Description: 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.

Description: In developmental studies of fish species (especially physostomians) it could be demonstrated, that the lack of haemoglobin during larval and juvenile stages is a relatively common phenomenon. Generally it is linked with body translucency. In representatives of the families Galaxiidae, Osmeridae and Clupeidae, partly reared, partly observed immediately after being caught in the wild, it turned out, that this condition coincides with a considerable delay in swimbladder inflation. To determine the moment of its first inflation, larvae placed in a hermetic chamber were observed under a dissecting microscope. While lowering the pressure, the expanding swimbladder showed whether or not its content is really gaseous. The reason postulated to be responsible for the delayed inflation is that larvae lacking haemoglobin do not have the possibility of oxygen transport to their buoyancy organ by means of the blood. Apart of this, capillarity force calculations and body force estimations show that with decreasing size the constraints linked with surface tension increase overproportionally. While in larger sized larvae like trout we could demonstrate inflation by swallowing air, in species with small larvae this was not the case. Below a certain size, even in physostomians, the ductus pneumaticus is no alternative to the blood pathway for swimbladder inflation.