RNA:DNA ratios and growth of herring (Clupea harengus) larvae reared in mesocosms

Autumn-spawned North Sea herring larvae (Clupea harengus L.) were released in two outdoor mesocosms of 2500 m³ (A) and 4000 m³ (B). The mesocosms were monitored for temperature, salinity, oxygen, chlorophyll a, zooplankton and herring larvae abundance. The density of suitable prey for first feeding larvae (mainly copepod nauplii) was initially low in Mesocosm A (<0.11^-1) compared to in Mesocosm B (>11^-1). Half-way through the experiment the situation was reversed, with higher densities of prey in Mesocosm A (>31^-1) as compared to Mesocosm B (～11^-1). The average temperature declined steadily in both mesocosms from 18°C at release to 11–12°C by the end of the experiment 60 d later. The RNA:DNA values of individual herring larvae were related to protein growth rates and temperature adjusted according to Buckley (1984). A corresponding DNA growth index (Gdi) was given as: Gdi=0.68 TEMP+3.05 RNA:DNA-9.92. The RNA:DNA based growth indices were significantly correlated with other somatic growth estimates. The average estimated protein growth rate in the two mesocosms followed the same temporal pattern as the somatic growth rate, but with a lag of 2 d or more. Residual analysis of the regression of ln RNA versus ln DNA also showed the same temporal pattern as the RNA:DNA ratios, but the shift in condition as estimated by this method occurred more in synchrony with the other somatic growth measures. Larvae in Mesocosm A had RNA:DNA values similar to the starvation control kept in the laboratory the first days after release, confirming that larvae in Mesocosm A initially were in poor nutritional condition. On the other hand, the majority of the herring from Mesocosm B were characterised as starving or in poor nutritional condition towards the end of the experiment. The assessment of growth and nutritional condition were in accordance with independent survival estimates which suggested that the majority of the total mortality occurred during the first 15 d in Mesocosm A and there-after in Mesocosm B.     

The effect of food availability,age or size on the RNA/DNA ratio of individually measured herring larvae: laboratory calibration

RNA/DNA ratios in individual herring (Clupea harengus) larvae (collected from Kiel Bay, Baltic Sea, in 1989) were measured and proved suitable for determining nutritional status. Significant differences between fed and starving larvae appeared after 3 to 4 d of food deprivation in larvae older than 10 d after hatching. The RNA/DNA ratio showed an increase with age or length of the larvae and was less pronounced in starving larvae compared to fed larvae. The individual variability of RNA/DNA ratios in relation to larval length of fed larvae and of larvae deprived of food for intervals of 6 to 9 d is presented. Based on the length dependency and the individual variability found within the RNA/DNA ratios, a laboratory calibration is given to determine whether a larva caught in the field has been starving or not. An example for a field application is shown.     

Effect of the receding ice-edge on the condition of mid-water fishes in the northwestern Weddell Sea: results from biochemical assays with notes on diet

Electrona antarctica and Bathylagus antarcticus are dominant fishes in the Southern Ocean pelagic ecosystem with disparate life histories, making them excellent subjects for comparative studies. Twenty-one indices of nutritional condition were compared in both species by observing changes in each index as it varied spatially and temporally in association with the marginal ice-zone bloom in the northwestern Weddell Sea. E. antarctica and B. antarcticus exhibited very different patterns of response to the bloom. Nutritional condition increased in 11 of 21 measures in E. antarctica, suggesting that it was in poorer condition at ice-covered stations and in greatly improved condition in post-bloom, open-water stations. The data, combined with a few observations of larvae, indicate that E. antarctica increases feeding before the spring bloom, rapidly building stores of lipid, which are probably used for spawning. In contrast, only three measures of condition were variable in B. antarcticus, and lipid actually decreased from ice-covered to open water. RNA:DNA values increased in open water, coinciding with an increase in food volume present in their guts. These observations, coupled with observations of larvae in ice-covered stations, suggest that B. antarcticus possesses sufficient energetic stores to spawn prior to the bloom, so that larvae are able to fully utilize available resources associated with the spring bloom. Increased somatic growth, as indicated by RNA:DNA, may not begin until later, when the bloom was beginning to decline. Earlier work, which suggested that the deeper living, non-migrating species, like B. antarcticus, would not be affected by the bloom until after shallower, migratory species, like E. antarctica, is supported. Of the assays tested, RNA:DNA appeared to be the most sensitive. The combined assessment of many measures including biochemical, compositional, dietary, and age estimates from single specimens is possible if the samples are treated with sufficient care. The reliance upon a single measure to help interpret the ecology of a species, especially in nekton-sized species, is not as effective as techniques used in combination. Received: 12 January 2000 / Accepted: 16 August 2000     

Diet of 0-group stages of capelin (<Emphasis Type="Italic">Mallotus villosus</Emphasis>), herring (<Emphasis Type="Italic">Clupea harengus</Emphasis>) and cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>) during spring and summer in the Barents Sea

Recruitment of capelin in the Barents Sea fail when juvenile herring and cod are abundant and the potential for feeding competition of wild sympatric capelin and herring larvae and small cod juveniles were investigated. The frequency of gut evacuation after capture of capelin larvae were also studied in mesocosms. Small capelin larvae (<35 mm length) fed on small prey including phytoplankton, invertebrate eggs and nauplii, bivalves, other invertebrate larvae and small copepods. Calanus copepodites were only observed in large capelin larvae (>26 mm length). Calanus copepodites were the major food sources for contemporary herring larvae (25–35 mm length) and Calanus and euphausiids were the major prey for small juvenile herring (37–60 mm length) and cod (18–40 mm length). Capelin larvae reared in mesocosms evacuated the guts shortly after capture. Capelin larvae had a smaller mouth and fed on smaller prey than herring and cod of the same length. This implies that the small capelin larvae, in contrast to sympatric small herring and cod, are not tightly linked to the food chain involving Calanus and euphausiids. Thus, exploitative competition between capelin larvae and planktivorous fish that rely on Calanus and euphausiids in the Barents Sea may be relaxed.     

Impacts of copepods on marine seston,and resulting effects on<Emphasis Type="Italic"> Calanus finmarchicus</Emphasis> RNA:DNA ratios in mesocosm experiments

We investigated the impact of copepods on the seston community in a mesocosm set-up, and assessed how the changes in food quantity, quality and size affected the condition of the grazers, by measuring the RNA:DNA ratios in different developmental stages of Calanus finmarchicus. Manipulated copepod densities did not affect the particulate carbon concentration in the mesocosms. On the other hand, chlorophyll a content increased with higher copepod densities, and increasing densities had a positive effect on seston food quality in the mesocosms, measured as C:N ratios and 3:6 fatty acid ratios. These food quality indicators were significantly correlated to the nutritional status of C. finmarchicus. In contrast to our expectations, these results suggest a lower copepod growth potential on higher quality food. However, in concordance with earlier studies, we found that when copepods were in high densities the large particles (>1000 µm³) decreased and that the smaller particles (<1000 µm³) increased in number. These patterns were closely linked to the condition of C. finmarchicus, which were of better condition (RNA:DNA ratios) with increasing biovolumes of large particles, and, conversely, lower RNA:DNA ratios with increasing biovolumes of smaller particles. Consequentially, the selective grazing by copepods stimulated increased biovolumes of smaller plankton, and this increase was responsible for the increased food quality, in terms of C:N and 3:6 ratios. Thus, we conclude that the decreasing growth potentials of C. finmarchicus were a result of a decrease of favourably sized food particles, induced by copepod grazing.Communicated by O. Kinne, Oldendorf/Luhe     

Essential fatty acid (docosahexaenoic acid, DHA) availability affects growth of larval herring in the field

Larval fish growth and survival depends not only on prey quantity, but also on prey quality. To investigate effects of prey fatty acid concentration on larval herring growth, we collected different prey organisms and larval herring (Clupea harengus L.) in the Kiel Canal during the spring season of 2009. Along with biotic background data, we analysed fatty acids both in prey organisms and in the larvae and used biochemically derived growth rates of the larvae as the response variable. Larval herring reached their highest RNA/DNA derived growth rates only at high docosahexaenoic acid (DHA) concentration. When the ratio of copepodids to lesser quality cirriped nauplii was low, larval growth and larval DHA concentration were both significantly negatively affected. This was true even as prey abundance was increasing. This finding indicates that even in mixed, natural feeding conditions, growth variations are associated with DHA availability in larval fish.     

Relation of RNA/DNA ratios to growth for the scallop Euvola (Pecten) ziczac in suspended culture

We examined the relation of RNA/DNA ratios to growth for three size groups of the tropical scallop Euvola ziczac maintained in suspended culture at 8, 21 and 34 m in depth in the Golfo de Cariaco, Venezuel. Various growth parameters indicated that production decreased with depth. This was more likely due to a decrease in seston quality with depth than to temperatures or seston abundance (which were similar at the various depths studied). The RNA/DNA ratio was correlated with the G-index of muscle growth for juveniles (r ²=0.55). A much weaker correlation was observed for the maturing scallops (r ²=0.18), probably because of the interaction between reproductive and somatic growth. In fully mature scallops, somatic growth was negligible and the RNA/DNA ratios appeared to be inversely related to the level of physiological stress of the scallops. Whereas RNA/DNA ratios are difficult to interpret for maturing E. ziczac, because an increased ratio can be due to either increased gonadal or somatic growth, they are useful in predicting growth in juveniles and physiological stress in fully mature scallops.     

Otolith ring deposition in relation to growth rate in herring (Clupea harengus) and turbot (Scophthalmus maximus) larvae

Larvae of Clyde spring-spawning Clupea harengus L. and hatchery-produced Scophthalmus maximus (L.) were reared from hatching through metamorphosis in 1980 and 1981 in laboratory tanks and in large enclosures under various light, temperature, and feeding regimes in order to study otolith ring deposition and growth under different conditions. Ring deposition and growth rates were significantly affected by rearing conditions in both species. The ring deposition rates observed under the conditions tested ranged from 0.34 to 0.92 rings d^-1 in herring larvae, and from 0.07 to 1.0 rings d^-1 in turbot larvae. Growth rates ranged from 0.11 to 0.42 mm d^-1 in herring and from 0.05 to 0.27 mm d^-1 in turbot. The number of otolith rings was dependent on the growth rate of the individual larva. At the population level, higher ring deposition rates were observed in faster growing populations. In herring larvae, the relationship between average growth rate and average ring deposition rate was logarthmic, reaching an asymptote at 1 ring d^-1 for growth rates approaching 0.40 mm d^-1. The relationship was linear for turbot larvae for the range of growth rates observed.     

Morphological and histological changes during the growth and starvation of herring and plaice larvae

Reared herring (Clupea harengus L.) and plaice (Pleuronectes platessa L.) were examined for morphological and histological changes during growth and starvation. The growth rate of herring larvae of 0.22 mm/day was less than that reported for wild stock, but this difference was attributed to survival of runts in laboratory. Larval plaice had a growth rate of 0.16 mm/day. The relative condition factor (antilogarithm of intercept of length-weight line) was used to assess condition throughout the larval stages. Starvation resulted in a progressive collapse of the larval body, especially of the ventral body surface around the pectoral girdle of both species (assessed by the pectoral angle) and of the spacing between the organs of the head in herring. There was a breakdown of the herring gut with decreases in epithelial cell height and catabolism of the connective tissue coat and a marked reduction in the transverse sectional area of the plaice liver. The changes in the pectoral angle in both herring and plaice and the eye height to head height ratio in herring should be useful to fishery biologists for assessing nutritional condition, even on board ship.     

Measuring RNA:DNA ratios in individual <Emphasis Type="Italic">Acartia tonsa</Emphasis> (Copepoda)

Acartia tonsa Dana is a dominant copepod in coastal waters and is therefore an important link in the food web between microplankton and higher trophic levels. RNA:DNA ratios have been used to describe growth and nutritional condition of field-collected copepods and to show strong correlation between RNA:DNA ratios and group egg production (EP). A method was developed using a sensitive, nucleic acid-specific fluorescent dye, and automated microplate fluorometer to measure DNA, RNA, and the RNA:DNA ratio of individual A. tonsa. DNA, RNA, and RNA:DNA ratios and EP were all significantly higher in copepods fed Thalassiosira sp. compared to starved copepods. There was a general trend toward an increase in RNA:DNA ratios with increase in EP, but due to the high degree of variation in both RNA:DNA ratios and EP among individual copepods there was no significant correlation between RNA:DNA ratios and EP. Significant differences in RNA:DNA ratios between fed (7.2) and starved (3.3) copepods were found after 2 days. This assay may be applied to other species of copepods sampled in the field to provide an index of the health of planktonic food webs.     

Feeding ecology of herring (Clupea harengus) larvae in the turbid Blackwater Estuary

Most studies on feeding by herring larvae (Clupea harengus) have taken place in clear, open waters, but several herring stocks around the world spawn in inshore and estuarine regions. An example is the spring-spawning Blackwater Estuary (Essex, England) stock. Samples were collected in this estuary to examine prey selectivity and feeding levels in relation to biological and environmental conditions. Herring larvae negatively selected copepod nauplii, but positively selected the copepodite and adult stages of Acartia spp. Gastropod larvae were also positively selected. Particles >150 μm width were preferred, whilst particles smaller than this value were preferentially rejected. Concentrations of potential prey items in the water were in the range of 6.0 to 49.7 organisms l⁻¹ with a median concentration of 15.0 organisms l⁻¹ (n = 26). These values are towards the low end of prey concentrations quoted in the literature as being required to sustain herring larval growth and survival. However, theoretical considerations suggest that, in this environment, levels of tidally-induced turbulence enhance encounter rates between larval herring and their prey. On the other hand, turbidity is also related to tidal current speed and might reduce feeding success by decreasing underwater light levels. Measurements at two sites in the estuary confirmed that tidally-induced turbidity reduced the effective water depth in which herring larvae could visually feed by up to 50% at times of peak current speed. However, with the gut-content data available in the present study, it was not possible to discern any clear relationships between feeding success and the state of the tide. Feeding success appeared to be more strongly influenced by surface light-levels. Received: 24 June 1998 / Accepted: 17 February 1999     

RNA:DNA ratio during the critical period and early larval growth of the red drum Sciaenops ocellatus

Eggs from two separate spawning stocks of the red drum Sciaenops ocellatus (Linnaeus) were hatched, and the larvae were reared in the laboratory for 2 wk under closely controlled conditions. Total RNA, DNA, and soluble protein were measured in each population daily in triplicate pooled samples of larvae from each of three tanks. Growth rate in mm d^-1 was determined for each population at 2 d intervals. Growth rate explained 72 and 95% of the variation in the RNA:DNA ratios of the two populations individually, and 86% of the variation in the RNA:DNA ratio when data from the two populations were combined. The RNA:DNA ratio appeared to be most effective as an indicator of growth in rapidly growing larvae, and to lose some resolution when growth was intermittent. The rates of deposition of RNA, DNA, and protein into tissue were all highly correlated with growth rate and with each other. Mean population RNA:DNA ratios of red drum yolk-sac larvae decreased from Day 1 post-hatch until larvae initiated successful feeding behavior, and then increased steadily throghout the remainder of the experimental period. This pattern of change in the RNA:DNA ratios during the yolk-sac stage appears to be an intrinsic developmental pattern of red drum ontogeny. The lowest values for the RNA:DNA ratio were observed just prior to the initiation of feeding or during the critical period, indicating that red drum larvae experience a decrease in capacity for protein synthesis as they initiate feeding. Intrinsic variation in the RNA:DNA ratio during development suggests that caution be used when comparing the RNA:DNA ratios of yolk-sac larvae to a critical ratio calculated from Buckley's general model.     

Assessment of the nutritional condition of larval and early juvenile tuna and Spanish mackerel (Pisces: Scombridae) in the Panamá Bight

The nutritional condition of first-feeding and late larval/early juvenile scombrids was investigated in waters of the northwestern Panamá Bight from May through early November 1988. Wild-caught larvae and juveniles of three taxa, black skipjack tuna (Euthynnus lineatus), bullet and/or frigate tuna (Auxis spp.) and sierra (Scomberomorus sierra), were examined histologically to determine nutritional condition. The incidence of malnourishment in wild-caught preflexion (first feeding—prior to notochord flexion) larvae of all taxa was high. Starvation rates for E. lineatus and Auxis spp. preflexion larvae ranged from 62 to 63% d^-1, while the percentage of larvae actually dying of starvation was estimated at 41 to 43% d^-1. The nutritional point-of-no-return for preflexion larvae was estimated at 1 to 2 d maximum. The cellular condition of liver hepatocytes, particularly the relative amount of vacuolation related to storage of glycogen and lipid, proved to be a sensitive indicator of nutritional condition. In laboratory trials, late larval (postflexion) and early juvenile black skipjack exhibited a nutritional point-of-no-return of 2 to 3 d. Although postflexion larvae were moderately vulnerable to malnourishment in laboratory trials, <13% of wild-caught postflexion larvae exhibited even mild nutritional stress, and no postflexion larvae or juveniles showed signs of severe malnourishment. This pattern of starvation incidence suggests that tropical scombrids undergo stagespecific starvation mortality. Preflexion larvae can suffer significant daily losses due to starvation, while postflexion larvae and early juveniles seem to experience a rapid improvement in feeding ability and/or food availability.     

Condition of larval and juvenile red drum (Sciaenops ocellatus ) from estuarine nursery habitats

RNA:DNA ratios of larval and juvenile red drum (Sciaenops ocellatus) collected from nursery habitats in the Aransas Estuary, Texas, in 1994 were quantified using a highly sensitive ethidium-bromide fluorometric technique. RNA:DNA ratios of wild red drum were evaluated by comparing individual values to a linear regression model derived for starved laboratory-reared red drum. Wild red drum were in relatively good condition with <5% of the RNA:DNA ratios within or below the 95% prediction interval of 4 to 5 d starved red drum. A multiple-regression model explained 54% of the variability in the RNA:DNA ratio of wild red drum, and identified length and water temperature (midday) as significant factors. RNA:DNA ratios increased with fish length [≃1.1 mm⁻¹, over the size range investigated (5␣to 20 mm)]. The effect of temperature on the RNA: DNA ratio was assessed on different sampling trips, and ratios increased with increasing temperature. Abundance of larval and juvenile red drum in the Aransas Estuary varied as a function of both habitat (shoal grass Halodule wrightii, turtle grass Thalassia testudinum) and site (Aransas Bay, Redfish Bay); however, no differences in RNA:DNA ratios were detected between habitats or between sites. It is postulated that the nutritional condition of newly settled red drum from the Aransas Estuary in 1994 was relatively high, and that starvation was of minor importance. Received: 19 August 1996 / Accepted: 23 August 1996     

The use of RNA:DNA ratios to predict growth limitation of juvenile summer flounder (Paralichthys dentatus) from Delaware and North Carolina estuaries

Nutritional indices were used to develop biochemical correlates of feeding and growth rates for juvenile summer flounder, Paralichthys dentatus (Linnaeus), from North Carolina (NC) and Delaware (DE). Six parameters (Fulton's condition K=10⁴xweight/(length³), wet weight/dry weight, [protein], [RNA], [DNA], and RNA:DNA) were related to feeding and growth rates of fish from previously reported 10 to 14-d experiments at temperatures ranging from 2 to 20 °C with varying feeding levels (0 to 100% and libitum). RNA:DNA ratios were the best predictors of growth rates, but inclusion of a temperature term improved the relationship between RNA:DNA ratios and growth rate for Delaware fish. Feeding rates were poorly correlated with all parameters. RNA:DNA ratios of fish in the laboratory changed significantly within 1 d of starvation and refeeding at 16 °C. RNA:DNA of juvenile summer flounder collected from one site in Indian River Bay, DE and two sites in the Newport River Estuary, NC, between January and June 1992 were used to estimate in situ growth rates following settlement. Predicted growth rates in both estuaries were close to maximum (suggesting ad libitum feeding) until early May. Growth rates of juveniles from Delaware were <0% d^-1 from December through early March, and were higher (0.6 to 3% d^-1) from April through early June. However, growth rates of DE juveniles during May were <50% of maxinum. North Carolina juveniles had growth rates of 2 to 5% d^-1 from February through early April. Juveniles from one of the Newport River sites (a marsh habitat) were also severely growth limited (<20% of maximum) after April. Prolonged periods of sub-optimal growth may be important to survival and recruitment of juvenile summer flounder in northern mid-Atlantic estuaries. A model is presented which illustrates the potential impact that small changes in temperature and growth limitation can have on recruitment success in both delaware and North Carolina estuaries.     

Fluctuating asymmetry and nutritional condition of Baltic cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>) larvae

The level of fluctuating asymmetry (FA), defined as random deviations from perfect bilateral symmetry, is assumed to reflect the developmental instability (DI) of an organism. Because environmental and genetic stress may increase DI, FA has been used to assess the level of stress experienced by, for example, fish. In this study, left–right asymmetry of lapillar otoliths was related to nutritional condition as estimated from RNA/DNA ratios, in order to investigate the utility of FA to detect feeding-related stress in Baltic cod, Gadus morhua L., larvae. Cod larvae in intermediate and good nutritional condition showed similar values of FA, and these were more symmetric than for larvae in poor condition. As increased levels of FA were restricted to larvae in a condition comparable to that of larvae experiencing at least 3 days of starvation in laboratory experiments, it is suggested that FA is an insensitive indicator of short-term feeding success of larval cod. However, FA can be used to reveal severely starved larva populations and probably also populations that have been subjected to prolonged sub-optimal feeding conditions.     

RNA/DNA ratio and expression of 18S ribosomal RNA, actin and myosin heavy chain messenger RNAs in starved and fed larval Atlantic cod (Gadus morhua)

Levels of total RNA, total DNA, 18S ribosomal RNA (rRNA), poly(A) messenger RNA (mRNA), and two mRNAs coding for abundant myofibrillar proteins were estimated in laboratory-reared Atlantic cod larvae (Gadusmorhua Linnaeus) under conditions of feeding and starvation. DNA probes specific for cod 18S rRNA, β-actin mRNA and myosin heavy chain mRNA were developed. In two experiments on newly hatched larvae in fed and starved treatments, changes in 18S rRNA and mRNA were similar to changes in total RNA during the first weeks after hatching. RNA levels in fed and starved larvae in both experiments were stable, or increased, over the first 3 d after hatching, and then decreased to minima at 9 d. RNA levels increased after 9 d, with the degree and timing of the increase varying among the individual classes of RNA. Complete mortality of starved larvae in both experiments was observed shortly after 11 d, corresponding to exhaustion of endogenous yolk reserves. Total RNA content, RNA/DNA ratio, 18S rRNA levels, total mRNA pool, and actin and myosin heavy chain mRNA levels showed significant differences in fed and starved first-feeding larvae after yolk exhaustion. In another experiment with 3- to 4-week-old cod larvae, 18S rRNA levels were significantly lower in starved versus fed larvae after 3 d. Total RNA responded to feeding and starvation within a similar time as 18S rRNA and the mRNAs examined. Analysis of bulk nucleic acids using fluorometric dyes was simpler and faster than analysis of individual RNAs using hybridization probes, and provides valuable information on recent growth and condition of individual larvae. However, analysis of specific RNAs can provide information on expression of the corresponding genes and reveal the changes underlying trends seen in bulk RNA. Received: 9 February 1996 / Accepted: 7 June 1999     

Nitrogen balance in marine fish larvae: influence of developmental stage and prey density

The utilization and fate of nitrogen in larvae of plaice (Pleuronectes platessa), blenny (Blennius pavo) and herring (Clupea harengus), from the stage of first-feeding to metamorphosis, was examined under laboratory conditions. Rates of ammonia excretion, primary amine defaecation, and growth in terms of protein-nitrogen were monitored throughout larval life. Data were used to calculate daily ration, the coefficient of nitrogen utilization (absorption efficiency), and gross and net growth efficiencies. The developmental pattern of nitrogen balance was similar for plaice and blenny larvae. These species showed increasing growth efficiency (k₁: 55 to 80%) with decreasing weight-specific waste nitrogen losses with age. Absorption efficiencies. were high (83 to 98%) in plaice and blenny larvae, and tended to increase with development in the former species. Ration relative to body weight decreased with growth in both species. Herring larval development, although at a slower rate than blenny and plaice, appeared normal up to 33 d, after which high mortality occurred. Absorption efficiency in this species tended to decline (83 to 43%) with age, until metabolic costs exceeded the absorbed ration and growth ceased. Artemia sp. nauplii proved a suitable food source for the rearing of plaice and blenny larvae, but this diet may have long-term toxicity or deficiency effects on herring. Availability and density of food affected nitrogen balance in the larvae of all three species. Feeding stimulated the output of wastes in excretion and defaecation by a factor of up to ten times the 12-h non-feeding basal rates. Waste nitrogen output reached a peak some 2 to 3 h after commencement of feeding and returned slowly to the baseline in 5 to 10 h after cessation of feeding. There was an asymptotic increase in ration, ammonia output and growth of larvae as prey density increased. Ration saturated at a higher prey density (>4 prey ml^-1) than either growth or excretion rate (1 prey ml^-1). Thus the efficiency with which food is absorbed and utilized for growth must eventually decline in response to high prey density. The idea that larval fish are adapted to maximize ingestion and growth rate, rather than optimize growth efficiency and thus to respond to prey occurring in either low density or in occasional patches, is supported by these results.     

Temporal and spatial variation in the growth rates of Baltic herring (<Emphasis Type="Italic">Clupea harengus membras</Emphasis> L.) larvae during summer

It has been suggested that larval survival determines the year-class strength in most marine fish species. During their growth and development, the ability of the larvae to catch prey and avoid predation will increase. However, the factors affecting short-term changes in the growth of Baltic Sea herring have been little studied in the field. We collected Baltic herring (Clupea harengus membras L.) larvae from five different towing areas in the Archipelago Sea (SW Finland) during May and June 1989, right after the main spawning season. Twenty thousand two hundred and ten larvae were analysed and the area-specific growth rate (i.e. increase in standard length) was estimated by tracing the larval cohorts from the length-frequency data. This represents the first Baltic herring study with daily sampling during a long study period. The growth rate was related to environmental factors, such as temperature, number of zooplankters, and wind speed and direction. Large variation in larval growth rate occurred between areas: lowest and highest growth rates were 0.18 and 0.52 mm·day^-1. Temperature was an important variable controlling larval-fish growth rate. An increase of one 1°C in average water temperature corresponded to an increase in growth rate of 0.043 mm·day^-1. This may have been caused either by a direct temperature effect (changes in metabolic rate) or by the indirect effect of changes in food availability. We also found the densest herring populations in the areas with highest average water temperature. However, temperature and larval growth rate both increased towards the inner archipelago.     

Food availability and physiological state of sea urchin larvae (<Emphasis Type="Italic">Strongylocentrotus purpuratus</Emphasis>)

Food availability is highly variable in the ocean. Many species of marine invertebrates have a larval form that depends upon exogenous nutrients for growth, yet there are few biochemical and physiological indices for determining changes in the nutritional status of larvae. In this study, the effects of food availability on biochemical compositions and metabolic processes of larvae of the sea urchin, Strongylocentrotus purpuratus, were determined. Larvae were cultured under different food concentrations (fed-to-excess and unfed) and a suite of biological processes assayed, ranging from measurements at the level of the whole organism to that of specific molecules. These data were normalized to DNA content (an index of cell number) to allow comparisons of physiological rates in larvae of different sizes. Changes in the following were measured during larval growth: free amino acid pool, protein, lipid classes (cholesterol, free fatty acids, hydrocarbons, phospholipids, triacylglycerol), enzyme activities (Na⁺, K⁺-ATPase and citrate synthase), and respiration rates. In growing larvae, the two key components that showed differential cell-specific content relative to unfed larvae were glycine in the free amino acid pool and phospholipids. Additionally, several lipid classes were detectable only in fed larvae (cholesterols, free fatty acids, and hydrocarbons). While triacylglycerols were present in eggs and utilized during pre-feeding development, they were not re-accumulated at detectable levels in feeding larvae. Respiration rates, protein content, and enzyme activities were all similar on a cell-specific basis, showing that these variables did not provide useful indices of differences in physiological state between fed and unfed larvae. In contrast, measurements of the cell-specific content of glycine and certain lipid classes did provide useful indices of physiological state of larvae. Application of these indices could potentially allow for determinations of nutritional state of larvae in the ocean.