Incorporating depth-dependent differences in fishing efficiency among vessels in the research survey time series for Atlantic cod (Gadus morhua) in the southern Gulf of St. Lawrence / by D.P. Swain, G.A.... : Fs97-4/2317E-PDF

"The southern Gulf of St. Lawrence groundfish abundance survey was conducted using the E.E. Prince in 1971-1985, the Lady Hammond in 1986-1991, and the Alfred Needier since 1992. A comparative fishing experiment was conducted in 1985 between the E.E. Prince and the Lady Hammond, and in 1992 between the Lady Hammond and the Alfred Needier. We calculated conversion factors between the E. E. Prince and the Alfred Needier using these two comparative fishing experiments. Catches by the E.E. Prince multiplied by 1.3 are comparable to those by the Alfred Needier. Previous work indicated depth-dependent differences in fishing efficiency between the Lady Hammond and the other two vessels. We examined effects of these differences on perceptions of annual variation in cod depth distribution. The application of depth-dependent conversion factors did not alter annual patterns in the strength of the effect of depth on cod catch rates or alter conclusions about the density-dependence of cod depth distribution. We compared time series of cod catches adjusted (when necessary) to be comparable to catches by either the E.E. Prince or the Alfred Needier. Estimates of annual variation in relative abundance varied little between the two series. We recommend adjusting 1971-1991 catches to be comparable to those by the Alfred Needier. This has the advantage that no further adjustments are required as additional data are collected each year by the Alfred Needier"--Abstract, page iii.

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