IMPROVING TIME-SERIES OF ABUNDANCE INDEXES APPLYING NEW KNOWLEDGE

Recent information on gear selectivity and fish scattering cross secti on are used to comet time series of abundance estimates of cod and had dock from surveys in the Barents Sea. Both swept area estimates and ac oustic estimates from the period 1981-1993 an corrected for fish size dependent capture efficiency, and the acoustic estimates are also corr ected for a change in the scattering cross section-fish length relatio nship. Ideally, such work ought to be carried out station by station, correcting the length and species distribution from every trawl haul. Because of difficulties in the availability of raw data for the earlie r part of the time series, a short cut procedure was established which yields new estimates by using the old total numbers at age as input. The results are compared with those obtained with the correct and comp rehensive procedure for some years and the differences were negligible . The general effect of the corrections is an increase in the estimate s of the younger age groups, ages 1 and 2 years, in relation to older fish. The corrections did not improve the fit between numbers at age a s estimated from virtual population analyses (VPA) and the numbers at age obtained from surveys. Hence, the main advantage seems to be the r eduction of a serious bias in the annual length (age) distributions, b ringing the survey estimates of fish aged 1 and 2 years more in line w ith the expectations from population models. Comparison of the various statistics in the linear regression analyses between VPA-numbers and survey numbers may indicate that the natural mortality rate applied by the Arctic Fisheries Working Group for haddock is too low. (C) 1997 E lsevier Science B.V.