THE DETECTION AND DISTRIBUTION OF 



LARVAL ARCTO-NORWEGIAN COD, GADUS MORHUA, 



FOOD ORGANISMS BY AN IN SITU PARTICLE COUNTER 



S. TlLSETH AND B. ELLERTSEN 1 



ABSTRACT 



An in situ particle counter system was developed to count measure food particles in numbers per liter within 

 the size range 150-600 /urn, the sizes of copepod nauplii captured by first feeding cod larvae. Patches of 

 particles/nauplii of 50- 1 00 per liter were found in the spawning and larval first feeding area. Different sizes of 

 copepod nauplii showed diel vertical migration, and this influenced the formation of patches. Mixing of the 

 water column by wind forces created a homogeneous vertical distribution of particles. Gut content analysis of 

 cod larvae during these hydrographical conditions indicated reduced accessibility of food organisms to 

 larvae. 



During the last few years fisheries scientists have 

 done a great deal of laboratory work on the behavior 

 of fish larvae and their energy requirements for 

 growth and survival (Hunter 1972; Laurence 1974; 

 Lasker and Zweifel 1978; Houde 1978; Werner and 

 Blaxter 1980). A review of these data (Hunter 1981) 

 shows that differences exist between the required 

 density of prey particles for first feeding larvae to 

 survive and the densities found in the sea. Since 

 pelagic fish larvae are successful in their environ- 

 ment, it is recognized that there must be patches of 

 suitable concentrations of food organisms for first 

 feeding larvae (Lasker and Zweifel 1978). This has 

 been demonstrated for the northern anchovy, 

 Engraulis mordax, in laboratory experiments by 

 Hunter and Thomas (1974) and in a series of field 

 investigations by Lasker (1978). Houde and Schek- 

 ter ( 1 978) have shown increased survival of larval bay 

 anchovy, Anchoa mitchilli, and sea bream, 

 Archosargus rhomboidalis, when exposed to sim- 

 ulated food patches in a laboratory experiment. 



This work has been stimulated by Hjort's (1914) 

 hypothesis which simply stated that larval mortality 

 rates may be due to variable feeding conditions at a 

 critical stage, which in turn causes variations in year- 

 class strength. It has been difficult to test this simple 

 hypothesis in field surveys because of the inade- 

 quacy of the sampling gear used (May 1974). To 

 obtain a better understanding of the relationship be- 

 tween estimates of food densities required by fish 

 larvae in the laboratory and densities found in the 



'Institute of Marine Research, Directorate of Fisheries, 5011 

 Bergen-Nordnes, Norway. 



open sea, samples should be taken which are relevant 

 to larval searching behavior. This would require an 

 enormous number of plankton samples. It would be 

 time-consuming to obtain these samples with con- 

 ventional plankton gear. Furthermore, water move- 

 ment and dispersion would make it difficult to obtain 

 time and space relationships for studying the forma- 

 tion and dynamics of plankton patches (Steele 1978). 

 One way of studying these relationships is by using in 

 situ instruments (Boyd 1973; Pugh 1978; Tungate 

 and Reynolds 1980). 



In this study an instrument designed to count and 

 measure particles in situ in the size range of food 

 organisms most frequently captured by cod larvae 

 was used. Investigations were made on the spawning 

 and first feeding grounds of the Arcto-Norwegian 

 cod, Gadus morhua Linnaeus, during two successive 

 years (1980-81). During the first survey, inves- 

 tigations were made in a sheltered fjord where cod 

 larvae are known to appear in high numbers 

 (Ellertsen et al. 1977) and where the current system 

 has been described (Furnes and Sundby 1981). The 

 objective was to find and study the formation of mi- 

 crozooplankton patches and to study larval cod feed- 

 ing under different environmental conditions with 

 regard to food density, water turbulence, etc. In the 

 following year, the main first feeding area, an open 

 ocean bay, was surveyed in order to find and study 

 the vertical and horizontal distribution of micro- 

 zooplankton patches in this exposed area. 



The present study is part of a project, started in 

 1975, dealing with growth, mortality, and drift of cod 

 larvae in the Lofoten area (Ellertsen et al. 1976). 



Manuscript accepted June 1983. 



FISHERY BULLETIN: VOL. 82. NO. 1, 1984. 



141 



