Population differentiation of Atlantic cod (and other fish species, for that matter) has long been appreciated as the following quote will attest:

The cod-fish which occupy the banks lying between the latitudes of 41 and 45, are very different on the different banks, and are kept so distinct, and are so similar on the respective banks that a man aquainted with the fishing business will separate those caught on one bank from those caught on another with as much ease as we separate the apple from the pear"

Honorable General Lincoln, 1791.

Evolutionary biologists and population geneticists have long held an appreciation for the considerable degree of variation within a species (e.g.Darwin's "origin" ). Yet, despite physiology being the summation of many traits and closer to what actually determines fitness than any single locus trait, physiologists have generally avoided the study of physiological variation within a species .

Genetic analyses have suggested small variability of Atlantic cod throughout its tremendous North Atlantic range (Mork et al. 1985; Smith et al. 1989), people familiar with the cod fishery have long recognized discrete stocks based on readily visible morphological criteria (see opening quote).

I was interested in determining whether significant variation existed in the physiology and performance of cod occupying geographically close, disparate habitats.

The Atlantic cod population in the Bras d'Or Lakes is an excellent candidate for testing the idea of population level physiological divergence. A number of factors suggest that faunal interchange with the Atlantic Ocean may be limited and that cod in the Bras d'Or Lakes may be somewhat isolated reproductively, allowing for rapid directional selection in their low salinity, less stable thermal, environment.

The blood chemistry of Cod from the brackish Bras d'Or lakes BDC, which had previously been shown to be quite different from that of Scotian Shelf cod SSC during and after an exercise regime, changed to resemble their oceanic relatives after two months of acclimation to full-strength salinity. In contrast, the blood chemistry of cod from the Scotian Shelf showed little change following two months of acclimation to brackish water. For plasma ion levels, these differences show that the degree of osmoconformity to changes in environmental salinity is a population-specific trait. These physiological differences did not translate into performance differentials: exercise performance was statistically uniform across all combinations of population and salinity. Exercise performance was more varied in fish swimming in a non-native salinity. Some physiological characteristics, including important factors like the metabolic rate and the point at which anaerobic metabolism is recruited to support the swimming effort, were population specific.

Environmental salinity also influenced metabolic rate, but in a population specific manner. Vigorous swimming was more energetically expensive at full strength salinity, yet resting metabolic rate was lower in this environment.   Environmental salinity also influenced the relative appearance of protons and metabolic acid in the extracellular fluid compartment, with full strength salinity favoring the appearance of lactate

Multivariate statistical analysis of this data set helped provide further insight: in contrast to other fish species, differences in blood oxygen transport appear to account for some of the swimming performance differences seen in Atlantic cod. The two experimental populations were cleanly separated by this analysis, confirming our earlier contention that cod populations can be physiologically distinct.

For more information see:

Nelson, J.A., Y. Tang and R.G. Boutilier. 1996. The effects of salinity change on the exercise performance of two Atlantic cod (Gadus morhua) populations inhabiting different environments. Journal of Experimental Biology. 199:1295-1309.

and

Nelson, J.A., Y. Tang, and R.G. Boutilier. 1994. Differences in exercise physiology between two Atlantic cod (Gadus morhua) populations from different environments: Physiological Zoology 67: 330-354.