18
J. E. SHELBOURNE
view that this was due to hatchery output rather than natural fluctuations. It is a difficult matter to obtain conclusive proof of hatchery
effectiveness by survey methods, without firat being able to distinguish
hatchery recruits from natural brood. Marine fish larvae still in the
yolk-sac stage of development are not easy to mark. Rollefsen (1940)
tried to overcome this problem by using easily-recognized hybrid fry
of P1. platessa x PI. $esw 8 for field experiments. They were
released into the innermost part of Borgenfjord, near Trondhjem,
during the years 1936 to 1939; the relative frequency of hybrids to
young plaice in the littoral region was ascertained a few months after
each liberation.
Liberated hybrids
yo hyb& in
( rnizzions )
later catches
1936
2
1.6
1936
13
30.0
1937
7
0.5
1938
10
98.0
1939
7
40.0
The evidence, as presented, does not enable us to assess the survival
rate of hybrids or plaice fry. The war unfortunately interrupted any
further development of this experimental attack.
From I896 to 1901 inclusive, plaice fry reared at the Dunbar and
Bay of Nigg hatcheries in Scotland were transported overland for
release into the upper reaches of Loch Fyne (Dannevig, 1896). Pushnet surveys for metamorphosed plaice were made at five specially
selected stations within the loch during the summer months following
ercch annual liberation, and these surveys were continued for 6 y e w
(1 903-08) following the last release. Results were published by Fulton
(1908); for the purpose of this brief history, I reproduoe the bseio
data only (Table IV). Fulton’s more detailed analysis of statistim
continued to support the main conclusion to be drawn from the above
results-that the mean yield of young plaice per hour during the g e m
when larvae were released was double the yield during an equivalent
period following the last liberation. This difference waa thought fo
demonstrate the effectiveness of hatchery output. But as Fulton
himself pointed out, plaice stocks were known to be diminishing
around the British coasts during the survey period, and this may have
had a bearing on the decreased yield during the second half of the
experiment. A separate set of observations in a nearby control area,
together with a third prolonged survey accompanied by r e l e w of
fry, would have substantially reinforced the data.
J. E. SHELBOURNE
view that this was due to hatchery output rather than natural fluctuations. It is a difficult matter to obtain conclusive proof of hatchery
effectiveness by survey methods, without firat being able to distinguish
hatchery recruits from natural brood. Marine fish larvae still in the
yolk-sac stage of development are not easy to mark. Rollefsen (1940)
tried to overcome this problem by using easily-recognized hybrid fry
of P1. platessa x PI. $esw 8 for field experiments. They were
released into the innermost part of Borgenfjord, near Trondhjem,
during the years 1936 to 1939; the relative frequency of hybrids to
young plaice in the littoral region was ascertained a few months after
each liberation.
Liberated hybrids
yo hyb& in
( rnizzions )
later catches
1936
2
1.6
1936
13
30.0
1937
7
0.5
1938
10
98.0
1939
7
40.0
The evidence, as presented, does not enable us to assess the survival
rate of hybrids or plaice fry. The war unfortunately interrupted any
further development of this experimental attack.
From I896 to 1901 inclusive, plaice fry reared at the Dunbar and
Bay of Nigg hatcheries in Scotland were transported overland for
release into the upper reaches of Loch Fyne (Dannevig, 1896). Pushnet surveys for metamorphosed plaice were made at five specially
selected stations within the loch during the summer months following
ercch annual liberation, and these surveys were continued for 6 y e w
(1 903-08) following the last release. Results were published by Fulton
(1908); for the purpose of this brief history, I reproduoe the bseio
data only (Table IV). Fulton’s more detailed analysis of statistim
continued to support the main conclusion to be drawn from the above
results-that the mean yield of young plaice per hour during the g e m
when larvae were released was double the yield during an equivalent
period following the last liberation. This difference waa thought fo
demonstrate the effectiveness of hatchery output. But as Fulton
himself pointed out, plaice stocks were known to be diminishing
around the British coasts during the survey period, and this may have
had a bearing on the decreased yield during the second half of the
experiment. A separate set of observations in a nearby control area,
together with a third prolonged survey accompanied by r e l e w of
fry, would have substantially reinforced the data.