From P. A. Henderson and R. M. Seaby
Fred Pearce suggests that the warming of the North Atlantic could be the
cause of the decline in cod numbers and could result in the species retreating
farther north (20 November, p 14).
We have been sampling the fish of the Bristol Channel every month for the
past 20 years and since 1986 there has indeed been a marked increase in mean
water temperatures. However, there has also been a surprising increase in the
abundance of cod between 1 and 3 years old within the estuary.
Between 1980 and 1986, we considered cod to be an infrequent visitor, now it
is an important member of the fish community. Our observations are supported by
reports we have received from commercial fishermen working in the Bristol
Channel and off Devon.
If the warming of our seas is reducing cod numbers, why has the abundance of
cod in southwestern British waters increased? We have no answer, but would
suggest that the natural mortality patterns of marine fish such as cod suggest
that diseases caused by viral, bacterial and protozoan pathogens are important
in determining the survival rate of larval and juvenile fish.
Advertisement
Given the well-proven impact of disease and parasites on terrestrial animal
and plant populations, including humans, it is surprising how little attention
fisheries biologists have devoted to disease. While we would not dispute that
populations have been much reduced by fishing, could it not be that some cod
populations are experiencing an epidemic that is reducing the survival of the
young?
Lymington, Hampshire
