FISHERY BULLETIN: VOL. 82, NO. 1 



£ 



£ 

 a. 



01 



Q 

 10 



20 



1033 



Eggum 26Apr.81 



2 n. miles 



1535 H 



30 ^ 



40 



10 



30 



Eggum 26 Apr. 81 



2 n. miles 



15 40 H 



g-IOH 

 Q 



20 



30- 



40 J 



FIGURE 13.— Particle (A) and nauplii (B) distributions (per liter) in the upper 40 m on the 



Eggum section, 26 April 1981. 



DISCUSSION 



Food particles found in the alimentary tract of larval 

 cod consist, with few exceptions, of copepod nauplii 

 in the size range of 140-520 tun. This observation did 

 not differ significantly from that of Ellertsen et al. 

 (1977), who found the size variation to be within 140- 

 600 /Am. The in situ instrument was set to detect par- 

 ticles in this size range. Investigations have shown 

 that in May copepod nauplii outnumber all other par- 

 ticles in this size range in the Lofoten area (Ellertsen 

 et al. 1977; Wiborg 1948a, b). The main objective 

 when designing this instrument was to obtain a quick, 

 reliable impression of naupliar distributions without 

 laborious, time-consuming countings by microscope. 

 The tests performed to compare the in situ instru- 

 ment system and the plankton pump samples 

 showed good agreement between the two methods. 

 The critical food concentrations for first feeding cod 

 larvae are not precisely known. They are thought to 

 be on the order of 40-200 nauplii/1 based on studies 

 of swimming activity, larval search volume, and 

 oxygen requirements of first feeding cod larvae 



(Solberg and Tilseth 1984). Patches of particles/ 

 nauplii with the required densities for first feeding 

 cod larvae to survive were found in the spawning and 

 first feeding area by these methods. 



The results presented in this paper show some of 

 the dynamics in the formation and distribution in 

 time and space of microzooplankton patches. The 

 vertical distribution and density of nauplii changes 

 due to the diel vertical migration of these organisms 

 (Figs. 5, 6). 



The concentration of particles/nauplii in a patch 

 was dependent on the hydrographic situation and on 

 the distribution and concentration of micro- 

 zooplankton in the water column (Figs. 5, 6). Conse- 

 quently the vertical distribution of particles and 

 nauplii will be dependent on factors such as hydro- 

 graphic conditions and time of day when the ob- 

 servations are made. 



Increased wind force caused mixing of the surface 

 layers and led to a homogeneous vertical particle dis- 

 tribution. No surface patch was observed at night 

 during windy conditions, and the mean particle con- 

 centration in the water column dropped steadily dur- 



152 



