BIOLOGY OF FISHES
factor influencing fish growth. In fishes of temperate waters, age determination is relatively easy, because of the annual variation of faster growth in the summer and slower
growth in the winter. These annual growth variations are recorded in the form of growth marks (called annuli) on the
bones and scales of fishes (much like the growth rings visible
on a tree stump). For fishes of tropical waters or those in the
deep sea, the temperature of the water (hence growth) is
constant all year round; consequently, these fishes may be
very difficult to age.
The "ear bones" (otoliths) of fishes are often used in age
determination, as they usually show the annuli better than
other bones or scales do. In long-lived species, the accurate
determination of age for fish older than 10 or 20 years is
complicated, because growth is much slower in mature fish
than it is in juveniles. After maturity the annuli are more
crowded together and difficult to count. For these older
fish, it may be necessary to weigh or section the otolith to
determine the growth increments. The ratio of radioactive
isotopes of lead and radium in the otolith has been used to
confirm age determinations from sectioned otoliths of
rockfish (a North Pacific scorpaenid fish); these studies
found that this cold-water species may attain an age of 80
years! Not bad for a fish that grows to only 46 cm.
Generally the larger fish species will live longer than the
smaller ones, provided that the large species is also a slow
grower. The dolphinfish (Coryphaena hippurus) , which attains a length of 2 metres, is an exceptionally fast-growing
species; it is mature at an age of one year, when it is 55 cm
fork length, and it probably does not live more than 4 years.
The enormous tunas and billfishes are also very fast growing
and apparently do not live much past the age of 15. Tagging
studies of sharks have confirmed an age of at least 25 years
for some species.
Shapes, sizes, swimming and speed
Fishes exhibit a tremendous variety of shapes: compressed (flattened from side to side) as in butterflyfish and
moonies; elongate, like snoek and cutlassfish; robust and
streamlined, as in tunas; depressed (flattened from back-tobelly) like monk and stingrays; spherical as in porcupine
fish, or box-shaped, e.g. boxfish. The triangular boxfish
(Tetrosomus concatenatus) is triangular in cross-section.
The ocean sunfish (Mola mola) looks like a truncate disc or
the front end of a fish that has been chopped in two (hence
the name "head-fish" by which it is sometimes known).
The largest fish in the world is the whale shark, which is
known to attain a length of at least 12 m and perhaps as
much as 18 m. Like the blue whale, which grows considerably larger, the whale shark also feeds on plankton. The
12
smallest fish in the world is probably Trimmatom nan us, a
tiny goby recently described by Winterbottom and Emery
(1981). This species, which has mature females at 8 mm SL,
is at present known only from Chagos Archipelago in the
western Indian Ocean.
Most fishes swim by movements of the body, tail (caudal
peduncle) and caudal fin. In the fastest fishes (tunas and
other scombrids), the body flexes hardly at all, and the
caudal fin provides all of the thrust. In eels and some other
elongate fishes, the whole body is flexed in an undulatory
wave that passes from head to tail, and this backward-moving wave drives the eel forward. The majority of fishes fall
in between these two extremes: their bodies are more flexible than tunas, but less flexible than eels.
Scombrids have several anatomical adaptations that contribute to their remarkable swimming abilities. The stiff,
deeply lunate caudal fin is "designed" for rapid movements
and least drag. A speeding tuna can drive its caudal fin at
the incredible rate of 10 beats/second! The massive body
muscles are connected to tendons that run along the narrow
peduncle (which acts as a pulley) to attach at the base ofthe
caudal fin rays. Horizontal keels on the peduncle
strengthen it and are thought to reduce turbulence from the
rapid lateral movements as the tail beats back and forth.
The body is very streamlined, with grooves and recesses to
receive the first dorsal and pectoral fins when they are
folded back to reduce drag. The sustained high-speed swimming characteristic of scombrid fishes requires a high
metabolic rate, which means a high oxygen consumption.
The gills of scombrids have a much greater surface area and
are therefore much more efficient at extracting oxygen than
are the gills of non-scombrid fishes.
Another adaptation for an increased metabolic rate is the
warm-bodiedness of the advanced scombrids (bonitos and
tunas). Unlike the typical "cold-blooded" fishes, the tunas
and the lamnid sharks (mako, porbeagle and great white
sharks) are able to maintain their inner muscular temperature at several degrees above their ambient water temperature. In tunas, the blood leaving the body muscles passes
through a special network of capillaries that function as a
countercurrent heat exchanger to transfer the metabolic
heat generated by the muscles to the cold blood running
into the muscles. The swimming muscles are thus able to operate at the higher temperatures necessary for the higher
metabolic rate required for sustained high-speed swimming. Of all fishes the bluefin tunas have the most efficient
heat exchangers, and they are the scombrids that penetrate
farthest into cold waters, often occurring along the coasts of
Norway and Alaska and from 40° to 50
0
S latitudes.
There are other fast-swimming fishes, besides the scombrids: kingfishes (Carangidqe), dolphinfish (Coryphaena),
marlins and sailfishes (Istiophoridae) and the lamnid
sharks. All of these speedy fishes have stiff, lunate caudal
fins and narrow peduncles. This type of caudal fin is the
factor influencing fish growth. In fishes of temperate waters, age determination is relatively easy, because of the annual variation of faster growth in the summer and slower
growth in the winter. These annual growth variations are recorded in the form of growth marks (called annuli) on the
bones and scales of fishes (much like the growth rings visible
on a tree stump). For fishes of tropical waters or those in the
deep sea, the temperature of the water (hence growth) is
constant all year round; consequently, these fishes may be
very difficult to age.
The "ear bones" (otoliths) of fishes are often used in age
determination, as they usually show the annuli better than
other bones or scales do. In long-lived species, the accurate
determination of age for fish older than 10 or 20 years is
complicated, because growth is much slower in mature fish
than it is in juveniles. After maturity the annuli are more
crowded together and difficult to count. For these older
fish, it may be necessary to weigh or section the otolith to
determine the growth increments. The ratio of radioactive
isotopes of lead and radium in the otolith has been used to
confirm age determinations from sectioned otoliths of
rockfish (a North Pacific scorpaenid fish); these studies
found that this cold-water species may attain an age of 80
years! Not bad for a fish that grows to only 46 cm.
Generally the larger fish species will live longer than the
smaller ones, provided that the large species is also a slow
grower. The dolphinfish (Coryphaena hippurus) , which attains a length of 2 metres, is an exceptionally fast-growing
species; it is mature at an age of one year, when it is 55 cm
fork length, and it probably does not live more than 4 years.
The enormous tunas and billfishes are also very fast growing
and apparently do not live much past the age of 15. Tagging
studies of sharks have confirmed an age of at least 25 years
for some species.
Shapes, sizes, swimming and speed
Fishes exhibit a tremendous variety of shapes: compressed (flattened from side to side) as in butterflyfish and
moonies; elongate, like snoek and cutlassfish; robust and
streamlined, as in tunas; depressed (flattened from back-tobelly) like monk and stingrays; spherical as in porcupine
fish, or box-shaped, e.g. boxfish. The triangular boxfish
(Tetrosomus concatenatus) is triangular in cross-section.
The ocean sunfish (Mola mola) looks like a truncate disc or
the front end of a fish that has been chopped in two (hence
the name "head-fish" by which it is sometimes known).
The largest fish in the world is the whale shark, which is
known to attain a length of at least 12 m and perhaps as
much as 18 m. Like the blue whale, which grows considerably larger, the whale shark also feeds on plankton. The
12
smallest fish in the world is probably Trimmatom nan us, a
tiny goby recently described by Winterbottom and Emery
(1981). This species, which has mature females at 8 mm SL,
is at present known only from Chagos Archipelago in the
western Indian Ocean.
Most fishes swim by movements of the body, tail (caudal
peduncle) and caudal fin. In the fastest fishes (tunas and
other scombrids), the body flexes hardly at all, and the
caudal fin provides all of the thrust. In eels and some other
elongate fishes, the whole body is flexed in an undulatory
wave that passes from head to tail, and this backward-moving wave drives the eel forward. The majority of fishes fall
in between these two extremes: their bodies are more flexible than tunas, but less flexible than eels.
Scombrids have several anatomical adaptations that contribute to their remarkable swimming abilities. The stiff,
deeply lunate caudal fin is "designed" for rapid movements
and least drag. A speeding tuna can drive its caudal fin at
the incredible rate of 10 beats/second! The massive body
muscles are connected to tendons that run along the narrow
peduncle (which acts as a pulley) to attach at the base ofthe
caudal fin rays. Horizontal keels on the peduncle
strengthen it and are thought to reduce turbulence from the
rapid lateral movements as the tail beats back and forth.
The body is very streamlined, with grooves and recesses to
receive the first dorsal and pectoral fins when they are
folded back to reduce drag. The sustained high-speed swimming characteristic of scombrid fishes requires a high
metabolic rate, which means a high oxygen consumption.
The gills of scombrids have a much greater surface area and
are therefore much more efficient at extracting oxygen than
are the gills of non-scombrid fishes.
Another adaptation for an increased metabolic rate is the
warm-bodiedness of the advanced scombrids (bonitos and
tunas). Unlike the typical "cold-blooded" fishes, the tunas
and the lamnid sharks (mako, porbeagle and great white
sharks) are able to maintain their inner muscular temperature at several degrees above their ambient water temperature. In tunas, the blood leaving the body muscles passes
through a special network of capillaries that function as a
countercurrent heat exchanger to transfer the metabolic
heat generated by the muscles to the cold blood running
into the muscles. The swimming muscles are thus able to operate at the higher temperatures necessary for the higher
metabolic rate required for sustained high-speed swimming. Of all fishes the bluefin tunas have the most efficient
heat exchangers, and they are the scombrids that penetrate
farthest into cold waters, often occurring along the coasts of
Norway and Alaska and from 40° to 50
0
S latitudes.
There are other fast-swimming fishes, besides the scombrids: kingfishes (Carangidqe), dolphinfish (Coryphaena),
marlins and sailfishes (Istiophoridae) and the lamnid
sharks. All of these speedy fishes have stiff, lunate caudal
fins and narrow peduncles. This type of caudal fin is the