Cod highly infected with the parasitic nematode Contracaecum osculatum demonstrates stunted growth: A controlled laboratory experiment with ad libitum feeding

Abstract

Predatory fish in the wild and in particular Eastern Baltic cod Gadus morhua are facing varying and unpredictable feeding opportunities. This implies a high assimilative capacity including nutritional state-related hyperphagia to maintain long-term positive energy balance. Heavy infection of cod livers by the parasitic nematode Contracaecum osculatum is suspected to jeopardize the assimilative capacity and thus the vital rates, growth, reproduction and mortality of the cod population. We examine the effect of C. osculatum on growth, condition and liver weight of cod in laboratory experiments with ad libitum feeding. We find that highly infected cod displays reduced hepatosomatic index and reduced or negative growth and change in condition factor. We suggest that in the wild, cod also with lower infection levels may be constrained by an impaired liver capacity as they here also face feeding constraints. Measurements of growth hormone expression together with knowledge about the endocrine regulation of appetite and growth are used to explain the data. We find strong indications that for cod with high infection level this plays a significant role for all the three vital population rates, and that these effects should be considered in the management of the cod stock in the eastern Baltic Sea.