Research Document - 2009/026
Atlantic Halibut on the Scotian Shelf and Southern Grand Banks (NAFO Divisions 3NOPs4VWX5Zc) – Industry/DFO Longline Survey and Tagging Results to 2008
By M.K. Trzcinski, S.L. Armsworthy, S. Wilson, R.K. Mohn, M. Fowler, and S.E. Campana
Abstract
The Industry/DFO (Department of Fisheries and Oceans) Atlantic halibut longline survey (halibut survey) has been in operation for 11 years. During this time, there has been no indication of a change in the distribution of halibut within the management unit area, NAFO (Northwest Atlantic Fisheries Organization) divisions 3NOPs4VWX5Zc. Overall, the halibut survey indicates that catch rates of halibut ≥81cm is either stable or possibly increasing and that there has been no depletion of large fish from the population since 1998. The number of pre-recruits (fish < 81cm) caught in the halibut survey declined from 2001 to 2004, but pre-recruits increased in 2005 and have remained high over the past 3 years in both the DFO research vessel (RV) survey and the halibut survey. The halibut survey uses a fixed station design, and the coverage of these stations has been erratic over the Scotian Shelf and on the southern Grand Banks in particular. Catch rates in 4VWX have been stable with a slight increase in 2007 and 2008. When the entire stock unit is considered and the survey standardized by estimating station effects, catch rates have been increasing since 2003. The commercial index catch rate shows a decline since 1998. However, this index is less standardized than the halibut survey, and not all sources of variability have been considered at this time. A tagging study was initiated in 2006: 526 tags and 848 tags were released in 2006 and 2007, respectively, on all sizes of fish caught by longline gear (50–200cm). Exploitation rate on the exploitable biomass (≥81cm) was estimated to be 17.7% (90% CI: 15.7-19.8%) in 2006, and 20.1% (90% CI: 17.7-22.7%) in 2007. An attempt has been made to calculate a simple reference point to determine the catch that would result in a zero change in the RV and halibut survey abundance indices. These measures, however, did not respond to changes in landings. The biomass and productivity of Atlantic halibut were estimated for the Scotian Shelf and Southern Grand Banks stock using halibut survey and tagging data. Over the last decade, 76% of the production has been taken in the catch, which for most stocks would be considered high. The current exploitation rate from tagging is about double natural mortality and F0.1, which would be considered high for most stocks. Without biological reference points, it is impossible to know when the stock is recovered or what level of precaution is necessary. Although the exploitation rate is double natural mortality and F0.1, given that the abundance indices from the halibut survey have been increasing recently and that there are good signs of recruitment, a 15% increase in the Total Allowable Catch (TAC) for the 2009/2010 fishing season is not expected to increase the risk to the stock as compared to the previous 4 years. However, the longer-term consequences of utilizing the relatively high catch to surplus production ratio (3.2:1) should be evaluated in the context of stock management objectives, reference points, and a risk management framework.
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