DETERMINATION OF LARVAL FISH SURVIVAL FROM FEEDING AND DISTRIBUTION OBSERVATIONS

Abstract

The series of papers round which this thesis is based highlight the inherent problems in assessing larval fish feeding success from sampling programmes and gut contents analysis, and show through novel observations, that counting the number of food items in the gut is not necessarily a good indication of feeding success. Food type, digestibility and size, gut passage rates and the particular lipid classes the food contains, must all be considered, illustrating the difficulties in interpreting larval fish energetics from field studies. The resistance to digestion demonstrated for copeped eggs, means that a potentially rich source of energy cannot be utilised. This may have severe nutritional consequences for the early larvae of many important commercial fish species which can feed heavily on these eggs, possibly influencing ultimate recruitment levels. While much can be learned from coarse grid, depth integrated plankton sampling, interpretation of relationships between food availability and larval condition was shown to require precise sampling of the larvae in their actual feeding environment. Intense integrated and vertically stratified plankton sampling allowed the most detailed observations on food selection by fish larvae ever carried out and demonstrated their considerable foraging adaptability, even in a changing food environment. Food availability is undoubtedly an important factor in larval fish survival because of its affect on condition and growth rates, but many other factors are now known to be involved and it is accepted that recruitment in a species may be decided at almost any stage of early development. The traditional approach to understanding the environmental impact on larval survival and fish recruitment has been by observational field ecology and correlation techniques. However, more recently, advances in mathematical modelling techniques in which we are involved, are allowing the representation and simulation of both physical and biological oceanographic processes. Progress in understanding how selective forces generally shape recruitment will depend increasingly on these exciting new techniques.