Preliminary assessment of the South Georgia ray populations

We modelled the ray population at South Georgia using a surplus production model implemented in a Bayesian framework. Catch and CPUE data were reconstructed for the time series 1985 – 2006 from a variety of sources, and included the consideration that since 2004 the practice of cutting rays off lines at the water surface should have increased survivorship of discarded rays, although this survivorship has been found to decrease as depth of fishing increases. Our analysis is concentrated on catches deeper than 800m, and we have assumed that the majority of catches were of the Amblyraja variant species previously described.
The assessment indicated that the data were informative with respect to carrying capacity but not with respect to r, the intrinsic rate of growth of the ray population. We chose a conservative, low value of r thought to be representative of long-lived rajids, but conducted sensitivity tests with higher and lower values. The assessment suggests that the level of mortality currently experienced by the population is sustainable, and that currently the population is well above Bmsy. Whilst current catches appear not to be significantly impacting the ray population, trends to deeper water fishing and increasing numbers of autoliners (whose catch rates of rays is greater than Spanish gear longliners) should continue to be observed closely. Future analyses should make use of the increasing accuracy and duration of CPUE data and the results of an ongoing mark-recapture experiment for rays at South Georgia.