TILSETH and ELLERTSEN: FOOD ORGANISMS OF LARVAL COD 



defined in the present paper as the concentration 

 of particles within the size range of 150-600 /xm 

 from the surface to 50 m depth in 1 m depth 

 intervals. 



An object found in the Hiac sensor was measured so 

 that the largest projected area was converted to a cir- 

 cle of the same area. By calibration, the object was 

 given a length similar to the diameter of this circle. 



The contours of Artemio nauplii were drawn by 

 using a microscope drawing tube. Their areas were 

 estimated by planimeter and converted to areas of 

 circles and their diameters calculated. Their size dis- 

 tribution was then divided into four 50 jum length 

 groups of 200 to 400 /xm. Four of the Hiac Particle 

 Counter channels were set according to the sensor 

 calibration diagram to the corresponding size 

 groups. 



The instrument system was tested and calibrated in 

 the laboratory by comparing microscope and Hiac 

 measurements of the size-frequency distribution of a 

 sample of laboratory hatched Artemia nauplii. Tests 

 were also made at sea when the research vessel was 

 anchored. The in situ instrument data were com- 

 pared with plankton pump samples taken simul- 

 taneously. These samples were taken by a submer- 

 sible electric pump (Flygt 2051, 250 1/min) which 

 pumped samples on deck through a 50 m long by 5 cm 

 diameter hose. Samples were taken at 0, 2.5, 5, 7.5, 

 10, 12.5, 15, 20, 25, 30, and 40 m depths. This is 

 defined as a zooplankton pump profile. Seawater was 

 collected in calibrated tanks (23.7 1), and zoo- 

 plankton were filtered through 90 itm mesh plankton 

 nets. Zooplankton were identified and counted by 

 microscope, the whole sample (23.7 1) was counted. 

 Results of the samples from these 11 depths were 

 statistically compared with the in situ counts from 

 corresponding depths by paired Mests. 



Field Investigations 



The main objectives of field investigations were to 

 use the in situ instrument system to find particle 

 patches and to identify larval cod food organisms and 

 study their vertical distribution. Observations were 

 made in the Lofoten area (Fig. 2). The effect of wind 

 driven turbulence on the distribution of particles and 

 the consequences on larval cod feeding incidence 

 were studied in the Austnesfjord (Fig. 3), which is in 

 the main spawning area of the Arcto-Norwegian cod. 

 Stations and sections in the Austnesfjord are shown 

 in Figure 3. A section is a transect with a series of 

 stations. Austnesfjord was chosen because cod lar- 

 vae are known to appear in high numbers (Ellertsen 

 et al. 1977), and the dynamics of the current system 



are known (Fumes and Sundby 1981). During the 

 1980 cruise, a Wolfe wind recorder was placed on 

 land in the fjord to continuously measure wind 

 velocity and direction. 



In 1981, observations were also made in the main 

 first feeding area, an open ocean bay (Fig. 2), for cod 

 larvae. The objectives were to find these food parti- 

 cle patches for cod larvae and to investigate the 

 extent and densities of these patches in this 

 exposed area. 



Distribution of cod larvae in the first feeding areas 

 was studied from the Juday net (80 cm, 375 jum 

 mesh) samples taken in vertical hauls from 30 to m. 

 In the Austnesfjord, three stations were taken on 

 eight sections (Fig. 3). The vertical distribution of 

 cod larvae in the Austnesfjord was investigated only 

 when the ship was anchored. A total of 42 samples 

 were taken by a submersible electric pump (Flygt 

 B2125, 3.4 mVmin) at 5, 10, 15, 20, 25, 30, and 35 m 

 depths every 3 h from 1600 h 13 May to 1000 h 14 

 May 1980. Fifteen cubic meters of seawater was sam- 

 pled at each depth. Seawater was pumped through a 

 40 m long by 15 cm diameter hose and filtered 

 through a Juday net (40 cm, 180 /xm mesh) into a 

 large tank on deck. Cod larvae were preserved in 4% 

 Formalin in 10%o seawater solution. Gut contents of 



FIGURE 2.— Map of the Lofoten area with stations and sections 21 

 April-8 May 1 98 1. The figures on the stations refer to number of cod 

 larvae/m 2 surface. 



143 



