While there is growing evidence suggesting the importance of IGF-I in growth and development in fish, interpretation of these studies is hampered by the use of heterologous research reagents. Indeed, we have preliminary evidence suggesting that the heterologous immunoassays used to investigate the circulating levels of IGF-I in fish have been underestimating concentrations by up to 200-fold. Moreover the lack of sensitivity of the assays may not have permitted detection of subtle, yet important changes in IGF-I levels in fish resulting from growth, developmental, nutritional or environmental factors. In addition, we have recently ascertained that recombinant human IGF-I is cleared from the circulation of juvenile barramundi almost twice as rapidly as recombinant barramundi IGF-I, providing the first in vivo evidence that there are functional differences between mammalian and fish IGFs. Thus the use of homologous research reagents may be essential for correctly interpreting results from not only in vivo, but also in vitro investigations into the biological actions of IGF-I in fish.

If IGF-I plays as an important role in fish as has been found in mammals, the benefits of homologous IGF research reagents to the aquaculture industry could be substantial. The development of a diagnostic assay for IGF-I as a reliable, reproducible marker of growth performance or response to environmental stress could in turn impact significantly on fish farm management. Clearly, the over-riding aim of every commercial fish farm is to obtain maximum output of a quality product in the most cost efficient manner. Hence the development of an endocrinological assay as an indicator of growth potential or as an early marker of changing health status, may significantly aid achieving this goal. Likewise, the production of recombinant fish IGF-II and the development of a specific fish IGF-II immunoassays may prove to be valuable tools for defining the role of IGF-II in fish growth and development.