TILSETH and ELLERTSEN: FOOD ORGANISMS OF LARVAL COD 



after this hydrographic condition had occurred. Dur- 

 ing the following hours the larval feeding incidence 

 increased again, most rapidly in larvae sampled at 

 15-30 m, indicating that food particle concentration 

 did not become critical. (Note that the particle con- 

 centration in Figure IOC only represents particles 

 within 300-500 /Am size range.) However, the feeding 

 ratio did not increase significantly, indicating a more 

 difficult accessibility of food particles to the larvae. 

 Similar observations were made by Lasker (1975, 

 1978), where stability of the water column in the 

 upper 30 m was necessary for food organisms to 

 aggregate in concentrations high enough to exceed 

 the threshold for feeding stimulus of first feeding 

 northern anchovy larvae. This observed reduced 

 feeding in cod larvae cannot be explained by a diel 

 feeding rhythm. Cod larvae are visual feeders; the 

 light intensity threshold for feeding is 0.1 lx 

 (Ellertsen et al. 1980). The light intensity in the 

 upper 40 m does not drop below this level in Lofoten 

 in May, and cod larvae are found with newly captured 

 nauplii in the gut at all hours (Gj0saeter and 

 Tilseth 1981). 



The number of cod larvae found in the main first 

 feeding area was too small to do a comparison on lar- 

 val feeding conditions. However, patches with 

 particle/nauplii concentrations of more than 50/1 

 were observed on every section made in this area. 

 Sizes of these patches were, on the other hand, small 

 compared with the volume of water surveyed. The 

 life span of these patches is probably very short 

 because of the influence of biological and physical 

 factors, especially when the upper 50 m of the water 

 column is unstable. This is the normal situation in the 

 Lofoten area in May (Furnes and Sundby 1981). 

 Therefore, prey organism patches with concen- 

 trations above the critical level for first feeding cod 

 larvae would probably be broken down, due to 

 increased water turbulence when the wind forces 

 increase. A series of storms during the larval cod first 

 feeding period could thereby have serious effects on 

 larval feeding conditions and consequently on sur- 

 vival and recruitment. 



ACKNOWLEDGMENTS 



The in situ instrument system was developed in 

 collaboration with The Foundation of Scientific and 

 Industrial Research at the Norwegian Institute of 

 Technology. We thank I. Mohus, B. Holand, and I. O. 

 Eriksen who were responsible for this work. We also 

 thank 0. Ulltang at the Institute of Marine Research 

 for his advice and help on statistics. 



LITERATURE CITED 



Boyd, C. N. 



1973. Small scale spatial patterns of marine zooplankton 

 examined by an electronic in situ zooplankton detecting 

 device. Neth. J. Sea Res. 7:103-11 1. 

 Ellertsen, B., E. Moksness, P. Solemdal, T. Str0mme, S. 

 Tilseth, and V. 0iestad. 



1976. The influence of light and food density on the feeding 

 success in larvae of cod (Gadus morhua L.); field and 

 laboratory observations. ICES, C. M. 1976/F:34, 31 

 p. [Processed.] 



Ellertsen, B., E. Moksness, P. Solemdal, T. Str0mme, S. 

 Tilseth, T.Westgard, E. Moksness, and V. 0iestad. 



1977. Vertical distribution and feeding of cod larvae in rela- 

 tion to occurrence and size of prey organisms. ICES, C. 

 M. 1977/L:33, 31 p. [Processed.) 



Ellertsen, B., P. Solemdal, T. Str0mme, S. Tilseth, T. 

 Westgard, E. Moksness, and V. 0iestad. 



1980. Some biological aspects of cod larvae (Gadus morhua 

 L.). Fiskeridir. Skr. Ser. Havunders. 17:29-47. 



Eriksen, J. O. 



1981. "Micro-count". Particle datalogger. Program man- 

 ual. Sintefrep. STF 48 F 81019, 203 p. [Processed.] 



Eriksen, J. O., and I. Mohus. 



1981. "Micro-count". Particle datalogger. User's man- 

 ual. Sintefrep. STF 48 F 81017, 54 p. [Processed.] 

 Furnes, G.K., and S. Sundby. 



1981. Upwelling and wind induced circulation in Vestfjor- 

 den. In R. Saetre and M. Mork (editors), Proceedings 

 from the Norwegian Coastal Current Symposium, Geilo, 

 Norway, 9-12 Sept. 1980, Vol. I, p. 152-177. Univ. 

 Bergen, Norway. 

 GJ0SAETER, H., AND S. TlLSETH. 



1981. Primary growth increments in otoliths of cod larvae 

 (Gadus morhua L.) of the Arcto-Norwegian cod stock. 

 Fiskeridir. Skr. Ser. Havunders. 17:287-295. 

 H.JORT, J. 



1914. Fluctuations in the great fisheries of northern Europe 

 viewed in the light of biological research. Rapp. P.— V. 

 Reun. Cons. Perm. Int. Explor. Mer 20:1-228. 

 HOUDE, E. D. 



1978. Critical food concentrations for larvae of three species 

 of subtropical marine fishes. Bull. Mar. Sci. 28:395- 

 411. 



HOUDE, E. D., AND R. C. SCHEKTER. 



1978. Simulated food patches and survival of larval bay 

 anchovy, Anchoa mitchilli, and sea bream, Archosargus 

 rhomboidalis. Fish. Bull., U.S. 76:483-487. 

 Hunter, J. R. 



1972. Swimming and feeding behavior of larval anchovy 

 Engraulis mordax. Fish. Bull., U.S. 70:821-838. 



1981. Feeding ecology and predation of marine fish lar- 

 vae. In R. Lasker (editor), Marine fish larvae, morphol- 

 ogy, ecology, and relation to fisheries, p. 33-77. Univ. 

 Wash. Press, Seattle. 

 Hunter, J. R., and G. L. Thomas. 



1974. Effect of prey distribution and density on the searching 

 and feeding behaviour of larval anchovy Engraulis mordax 

 Girard. In J. H. S. Blaxter (editor), The early life history of 

 fish, p. 559-574. Springer- Verlag, Berl. 



Lasker, R. 



1975. Field criteria for survival of anchovy larvae: The rela- 

 tion between inshore chlorophyll maximum layers and 

 successful first feeding. Fish. Bull., U.S. 73:453-462. 



1978. The relation between oceanographic conditions and 



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