GEOGRAPHIC RESPONSES OF GROUNDFISH TO VARIATION IN ABUNDANCE - METHODS OF DETECTION AND THEIR INTERPRETATION

Recent published studies have used data from bottom trawl surveys of g roundfish populations to test whether distributional area and abundanc e are correlated. Two studies that used different indices to represent the distributional area of Georges Bank haddock (Melanogrammus aeglef inus) yielded conflicting results. To determine whether this is an exa mple of different distributional indices measuring different things, b oth indices were regressed against estimates of abundance of haddock f rom a different but neighbouring location on the southwestern Scotian Shelf. Positive correlations were observed for immature age-classes us ing both indices whereas only one of the two indices resulted in posit ive correlations for mature age-classes. The following factors contrib uted to the lack of agreement among distributional indices: (1) age-ag gregated indices potentially obscure correlations between distribution al area and abundance for individual age-classes; (2) distributional i ndices that depend on the magnitude of catch rates confound variation in the large-scale horizontal distribution of stocks with diurnal vari ation in the three-dimensional distribution of schools; (3) distributi onal indices that scale positively with abundance generate spurious co rrelations. The results suggest that the outcome of any test of whethe r distributional area and abundance are correlated depends on the inde x chosen to represent distributional area.