Spatial structure and movement of blue cod Parapercis colias in Doubtful Sound, New Zealand, inferred from delta C-13 and delta N-15

Migrations and home range shifts by adult blue cod Parapercis colias provide an important means of dispersal among resident subpopulations within the Doubtful-Thompson Sound complex, a fjord system in southwestern New Zealand. In the present study we used stable isotope analysis (delta C-13 and delta N-15) to investigate regional differences in the carbon sources for, and likely migration patterns among, subpopulations of P. colias. We found consistent differences in delta C-13 and delta N-15 of blood and muscle collected from outer coast (mean +/- SE: blood, delta C-13 -18.4 +/- 0.2, delta N-15 11.5 +/- 0.2; muscle, delta C-13 = - 17.9 +/- 0.1, delta N-15 = 13.0 +/- 0.1) and inner fjord (mean SE: blood, VC = 9.9 +/- 0.5, delta N-15 = 9.5 +/- 0.3; muscle, delta C-13 = -19.3 +/- 0.4, delta N-15 = 11.6 +/- 0.2) locations, likely reflecting long-term (> 1 yr) residency. Inner fjord fish had evidence of significant input of recycled carbon from chemoautotrophic bacteria. On the outer coast, delta N-15 and delta C-13 reflected a diet likely supported primarily by new production from macroalgae and phytoplankton. To provide information on isotopic turnover rate of blood and muscle we translocated fish from the outer coast to an inner fjord site and measured changein delta N-15 and delta C-13 after 160 d. These data showed a depletion of C-13 and N-15 with shift in habitat and provided a basis for classifying likely migrants (< 160 d) to each study site. The conclusion that populations of blue cod within Doubtful Sound are made up of long-term residents with some subsidy from the outer coast has implications for the efficacy of 2 newly established marine reserves in Doubtful Sound and others in Fiordland.