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Rapp. P.-v. R éun. Cons. int. Explor. M er, 191: 330-338. 1989
Nekton and plankton: some comparative aspects of larval
ecology and recruitment processes
Hein Rune Skjoldal and W ebjørn Melle
Skjoldal, Hein R une, and Melle, W ebjørn. 1989. Nekton and plankton: some
comparative aspects of larval ecology and recruitment processes. - Rapp. P. v. Réun.
Cons. int. Explor. M er, 191: 330-338.
The larval ecology of fish and zooplankton differs in many respects. The fecundity
of fish is 3-4 orders of magnitude higher than for zooplankton. Despite this, zooplank­
ton eggs and larvae are in general much smaller than fish larvae. Zooplankton larvae
are predominantly herbivores whereas fish larvae feed on larger particles and are
predominantly carnivores. Egg production in zooplankton is closely coupled with
feeding conditions of the females, whereas that of fish is more distantly related both
temporally and spatially. The ability to endure starvation is roughly equal for larvae
of fish and zooplankton despite the smaller size of the latter. A m ajor difference in
terms of predation is that fish eggs and larvae are large enough to be preyed upon
by plankton-feeding fish, whereas eggs and larvae of zooplankton are not. Loss of
individuals during larval drift is much more likely for fish with spawning migration
as part of the life cycle than for zooplankton. Fish larvae seem therefore more prone
to suffer losses due to food limitation, predation, and variable currents than do larvae
of zooplankton. This is in accordance with higher mortality rates of fish larvae than
of zooplankton larvae. The relative contributions by the three m ajor factors (food
limitation, predation, and vagrancy) causing this high mortality are difficult to
separate because they are interrelated and variable. In the fish recruitment variability
problem we are probably looking for relatively small differences in mortality rate to
explain large variations in num ber of recruits.
Hein Rune Skjoldal and Webjørn Melle: Institute o f Marine Research, P.O. B ox 1870,
Nordnes, N-5024 Bergen, Norway
Introduction
By definition plankton and nekton are differentiated by
their respective abilities to determine their horizontal
distribution. Fish are a m ajor group of nekton which
use their ability to swim for purposes such as feeding
and reproduction. Many species migrate to restricted
spawning areas which are located in relation to the
water circulation pattern so as to ensure transport of
recruits back into m ajor feeding areas (Harden-Jones,
1968; Sherman et al. , 1984; Dragesund and Gjøsæter,
1988). Z ooplankton, in contrast, spawn over a much
wider part of their distribution area than do fish, and
there is little evidence for behaviourally localized
spawning areas.
The large variability in recruitment of commercial
fish stocks has received long-lasting scientific attention.
Despite this, the mechanism involved in recruitment
processes have not yet been properly resolved. Current
research activities are guided by three m ajor groups of
hypothesis: that the recruitment variability is due to (1)
food limitation, (2) predation, or (3) physical oceanic
variability (Sissenwine, 1984).
330
Nekton and zooplankton have one thing in common,
both groups being part of the plankton in the larval
stage of their life cycles. They are therefore subject in
part to the same ecological recruitment processes. Here
we com pare their reproductive and larval ecology,
emphasizing similarities and differences in basic eco­
logical properties of nekton and zooplankton. We have
done this in a very general way, using fish and crus­
taceans (with a bias towards copepods) as typical rep­
resentatives for the two groups. Where possible we have
chosen our examples from the Barents Sea ecosystem.
Spawning behaviour
Many marine fish populations have more or less welldefined spawning areas where the adults aggregate after
a spawning migration to mate and spawn. The location
of spawning areas is the result of adaptation to the
physical regime which disperses and transports the lar­
vae and juveniles into areas which are favourable for
growth, survival, and further reproduction (HardenJones, 1968; Sherman et al., 1984; Sinclair, 1988). The
dominant commercial fish stocks in the Barents Sea
provide many clear examples of this (Dragesund and
Gjøsæter, 1988).
The limited swimming capacity of zooplankton prevents them from performing extensive horizontal
spawning migrations to well-defined spawning areas like
fish. Instead they spawn over most or all of their area
of distribution. It is likely, however, that specific repro­
ductive behaviour has evolved as a mechanism for aggregation to improve the chances of encounter between
the sexes. Swimming speed is related to body size,
and this sets upper limits to the distance and size of
aggregations. The physical dispersive forces in the ocean
are generally much stronger in the horizontal than in
the vertical direction. Aggregation in the vertical would
therefore seem to be a mechanism whereby zooplankton
could meet for reproductive purposes. Many zooplank­
ton species living in spatially restricted environments
such as estuaries or coral reefs have evolved behavioural
patterns which contribute to their retention (Sinclair,
1988).
A seasonal vertical migration upwards in spring from
overwintering in deeper water is a common phenom ­
enon for many open water zooplankton species (e.g.
Østvedt, 1955). Aggregation in the surface layer is a
possible mechanism for concentrating the population,
thus enhancing the probability that males and females
meet. This probability would decrease with decreasing
abundance per area. Reproduction would therefore be
less intense in the border areas of distribution than in
the central areas.
Swarming behaviour could play a more important
role in reproduction of zooplankton than has generally
been recognized. Several species of copepods have been
observed to form dense local monospecific swarms con­
sisting mainly of adults (U eda et al., 1983). Calanus
finmarchicus can form dense red patches in the surface
layer (Wiborg, 1976). Krill occur regularly in schools
or swarms of different sizes and probably for several
different purposes (H am ner et al., 1983). Swarming in
relation to reproduction was indicated for Meganyctiphanes norvegica by Nicol (1984).
In fishes mating and spawning usually occur simul­
taneously. For most crustacean zooplankton, on the
other hand, mating involves transfer of a spermatophore
that is used to fertilize the eggs when subsequently
spawned.
Egg size and fecundity
The fecundity of fish can be very high, and, as a conse­
quence, the size of the eggs is small relative to the size
of the adult fish. The mean ratio of egg to adult volume
is about 10-7 for fish species from the Barents Sea as
shown in Figure 1. The difference in size between egg
and adult is considerably less for zooplankton, with a
Species.C alanus
g- <
o
gla c ia lis .
C.
hyperboreus.
M etrid ia
,__________ .
,
longa.
P s eu d o calan u s
E u chaeta
=
-*
.____________ .
f i n m ar ch ic us .
C.
.___________.
sp.
.
norvegica.
.-------------------- .
T h y s a n o e s s a r a sc h ii.
T.
^ ----------------------- .
i n er m is .
,
T. l o n g i c a u d a t a .
w <
?v----------------- .
,
M allotus v illo s u s .
.-------------------------------- .
C lu p e a h a r e n g u s .
._______________________ .
Boreogadus saida.
Pollachius
------------ -
viren s.
,
Melanogram m us a e glefinus.
.
Gadus m orhua.
.____________________________.
i
i
i
1
i
1--------1------- 1--------1--------1------- 1
1Ö6 1Ö5 1o‘ 10"3 1Ô2 101 10° 101 102 103 10‘
V o l u m e (ml )
Figure 1. Egg volume (left point) related to body volume of
mature females (right point) of zooplankton and fishes, pv =
volume including perivitelline space. Based on data from Bogorov (1959), Zelikman (1961), Pertsova (1966), H em pel and
Blaxter (1967), Blacker (197i), Schopka (1971), G jøsæter and
Monstad (1973), Williams and Lindley (1982), Bergstad et
al. (1987), Kjesbu (1988), P. D alpadado, H. Gjøsæter, T.
Jakobsen, T. Jørgensen, W. Meile, and K. F. Wiborg (unpubl.
data from scientific cruises and commercial catches, Institute
of Marine Research, Bergen).
mean ratio of egg to adult volume of 10-3 for copepods
and 2 • 10" 4 for krill (Fig. 1). Despite being small relative
to the adult, fish eggs are still almost 2-3 orders of
magnitude larger in volume than eggs of krill and cope­
pods (Fig. 1). On a logarithmic scale, the variation
within each of the 3 groups, copepods, krill, and fish,
is relatively limited. There is, however, a fairly strict
relationship of increasing egg size with increasing size of
species for copepods and krill as well as other crustacean
taxa (Mauchline, 1988).
D ata on fecundity of zooplankton compiled by Paffenhöfer and Harris (1979) show a variation from <10
to 1 .4 -104 eggs per female. For fish the fecundity can
be as high as 3 - 107 (Blaxter, 1969). The ratio between
gonad weight and body weight for fishes ranges from
0.04 to 0.65 (Gunderson and Dygert, 1988). A similar
range of 0.02 to 0.50 has been reported for the ratio
between brood volume and body volume of copepods
(Mauchline, 1988). Thus the reproductive effort is
roughly similar in fishes and zooplankton. Given the
difference in size between eggs and adults, the fecundity
is therefore 3 -4 orders of magnitude higher for fish than
for zooplankton. Since fish in general spawn repeatedly
over several years whereas most zooplankters spawn in
only one season, the difference in total fecundity over
the life cycle is even greater.
331
Feeding ecology
The feeding ecology of fish larvae has been the subject
of numerous studies (reviewed by H unter, 1981 ; Turner,
1984). Zooplankton is the predom inant food for most
species of fish larvae, with copepod and barnacle nauplii, tintinnids, copepodites, cladocerans, gastropod lar­
vae, pteropods, and appendicularians as common food
items (Turner, 1984). Phytoplankton can also constitute
an im portant part of the diet of young larvae, par­
ticularly of clupeoid species (Govoni et a l., 1983). There
is evidence that phytoplankton also may play a role in
the first feeding of cod larvae (Klungsøyr et a l., 1989).
Much less has been done concerning the feeding
ecology of larval stages of zooplankton. Nauplii of the
few copepod species that have been investigated have
been found to feed primarily on phytoplankton (Turner,
1984). A common feature has been that phytoplankton
in the smallest size range have been grazed with low
efficiency, grazing being predominantly on algae of
medium or large size (e.g. Berggren et a l., 1988). The
larval stages of krill are also assumed to feed primarily
on phytoplankton (Mauchline and Fisher, 1969).
T he size-efficiency hypothesis (Brooks and Dodson,
1965) postulated that large size produced a competitive
advantage in that a large consumer would be able to
utilize both small and large prey items. It has been
shown that the total size range of prey increases with
increasing size of fish larvae (H unter, 1981; Govoni et
al., 1983). Pearre (1986) found, however, that more
realistic ratio-based indices for trophic niche breadth
were in general constant as the fish grew and showed
no consistent trend with size of fish species. He further
concluded that, combined with a constant or declining
prey biomass in increasing geometric size classes (Shel­
don et a l , 1972; Platt and D enm an, 1977), a constant
trophic niche breadth would imply a constant or declin­
ing prey biomass as predators become larger.
Extensive studies of Canadian freshwater lakes have
revealed a consistent pattern in size distribution with
two pronounced peaks in the phytoplankton and mesozooplankton size ranges respectively (Sprules et al.,
1983). In marine benthic communities there appears to
be a similar consistent biomass distribution with peaks
corresponding to bacteria, meiofauna, and macrofauna
respectively (Schwinghamer, 1983). Sprules et al. (1983)
suggested that a peaked size spectrum similar to that of
lakes may also be the case for marine waters with large
seasonal variations in climate and productivity. Peaky
size distributions may be characteristic for young organ­
ism assemblages, whereas the flat spectrum may be a
feature of old and m ature communities (McCave, 1984).
The limited num ber of size spectra for tem perate and
high latitude marine environments tend to support a
general p attern with peaks in the phytoplankton and
mesozooplankton size ranges (Schwinghamer, 1983;
Hargrave et al., 1985; Witek and Krajewska-Soltys,
332
1987). The trough between these peaks is in the region
of about 50-200 urn. This coincides with the size region
of prey for most first-feeding fish larvae (H unter, 1981).
The general nature of the minimum in particle con­
centration around 100 urn needs further confirmation.
It is possible, however, that fish larvae, due to their
size, feed in a minimum region of the particle size
spectrum.
Superimposed on any such general size spectrum,
there will no doubt be large temporal variation. The
spring phytoplankton bloom is a dramatic wave in pri­
mary production. Overwintering zooplankton spawn,
and the resulting new generations develop as distinct
cohorts (e.g., Krause an d T rahm s, 1982). The dynamics
of these events lead to marked changes in the size
distributions, as exemplified by data from St Georges
Bay, Nova Scotia (Hargrave et al., 1985). The timing
of occurrence of suitable prey is therefore of great
im portance in addition to average abundance levels.
There is a marked difference between plankton and
fish in the dependence of egg production on the feeding
regime. Egg production of many herbivorous zooplankters such as copepods and krill is closely related to the
current feeding conditions and food intake (Kiørboe et
a l., 1985; Ross and Q uentin, 1986). This acts to increase
the chances that zooplankton larvae, which feed roughly
on the same food as the adults (Berggren et al., 1988),
will hatch at a time and place when and where the
phytoplankton concentration is high. The egg pro­
duction of fish, on the other hand, is much more
distantly related to the present feeding conditions both
temporally, spatially, and qualitatively. This produces
a much looser coupling between the occurrence of larvae
and their food for fish than for zooplankton.
M etabolism and starvation
The length of time an animal can endure starvation is a
function of metabolic rate and am ount of available body
reserves. The am ount of reserves possessed by larvae
at hatching varies among groups and species. Nauplii of
the larger calanoid copepods do not feed during the first
naupliar stages. Calanus finmarchicus and C. helgolandicus start to feed in stage III (Marshall and Orr,
1966) and the larger species C. hyperboreus in stage V
(Conover, 1962). Several carnivorous copepods such as
e.g. Euchaeta norvegica do not feed in the naupliar
stages at all (Matthews, 1964). The duration of the non­
feeding naupliar period is, at 5°C, about 1.5 weeks for
C. finmarchicus and about 3 weeks for C. hyperboreus
(Tande, 1988). Naupliar stages III and IV of C. pacificus
experienced 50% mortality after 4-5 d of starvation at
15°C (Fernandez, 1979). For smaller nauplii the time
they can endure starvation can be even shorter (Dagg,
1977).
The nauplii and metanauplius larvae of krill have
non-functional mouthparts and feeding starts in the first
calyptopis stage (Mauchline, 1980). In Antarctic krill
Euphausia superba, the development of the non-feeding
stages lasts about 20 d at 0°C, whereas the first feeding
stage (calyptopis I) survives about 6 d without food
(Ikeda, 1984). Z oea larvae of several benthic crus­
taceans have 50% survival after 3-15 d of starvation
(Lang and Marcy, 1982).
The survival time of fish larvae under starvation shows
considerable variation among species. McGurk (1984)
summarized information on larvae of 25 species of mar­
ine fishes. The time from fertilization to the age of
irreversible starvation was strictly correlated with the
time from fertilization to absorption of the yolk (r =
0.98) and inversely correlated with tem perature (r =
-0 .9 1 ). The time from hatching to absorption of the
yolk ranged from 1.5 to l i d . whereas the time from
absorption of the yolk to the age of irreversible star­
vation ranged from 0.5 to 15 d (McGurk, 1984). For
northern species these times at am bient tem perature
were about 6 and 5 d for cod and haddock and 9-10 d
for herring. It appears from the data reviewed that the
capacity of starvation for fish larvae is roughly similar
to that of zooplankton larvae.
Weight-specific metabolic rate generally shows a clear
inverse relationship with body size (e.g., Ikeda, 1985).
Using such a relationship and assuming that half the
body mass could be metabolized prior to death, Threlkeld (1976) developed a simple model to predict survival
time during starvation. According to this model, at
20°C, a copepod nauplius of 1 u.g dry weight would
survive for 3 d without food whereas a fish larvae of
100 |j,g would survive for about 9 d. Fishes have annual
production/biomass (P/B ) ratios that are on average 4 5 times higher than those of invertebrates of the same
size, reflecting a generally higher metabolic activity
(Banse and M osher, 1980). This difference in metabolic
activity, if shown also by the larval stages, would
counteract the larger size of fish larvae, resulting in the
fairly equal starvation potential for larvae of the two
groups.
Predation
In recent years there has been increasing attention to
predation as a possible cause of recruitment variability
in fish stocks (H unter, 1981; Sissenwine, 1984; Bartey
and H oude, 1987). The predation impact on fish eggs
and larvae has been assessed for one or a few potentially
important predators (e.g., Möller, 1980; D aan et al.,
1985), but data on total mortality due to predation are
lacking. For zooplankton eggs and larvae our knowledge
on predation mortality is even more limited.
The difference in size of eggs and larvae between
zooplankton and fish makes them vulnerable to pre­
dation from different parts of the predator size spec­
trum. Cyclopoid copepods could be important as
predators on copepod eggs and nauplii. They are widely
distributed and occur often in high abundance. Species
of Oithona are generally considered to be omnivores or
carnivores (Turner, 1984). Oithona similis and O. nana
have been found to prey on copepod nauplii (Marshall
and O rr, 1966; Lampitt and Gamble, 1982). The vertical
distribution of Oithona copepodites in the Barents Sea
is often similar to that of copepod nauplii (Ellertsen et
al., 1981). O ther forms among the smaller plankton,
such as the dinoflagellate Noctiluca (Daan, 1987), may
also predate on zooplankton eggs and larvae.
Eggs and larvae of zooplankton are an important
com ponent of the diet offish larvae (Turner, 1984). The
abundance of fish larvae is considered to be generally
too low to affect the density of their prey (Cushing,
1983), and they have therefore limited effect on the
mortality of zooplankton eggs and larvae.
Predators on fish eggs and larvae belong to a variety
of animal groups, such as ctenophores, medusae, chaetognaths, polychaetes, crustaceans, squids, fish, and
birds (Hunter, 1981; Bailey and H oude, 1987). A major
difference between plankton and fish is that the eggs
and larvae of the former are generally too small to be
preyed upon with any efficiency by planktivorous fishes
(Hardy, 1924; D aan, in press). Fish eggs and larvae, in
contrast, are big enough to come into the predation
realm of pelagic fish. This may give rise to significant
differences in the predation pressure exerted on eggs
and larvae of fish and zooplankton respectively. Fuiman
and Gamble (1988) considered predation by fish to be
more im portant than predation by invertebrates as a
source of mortality of fish eggs and larvae. The schooling
and migratory behaviour of planktivorous fishes allows
them to search through extensive areas and concentrate
in areas with abundant food. This probably enables
them to exploit their food resource more efficiently,
resulting in relatively high predation pressure on their
prey. Aggregation of pelagic fish in areas with high
abundance of fish eggs and larvae could be a direct
response, but it could also be indirectly cued as a
response to high abundance of zooplankton in the same
area.
The vulnerability of eggs and larvae to predators are
affected by a wide range of factors, both intrinsic and
external (H unter, 1981; Bailey and H oude, 1987) of
which patchiness in distribution and anti-predatory
behaviour are most im portant. McGurk (1986) showed
that the rate of mortality of fish eggs and larvae was
positively correlated with the degree of patchiness in
distribution. This is contrary to what has been anti­
cipated from theoretical consideration of spatial distri­
bution, search time, and satiation (Gulland, 1987), and
could be due to the behaviour of predators and prey
(McGurk, 1987). It is possible that, for instance, plank­
tivorous fish concentrate their foraging effort in areas
with high and patchy distributed prey abundance.
333
Populations of fish in the Barents Sea can be used to
Patchiness occurs at various spatial scales. On a large
illustrate this point. Capelin (Mailotus villosus) is a
scale it seems clear that the restricted spawning areas
dominant planktivorous fish which has a large scale
of fish result in higher patchiness for fish than is the
case for zooplankton. Within a spawning area there is seasonal migration northwards following, with a time
lag, the receding ice edge. This behaviour allows capelin
probably also finer scale patchiness due to behaviourally
to exploit the secondary production of a considerable
determ ined aggregations of spawning fish. For
part of the Barents Sea and to maintain a large stock
zooplankton where mating and spawning are temporally
under favourable conditions (Sakshaug and Skjoldal,
separated, patchiness induced by swarming is probably
1989; Skjoldal and Rey, 1989). Capelin matures at an
reduced at the time of spawning. It is therefore a reason­
age of about 4 yr and migrates to the coasts of northern
able assumption that patchiness of eggs and larvae is
Norway and Kola to spawn (Tjelm eland, 1987). Spawn­
greater for fish than for zooplankton, both on a fine and
ing areas along a wide stretch of coast of the southern
large scale.
Barents Sea and widely distributed feeding areas may
Mortality rates of fish eggs and larvae are high, typi­
represent a situation where the range is short and the
cally in the range 0.1—1.0 d 1, which are 5 to 10 times
higher than predicted from their dry weight by the target wide. This may explain the relatively low fec­
undity of capelin, which, on the other hand, makes
general model of Peterson and Wroblewski (1984)
(M cGurk, 1986, 1987). There is a general decrease in this species more vulnerable to predation. Increased
predation from the strong 1983 year classes of herring
mortality with increasing development from egg through
and cod probably played a m ajor role in the recruitment
larval stages that corresponds to a decrease in the degree
of patchiness (M cG urk, 1986, 1987). D ata on mortality failure and collapse of capelin stock from 1984 to 1986
(Mehl, 1987; Skjoldal and Rey, 1989).
of pelagic crustaceans tend, on the other hand, to fall
Cod is predominantly a piscivorous fish (Mehl, 1987)
below the prediction of Peterson and Wroblewski’s
(1984) model (M cGurk, 1987). Reported mortality rates which requires an abundant pelagic fish resource such
of copepod nauplii tend to fall in the range 0.1-0.7 d “ 1 as capelin in order to sustain a large population. The
(Heinle, 1966; Mullin and Brooks, 1970; Kimmerer feeding areas of cod will therefore be determined to a
and McKinnon, 1987). M cGurk's (1987) regression for large extent by the patchy and variable distribution of
pelagic crustaceans predicts a mortality rate of 0.12 d ‘ 1 capelin. The m ajor spawning area of cod is located in
for a nauplius of 2 ^ig dry weight (e.g. Calanus). With Lofoten, and larvae are transported over a long distance
a fecundity of 250-2000 eggs per female (Paffenhöfer, in a rather complex circulation system into the optimal
1970; Marshall and O rr, 1972) and a duration of egg feeding areas in the Barents Sea (Bjørke and Sundby,
and naupliar stages of 30-60 d at 0-5°C for Calanus 1987; Dragesund and Gjøsæter, 1988). The probability
(Marshall and O rr, 1972; Tande, 1988), a minimum of vagrancy is high due to variations in the current
surviving num ber of two individuals corresponds to pattern which may transport larvae out over deep water
mortality rates of 0.08-0.23 d ' 1. Increased mortality in the Norwegian Sea or into cold waters of the Svalbard
rate for adult copepods has been ascribed to size selec­ region. This may be one reason for the high fecundity
of cod.
tive grazing by fish (Landry, 1978).
Zooplankton species and populations need also to
The available information suggests that eggs and lar­
vae of zooplankton tend in general to have lower m or­ maintain themselves within geographical areas where
tality rates than those of fish. The size difference which the living conditions are favourable. However, the situa­
brings fish eggs and larvae into the size range preyed tion for zooplankton is different since they spawn over
upon by visual plankton-feeding fish, could be a major a wide area. Their abundance and distribution will
therefore be governed by their population dynamics and
factor responsible for this difference.
the circulation pattern. Individuals, either larvae or
adults, which drift into areas unfavourable to repro­
Larval transport and distribution
duction and growth, are expatriated vagrants that are
The difference between fish and plankton in having and lost to the population. The expatriates are likely to
not having restricted spawning areas, has important originate from outskirts of the area of distribution
implications in terms of larval drift and distribution. where the living conditions are suboptimal. We consider
The drift of fish larvae from spawning to nursery areas it unlikely that the fecundity of zooplankton would be
is the reversal in the life cycle of the spawning migration increased to counteract the effect of such expatriation.
by adult fish. Sinclair (1988) has stressed the importance
of spatial losses of recruits from a population in his
G eneral discussion
“m em ber/vagrant” hypothesis. The spatial extent of the
nursery areas may be an im portant aspect determining The foregoing analysis of reproduction and larval eco­
the likelihood that larvae transported by currents will logy has revealed several differences between fish and
strike home and be “m em bers” or whether they miss zooplankton when these are considered as general groups
(Table 1). Zooplankton larvae appear to be in a more
their target areas and become “vagrants” .
334
Table 1. General characteristics of reproduction and larval ecology of fish and zooplankton. (It should be noted that there are
many exceptions to these generalities.)
Fish
Zooplankton
Horizontal to restricted
spawning areas
Vertical aggregation
and swarming
Mating and spawning
In one act
Temporally
separated
Patchiness in distribution
of eggs and larvae
High
Lower
i ( r 7- i o - 5
10 4-10 3
Egg size
(diameter: mm)
1.0-2.0
0.1-0.5
Fecundity (eggs per
female)
104-1 07
10:-103
Spawning behaviour
Migration
Egg size and fecundity
Egg to adult volume
ratio
Larval feeding ecology
Trophic type
Carnivores
Herbivores
Size of food particles
30-200 fim
5-50 vim
General food abundance
Lower
Higher
Food of larvae and
adults
Different
Same
Egg production and
feeding conditions of
females
Distantly related both
temporally and
spatially
Closely coupled
Food predictability
Low
Higher due to the
synchronizing
effect of the above
coupling
2-30 d
Roughly
comparable to fish
larvae
Higher
Lower
Relatively large
predators, including
planktivorous fish
Small predators
(cyclopoid
copepods, etc.)
Susceptibility to
predation loss
High
Lower
Total mortality rate
High
Lower
High. Loss of recruits
by vagrancy is likely
Less. Expatriation
of both juveniles
and adults from
outskirts of
distributional area
Starvation and metabolism
Larval starvation
capacity
Metabolic activity
Predation
Predator field
Larval drift and transport
Spatial constraints on
life cycle closure
favourable situation than are fish larvae in many
respects. They have a more predictable and abundant
food source, are relatively resistant to starvation, are
less prone to be preyed upon, and are at less risk of not
hitting home to nursery areas. This provides at least a
partial explanation for the lower mortality of zooplank­
ton larvae com pared to fish larvae.
A central question in the fish recruitment variability
problem is which factors cause the high mortality in the
early stages of fish. To allocate the responsibility by
335
each of the main factors, starvation, predation, or trans­
port loss, is difficult due to the fact that they are inter­
related and their relative im portance probably variable.
The high fecundity of fish can be viewed as an adap­
tation to counter high mortality of the early stages
of development. O n the other hand, an increase in
fecundity for a fish of a given size represents less ener­
getic investment in each egg, and larvae that are more
susceptible to starvation or food limitation (Ware,
1975). Thus high fecundity can also be viewed as causing
high mortality. Prolongation of the larval period, either
inherently or due to food limitation and poor growth,
can result in higher mortality due to predation. In the
context of spatial life cycle closure there could be inter­
relationships between the duration of the planktonic
stages, total mortality and fecundity.
Much attention has been given to mortality in the
very earliest stages in the life history of fish since the
critical period concept was introduced by H jort (1914).
There is little evidence, however, in support of an
exceptionally high and variable mortality at this period.
There are in contrast many examples of rather constant
or declining rate of mortality with development during
the egg and larval stages (McGurk 1986, 1987; Fossum
1987).
Large variability in year-class strength need not imply
large variation in rate of mortality. If the mortality rate
is high over a long period, a relatively small variation
in mortality rate will produce a large variation in the
number of survivors. Recruitm ent studies on cod in
the Lofoten area have revealed a relatively constant
mortality rate of about 0.15 d _1 for eggs and larvae
during a period of 40 d after spawning (Fossum, 1987).
This mortality rate would result in 120 surviving larvae
out of 1 million eggs after 2 months. Varying the m or­
tality rate to 0.10 and 0.20 d _1 would leave 2500 and 6
survivors, respectively (Fig. 2). Thus, a difference in
mortality rate by a factor of 2 results in more than
two orders of magnitude difference in the number of
surviving larvae 1 month after resorption of the yolk
sac.
A m oderate variation in growth rate due to feeding
conditions or tem perature can have an equally drastic
effect. Increasing and decreasing the growth rate by
50% and 33% , respectively, results in changes of devel­
opm ent time from 60 d to 40 and 90 d respectively. With
a constant mortality rate of 0.15 d ^ 1 this produces
m ore than three orders of magnitude difference in num­
ber of survivors (Fig. 2). The logarithmic nature of the
exponential relationship is also evident if one considers
zooplankton. With a generation time of 3 0 d, the fec­
undity must be 40 and 800 eggs per female to sustain
mortality rates of 0.1 and 0.2 d ' 1 (Fig. 2).
The high rate of mortality at the egg and larval stages
of fish seems well docum ented. If the high mortality
rate applies to a relatively long period during the egg
and larval stages, as it appears to do, we are indeed
336
to N 0.2
Time ( days )
Figure 2. Effects of mortality rate on exponential survivorship
curves in three hypothetical cases: 1) Survivors out of 1 million
eggs after 60 d at mortality rates of 0.1, 0.15 and 0.2 d " 1. 2)
Survivors after 40 and 90 d (representing a 2.2-fold change in
growth rate) at a mortality rate of 0.15 d “ 1. 3) Required egg
numbers to produce 2 survivors after 30 d at m ortality rates of
0.1 and 0.2 d ' 1.
looking for relatively small differences in mortality rate
to explain the large variability in recruitment. We sug­
gest that predation plays an overriding role for the high
mortality rates (Fig. 3). The feeding conditions are
considered to influence mortality mainly through the
exposure time for predation. The same is probably the
case for at least some of the spatial aspects which
influence recruitment. Resolving the recruitment prob­
lem will require high accuracy and precision in carefully
designed field sampling programs. The interrelations
between the causes of mortality (Fig. 3) and their varia­
bility put emphasis on the need for a more broad system
ecological approach in future recruitment variability
PREDA TION
T o ta l
R e c ru its
no. o f
eggs
'
g ro w th
r
FOOD
L IM IT A ­
TION
d is t r i­
b ution
SPA TIAL
ASPECTS
Figure 3. A schematic representation of interrelationships
between predation, food limitation, and spatial aspects as
causes of mortality in fish eggs and larvae. Food limitation is
thought to act mainly through reduced growth rate and
increased time of predation. T he spatial aspects of larval trans­
port affect recruitm ent directly through loss by vagrancy and
indirectly through loss by predation.
studies where less attention should be given to prove or
disprove any specific mortality hypothesis.
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