33::
J . H . 8. BIAXTRIL AND Y. (1. T. HOLLIDAY
to maintain the status quo, or even cause rapid dispersion. The relative
importance of shoal size and shoal numbers depends on the type of
fishery and the searching power available to detect the presence of
fish. Large shoals are usually more valuable to fisheries using cncircling
gear, while more numerous but smaller shoals, but not too small, are
more valuable to trawl and drift net fisheries.
R. Migrations
1. General
The migrations of clupeids have been studied by plotting distribution
of the fish, and by internal and external tagging. Such work started on
Clupea pallasii about 1930, on Sardinops cuerulea a little later and on
&?lapea harengus just after the Second World War. How the migrations
are controlled is not well known. Cycles of hormone activity (see
Sections VI, P ; VI, X) suggest that there is a cycle of internal endocrine
secretion, which may well give the fish an urge to migrate ; the question
of whether this cycle is an endogenous one, or one brought about by
environmental conditions, such as tcmperature or day length, is also
discussed in these sections. The problem to be discussed here is how the
urge to migrate is directed along certain fairly well-defined routes. Is
this essentially an undirected process, m a result of kineses (random
movements where the speed of movement or turning frequency depends
on environmental conditions), or is it an active, orientated movement
(taxis) on the part of the fish, or is it a mixture of the two?
An undirected process could come about m follows. The fixh might
overwinter and remain relativoly inactive in conditicmn of low b m -
perature and poor food. The end of tJih pcriofl wotlltj qq)c:irr itlg t h : fidi
became more activc in roHponsc: to riHing txmpcrciturc: or innnitirm.
Their movements would then be limited by unfavourable water m w e s
(of low temperature or sihinity and with poor food) as well aa by the
depth of water or by land masses. The fish would tend to aggregate
where food was good, but as the season went by it would be necessary
to postulate some factor which would direct them to the spawning
ground. Assuming that a feeding phase were followed by a further
spawning migration, this might be triggered off by the €at content
reaching a certain value. The direction of the spawning migration
might be partly controlled by non-optimal water masmH and b y aggregation, in the case of herring, in arcas of ~ u i t a t h rit:pt,h arid nuhhtratc:
for depositing eggs. After ripawning, f h: urgc f,o migrih: woi1J4 bill,&Jt:
and the fish .~.vould tmd to drift, off with tho provtiilirig c:urrcvif.#.
The active Pr0CCH.S would poatuhtsc nuch fnctorrt art ntift o r ;r:rrt,st,ic,n
J . H . 8. BIAXTRIL AND Y. (1. T. HOLLIDAY
to maintain the status quo, or even cause rapid dispersion. The relative
importance of shoal size and shoal numbers depends on the type of
fishery and the searching power available to detect the presence of
fish. Large shoals are usually more valuable to fisheries using cncircling
gear, while more numerous but smaller shoals, but not too small, are
more valuable to trawl and drift net fisheries.
R. Migrations
1. General
The migrations of clupeids have been studied by plotting distribution
of the fish, and by internal and external tagging. Such work started on
Clupea pallasii about 1930, on Sardinops cuerulea a little later and on
&?lapea harengus just after the Second World War. How the migrations
are controlled is not well known. Cycles of hormone activity (see
Sections VI, P ; VI, X) suggest that there is a cycle of internal endocrine
secretion, which may well give the fish an urge to migrate ; the question
of whether this cycle is an endogenous one, or one brought about by
environmental conditions, such as tcmperature or day length, is also
discussed in these sections. The problem to be discussed here is how the
urge to migrate is directed along certain fairly well-defined routes. Is
this essentially an undirected process, m a result of kineses (random
movements where the speed of movement or turning frequency depends
on environmental conditions), or is it an active, orientated movement
(taxis) on the part of the fish, or is it a mixture of the two?
An undirected process could come about m follows. The fixh might
overwinter and remain relativoly inactive in conditicmn of low b m -
perature and poor food. The end of tJih pcriofl wotlltj qq)c:irr itlg t h : fidi
became more activc in roHponsc: to riHing txmpcrciturc: or innnitirm.
Their movements would then be limited by unfavourable water m w e s
(of low temperature or sihinity and with poor food) as well aa by the
depth of water or by land masses. The fish would tend to aggregate
where food was good, but as the season went by it would be necessary
to postulate some factor which would direct them to the spawning
ground. Assuming that a feeding phase were followed by a further
spawning migration, this might be triggered off by the €at content
reaching a certain value. The direction of the spawning migration
might be partly controlled by non-optimal water masmH and b y aggregation, in the case of herring, in arcas of ~ u i t a t h rit:pt,h arid nuhhtratc:
for depositing eggs. After ripawning, f h: urgc f,o migrih: woi1J4 bill,&Jt:
and the fish .~.vould tmd to drift, off with tho provtiilirig c:urrcvif.#.
The active Pr0CCH.S would poatuhtsc nuch fnctorrt art ntift o r ;r:rrt,st,ic,n