Ageing studies done 50 years apart for an inshore fish species from southern Australia—contribution towards determining current stock status

Abstract

The status of the stocks of Southern Garfish (Hyporhamphus melanochir) in South Australia is currently uncertain. Knowing the extent to which populations have become truncated from fishing activity would help resolve this. Estimates of population structure are available for this species from market sampling done across the two-year periods of 1954 and 1955 and 2006 and 2007. During the latter, fish age was estimated from transverse sections of otoliths using a modern quality assessment and quality control (QA/QC) protocol. The resulting age structures were dominated by the 1+ and 2+ age classes, with older fish contributing less than 16% of harvested numbers. Also, there were few fish >30 cm CFL in size. In comparison, in 1954 and 1955, fish age was determined from whole otoliths, whilst QA/QC procedures were poorly developed. The resulting age structures were dominated by 3+ and 4+ age classes, whilst >25% of the numbers were >30 cm CFL in size. Before concluding that populations were now truncated, it was necessary to reconcile the difference in ageing methods between studies. This was done by assessing the tractability of obtaining accurate estimates of fish age from whole otoliths. Otoliths from a broad age range collected in 2006 and 2007, when immersed in water and displaying the distal face, displayed interpretable patterns of increments. Furthermore, the growth of these otoliths in length slowed but had not stopped even for relatively old fish, indicating that the anterior edge remained a growing edge. A comparison of counts between whole and sectioned otoliths from the same fish gave comparable estimates of age. The studies indicated the tractability of obtaining accurate estimates of age from whole otoliths, when using modern laboratory equipment. However, since cruder techniques were used in the 1950s the accuracy of those ages remained in question. Fortuitously, a small set of otoliths collected in 1955 were available to this study. These were sectioned, counted and compared to counts of the whole otoliths done in the 1950s. Again the counts were comparable, thus conferring confidence about age estimates from the 1950s. The populations of Southern Garfish in 2006 and 2007 were considerably truncated relative to the 1950s, as a consequence of heavy commercial exploitation over the past 50 years. Historical datasets on population structure have made a significant contribution towards understanding current fishery status.