Changes in swimming depth and direction of silver eels(Anguilla anguillaL.) from the continental shelf to the deep sea

Abstract

Sixteen silver eels (Anguilla anguilla L.) were tagged with pressure-sensing ultrasonic transmitters and tracked for 1 to 152 hours, 12 in the western Mediterranean and 4, treated with pituitary extracts, in the Sargasso Sea. When released on the shelf of the Mediterranean near Gibraltar the eels preferred a course which led them continuously to deeper water. Four specimens released on the Morrocan shelf, reached depths of 350m, continued to swim north or changed their earlier easterly course towards the north. Three eels in the Sargasso Sea showed progress in a southwesterly direction. No influence of tidal stream on the swimming course and speed is obvious except for one eel, released in the Straits of Gibraltar. In the Mediterranean, the eels exhibited conspicuous daily vertical migration downward during dusk and upward during dawn with a mean swimming depth of 196m during darkness, 344m during daylight. Depth preference during night probably varied with the phase of the moon. Maximum swimming depth near Gibraltar as well as in the Sargasso Sea was nearly 700m. The eels in the Mediterranean swam at a mean speed of 0.3 m. s-' and were slower than most eels tracked in northern waters. Differences in the speed and in the directed swimming in the course of the lunar or the solar day are visible.

Vertical migrations may control maturation in migrating female Anguilla dieffenbachii

“…Tesch 1978, 1989, 1995, Tesch et al 1991, Jellyman & Tsukamoto 2002, Westerberg et al 2007, Aarestrup et al 2009, with daytime diving and nighttime ascents, sometimes to the surface (Sasai et al 2001, Westerberg et al 2007. In shallow or continental waters, multiple dives have been observed within a single night (Tesch et al 1991, Westerberg et al 2007), but a single descent/ascent pattern becomes established in oceanic water (Tesch 1978, Jellyman & Tsukamoto 2002, Aarestrup et al 2009.…”

“…In shallow or continental waters, multiple dives have been observed within a single night (Tesch et al 1991, Westerberg et al 2007), but a single descent/ascent pattern becomes established in oceanic water (Tesch 1978, Jellyman & Tsukamoto 2002, Aarestrup et al 2009. Typically, ascents do not bring eels close enough to the surface for celestial navigation cues (Tesch 1989, Jellyman & Tsukamoto 2005, and geomagnetism is thought to be the more likely means of navigation (Tesch et al 1992, Moore & Riley 2009). Whatever the reason for these dives, they must be ben- eficial as they represent additional swimming distance to a starving (non-feeding) fish.…”

Evaluation of surgery procedures for tagging eel Anguilla anguilla (L.) with biotelemetry transmitters

“…Externally attached transmitters have been frequently used to tag large eels during their movements in lakes, estuaries, tidal creeks and the first stages of their marine migration on the continental shelf (Tesch, 1974(Tesch, , 1989Helfman et al, 1983;McGovern & McCarthy, 1992). They are known to modify the hydrodynamism of the fish (Tesch, 1974) and may be unsuitable to study resident life stages in streams, where eels exhibit cryptic life habits and hide in narrow cavities in between rocks.…”

A pressurising system for long-term study of marine or freshwater organisms enabling the simulation of cyclic vertical migrations

“…Numerous species of aquatic organisms experience (at least during some stages of their life cycles) those rhythmic pressure changes associated with vertical movements and there are usually two distinct patterns of vertical movements: a diurnal pattern and a semi-diurnal pattern related to tides and involved in the selective tidal stream transport (Greer-Walker et al, 1978). Plaice, eel and cod are some of the fish species with the best-characterised patterns of cyclic vertical movements during spawning migrations, although they are quite different in terms of cycle duration and level of pressure exposure (Metcalfe et al, 1994;Tesch, 1979Tesch, , 1989McCleave and Arnold, 1999;Godø and Michalsen, 2000;Stensholt, 2001). …”