Uptake of radiolabelled glycine has been used in the studies of growth of adult fish (Goolish and Adelman 1983; Busacker and Adelman 1987), but not in studies of fish juveniles, as far as we know. According to our results, a major problem in using this method is the sampling of scales as this clearly affected the growth values obtained with herring juveniles. As it was not known, which scales best represent the growth process of juvenile Baltic herring, the scales were removed from the area, which has been used in the studies of age and growth in the herring previously. Eklund (1999) determined the age of the Baltic herring from scales and otoliths and found that in young fish (age 2–4 years) the otolith age and scale age are equal, if scales are removed from the upper caudal part of the fish. However, in our experiment this did not guarantee that all scales sampled were suitable for the growth analysis.
In juvenile fish, the scales are thin and small and the counts of radioactivity probably too low to be detected reliably, if at all, in single scales. For this reason, we increased the total area of the scales tenfold, by combining a set of ten scales from each fish into a sample. This procedure was clearly necessary for the detection of radioactivity, but it brought about another problem: how to select the scales, when there is some variation in size even among the scales of an individual fish?
The scale sampling method M2, i.e., choosing scales of uniform size (Busacker and Adelman 1987), resulted in a significantly smaller scale size than the random sampling used in method M1. The obvious reason was that small scales were more numerous than big ones and they were selected because of their higher frequency. Small scale size, in turn, may have been a result of degeneration of the scales (e.g., Tesch 1971), which takes place in given physiological conditions. Scales are rich in calcium, which is needed especially at times of active growth. If necessary, fish can absorb the scale material, use it for the formation of bones and other body structures, and rebuild the scales later, when the conditions have changed. So, in addition that scales protect the fish body, they form a calcium reservoir, which fish can utilise according to their physiological condition (Kapoor and Khanna 2004). Most likely, scales are not a stable structure particularly in fish juveniles, which grow fast but have no large depots of calcium required in the growth. The dynamic character of scales should thus be taken into account, when collecting them for the growth analysis of fish juveniles. According to our results, ≥10 scales collected in random and combined into a sample diminishes this problem, as the probability of getting growing scales increases in this way.
The size of the scales obtained using method M1 was equal in different salinity groups (Fig. 2a), but the uptake of 14C-glycine was significantly more active in 8 and 12 psu than in the two other salinities (Fig. 2b). However, also in these (5.7 and 15 psu) incorporation of radioactive glycine was increased compared with the control scales, indicating that some growth took place in these salinities as well. To conclude, measurement of the growth rate using the present method seems appropriate in the studies of fish juveniles, as long as the problems in sampling of scales are taken into account.
Salinity affected significantly also the accumulation of fat in the body cavity during the experiment. At the end of it, fish in the highest salinities (12 and 15 psu) had significantly more mesenteric fat than fish in lower salinities (5.7 and 8 psu). There were differences among individual fish in all salinities except 12 psu, where all fish had rich fat deposits and their physical condition was significantly better than in other salinities. The differences in the fat reserves and fish condition may indicate an optimum of feed conversion rate (Jarvis et al. 2001) in salinity of 12 psu or close to it. This could be linked to the lipid metabolism, which can be altered by salinity through enzymatic activity (Cordier et al. 2002).
Sensitivity to handling stress causes high mortality in herring kept in captivity (Blaxter and Holliday 1963). This is the obvious reason why no long-term experiments have been done with adult or juvenile Baltic herring previously. In our study, the expected high stress-induced mortality, together with the limited number of experimental fish, prohibited the use of replicate experimental tanks. Also, the study had to be interrupted because of fish mortality, which occurred in all test tanks irrespective of the salinity. In the first place, mortality indicates the low tolerance of the herring of life in captivity. In fish kept in non-optimal salinity conditions, the reason for high mortality is usually the difficulty of maintaining ionic balance, which takes place mainly through the gill epithelium (Evans 1980; 1993). That fish mortality was lowest in salinity of 15 psu in our experiment suggests that the optimum salinity of osmoregulation in the Baltic herring is higher than the salinity where fish live in nature. Interestingly, Holliday and Blaxter (1961) found that Atlantic herring, which lives in oceanic salinities (>30 psu) recovered from the handling stress and loss of scales best in salinity of <15.8 psu. In this salinity, sea water was isotonic with the herring blood (Holliday and Blaxter 1961). It is not known, however, whether the same value applies to the Baltic herring, too.
Fish growth is controlled by several external factors of which temperature and food are traditionally considered as the major ones (e.g., Weatherley and Rogers 1978). Also oxygen can influence the growth of fish, and probably the scale growth as well (Ottaway and Simkiss 1977). However, hypoxia usually becomes visible as a change of fish behaviour; i.e., fish loose their activity, stay close to the water surface and even tend to breathe air (Kramer 1987). Such effects were not observed in our experiments, although the condition of fish was checked daily. Moreover, the oxygen saturation level in the test tanks was regularly controlled and it proved to be high in all tanks through the experiment. With exception of salinity, the environmental conditions and treatment of fish were similar in all tanks and consequently, the observed differences in mortality, accumulation of fat, fish condition and growth rate were most likely due to the salinity.
In the Archipelago Sea, herring are known to feed still in December (Rajasilta 1992) so that the experimental conditions matched relatively well the feeding cycle of the herring in their natural environment. Somatic growth of herring, instead, takes place mainly in summer and autumn, so that the time of the experiment was not optimal with regard to herring growth in the sea. Additional light with a photoperiod of 12 h light: 12 h dark apparently corrected this problem sufficiently, as fish were feeding and some growth took place in all salinities, but water temperature (+6°C) was obviously too low for rapid growth. The mean length and weight of fish increased with increasing salinity (Fig. 4), but as fish size was not determined in the beginning of the experiment, in order to avoid stress-induced mortality, the potential differences in growth cannot be asserted using length and weight data.
Many studies have shown that salinity affects euryhaline fish in multiple ways; e.g., their behaviour, intake of food, food conversion rate, somatic condition, fat reserves, growth and survival (e.g., Peterson-Curtis 1997; Boeuf et al. 1999; Peterson et al. 1999; Atwood et al. 2001; Jarvis et al. 2001; Cordier et al. 2002). Euryhaline species usually tolerate a wide range of salinities as adults, but they have a given optimum, where they perform best growth. In clupeids, the effect of salinity on growth has been studied with juvenile Atlantic menhaden, Brevoortia tyrannus. In experimental conditions, this species achieved better growth in salinity of 5–10 ppt than in 28–34 ppt, where adult live (Hettler 1976). In marine turbot, Scophthalmus maximus, brackish water (8–20 psu) enhanced somatic growth (Boeuf et al. 1999), whereas in juvenile sturgeon, Acipenser brevirostrum, the growth rate was highest in fresh water.
Fish growth is a complicated physiological process, where several hormones are involved (Holloway and Leatherland 1998; Mommsen 2001). It is possible that salinity affects directly on growth, by stimulating the endocrinological system, which regulates the production of growth hormone and other growth promoting substances such as the insulin-like growth factors. These, in turn, can have an effect on the osmoregulatory capacity of fish (Seidelin et al. 1999; Inoue et al. 2003). Growth hormone, for instance, increases the tolerance for non-optimal salinites in fish (Sakamoto and McCormick 2006). Alternatively, faster growth can be a result of decreased costs of osmoregulation (Hettler 1976; Imsland et al. 2003). For instance in turbot, growth was improved when fish were reared in water that was isoosmotic to fish blood (Gaumet et al. 1995; Imsland et al. 2003). In our study, a direct effect of salinity on growth was obvious, although this effect may well have been strengthened by lower costs of osmoregulation in the higher salinities.
In estuarine nursery grounds, where salinity may vary considerably during the growth season, fish juveniles seek for such salinities that enhance their growth (Lankford and Targett 1994). In most of the Baltic Sea, salinity is constantly low at such depths where herring juveniles live (Raid 1985; Urho and Hildén 1990; Axenrot and Hansson 2004) so that they cannot find a higher salinity even though it would provide them better growth. However, from the nursery grounds of the south-western Baltic, which are close to the Danish straits, the Baltic herring migrate out to the Skagerrak and Kattegat area (Jönsson and Biester 1981; Aro 1989), where salinity is 15–20 psu (Fig. 1). The south-western herring populations exhibit high growth rate and large body size (Popiel 1958) obtained in a higher salinity compared to that in the Baltic Sea, with the exception of the deep bottom areas. This example from field conditions thus supports our results and suggests that the optimum salinity for the Baltic herring growth is somewhat higher than the present salinity of the Baltic Sea; possibly within 8–12 psu.
Present herring juveniles were about 5 months old when starting the experiment. At this phase, herring juveniles already resemble adults by their general physiology (Blaxter and Holliday 1963). Our results can thus represent also the adult Baltic herring, but this is not necessarily so. In adult fish, excretion of sexual hormones can modulate the action of the hormones that control growth and osmoregulation (Holloway and Leatherland 1998). Therefore, more studies are needed to find out the optimum salinity of the Baltic herring in different phases of its life cycle. Our study demonstrates that although experimental work with the Baltic herring faces difficulties due to sensitivity of the species to handling and life in captivity, the effect of stress can be reduced by increasing water salinity to 12–15 psu. In such conditions, it could be possible to carry out the experiments with replicates and other arrangements meeting the requirements of normal experimental design, which we failed to do in the present work.