Natural oceanic variation

Recognition of the importance of environmental changes in determining the production of fish stocks has been increasing in recent years. Part of this has been as the result of studies on the impact of El Niño on fish abundance and distribution in the Pacific and the concomitant observation that some stocks of fish in various parts of the world appear to be increasing and decreasing in synchrony. It is becoming increasingly clear that fish production models must explicitly treat environmental variability along with fishing pressure, particularly as environmental forecasts become increasingly accurate. Some species are far more susceptible to environmental influences than others. In some cases, relatively small changes in ocean climate can lead to major changes in a species' abundance.

Physical variables

The principle oceanographic variables affecting plant and animal life in the ocean are temperature, winds, currents, salinity, and other physical parameters. Fish distribution generally follows the environment, with most species having fairly narrow zones of preference or of reproduction ability. Since forecasts of global warming indicate that water temperatures will increase least at the equator and more toward the poles, it is expected that poleward shifts in distribution will occur. High latitude warming should lead to greater fish production in those regions due to longer growing periods, increased growth rates, and greater overall productivity. Some species cannot tolerate the increased warmth during critical parts of their life cycle and their abundance can be expected to decline. Some species of Pacific salmon may be in this group as their winter habitat may disappear.

Impact on sessile animals

Marine life in coastal areas will be affected sooner and more intensely than that in the deep ocean. Those species that move little or not at all, such as clams, mussels, and oysters, will not be able to follow a changing environment. However, it is possible that their progeny may flourish in new areas. If suitable areas do not develop, perhaps because there is no suitable sea-bottom, or currents to carry the eggs, the species may be severely restricted or eliminated.

Food web changes

Changes are always occurring in the oceans at many scales, and marine life has evolved with them. Changes in oceanographic features would alter marine habitats. However, the global warming forecasts are for relatively fast changes that would endure for centuries. This would likely lead to some discontinuities in the oceanic food web, particularly if some aspects of an ecosystem move but others are constrained by factors such as salinity or bottom topography. Environmental changes also can lead to different advantages for predator and prey species. Strong winds or currents can lead to more mixing of water layers. Prey that stratify at a certain depth may become more dispersed when there is mixing and thus less available to predators. Their abundance may increase while that of the predators may decrease. There are many such interactions and complexities. For example, a cod egg or larva that is not consumed by a filter-feeding herring may one day grow large enough to eat herring as a major part of its diet.

Boundary currents

As in any complex situation there are always exceptions. Among the most notable in this subject are species that exist in eastern boundary currents and in the Northwest Atlantic. Displacement toward the equator (not poleward) may occur in eastern boundary currents as a result of upwelling and ecological processes related to tradewind acceleration. In the Northwest Atlantic, there may be no warming and cooling may occur due to changes in wind and currents. This may have negative effects on species that do better when it is warm.

Storminess

Increased storminess that may come with climate change affects coastal ecosystems in several ways. For example, increases in water runoff from more intense precipitation can reduce salinity along the coast, bring more nutrients in the runoff, and increase sediments in the water. The changes can lead to changes in production and distribution of species, including changes in migratory paths. When combined with the expected warming of coastal waters, there may be considerable ecological changes. For example, turbidity, by itself, provides protection of some prey species. Changes in sedimentation can lead to important changes throughout the ecosystem. Similarly, a rush of freshwater will kill several predators of oysters, which are less tolerant of low salinity than are oysters.

Coastal areas

Temperature changes alone do not account for changes in marine ecosystems and fisheries. One must consider all the diverse elements of the marine environment. Species have many ways to deal with changes in their habitat and migrating predators and prey. However, most species depend on near-surface layers and shallow waters in estuaries at some point in their lives. These are the areas that will have the greatest changes. There is a need to particularly monitor these species in order to ensure their wise stewardship.

Other stresses

Similarly, environmental changes cannot be considered in the absence of human impacts, such as from pollution and fishing. There is likely a synergistic effect. For example, a cooling environment in the 1990s quite likely led to reduced cod recruitment off eastern Canada. However, it came at a time of heavy fishing pressure that had led to a small biomass and few older, more productive fish. Together the factors caused the stock to become vulnerable to collapse. Climate change is happening at a time when many stocks are under heavy fishing pressure. For those stocks where the climate impact may be negative, resource managers should be particularly vigilant.