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 differentiation 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' population 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.

Larval fish distribution, growth and feeding in Patagonian fjords: potential effects of freshwater discharge

“…Fishes inhabiting fjord ecosystems have developed several adaptive mechanisms to feed and reproduce in such a variable environment, even with a tendency in some fish species for population differentiation in the inner and outer part of a fjord (Goodson et al 1995;Olsen et al 2002;Kaartvedt et al 2009). In fjord ecosystems, freshwater runoff and forceful mountain winds set up estuarine circulation patterns that may transport zoo-and ichthyoplankton into fjords, and possibly lead to entrapment of plankton there.…”

Panmixia in Zoarces viviparus: implications for environmental monitoring studies

“…To investigate this further, it would be of great interest to apply the near‐neutral microsatellite markers to the populations in Kattegatt and Danish fjords. Olsen et al (2002) found a change in population distribution of Z. viviparus in Mariager Fjord, where genetic data obtained in studies from the 1979s were re‐examined 1998. The population differentiation found in the 1970s had disappeared in the 1990s, which was thought to be related to eutrophication and oxygen depletion happening during that time.…”

Genetic variation of Zoarces viviparus: six populations revisited after about 35 years

“…The stability of the genetic distribution was very high as homogeneity between the samples, caught now and 36–38 years ago, was observed and thereby the same allelic frequencies, indicating either that the migration among the areas is low or that selection is going on. However, after a severe oxygen deficiency in Mariager Fjord in 1998, a migration from the mouth of the fjord and into the fjord was seen, so the eelpouts migrated at least 30 km (Olsen et al 2002). Another study indicated that eelpout is very sensitive to elevated sea water temperatures during summer time, e.g.…”

“…Mainly one of the fjords, Mariager Fjord, has been studied several times. Samples were analysed by using the analytic method described by Schmidt (1917) and by isozymes (Christiansen et al 1981, 1988; Olsen et al 2002). Christiansen et al (1981, 1988) found that the morphological traits were nearly unchanged for at least 60 years and some of the isozymes showed the same gradient as the morphological traits which indicated that the fjord harboured at least two populations of eelpouts, one in the inner part and one in the outer part of the fjord.…”

“…Christiansen et al (1981, 1988) found that the morphological traits were nearly unchanged for at least 60 years and some of the isozymes showed the same gradient as the morphological traits which indicated that the fjord harboured at least two populations of eelpouts, one in the inner part and one in the outer part of the fjord. However, after a tremendous oxygen deficiency in 1998 where nearly all life in the fjord was destroyed, both the morphological traits and the isozymes were revealing gradients indicating that the fjord was re‐colonised from the population located outside of the fjord (Olsen et al 2002).…”