— l84 — 



(2) Importance of the hydrographical conditions. Just as the Chart of di- 

 stribution Plate X clearly shows that the conditions as to depth play an extremely essen- 

 tial part in the occurrence of the eel larvae, it also shows that other conditions must be 

 of importance likewise. From the knowledge we have of the biology of other fishes we 

 might well conclude that the temperature and salinity, i. e. hydrographical conditions, have 

 their importance. So far as the eel is concerned, the temperature and salinity of the 

 surface layers can naturally be of no importance as this fish is certainly born in deep 

 water. We must therefore investigate the hydrographical conditions in deeper water at the 

 places where the' eel larvae are found, and thus seek to understand what the conditions 

 are which in addition to the depth control the distribution. As the chart shows, we have 

 made investigations at many places where the depth was lOOO meters or more and 

 where we had therefore the right conditions as to depth, and yet found no eel larvae. 

 We may now investigate at which of the stations marked in red the lowest and highest 

 temperatures (and salinities) were found in lOOO meters depth. The result is, that the 

 most northerly red station (W. of the Faeroes) has the lowest temperature, viz. 

 6.90°, and the lowest salinity, viz. 35-17 pro mille, whereas the most southerly red 

 station (right off" Brittany) shows both the highest temperature (9.47°) and salinity 

 35'64 pro mille), which have hitherto been detected in lOOO meters depth on the cruises 

 of the "Thor" in the Northern and Atlantic Oceans. As we know that the eel spawns 

 in quantities in the Mediterranean, where the lowest temperatures even at the greatest 

 depths (over 4000 meters) are ca. 13° C. ', it is evident from this alone that there 

 can be no question of finding places in the northern Atlantic, where our investiga- 

 tions were made, where the temperature in 1000 meters depth is too high to allow the 

 eel to reproduce. 



On the other hand, by means of the material collected by us we can determine the 

 lowest temperature (and salinity) in 1000 meters within the region in which the larvae 

 of the eel are found and in which we may say that this species reproduces. We have 

 already seen, that quite the lowest temperature and salinity were found at the most 

 northerly station, west of the Faeroes, and amounted to respectively 6'90° and 35-17 pro 

 mille. But at this station only a single eel larva was once found, and we had to go 

 somewhat further to the south, viz. to off the Hebrides, before we found the eel larvae in 

 any way regularly in our hauls with the young- fish trawl. At one of the latter stations 

 the temperature and salinity in 1000 meters depth were found to be respectively 7-70° 

 and 35'28 pro mille. From the hydrographical section annexed and from the tempe- 

 rature curves drawn on the Chart for lOOO m. depth ^ we see that both the temperature 

 and salinity rise uniformly towards the south, until we come finally to the maxima men- 

 tioned above (temp. 9.47°, salinity 35'64 pro mille) at the stations off Brittany. 



At the stations where most eel larvae per haul were taken, namely 70 in addition 

 to other Leptocephalus species, the temperature and salinity in 1000 meters depth were 

 respectively 8'8l° and 35'55 pro mille. 



We may conclude from these data, that the eel \3.r\r BlQ {Leptocephalus brevirostris), 

 i. e. the youngest known larval stages of the eel, were only found in those parts 



1 Cf. e. g. the "PoIa"-Expedition, 1890 — 93, in Ber. d. Kom. für Erforschung des östlichen Mittelmeeres 

 (Denkschr. d. Wiener Akad. d Wis.senschaften, Mat.-Naturwiss. Klasse, Bd. LIX — LXI, 1892 — 94). 



2 The hydrographical data are due to Cand. mag. J. N. Nielsen who carried out the observations on board 

 the "Thor" and most kindly informed me of the results for which I am gjeatly indebted to him. 



