RED WHITE MUSCLE DISTRIBUTION AND FIBER GROWTH DYNAMICS - A COMPARISON BETWEEN LACUSTRINE AND RIVERINE POPULATIONS OF THE SOUTHERN SMELT RETROPINNA-RETROPINNA RICHARDSON

The myotomal musculature of the common New Zealand smelt Retropinna re tropinna from a lacustrine and a riverine population was investigated in an attempt to correlate differences in the fish's maximum body size and migratory habits with differences in muscle morphology and growth dynamics of their component muscle fibres. Based on measurements of c ross sections through the fish at 70% fork length, a significant incre ase in red muscle percentage is recorded between pre- and post-migrato ry stages in the riverine population. Although lacustrine and pre-migr atory riverine smelt possess similar white:red muscle ratios, the red muscle increase in post-migratory, riverine smelt is explained by the requirement for sustained swimming during upstream migration. The grow th dynamics of red fibres also appear to be related to functional requ irements such as sustained swimming. The slower rates of growth and sm aller maximum size (dwarfing) exhibited by lacustrine smelt of basical ly riverine stock in comparison with diadromous, riverine smelt, are a ttributed to differences in white fibre growth dynamics. The recruitme nt of new fibres into the growing muscle mass ceases to be of signific ance at a shorter fork length in the dwarfed lacustrine form of smelt, with the subsequent result of larger mean diameters of white fibres a t all fork lengths compared with those of the riverine form. Hypertrop hy of white fibres gradually diminishes in both forms as the mean diam eters of white fibres approach 100 mum. Considering that only fish fro m the wild were examined and that none of the data are derived from sp ecimens subjected to forced swimming experiments in the laboratory, th e results convincingly demonstrate the usefulness of the fish's muscle plasticity under natural conditions in relation to the environment an d the behaviour of the fish.