Rapp. P.-v. Réun. 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 Webjørn Melle

Skjoldal, Hein Rune, and Melle, Webjørn. 1989. Nekton and plankton: some comparative aspects of larval ecology and recruitment processes. - Rapp. P. v. Réun. Cons. int. Explor. Mer, 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 major 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 major 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 number of recruits.

Hein Rune Skjoldal and Webjørn Melle: Institute of Marine Research, P.O. Box 1870, Nordnes, N-5024 Bergen, Norway

Introduction Nekton and zooplankton have one thing in common, By definition plankton and nekton are differentiated by both groups being part of the plankton in the larval their respective abilities to determine their horizontal stage of their life cycles. They are therefore subject in distribution. Fish are a major group of nekton which part to the same ecological recruitment processes. Here use their ability to swim for purposes such as feeding we compare their reproductive and larval ecology, and reproduction. Many species migrate to restricted emphasizing similarities and differences in basic eco­ spawning areas which are located in relation to the logical properties of nekton and zooplankton. We have water circulation pattern so as to ensure transport of done this in a very general way, using fish and crus­ recruits back into major feeding areas (Harden-Jones, taceans (with a bias towards copepods) as typical rep­ 1968; Sherman et al., 1984; Dragesund and Gjøsæter, resentatives for the two groups. Where possible we have 1988). Zooplankton, in contrast, spawn over a much chosen our examples from the Barents Sea ecosystem. wider part of their distribution area than do fish, and there is little evidence for behaviourally localized Spawning behaviour spawning areas. The large variability in recruitment of commercial Many marine fish populations have more or less well- fish stocks has received long-lasting scientific attention. defined spawning areas where the adults aggregate after Despite this, the mechanism involved in recruitment a spawning migration to mate and spawn. The location processes have not yet been properly resolved. Current of spawning areas is the result of adaptation to the research activities are guided by three major groups of physical regime which disperses and transports the lar­ hypothesis: that the recruitment variability is due to (1) vae and juveniles into areas which are favourable for food limitation, (2) predation, or (3) physical oceanic growth, survival, and further reproduction (Harden- variability (Sissenwine, 1984). Jones, 1968; Sherman et al., 1984; Sinclair, 1988). The

330 dominant commercial fish stocks in the Barents Sea Species.- provide many clear examples of this (Dragesund and Calanus finmarchicus. .______. Gjøsæter, 1988). C. glacialis. ,______. The limited swimming capacity of zooplankton pre- C. hyperboreus. , vents them from performing extensive horizontal g- < Metridia longa. .______. spawning migrations to well-defined spawning areas like o Pseudocalanus sp. . fish. Instead they spawn over most or all of their area Euchaeta norvegica. .------. of distribution. It is likely, however, that specific repro­ Thysanoessa raschii. ^ ------. ductive behaviour has evolved as a mechanism for aggre- = T. inermis. , ?v------. gation to improve the chances of encounter between -* T. longicaudata. , the sexes. Swimming speed is related to body size, and this sets upper limits to the distance and size of Mallotus villosus. .------. Clupea harengus. .______. aggregations. The physical dispersive forces in the ocean are generally much stronger in the horizontal than in w < Boreogadus saida. ------Pollachius virens. , the vertical direction. Aggregation in the vertical would therefore seem to be a mechanism whereby zooplankton Melanogrammus aeglefinus. . could meet for reproductive purposes. Many zooplank­ Gadus m orhua. .______. ton species living in spatially restricted environments i i i 1 i 1------1------1------1------1------1 such as estuaries or coral reefs have evolved behavioural 1Ö6 1Ö5 1o‘ 10"3 1Ô2 101 10° 101 102 103 10‘ Volume (ml ) patterns which contribute to their retention (Sinclair, 1988). Figure 1. Egg volume (left point) related to body volume of A seasonal vertical migration upwards in spring from mature females (right point) of zooplankton and fishes, pv = volume including perivitelline space. Based on data from Bogo- overwintering in deeper water is a common phenom­ rov (1959), Zelikman (1961), Pertsova (1966), Hempel and enon for many open water zooplankton species (e.g. Blaxter (1967), Blacker (197i), Schopka (1971), Gjøsæter and Østvedt, 1955). Aggregation in the surface layer is a Monstad (1973), Williams and Lindley (1982), Bergstad et possible mechanism for concentrating the population, al. (1987), Kjesbu (1988), P. Dalpadado, H. Gjøsæter, T. thus enhancing the probability that males and females Jakobsen, T. Jørgensen, W. Meile, and K. F. Wiborg (unpubl. data from scientific cruises and commercial catches, Institute meet. This probability would decrease with decreasing of Marine Research, Bergen). 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 mean ratio of egg to adult volume of 10-3 for copepods role in reproduction of zooplankton than has generally and 2 • 10" 4 for krill (Fig. 1). Despite being small relative been recognized. Several species of copepods have been to the adult, fish eggs are still almost 2-3 orders of observed to form dense local monospecific swarms con­ magnitude larger in volume than eggs of krill and cope­ sisting mainly of adults (Ueda et al., 1983). Calanus pods (Fig. 1). On a logarithmic scale, the variation finmarchicus can form dense red patches in the surface within each of the 3 groups, copepods, krill, and fish, layer (Wiborg, 1976). Krill occur regularly in schools is relatively limited. There is, however, a fairly strict or swarms of different sizes and probably for several relationship of increasing egg size with increasing size of different purposes (Hamner et al., 1983). Swarming in species for copepods and krill as well as other crustacean relation to reproduction was indicated for Meganycti- taxa (Mauchline, 1988). phanes norvegica by Nicol (1984). Data on fecundity of zooplankton compiled by Paf- In fishes mating and spawning usually occur simul­ fenhöfer and Harris (1979) show a variation from <10 taneously. For most crustacean zooplankton, on the to 1.4-104 eggs per female. For fish the fecundity can other hand, mating involves transfer of a spermatophore be as high as 3 - 107 (Blaxter, 1969). The ratio between that is used to fertilize the eggs when subsequently gonad weight and body weight for fishes ranges from spawned. 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 Egg size and fecundity (Mauchline, 1988). Thus the reproductive effort is roughly similar in fishes and zooplankton. Given the The fecundity of fish can be very high, and, as a conse­ difference in size between eggs and adults, the fecundity quence, the size of the eggs is small relative to the size is therefore 3-4 orders of magnitude higher for fish than of the adult fish. The mean ratio of egg to adult volume for zooplankton. Since fish in general spawn repeatedly is about 10-7 for fish species from the Barents Sea as over several years whereas most zooplankters spawn in shown in Figure 1. The difference in size between egg only one season, the difference in total fecundity over and adult is considerably less for zooplankton, with a the life cycle is even greater.

331 Feeding ecology 1987). The trough between these peaks is in the region of about 50-200 urn. This coincides with the size region The feeding ecology of fish larvae has been the subject of prey for most first-feeding fish larvae (Hunter, 1981). of numerous studies (reviewed by Hunter, 1981 ; Turner, The general nature of the minimum in particle con­ 1984). Zooplankton is the predominant food for most centration around 100 urn needs further confirmation. species of fish larvae, with copepod and barnacle nau- It is possible, however, that fish larvae, due to their plii, tintinnids, copepodites, cladocerans, gastropod lar­ size, feed in a minimum region of the particle size vae, pteropods, and appendicularians as common food spectrum. items (Turner, 1984). Phytoplankton can also constitute Superimposed on any such general size spectrum, an important part of the diet of young larvae, par­ there will no doubt be large temporal variation. The ticularly of clupeoid species (Govoni et al., 1983). There spring phytoplankton bloom is a dramatic wave in pri­ is evidence that phytoplankton also may play a role in mary production. Overwintering zooplankton spawn, the first feeding of cod larvae (Klungsøyr et al., 1989). and the resulting new generations develop as distinct Much less has been done concerning the feeding cohorts (e.g., Krause andTrahms, 1982). The dynamics ecology of larval stages of zooplankton. Nauplii of the of these events lead to marked changes in the size few copepod species that have been investigated have distributions, as exemplified by data from St Georges been found to feed primarily on phytoplankton (Turner, Bay, Nova Scotia (Hargrave et al., 1985). The timing 1984). A common feature has been that phytoplankton of occurrence of suitable prey is therefore of great in the smallest size range have been grazed with low importance in addition to average abundance levels. efficiency, grazing being predominantly on algae of There is a marked difference between plankton and medium or large size (e.g. Berggren et al., 1988). The fish in the dependence of egg production on the feeding larval stages of krill are also assumed to feed primarily regime. Egg production of many herbivorous zooplank- on phytoplankton (Mauchline and Fisher, 1969). ters such as copepods and krill is closely related to the The size-efficiency hypothesis (Brooks and Dodson, current feeding conditions and food intake (Kiørboe et 1965) postulated that large size produced a competitive al., 1985; Ross and Quentin, 1986). This acts to increase advantage in that a large consumer would be able to the chances that zooplankton larvae, which feed roughly utilize both small and large prey items. It has been on the same food as the adults (Berggren et al., 1988), shown that the total size range of prey increases with will hatch at a time and place when and where the increasing size of fish larvae (Hunter, 1981; Govoni et phytoplankton concentration is high. The egg pro­ al., 1983). Pearre (1986) found, however, that more duction of fish, on the other hand, is much more realistic ratio-based indices for trophic niche breadth distantly related to the present feeding conditions both were in general constant as the fish grew and showed temporally, spatially, and qualitatively. This produces no consistent trend with size of fish species. He further a much looser coupling between the occurrence of larvae concluded that, combined with a constant or declining and their food for fish than for zooplankton. prey biomass in increasing geometric size classes (Shel­ don et a l, 1972; Platt and Denman, 1977), a constant trophic niche breadth would imply a constant or declin­ Metabolism and starvation ing prey biomass as predators become larger. Extensive studies of Canadian freshwater lakes have The length of time an animal can endure starvation is a revealed a consistent pattern in size distribution with function of metabolic rate and amount of available body two pronounced peaks in the phytoplankton and meso- reserves. The amount of reserves possessed by larvae zooplankton size ranges respectively (Sprules et al., at hatching varies among groups and species. Nauplii of 1983). In marine benthic communities there appears to the larger calanoid copepods do not feed during the first be a similar consistent biomass distribution with peaks naupliar stages. Calanus finmarchicus and C. hel- corresponding to bacteria, meiofauna, and macrofauna golandicus start to feed in stage III (Marshall and Orr, respectively (Schwinghamer, 1983). Sprules et al. (1983) 1966) and the larger species C. hyperboreus in stage V suggested that a peaked size spectrum similar to that of (Conover, 1962). Several carnivorous copepods such as lakes may also be the case for marine waters with large e.g. Euchaeta norvegica do not feed in the naupliar seasonal variations in climate and productivity. Peaky stages at all (Matthews, 1964). The duration of the non­ size distributions may be characteristic for young organ­ feeding naupliar period is, at 5°C, about 1.5 weeks for ism assemblages, whereas the flat spectrum may be a C. finmarchicus and about 3 weeks for C. hyperboreus feature of old and mature communities (McCave, 1984). (Tande, 1988). Naupliar stages III and IV of C. pacificus The limited number of size spectra for temperate and experienced 50% mortality after 4-5 d of starvation at high latitude marine environments tend to support a 15°C (Fernandez, 1979). For smaller nauplii the time general pattern with peaks in the phytoplankton and they can endure starvation can be even shorter (Dagg, mesozooplankton size ranges (Schwinghamer, 1983; 1977). Hargrave et al., 1985; Witek and Krajewska-Soltys, The nauplii and metanauplius larvae of krill have

332 non-functional mouthparts and feeding starts in the first trum. Cyclopoid copepods could be important as calyptopis stage (Mauchline, 1980). In Antarctic krill predators on copepod eggs and nauplii. They are widely Euphausia superba, the development of the non-feeding distributed and occur often in high abundance. Species stages lasts about 20 d at 0°C, whereas the first feeding of Oithona are generally considered to be omnivores or stage (calyptopis I) survives about 6 d without food carnivores (Turner, 1984). Oithona similis and O. nana (Ikeda, 1984). Zoea larvae of several benthic crus­ have been found to prey on copepod nauplii (Marshall taceans have 50% survival after 3-15 d of starvation and Orr, 1966; Lampitt and Gamble, 1982). The vertical (Lang and Marcy, 1982). distribution of Oithona copepodites in the Barents Sea The survival time of fish larvae under starvation shows is often similar to that of copepod nauplii (Ellertsen et considerable variation among species. McGurk (1984) al., 1981). Other forms among the smaller plankton, summarized information on larvae of 25 species of mar­ such as the dinoflagellate Noctiluca (Daan, 1987), may ine fishes. The time from fertilization to the age of also predate on zooplankton eggs and larvae. irreversible starvation was strictly correlated with the Eggs and larvae of zooplankton are an important time from fertilization to absorption of the yolk (r = component of the diet offish larvae (Turner, 1984). The 0.98) and inversely correlated with temperature (r = abundance of fish larvae is considered to be generally -0.91). The time from hatching to absorption of the too low to affect the density of their prey (Cushing, yolk ranged from 1.5 to lid. whereas the time from 1983), and they have therefore limited effect on the absorption of the yolk to the age of irreversible star­ mortality of zooplankton eggs and larvae. vation ranged from 0.5 to 15 d (McGurk, 1984). For Predators on fish eggs and larvae belong to a variety northern species these times at ambient temperature of animal groups, such as ctenophores, medusae, chae- were about 6 and 5 d for cod and haddock and 9-10 d tognaths, polychaetes, crustaceans, squids, fish, and for herring. It appears from the data reviewed that the birds (Hunter, 1981; Bailey and Houde, 1987). A major capacity of starvation for fish larvae is roughly similar difference between plankton and fish is that the eggs to that of zooplankton larvae. and larvae of the former are generally too small to be Weight-specific metabolic rate generally shows a clear preyed upon with any efficiency by planktivorous fishes inverse relationship with body size (e.g., Ikeda, 1985). (Hardy, 1924; Daan, in press). Fish eggs and larvae, in Using such a relationship and assuming that half the contrast, are big enough to come into the predation body mass could be metabolized prior to death, Threl- realm of pelagic fish. This may give rise to significant keld (1976) developed a simple model to predict survival differences in the predation pressure exerted on eggs time during starvation. According to this model, at and larvae of fish and zooplankton respectively. Fuiman 20°C, a copepod nauplius of 1 u.g dry weight would and Gamble (1988) considered predation by fish to be survive for 3 d without food whereas a fish larvae of more important than predation by invertebrates as a 100 |j,g would survive for about 9 d. Fishes have annual source of mortality of fish eggs and larvae. The schooling production/biomass (P/B) ratios that are on average 4- and migratory behaviour of planktivorous fishes allows 5 times higher than those of invertebrates of the same them to search through extensive areas and concentrate size, reflecting a generally higher metabolic activity in areas with abundant food. This probably enables (Banse and Mosher, 1980). This difference in metabolic them to exploit their food resource more efficiently, activity, if shown also by the larval stages, would resulting in relatively high predation pressure on their counteract the larger size of fish larvae, resulting in the prey. Aggregation of pelagic fish in areas with high fairly equal starvation potential for larvae of the two abundance of fish eggs and larvae could be a direct groups. response, but it could also be indirectly cued as a response to high abundance of zooplankton in the same area. Predation The vulnerability of eggs and larvae to predators are affected by a wide range of factors, both intrinsic and In recent years there has been increasing attention to external (Hunter, 1981; Bailey and Houde, 1987) of predation as a possible cause of recruitment variability which patchiness in distribution and anti-predatory in fish stocks (Hunter, 1981; Sissenwine, 1984; Bartey behaviour are most important. McGurk (1986) showed and Houde, 1987). The predation impact on fish eggs that the rate of mortality of fish eggs and larvae was and larvae has been assessed for one or a few potentially positively correlated with the degree of patchiness in important predators (e.g., Möller, 1980; Daan et al., distribution. This is contrary to what has been anti­ 1985), but data on total mortality due to predation are cipated from theoretical consideration of spatial distri­ lacking. For zooplankton eggs and larvae our knowledge bution, search time, and satiation (Gulland, 1987), and on predation mortality is even more limited. could be due to the behaviour of predators and prey The difference in size of eggs and larvae between (McGurk, 1987). It is possible that, for instance, plank­ zooplankton and fish makes them vulnerable to pre­ tivorous fish concentrate their foraging effort in areas dation from different parts of the predator size spec­ with high and patchy distributed prey abundance.

333 Patchiness occurs at various spatial scales. On a large Populations of fish in the Barents Sea can be used to scale it seems clear that the restricted spawning areas illustrate this point. Capelin (Mailotus villosus) is a of fish result in higher patchiness for fish than is the dominant planktivorous fish which has a large scale case for zooplankton. Within a spawning area there is seasonal migration northwards following, with a time probably also finer scale patchiness due to behaviourally lag, the receding ice edge. This behaviour allows capelin determined aggregations of spawning fish. For to exploit the secondary production of a considerable zooplankton where mating and spawning are temporally part of the Barents Sea and to maintain a large stock separated, patchiness induced by swarming is probably under favourable conditions (Sakshaug and Skjoldal, reduced at the time of spawning. It is therefore a reason­ 1989; Skjoldal and Rey, 1989). Capelin matures at an able assumption that patchiness of eggs and larvae is age of about 4 yr and migrates to the coasts of northern greater for fish than for zooplankton, both on a fine and Norway and Kola to spawn (Tjelmeland, 1987). Spawn­ large scale. ing areas along a wide stretch of coast of the southern Mortality rates of fish eggs and larvae are high, typi­ Barents Sea and widely distributed feeding areas may cally in the range 0.1—1.0 d 1, which are 5 to 10 times represent a situation where the range is short and the higher than predicted from their dry weight by the target wide. This may explain the relatively low fec­ general model of Peterson and Wroblewski (1984) undity of capelin, which, on the other hand, makes (McGurk, 1986, 1987). There is a general decrease in this species more vulnerable to predation. Increased mortality with increasing development from egg through predation from the strong 1983 year classes of herring larval stages that corresponds to a decrease in the degree and cod probably played a major role in the recruitment of patchiness (McGurk, 1986, 1987). Data on mortality failure and collapse of capelin stock from 1984 to 1986 of pelagic crustaceans tend, on the other hand, to fall (Mehl, 1987; Skjoldal and Rey, 1989). below the prediction of Peterson and Wroblewski’s Cod is predominantly a piscivorous fish (Mehl, 1987) (1984) model (McGurk, 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). McGurk'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 major 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 Orr, 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 Orr, 1972; Tande, 1988), a minimum of vagrancy is high due to variations in the current surviving number 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 tive grazing by fish (Landry, 1978). of cod. The available information suggests that eggs and lar­ Zooplankton species and populations need also to vae of zooplankton tend in general to have lower mor­ 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 factor responsible for this difference. therefore be governed by their population dynamics and the circulation pattern. Individuals, either larvae or Larval transport and distribution adults, which drift into areas unfavourable to repro­ 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 General discussion “member/vagrant” hypothesis. The spatial extent of the nursery areas may be an important 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 “members” or whether they miss zooplankton when these are considered as general groups their target areas and become “vagrants”. (Table 1). Zooplankton larvae appear to be in a more

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

Spawning behaviour Migration Horizontal to restricted Vertical aggregation spawning areas and swarming Mating and spawning In one act Temporally separated Patchiness in distribution High Lower of eggs and larvae

Egg size and fecundity Egg to adult volume i ( r 7-io - 5 10 4-10 3 ratio Egg size 1.0-2.0 0.1-0.5 (diameter: mm) Fecundity (eggs per 104-107 10:-103 female)

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 Different Same adults Egg production and Distantly related both Closely coupled feeding conditions of temporally and females spatially Food predictability Low Higher due to the synchronizing effect of the above coupling

Starvation and metabolism Larval starvation 2-30 d Roughly capacity comparable to fish larvae Metabolic activity Higher Lower

Predation Predator field Relatively large Small predators predators, including (cyclopoid planktivorous fish copepods, etc.) Susceptibility to High Lower predation loss Total mortality rate High Lower

Larval drift and transport Spatial constraints on High. Loss of recruits Less. Expatriation life cycle closure by vagrancy is likely of both juveniles and adults from outskirts of distributional area favourable situation than are fish larvae in many partial explanation for the lower mortality of zooplank­ respects. They have a more predictable and abundant ton larvae compared to fish larvae. food source, are relatively resistant to starvation, are A central question in the fish recruitment variability less prone to be preyed upon, and are at less risk of not problem is which factors cause the high mortality in the hitting home to nursery areas. This provides at least a 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 importance probably variable. The high fecundity of fish can be viewed as an adap­ tation to counter high mortality of the early stages of development. On 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 to N 0.2 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 Time ( days ) very earliest stages in the life history of fish since the Figure 2. Effects of mortality rate on exponential survivorship critical period concept was introduced by Hjort (1914). curves in three hypothetical cases: 1) Survivors out of 1 million There is little evidence, however, in support of an eggs after 60 d at mortality rates of 0.1, 0.15 and 0.2 d "1. 2) exceptionally high and variable mortality at this period. Survivors after 40 and 90 d (representing a 2.2-fold change in There are in contrast many examples of rather constant growth rate) at a mortality rate of 0.15 d“1. 3) Required egg numbers to produce 2 survivors after 30 d at mortality rates of or declining rate of mortality with development during 0.1 and 0.2 d ' 1. the egg and larval stages (McGurk 1986, 1987; Fossum 1987). Large variability in year-class strength need not imply looking for relatively small differences in mortality rate large variation in rate of mortality. If the mortality rate to explain the large variability in recruitment. We sug­ is high over a long period, a relatively small variation gest that predation plays an overriding role for the high in mortality rate will produce a large variation in the mortality rates (Fig. 3). The feeding conditions are number of survivors. Recruitment studies on cod in considered to influence mortality mainly through the the Lofoten area have revealed a relatively constant exposure time for predation. The same is probably the mortality rate of about 0.15 d_1 for eggs and larvae case for at least some of the spatial aspects which during a period of 40 d after spawning (Fossum, 1987). influence recruitment. Resolving the recruitment prob­ This mortality rate would result in 120 surviving larvae lem will require high accuracy and precision in carefully out of 1 million eggs after 2 months. Varying the mor­ designed field sampling programs. The interrelations tality rate to 0.10 and 0.20 d_1 would leave 2500 and 6 between the causes of mortality (Fig. 3) and their varia­ survivors, respectively (Fig. 2). Thus, a difference in bility put emphasis on the need for a more broad system mortality rate by a factor of 2 results in more than ecological approach in future recruitment variability two orders of magnitude difference in the number of surviving larvae 1 month after resorption of the yolk sac. A moderate variation in growth rate due to feeding PREDA TION conditions or temperature can have an equally drastic effect. Increasing and decreasing the growth rate by Total 50% and 33%, respectively, results in changes of devel­ Recruits no. o f opment time from 60 d to 40 and 90 d respectively. With a constant mortality rate of 0.15 d ^ 1 this produces eggs ' growth distri­ r bution more than three orders of magnitude difference in num­ ber of survivors (Fig. 2). The logarithmic nature of the FOOD SPA TIAL LIMITA­ ASPECTS exponential relationship is also evident if one considers TION zooplankton. With a generation time of 30d, the fec­ undity must be 40 and 800 eggs per female to sustain Figure 3. A schematic representation of interrelationships mortality rates of 0.1 and 0.2 d ' 1 (Fig. 2). between predation, food limitation, and spatial aspects as The high rate of mortality at the egg and larval stages causes of mortality in fish eggs and larvae. Food limitation is thought to act mainly through reduced growth rate and of fish seems well documented. If the high mortality increased time of predation. The spatial aspects of larval trans­ rate applies to a relatively long period during the egg port affect recruitment directly through loss by vagrancy and and larval stages, as it appears to do, we are indeed indirectly through loss by predation.

336 studies where less attention should be given to prove or Fuiman, L. A., and Gamble, J. C. 1988. Predation by Atlantic disprove any specific mortality hypothesis. herring, sprat, and sand-eels on herring larvae in large enclosures. Mar. Ecol. Prog. Ser., 44: 1-6. References Govoni, J. J., Hoss, D. E., and Chester, A. J. 1983. Com­ parative feeding of three species of larval fishes in the north­ Bailey, K. M., and Houde, E. D. 1987. Predators and pre­ ern Gulf of Mexico: Brevoortia patronus, Leiostomus dation as a regulatory force during the early life of fishes. xanlhurus, and Micropogonias undulatus. Mar. Ecol. Prog. ICES CM, 1987/Mini No. 2 36 pp. Ser., 13: 189-199. Banse, K., and Mosher, S. 1980. Adult body mass and annual Gjøsæter, J., and Monstad, T. 1973. Fecundity and egg size of production/biomass relationships of field populations. Ecol. spring spawning Barents Sea capelin. FiskDir. Skr. Ser. Monogr., 50: 355-379. HavUnders., 16: 98-104. Berggren. 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