11. ECOLOGY 03‘ DISTRIBUTION
347
penetration into a normally uninhabited area for a season. Seasonal
changes in the local distribution of a species are often a constant
feature of a geographical area. Brinton (1962a) presents the most
extensive accounts available on such changes in the distributions of
euphausiids and shows that they are evident in the Kuroshio region,
the California Current, the Peru Current, the North Pacific Drift,
and even in the Equatorial mid-Pacific. These changes in distribution
often reflect changes in water mass distribution within an area, either
indicated by different distributions of temperature or water currents.
Changes are also related to varying amounts of food available and
probably to physiological changes within the euphausiids themselves
although little is known about this aspect as yet. Pre-spawning
aggregations of species such as Meganyctiphanes norvegica in the
North Atlantic have been noticed by many workers for a long time
and Mauchline (1960) suggested that these were effected by using
the photophores. Bigelow (1926) and Bigelow and Sears (1939)
examined seasonal changes in the distribution of this species and of
Thysanoessa inermis, T . raschii, T . longicaudata, Nematoscelis megalops
and Euphausia lcrohnii in the Gulf of Maine. Thysanoessa inermis is
the most abundant species numerically in this area but Bigelow considers that Meganyctiphanes norvegica, through its large size and
abundance, is more important in the economics of the area and he
showed that the populations live and breed in the northeastern part
of the Gulf in the late winter and spring but occur much more widely
and offshore during the summer months. Aggregations for breeding
with subsequent summer dispersions of the populations appear to be a
feature of the distribution of this species within any one geographical
area; it may also be a feature of many more species but, a t present,
adequate data are lacking.
Many species of euphausiids form large swarms either as “ surface
rafts ” or as subsurface aggregations and most records of these have
been summarized by Komaki (1967a). Nemoto (1957) notes that
Thysanokksa inspinata forms swarms and Sheard (1953) found that
Nyctiphanes australis forms (‘ surface rafts )’ as did Thysanoessa
gregaria. The North Atlantic species Meganyctiphanes norvegica,
Thysanoessa inermis, and T . raschii have all been observed to form
( ( surface rafts ”) even during daylight hours but little is known about
the reasons why these swarms are formed (Mauchline and Fisher, 1967).
Komaki (1967a) suggests that the surface swarms of Euphausia pacijica,
which occur in Japanese coastal waters during daylight hours, are
associated with cold water masses and that the mixing of these cold
water masses with the warmer coastal waters produces conditions for
347
penetration into a normally uninhabited area for a season. Seasonal
changes in the local distribution of a species are often a constant
feature of a geographical area. Brinton (1962a) presents the most
extensive accounts available on such changes in the distributions of
euphausiids and shows that they are evident in the Kuroshio region,
the California Current, the Peru Current, the North Pacific Drift,
and even in the Equatorial mid-Pacific. These changes in distribution
often reflect changes in water mass distribution within an area, either
indicated by different distributions of temperature or water currents.
Changes are also related to varying amounts of food available and
probably to physiological changes within the euphausiids themselves
although little is known about this aspect as yet. Pre-spawning
aggregations of species such as Meganyctiphanes norvegica in the
North Atlantic have been noticed by many workers for a long time
and Mauchline (1960) suggested that these were effected by using
the photophores. Bigelow (1926) and Bigelow and Sears (1939)
examined seasonal changes in the distribution of this species and of
Thysanoessa inermis, T . raschii, T . longicaudata, Nematoscelis megalops
and Euphausia lcrohnii in the Gulf of Maine. Thysanoessa inermis is
the most abundant species numerically in this area but Bigelow considers that Meganyctiphanes norvegica, through its large size and
abundance, is more important in the economics of the area and he
showed that the populations live and breed in the northeastern part
of the Gulf in the late winter and spring but occur much more widely
and offshore during the summer months. Aggregations for breeding
with subsequent summer dispersions of the populations appear to be a
feature of the distribution of this species within any one geographical
area; it may also be a feature of many more species but, a t present,
adequate data are lacking.
Many species of euphausiids form large swarms either as “ surface
rafts ” or as subsurface aggregations and most records of these have
been summarized by Komaki (1967a). Nemoto (1957) notes that
Thysanokksa inspinata forms swarms and Sheard (1953) found that
Nyctiphanes australis forms (‘ surface rafts )’ as did Thysanoessa
gregaria. The North Atlantic species Meganyctiphanes norvegica,
Thysanoessa inermis, and T . raschii have all been observed to form
( ( surface rafts ”) even during daylight hours but little is known about
the reasons why these swarms are formed (Mauchline and Fisher, 1967).
Komaki (1967a) suggests that the surface swarms of Euphausia pacijica,
which occur in Japanese coastal waters during daylight hours, are
associated with cold water masses and that the mixing of these cold
water masses with the warmer coastal waters produces conditions for