Population differentiation of eelpout Zoarces viviparus in a Danish fjord

Abstract

Studies carried out in the early and mid-1900s demonstrated the presence of 2 separate populations of eelpout Zoarces viviparus associated with the outer and inner parts, respectively, of Mariager Fjord in eastern Jutland, Denmark. In 1997, the fjord experienced a major fish kill resulting from a hypoxia/anoxia event. The purpose of this study was to examine whether 2 populations could still be identified in this fjord in 1998. The same morphological traits and genetic methods used in the earlier studies were employed for the present study. Neither the morphological characteristics nor any of the 7 investigated allozymes examined revealed any population differentiation between fish caught in the inner and outer fjord in 1998. It was concluded that there was no population differenti- ation in eelpout in this fjord in 1998. In addition, growth rates (estimated from otolith growth zones) appeared to be the same in fish caught in the inner and outer fjord in 1998. Estimated growth rates for fish in autumn/winter 1997 to spring/summer 1998 (assumed to be the growth season for the year of the hypoxia event) were lower than those estimated for autumn/winter 1996 to spring/summer 1997. Genetic data obtained in studies from the 1970s were re-examined and the earlier conclusion regarding the existence of 2 populations was confirmed to be robust. Thus, a change in population distributions of eelpout may have occurred in this fjord between the 1970s and the late 1990s. As the characteristics of the fish found throughout the fjord in 1998 resembled those of the 'outer fjord' pop- ulation identified in earlier studies, it is argued that the inner fjord population identified in earlier studies has been reduced or eliminated and that the outer fjord population has moved into the inner fjord. A comparison of catch statistics from the 1970s and 1998 suggests a major decrease in stock size during this same period.