APPENDIX E: HEINCKE — 8 — 



A. The apparatus and methods used 



1. The fishing for planktonic eggs and larvae. 



For this we have four different nets in use, two for vertical and two for horizontal 

 fishing, or in other words, for quantitative and qualitative hauls respectively. A detailed 

 description of these apparatus and the method we use in fishing with them, is given in 

 the work of Ehrenbaum and Strodtmann on the eggs and young forms of the 

 Baltic fishes (Wissen. Meeresunters. VI. Abteilung Helgoland. 1904. p. 57 et seq.) 



The most important of these nets is the quantitative egg-net, designed by 

 H ens en (see Hensen u. Apstein, "Ueber die Menge der im Winter laichenden Fische". 

 Wissensch. Meeresunters. II. Abteilung Kiel. 1897. p. 1 et seq.). It is furnished with silk- 

 gauze No. Ill and so measured, that it filters vertically a column of water of ca. Vs m^ 

 by an average velocity of 0-5 m. in the second. Then the actual catch of the net has to 

 be multiplied by 3, in order to obtain the number of the eggs and larvEe occurring under 

 one square meter of the surface. The great importance of this net is twofold. 

 Firstly, it fishes through the entire column of water from the bottom to the surface, and 

 thus gives information whether eggs and larvae occur at all at a definite spot in the sea. 

 Secondly, it gives information regarding the quality, i. e. the relative abundance (per square 

 meter of the surface) of the planktonic eggs and larvae in a fairly large, uniform region of 

 the sea. Numerous parallel hauls, which we have made with this net, have unmistakeably 

 confirmed again and again the assumption underlying all H en s en' s quantitative plankton 

 investigations, viz. that the fish-eggs and larvae floating in the sea within a region of limited 

 extension, are equally distributed. Consequently, the quantitative egg-net of 

 Hensen is, in fact, the only apparatus at present, which can give us any, 

 approximately true information with regard to the occurrence and quantity 

 of the fish-eggs and larvae of the different species. The information given 

 below for the various species, has been gained with this net particularly. 



A fault of this net is, that it takes very small quantities of eggs, on account of its small 

 dimensions. Thus, the occurrence of very sparsely distributed eggs of a species (1 or but 

 a broken piece, per square meter) may escape detection, if no parallel hauls or only a few, 

 are made with it. Also, the small number of the eggs and larvae taken, is naturally a dif- 

 ficulty otherwise. This is helped out by using the so-called large quantitative vertical 

 egg-net which has a diameter at the opening of 2-5 m. (the foremost ring is in the form 

 of a hinged closing hoop), and in a vertical haul filters the column of water contained 

 beneath ca. 2 square meters of the surface. The manipulation of this large net is cer- 

 tainly difficult and, especially in the superficial layers, untrustworthy, whereas it is of good 

 service where, as in the eastern parts of the Baltic, the planktonic eggs are quite absent 

 in the upper layers and only occur in the deeper. 



To take very large quantities of eggs and larvae, our qualitative horizontal nets 

 are of use. A suitable apparatus for the surface layers is the well-known and often de- 

 scribed Heligoland young-fish net, constructed by us. For horizontal fishing in the 

 deeper layers, at a certain distance about 5 to 10 m. from the bottom, we use the so-called 

 Heligoland otter young-fish net which is described in detail and figured, in the work 



