8.3.6. Food and Fiber: Fisheries and Aquatic Systems
Although there is considerable uncertainty about the physical changes and response
of the various freshwater and marine species, it is possible to suggest how
certain species may respond to projected climate changes over the next 50-100
years. The uncertainties highlight the importance of research to separate the
impacts of changing climate from natural population fluctuations and fishing
effects. Many commercial finfish populations already are under pressure (e.g.,
overexploited), and global change may be of minor concern compared with the
impacts of ongoing and future commercial fishing and human use or impacts on
the coastal zone. Further, changes in the variability of climate may have more
serious consequences on the abundance and distribution of fisheries than changes
in mean conditions alone (Katz and Brown, 1992), and changes in future climate
variability are poorly understood at this time.
Fish, including shellfish, respond directly to climate fluctuations, as well
as to changes in their biological environment (predators, prey, species interactions,
disease) and fishing pressures. Although this multiforcing sometimes makes it
difficult to establish unequivocal linkages between changes in the physical
environment and the responses of fish or shellfish stocks, some effects are
clear (see reviews by Cushing and Dickson, 1976; Bakun et al., 1982; Cushing,
1982; Sheppard et al., 1984; Sissenwine, 1984; and Sharp, 1987). These effects
include changes in the growth and reproduction of individual fish, as well as
the distribution and abundance of fish populations. In terms of abundance, the
influence occurs principally through effects on recruitment (how many young
survive long enough to potentially enter the fishery) but in some cases may
be related to direct mortality of adult fish.
Fish carrying capacity in aquatic ecosystems is a function of the biology of
a particular species and its interrelationship with its environment and associated
species. Specific factors that regulate the carrying capacity are poorly known
for virtually all species, but some general statements can be made with some
confidence. Fish are affected by their environment through four main processes
(Sheppard et al., 1984):
- Direct physiological effects, including metabolic processes influenced
by temperature, salinity, and oxygen levels-Fish often seek optimal temperature
or salinity regimes or avoid suboptimal conditions. Thus, ocean and freshwater
changes as a result of projected climate changes can lead to distributional
changes. In suboptimal conditions, performance is reduced, leading to starvation
or increased predation.
- Diseases-Certain environmental conditions are more conducive to diseases
than others (e.g., warm waters can trigger disease outbreaks; likewise, cold
temperatures can limit them).
- Food-The environment affects feeding rates and competition, as well
as abundance, quality, size, timing, spatial distribution, and concentration
- Predators-The environment affects predation through influences on
the abundance and distribution of predators.
Fish are influenced not only by temperature and salinity conditions but also
by mixing and transport processes (e.g., mixing can affect primary production
by promoting nutrient replenishment of the surface layers; it also can influence
the encounter rate between larvae and prey organisms). Ichthyoplankton (fish
eggs and larvae) can be dispersed by the currents, which may carry them into
or away from areas of good food production, or into or out of optimal temperature
or salinity conditions-and perhaps, ultimately determine whether they are lost
to the original population.
Climate is only one of several factors that regulate fish abundance. Managers
attempt to model abundance trends in relation to fishing effects in order to
sustain fisheries. In theory, a successful model could account for global warming
impacts along with other impacts without understanding them. For many species
of fish, the natural mortality rate is an inverse function of age: Longer-lived
fish will be affected by natural changes differently than shorter-lived fish.
If the atmosphere-freshwater-ocean regime is stable for a particular time, it
is possible to estimate the age-specific mortality rates for a species of interest.
However, at least some parts of the atmosphere-freshwater-ocean system are prone
to oscillations on a decadal scale, which may not be cyclical. These natural
changes occur globally; thus, they will have impacts on the freshwater and marine
ecosystems that support North American fish populations. Under natural conditions,
it may be expected that the different life histories of these fish will result
in different times of adjustment to a new set of environmental conditions.
Any effects of climate change on fisheries are expected to be most pronounced
in sectors that already are characterized by full utilization, large overcapacities
of harvesting and processing, and sharp conflicts among users and competing
uses of aquatic ecosystems. Climate change impacts, including changes in natural
climate variability on seasonal to interannual time scales, are likely to exacerbate
existing stresses on fish stocks. The effectiveness of actions to reduce the
decline of fisheries depends on our ability to distinguish among these stresses
and other causes of change and on our ability to effectively deal with those
over which we have control or for which we have adaptation options. This ability
is insufficient at present; although the effects of environmental variability
are increasingly recognized, the contribution of climate change to such variability
is not yet clear.
Recreational fishing is a highly valued activity that could incur losses
in some regions as a result of climate-induced changes in fisheries.
Recreational fishing is a highly valued activity within North America. In the
United States, for example, 45 million anglers participate annually; they contribute
to the economy through spending on fishing and related activities (US$24 billion
in 1991). The net economic effect of changes in recreational fishing opportunities
as a result of climate-induced changes in fisheries is dependent on whether
projected gains in cool- and warm-water fisheries offset losses in cold-water
fisheries. Work by Stefan et al. (1993) suggests mixed results for the United
States, ranging from annual losses of US$85-320 million to benefits of about
US$80 million under a number of GCM projections. A sensitivity analysis (U.S.
EPA, 1995) was conducted to test the assumption of costless transitions across
these fisheries. This analysis assumed that best-use cold-water fishery losses
caused by thermal changes were effectively lost recreational services. Under
this assumption, all scenarios resulted in damages, with losses of US$619-1,129