72
J. E. SHELBOURNE
more fish than they do at present, now fishing is once again intense.
It is a remarkable fact that although wartime stocks of plaice in the
North Sea multiplied to several times their pre-war abundance, the
reoruitment of young brood from the heavy spawnings of 1946 and
1946 were no higher than average. Beverton (1960) has described
the early environment of the young pelagic plaice larva as a kind of
turnstile, permitting only a certain number to pass through, irrespective
of the length of the queue behind. The limiting factors in this environmental turnstile are not known, but food and predator abundance
are undoubtedly involved. The absence of both these limitations m
the hatchery environment probably accounts for the gap between
our high experimental survivals and low estimates for the sea.
There is, as yet, no reliable estimate of the expected survival rate
of metamorphosed hatchery plaice between release into the sea at
4 months old, and their first birthday. The same applies to the natural
stock, although our Dutch colleagues believe the natural mortality
rate during subsequent life to be of the order 10% per year (L. Boerema,
personal communication). This figure was derived from North Sea
trawling data, and must be regarded as very tentative indeed. It
enables us to guess the possible magnitude of a hatchery liberation
calculated to double the abundance of plaice brood in the North Sea
in any one year. Several hundred million recruits, at least, would be
required. Such a colossal undertaking would demand a high degree of
international co-operation. At a less ambitious level, annual liberations
of 1 million young plaice might benefit the smaller bay fisheries of the
Irish Sea.
A support programme is now under way at the Fisheries Laboratory,
Lowestoft, in anticipation of a successful outcome to the mass-production project. It has four immediate objectives. Firstly, to develop
gear with which to follow the fate of a liberated hatchery stock.
Secondly, to make preliminary surveys of areas suitable for field
experiments. Thirdly, to establish the background conditions in
promising localit,ies-the magnitude of natural recruitment, natural
mortality, abundance of food, rate of growth, etc. Fourthly, from the
information coming in, to plan in detail the course of future experiments
to measure the effect of hatchery liberations on fish yield from open
waters.
A very important feature of this preliminary work concerns the
development of a mass-marking technique for 20-mm plaice. Vital
stains are giving promising results, as well as other methods, including
the total immersion of young fish in non-toxic concentrations of
fluorescing substances suoh as tetracycline hydrochloride (J. D. Riley,
J. E. SHELBOURNE
more fish than they do at present, now fishing is once again intense.
It is a remarkable fact that although wartime stocks of plaice in the
North Sea multiplied to several times their pre-war abundance, the
reoruitment of young brood from the heavy spawnings of 1946 and
1946 were no higher than average. Beverton (1960) has described
the early environment of the young pelagic plaice larva as a kind of
turnstile, permitting only a certain number to pass through, irrespective
of the length of the queue behind. The limiting factors in this environmental turnstile are not known, but food and predator abundance
are undoubtedly involved. The absence of both these limitations m
the hatchery environment probably accounts for the gap between
our high experimental survivals and low estimates for the sea.
There is, as yet, no reliable estimate of the expected survival rate
of metamorphosed hatchery plaice between release into the sea at
4 months old, and their first birthday. The same applies to the natural
stock, although our Dutch colleagues believe the natural mortality
rate during subsequent life to be of the order 10% per year (L. Boerema,
personal communication). This figure was derived from North Sea
trawling data, and must be regarded as very tentative indeed. It
enables us to guess the possible magnitude of a hatchery liberation
calculated to double the abundance of plaice brood in the North Sea
in any one year. Several hundred million recruits, at least, would be
required. Such a colossal undertaking would demand a high degree of
international co-operation. At a less ambitious level, annual liberations
of 1 million young plaice might benefit the smaller bay fisheries of the
Irish Sea.
A support programme is now under way at the Fisheries Laboratory,
Lowestoft, in anticipation of a successful outcome to the mass-production project. It has four immediate objectives. Firstly, to develop
gear with which to follow the fate of a liberated hatchery stock.
Secondly, to make preliminary surveys of areas suitable for field
experiments. Thirdly, to establish the background conditions in
promising localit,ies-the magnitude of natural recruitment, natural
mortality, abundance of food, rate of growth, etc. Fourthly, from the
information coming in, to plan in detail the course of future experiments
to measure the effect of hatchery liberations on fish yield from open
waters.
A very important feature of this preliminary work concerns the
development of a mass-marking technique for 20-mm plaice. Vital
stains are giving promising results, as well as other methods, including
the total immersion of young fish in non-toxic concentrations of
fluorescing substances suoh as tetracycline hydrochloride (J. D. Riley,