| Summary: | Explaining the variability in growth rates of fish is critical to predicting the large fluctuations in biomass that characterize fish populations. Aside from directly affecting stock biomass, variability in size-at-age determines future production, because larger fish have larger growth increments and are more fecund. In northern cod, a 50% drop in weight-at-age in recent years was particularly concerning because it coincided with record low abundance. Although this drop was accompanied by low temperatures, bioenergetic analysis from the present work suggested that northern cod growth is not physiologically limited by cold water, but that temperature affects growth indirectly through prey supply. An analysis of cod stocks across the North Atlantic suggested that this indirect effect of temperature lies behind the temperature-growth relationships in other cod stocks, as well. While weight-at-age of northern cod decreased across age-classes, analysis of size-specific growth rates indicated that growth has only decreased in the youngest age-classes, and that older age-classes are smaller because of environmental factors experienced at an earlier age. The observed pattern in growth was also consistent with the selective removal of larger fish of a given age by the fishery. While growth represents a benefit, it comes at a physiological cost because tissue synthesis is energetically demanding. A comparison of the maximum rate of oxygen consumption of fed and unfed cod during exhaustive exercise indicated that oxygen supply for growth and digestion has priority over oxygen supply to the swimming muscles, and that aerobic capacity is limited by oxygen uptake by the tissues rather than by the gills. In summary, growth rates of northern cod are correlated to temperature because of indirect effects such as prey supply, not because of physiological temperature limitation. Cod growth is most sensitive in early years, and this early growth is largely responsible for size later in life. Thesis (Ph.D.)--Dalhousie University (Canada), 1999.
|
|---|