Kritische Zeiten für Fischlarven - Bestimmng der Trypsinaktivität zur Beobachtung des Ernährungszustands von Fischlarven in Labor- und Felduntersuchungen - ein Überblick

The studies which form the background for this synoptic presentation aimed to contribute to the investigation of the reason for great yearly fluctuations in recruitment in marine fish stocks. In order to evaluate if mass mortality due to starvation in the larval stage contributes to these fluctuations, trypsin activity was suggested as an appropriate indicator to assess individually both the nutritional condition and feeding activity of fish larvae of all age stages. Based on the principle of conventional chromogenic methods a highly sensitive fluorometric measurement was developed, which allows the individual assessment of tryptic activity even in the youngest larval stages. In order to evaluate species-specific features of tryptic activity, larvae of six different marine species were reared under laboratory conditions (herring, Clupea harengus, turbot, Scophthalmus maximus, cod, Gadus morhua, sea bass, Dicentrarchus labrax, gilthead seabream, Sparus aurata, and Brazilian sardine, Sardinella brasiliensis). The exemplary application of the laboratory calibration in field surveys clearly demonstrates that the tryptic activity is appropriate to evaluate the nutritional condition of larvae from field samples. Examples for some species are presented (herring, cod and sprat). The most significant results from laboratory experiments: Larval age and larval size were chosen as the standard for comparison to relate to tryptic activity. Tryptic activity was found even in yolk sac stages of the investigated species. Tryptic activity increases with age in continuously fed larvae following a non-linear pattern which is supposed to be determined in the ontogenetic development of each species. Increase in tryptic activity persists until larvae develop a functional stomach. The amount of maximum tryptic activity at a given size and age is species specific: turbot larvae show the highest whereas herring larvae have the lowest values at a given age. The amount of trypsin which has its origin in the ingested food oganism is of minor significance for the measurement. Tryptic acticity reacts very sensitive on variations in food density. Not only fed and starved larvae can be separated but also larvae kept at high and low food concentration. Differences in tryptic activity between fed and starved larvae are statistically significant in the majority of the investigated species, but differences in larvae kept at high and low food densities are not always significant. In yolk sac stages and beyond, while adapting to exogenic food sources, differences in tryptic activity are insignificant in larvae with and without food supply. This stage can last until ten days after hatching, depending from the species. When relying completely on external food sources, differences in tryptic activity occurs within hours when fed and starving larvae are compared. However, significant differences arise only after 1- 3 days, depending on species, size and age. Species with their origin in temperate latitudes shows a slower decrease in tryptic activity compared to species from subtropical or tropical areas. If food deprivation persists, tryptic activity levels off to zero within days; the number of days is depending from the species, age and size. Individual variability of tryptic activity is large in continuously fed larvae, moderate in larvae kept on low food density and disappears in starving larvae. A diurnal rhythm in tryptic activity was identified. In addition, tryptic activity increases as a consequence of active feeding and decreases thereafter within hours to the pre-feeding level; however, those level is always well above the tryptic activity level of starving larvae. Starving larvae show a very weak diurnal oscillation in tryptic activity. 196 Laboratory calibration was used to estimate the number of starving larvae in field samples. For this purpose, tryptic activity was related to the larval size and the values for starving larvae were fitted to a linear regression. The upper 99% prediction limit of the regression analyses fitted to starving larvae was used as threshold value to assess the nutritional condition of field samples. Larval length was chosen as reference value for tryptic activity, since tryptic activity rather depends on the developmental stage (size) than on larval age; further it is easy to measure the length in larvae from field samples and moreover, the length is not much affected when larvae are in a bad condition (compared to larval weight or protein content). Ideally, the species from field surveys should have a calibration background under laboratory conditions; however the application of laboratory results appears to be valid within the same taxa (e.g. Clupeidae). An example is given in this presentation. Results from experiments with turbot larvae indicate that tryptic activity measurements can be used to optimize the food ration and the time of feeding in growing larvae in aquaculture. The exemplary application of the laboratory calibration in field surveys clearly demonstrates, the tryptic activity is appropriate to evaluate the nutritional condition of larvae from field samples. Examples for some species are presented (herring, cod and sprat). The most significant results from field studies: Nutritional condition of herring larvae from two different, but typical spawning locations (Hebrides-Orkney-Shetland in autumn season, English Channel in January) were evaluated and compared. Starving larvae in the range of 0-80% were found for the autumn spawners, however only small larvae around 10mm were concerned; about 27% of the larvae caught in the English Channel were rated as starving larvae, but larvae of all sizes were concerned. The results are in accordance with the usually expected environmental conditions in those areas (food availability and hydrographic conditions). In the context of a a large scale study on sprat recruitment (Sprattus sprattus) in the North Sea (SARP, Sardine-Anchovy-Recruitment-Program), a survey on the condition of sprat larvae throughout the whole spawning season was conducted (sprat are batch spawners). The tryptic activity of sprat larvae were assessed from May to August and compared to the abundance of juvenile sprat in the related area in autumn of the same year. There was a clear positive relation between the periods with larvae in a fair nutritional condition and the number of juvenile sprat and vice versa. Results on the abundance of food availability in the investigated area confirmed these observations. Nutritional condition of cod larvae from field samples from the Baltic were rated in accordance with the laboratory calibration. About 30% of starving larvae of all sizes were calculated. The results show, that not only the youngest larval stages are concerned from weak nutritional conditions. Investigations on the diurnal rhythm of tryptic activity in field samples of sardine larvae showed a clear periodical day and night pattern. Low tryptic activities were measured in samples from afternoon to dusk, whereas high activities were measured in samples from midnight to dawn. In accordance with related laboratory results it can be concluded, that feeding activity is responsible for the clear pattern. Concluding, the results suggest that mortality due to starvation is of significance in marine fish larvae

A comparison of the nutritional condition of herring larvae as determined by two biochemical methods - tryptic enzyme activity and RNA/DNA ratio measurements

Two biochemical methods for measuring larval fish condition – tryptic enzyme activity and RNA/DNA ratio measurement - were applied to laboratory-reared and wild-caught herring larvae. The comparison of both methods when applied to laboratory-reared herring larvae showed that tryptic enzyme activity and RNA/DNA ratio are linear and positively correlated under constant nutritional conditions. Wild-caught larvae were transferred to the laboratory and used to compare both indicators in relation to shortterm changes in food availability and long-term starvation periods (13 days). In the starvation experiments with the wild-caught larvae the lowest trypsin values were obtained after 3–4 days and a significant decrease in RNA/DNA ratios was obtained after 5–6 days. Prolongation of the starvation time did not result in a further significant change in either parameter. The results of the study demonstrate the usefulness of both methods in monitoring nutritional condition offish larvae in field samples

The use of tryptic enzyme activity measurement as a nutritional condition index: Laboratory calibration data and field application

Tryptic enzyme activity of herring (Clupea harengus), turbot (Scophthalmus maximus), and cod (Gadus morhua) larvae kept under defined laboratory conditions was determined. Tryptic enzyme activity was related to larval age, length, days of food deprivation, and feeding time. From 10 days after hatching onwards, significant differences in tryptic enzyme activity appeared when larvae were deprived of food for between 3 and 9 d. Diurnal feeding patterns were monitored by measuring individual tryptic enzyme activity. In short-term feeding, starving and re-feeding experiments, tryptic enzyme activity reflects the digestion processes in relation to food ingestion and re-establishment of tryptic enzyme level within hours of re-feeding. Individual tryptic enzyme activity levels in herring and sprat larvae (Sprattus sprattus) were determined in field samples and compared with laboratory calibration data in order to evaluate the nutritional condition of the field collected larvae from different sampling sites and different seasons. Continuous sampling of sardine larvae (Sardina pilchardus) on an oceanic drift station was used to show diurnal feeding rhythm by applying tryptic enzyme activity as an indicator

Proposal for the establishment of an EAS thematic group for aquaculture education, training and knowledge exchange (AQUA-TNET)

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Comparison of biochemical and histological methods for the evaluation of the in situ nutritional condition of marine fish larvae

To estimate the importance of starvation induced mortality for recruitment of marine fish larvae three distinct methods were applied to determine the nutritional condition of fish larvae in situ. In addition to highly sensitive fluorescence techniques for analysing RNA/DNA ratios and tryptic enzyme activities histological standard methods were used to compare the nutritional status of fish larvae of the genus Vinciguerria (Photichthyidae) caught in two ecologically different areas of the Indian Ocean: In the central Arabian Sea and on the continental shelf of Pakistan. A comparison of the results elaborated by the distinct methods shows a trend towards better nutritional conditions for fish larvae from the offshore region

Larval condition and growth of Sardinella brasiliensis (Steindachner, 1879): preliminary results from laboratory studies

Brazilian sardine, the most important resource along the southeastern Brazilian coast, presented great variations and declines in its stocks. The main factors contributing to this are: oceanographic structure changes; recruitment failures; excessive catches of juveniles and increase in fishery effort. In spite of this, no alterations in the density-dependent parameters were detected. Consequently, methods analysing the condition of the larvae coupled with methods determining growth using sagittae otolith increment width were applied to evaluate growth under experimental conditions. The results of the readings on the sagittae were compared with the age of the laboratory-reared sardine larvae and confirmed that increments are formed on a daily basis. Under poor feeding conditions, sardine larvae showed a low growth expressed by dry weight, RNA/DNA ratio and tryptic enzyme activity and by the narrow and low contrast increments in the otoliths. The results of the biochemical indices showed an unexpected decline in the feeding group coupled with a decrease in width of increment numbers 8 and 10. Other factors than food availability were affecting the condition of the larvae and might be indicative of physiological processes and ontogenetic changes occurring in sardine larvae

Nahrungs- und Futtermittel in der Fischaufzucht

Die Erfindung betrifft ein Nahrungs- oder Futtermittel mit einer Beimengung von Phytohaemagglutinin und/oder von wenigstens einer Isoform einer Phytohaemagglutinin-Untereinheit, insbesondere die Verwendung von Phytohaemagglutinin als Fischfutterzusatz in kommerziellen Brutfuttern zur Unterstützung der Reifung des Verdauungstraktes und damit zur Steigerung der larvalen Verdauungseffizienz. Des Weiteren betrifft sie die Verwendung von Phytohaemagglutinin zur Einsparung von Lebendfutter in der Fischzucht