From Pristine to Modified Ecosystems
33
Sea ranges from 5% to 39%, but for about half the species is in the vicinity of 20%. It
would be unreasonable to expect long-term occupation of habitat by species where such
mortality is additional to natural causes of death.
The extent to which the ecosystems of continental shelves can be modified by industrial fishing has taken fisheries and ecological science by surprise. Indeed, the imposed
modifications can go far beyond the fish species and the benthic invertebrates of the
seabed. The existence of an abundant large herbivorous mammal, Steller’s sea cow, must
have significantly structured the Gulf of Alaska shelf ecosystem, but they are all gone
now; the ecological consequences of the greatly reduced Southern Ocean biomass (by
perhaps 40 million tons) of large whales toward the end of the whaling period must have
had a significant effect on the biomass of krill; other examples are not hard to find.
The evidence is somewhat circumstantial, but there is general agreement that prior to
industrial fishing the stocks of demersal fish on temperate shelves were very much larger
than the stocks we became familiar with during the 20th century. Steele and Schumacher
(2000) suggest that the pristine cod and haddock stocks on the NE Atlantic shelves may
have been as much as an order of magnitude more abundant, while pristine pelagic fish
stocks were probably less abundant.
The progressive decline in the abundance of demersal fish has everywhere been associated both with excessive mortality of the larger species, and with a progressive reduction
in the average age of the populations of both large and moderate-sized species: a reduction in the average individual weight of commercially exploited fish off eastern Canada of
about 50% occurred in the period 1970–1990, whereas a few Icelandic kitchen middens
from the mediaeval period were found to contain larger absolute numbers of 100-cm
cod than the Icelandic trawl surveys took during entire seasons in recent years.
Stergiou (2002) refers to this process as the “tropicalisation” of fish species of temperate
seas. By this he means that the fish have now come to possess growth rates and age-atmaturity appropriate to warm, rather than cold, water fish. They come to have, as I have
suggested, life history traits unsuitable for the habitat in which they must continue to try
to survive (Longhurst, 2002). Typically, a series of years with conditions likely to result
in poor recruitment is associated with the final collapse of a fish stock already stressed by
fishing pressure. It would have seemed literally incredible only a few decades ago that cod
should be fished to ecological extinction on the Grand Banks of Newfoundland and that
their ecological and economic role should be replaced by lobsters and other crustacea.
But so it is. And, even as I am writing this section, the headlines proclaim the imminent
probability of a similar regime shift in the North Sea, because of tardy action by fishing
regulators, themselves caught in a morass of eurobureaucratic niceties. The “regime shifts”
that Steele and Schumacher (2000) suggested would occur as a consequence of heavy
fishing seem not, on balance, to have been radical enough to encompass everything that
has happened in the NE Atlantic.
If the landings of commercial fisheries reflect, in general, the populations of fish
currently occupying the continental shelves then, as has been shown repeatedly (e.g., Pauly
et al., 1998), the mean trophic level of fish has progressively decreased since 1950. This
change reflects a gradual transition from species assemblages containing a due proportion
of high-trophic-level piscivores to short-lived, low-trophic-level planktivores. All this we
shall have to bear in mind when discussing the regional ecology of every region, in both
high and low latitudes. Though the extent of the process is very poorly evaluated in most
places, the effects of fisheries can be found wherever you look.
It is slowly coming to be acknowledged that long-lived fish have life history traits
that render them peculiarly vulnerable to the additional adult mortality imposed by any
industrial fishery. The age structure of the population is rapidly truncated, reproduction
and recruitment begin to fail, and a terminal population decline may be induced if
the fishery persists. Tropical groupers such as Epinephalus and Mycteropercus are such
33
Sea ranges from 5% to 39%, but for about half the species is in the vicinity of 20%. It
would be unreasonable to expect long-term occupation of habitat by species where such
mortality is additional to natural causes of death.
The extent to which the ecosystems of continental shelves can be modified by industrial fishing has taken fisheries and ecological science by surprise. Indeed, the imposed
modifications can go far beyond the fish species and the benthic invertebrates of the
seabed. The existence of an abundant large herbivorous mammal, Steller’s sea cow, must
have significantly structured the Gulf of Alaska shelf ecosystem, but they are all gone
now; the ecological consequences of the greatly reduced Southern Ocean biomass (by
perhaps 40 million tons) of large whales toward the end of the whaling period must have
had a significant effect on the biomass of krill; other examples are not hard to find.
The evidence is somewhat circumstantial, but there is general agreement that prior to
industrial fishing the stocks of demersal fish on temperate shelves were very much larger
than the stocks we became familiar with during the 20th century. Steele and Schumacher
(2000) suggest that the pristine cod and haddock stocks on the NE Atlantic shelves may
have been as much as an order of magnitude more abundant, while pristine pelagic fish
stocks were probably less abundant.
The progressive decline in the abundance of demersal fish has everywhere been associated both with excessive mortality of the larger species, and with a progressive reduction
in the average age of the populations of both large and moderate-sized species: a reduction in the average individual weight of commercially exploited fish off eastern Canada of
about 50% occurred in the period 1970–1990, whereas a few Icelandic kitchen middens
from the mediaeval period were found to contain larger absolute numbers of 100-cm
cod than the Icelandic trawl surveys took during entire seasons in recent years.
Stergiou (2002) refers to this process as the “tropicalisation” of fish species of temperate
seas. By this he means that the fish have now come to possess growth rates and age-atmaturity appropriate to warm, rather than cold, water fish. They come to have, as I have
suggested, life history traits unsuitable for the habitat in which they must continue to try
to survive (Longhurst, 2002). Typically, a series of years with conditions likely to result
in poor recruitment is associated with the final collapse of a fish stock already stressed by
fishing pressure. It would have seemed literally incredible only a few decades ago that cod
should be fished to ecological extinction on the Grand Banks of Newfoundland and that
their ecological and economic role should be replaced by lobsters and other crustacea.
But so it is. And, even as I am writing this section, the headlines proclaim the imminent
probability of a similar regime shift in the North Sea, because of tardy action by fishing
regulators, themselves caught in a morass of eurobureaucratic niceties. The “regime shifts”
that Steele and Schumacher (2000) suggested would occur as a consequence of heavy
fishing seem not, on balance, to have been radical enough to encompass everything that
has happened in the NE Atlantic.
If the landings of commercial fisheries reflect, in general, the populations of fish
currently occupying the continental shelves then, as has been shown repeatedly (e.g., Pauly
et al., 1998), the mean trophic level of fish has progressively decreased since 1950. This
change reflects a gradual transition from species assemblages containing a due proportion
of high-trophic-level piscivores to short-lived, low-trophic-level planktivores. All this we
shall have to bear in mind when discussing the regional ecology of every region, in both
high and low latitudes. Though the extent of the process is very poorly evaluated in most
places, the effects of fisheries can be found wherever you look.
It is slowly coming to be acknowledged that long-lived fish have life history traits
that render them peculiarly vulnerable to the additional adult mortality imposed by any
industrial fishery. The age structure of the population is rapidly truncated, reproduction
and recruitment begin to fail, and a terminal population decline may be induced if
the fishery persists. Tropical groupers such as Epinephalus and Mycteropercus are such