others arrive only in time for the following spawning season. Our results have consequences for eel management.

Crossing Invisible Boundaries: the Effectiveness of the Langebaan Lagoon Marine Protected Area as a Harvest Refuge for a Migratory Fish Species in South Africa

The application of no-take areas in fisheries remains controversial. Critics argue that many targeted species are too mobile to benefit from area protection and that no-take areas are only appropriate for resident species. The degree of protection does not depend on the size of the no-take area but rather on the time fish reside inside its boundaries during key life-history events (i.e., spawning) and during periods of peak fishing activity. We evaluated the potential of a small no-take marine protected area (MPA) inside a coastal embayment as a harvest refuge for a mobile, possibly migratory, long-lived fish species. We used acoustic telemetry to track movements of 30 transmitter-tagged white stumpnose (Rhabdosargus globiceps) across and on both sides of the boundary of a small (34 km(2)) no-take area over a full year. Being landlocked on 3 sides, the location of the MPA inside the lagoon made it practical to detect all boundary crossings and to calculate the time individual fish used the MPA. We detected frequent movements across the boundary, with strong seasonal and individual variations. There were significant differences in MPA use patterns between fish from different release areas. The time spent in the MPA by individual fish during summer (mean 50%; max 98%) was out of proportion with the size of that area (4% of total habitat). Summer coincided with peak recreational fishing activity and with the spawning season of this species. The small MPA provided a refuge for a part of the spawning stock of white stumpnose. Our findings suggest that if strategically placed, a small no-take area can be effective in protecting mobile species and that models of spillover from no-take areas should account for seasonal and individual variation in area use and the spatiotemporal distribution of fish and fishers.

Early marine migration of European silver eel (Anguilla anguilla) in Northern Norway

To study the migratory behaviour in wild northern European silver eel Anguilla anguilla during sea entry and early marine migration, 32 individuals were tagged with acoustic transmitters and registered at four automatic listening station arrays from the mouth of the north Norwegian River Alta and throughout the Alta Fjord. The A. anguilla entered the fjord during all parts of the tidal cycle and did not seem to utilize the outgoing tidal currents. They migrated mainly during the night, in both the river mouth and the fjord. On average, they spent 2·7 days travelling from the river mouth to the outermost array, 31 km from the river mouth, corresponding to an average migratory speed of 0·5 km h(-1) . The A. anguilla generally migrated in the central part of the fjord and in the uppermost 10-25% of the water column, but with frequent dives to greater depths. Already 4 km after sea entry, A. anguilla were observed diving deeper than 130 m within 20-30 min periods. Hence, this study demonstrated that A. anguilla may perform an active diving behaviour during the early marine migration. The study took place in a pristine area with a minimum of anthropogenic interventions and by individuals from a population still uninfected by the introduced parasite Anguillicoloides crassus. The results may therefore be used as a baseline for future studies of the A. anguilla early marine migration.

Mid-winter activity and movement of Atlantic salmon parr during ice formation events in a Norwegian regulated river

A telemetry study in a Norwegian regulated river was conducted through a 12-day period in mid-winter 2003. The objective was to study activity (defined as number of movement per hour) and movement (defined as distance moved per hour) during different ice formation events. Twenty-four Atlantic salmon (Salmo salar L.) parr were radio tagged and continuously monitored by both manually tracking (N = 24) and by fixed recording stations (N = 15). Detailed data on climate, flow and ice formation and its spatial distribution were collected and used in the analyses. Fish activity was not found to be affected by their size (L F). There was a significant difference in activity between diel periods with highest activity during dusk (5–6 p.m.). Between high and low flow (mean ± SD, 21.1 m3 s−1 ± 1.7 SD and 11.1 m3 s−1 ± 1.7 SD, respectively) no significant difference in activity was found. During the experiment extensive anchor ice growth occurred mainly in the riffle part with thickness up to 50 cm. Juveniles tend to avoid riffle section during anchor ice formation and exploited ice covered areas, indicating critical and preferable habitats respectively. Further, a significant difference in movement was found between five selected ice events with highest mean movement during an anchor ice event and lowest mean movement during an ice break up with no anchor ice formation. No significant difference in activity or movement between parr exposed to frazil ice and parr not exposed were found.

Long-term effects of two sizes of surgically implanted acoustic transmitters on a predatory marine fish (Pomatomus saltatrix)

Fish telemetry is increasingly used to study fish behaviour in marine systems and it is crucial that the tagging does not affect fish behaviour and welfare negatively. Hence, the long-term effects of surgically implanted acoustic telemetry transmitters on survival, tag retention, healing and growth on Pomatomus saltatrix (known as tailor in Australia, elf in South Africa and bluefish in North America) were studied over 144 days. P. saltatrix are well suited for transmitter implants because no tagging-related short- or long-term mortality or transmitter expulsion was recorded. Small transmitters (9 × 28 mm, 3.3 g in water) did not affect the specific growth rate of tagged fish compared with an untagged control group. In contrast, the fish tagged with large transmitters (13 × 50 mm, 6.9 g in water) had a reduced specific growth rate. The small, but not the large transmitter, seemed suitable for tagging P. saltatrix of the body sizes represented in this study (215–621 g). The specific growth rate was negatively affected by increasing the transmitter-mass-in-water to body-mass ratio. It is recommended that this ratio should not exceed 1.4%.