Abstract
Previous studies have found strong evidences for Atlantic cod (Gadus morhua) egg retention in fjords, which are caused by the combination of vertical salinity structure, estuarine circulation, and egg specific gravity, supporting small-scaled geographical differentiations of local populations. Here, we assess the variability in egg specific gravity for selected local populations of this species, that is, two fjord-spawning populations and one coastal-spawning population from Northern Norway (66–71°N/10–25°E). Eggs were naturally spawned by raised broodstocks (March to April 2009), and egg specific gravity was measured by a density-gradient column. The phenotype of egg specific gravity was similar among the three local populations. However, the associated variability was greater at the individual level than at the population level. The noted gradual decrease in specific gravity from gastrulation to hatching with an increase just before hatching could be a generic pattern in pelagic marine fish eggs. This study provides needed input to adequately understand and model fish egg dispersal.
Highlights
IntroductionThe Atlantic cod in the Baltic Sea has developed large eggs of high water content (97 %) and thin chorion (thickness: \2 lm)
Egg buoyancy, Dq, is defined as the difference in specific gravity between the ambient sea water, qw, and the specific gravity of the egg, qe, that is, Dq = qw - qe (Sundby 1991)
The wild fish were collected in 2001–2003 as follows: fish in Porsangen fjord were collected by Danish seine in March/ April 2002, and uncertain fish that might belong to the Northeast Arctic cod (NEAC) were removed based on genetic markers (Dahle et al 2006)
Summary
The Atlantic cod in the Baltic Sea has developed large eggs of high water content (97 %) and thin chorion (thickness: \2 lm) As a result, they are neutrally buoyant at salinities of 12.3–18.3 (Nissling et al 1994), which make them float above the hypoxic deeper layers. Atlantic cod eggs in the high-saline North Atlantic basin are smaller and heavier with less water content (93 %) and thicker chorion (thickness: 5–9 lm) (Lønning et al 1984), neutrally buoyant at salinities of 29.5–33 (Solemdal and Sundby 1981). Despite these circumstances, they are pelagically distributed with highest concentrations in the surface layers because of the highsaline ambient water
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