The point of no return and pectoral angle of Japanese anchovy

Transcription

1 The point of no return and pectoral angle of Japanese anchovy (Engraulis japonicus) larvae during growth and starvation WAN Ruijing 1, LI Xiansen 1, ZHUANG Zhimeng 1*, JOHANNESSEN Arne 2 1. Key Laboratory of Ministry of Agriculture for Sustainable Utilization of Marine Fishery Resources, Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Qingdao , China 2. Department of Biology, University of Bergen, Bergen, Norway Abstract At a temperature of 23.0~24.8 C, the mixed feeding of Japanese anchovy larvae was initiated 24 h before the yolk-sac was exhausted. The point of no return (PNR) was reached on the 6th day after hatching. On the 4th day after hatching, pectoral angle appeared in both fed and unfed anchovy larvae although it was more evident and sharper in the starved and the PNR stage larvae than in the fed ones. According to our observations of larvae collected in the sea, the pectoral angles were evident not only in larvae of 3.62~7.44 mm in standard length, but also in larvae of 2.70 mm in standard length with remnants of yolk. The pectoral angles became diffuse when the larvae reached 7.84 mm and vanished at 9.86 mm. The pectoral angle can not be used as a criterion to distinguish healthy from starving larvae. Key words: Engraulis japonicus, larvae, starvation, point of no return (PNR), pectoral angle 1 Introduction Pelagic fish experience high mortality during early life history stages in general. This mortality is attributed to predation, starvation, physical transport and sublethal factors, although the relative roles of these remain unresolved (Leggett and DeBlois, 1994; Bailey and Houde, 1989; Anderson, 1988). As small variation in survival rates during early life history stages could lead to dramatic fluctuations in recruitment (Houde, 1989; 1987), the research about survival mechanisms is important in order to better understand processes about early recruitment dynamics of fish. A variety of hypotheses have been proposed. In the early studies, mortality due to starvation linked with food supply and feeding success was regarded as the major determinant of recruitment success according to the critical period hypothesis (Hjort, 1914), ocean stability hypothesis i.e. Lasker s event (Lasker, 1978; 1975), match-mismatch hypothesis (Cushing, 1990a, b, 1975), retention hypothesis (Iles and Sinclair, 1982), growth-mortality hypothesis (Anderson, 1988) and optimal environmental window hypothesis (Cury and Roy, 1989). All these hypotheses were concerned with the relationships among feed supply, starvation mortality and recruitment at fish early life stages. Recent studies, however, have not conclusively linked starvation with year-class strength (Leggett and DeBlois, 1994), although the larval starvation is likely to be a significant source of mortality in the first feeding stage (Hunter, 1976; Hjort, 1914). Blaxter and Hempel (1963) defined the point of no return (PNR) for herring larvae, as the threshold point where larvae can endure starvation. This point has also been called irreversible * Corresponding author, zhuangzm@ysfri.ac.cn 1

2 starvation. The PNR describes the tolerance of larvae to starvation, after which the larvae are predestined to die. A number of workers have observed changes in morphology of fish larvae during starvation. Shelbourne (1957) assessed the status of sea-caught plaice (Pleuronectes platessa) larvae in good and poor plankton patches. Blaxter (1971) related the condition factor (dry mass multiplied by negatively third-order squares of the length) of sea-caught herring (Clupea harengus) larvae to the condition factor of herring known periods of starvation in the aquarium. Ehrlich et al. (1976) used a more sophisticated measure of condition factor, the pectoral angle (the angle of the ventral surface of the body at the insertion of the pectoral fin) during periods of experimental starvation of herring and plaice larvae. Theilacker and Dorsey (1980) summarized experiments on marine fish larvae and recorded the time to reach the PNR for post-yolk-sac larvae of 12 species. McGurk (1984) reported the time from fertilization to irreversible starvation for Pacific herring larvae reared at different temperatures and compared these periods with those of 25 species of pelagic marine fish larvae. The time to reach the PNR from hatching for herring and plaice larvae has been reported by Blaxter and Ehrlich (1974) and Ehrlich et al. (1976), for herring (Clupea harengus) from three races, Clyde, Baltic and North Sea, cod (Gadus morhus) and flounder (Platichthys flesus) by Yin and Blaxter (1987; 1986), for marbled flounder (Pseudopleuronectes yokohamae) by Zhou et al. (1998), for red sea bream (Pagrosomus major) and Japanese flounder (Paralichthys olivaceus) by Bao et al. (1998), for halfbeak (Hyporhamphus sajori) by Wan et al. (2003) and for tonguefish (Cynoglossus semilaevis) by Zhuang et al. (2005). Japanese anchovy (Engraulis japonicus) is one of the most abundant pelagic fish in the Sea and the East China Sea and is considered as an important link in the ecosystem food chain (Tang and Ye, 1990; Zhu and Iversen, 1990). Japanese anchovy is not commercially harvested in the Huanghai Sea, but provide a major forage base to more than 40 commercially important species such as small yellow croaker, Spanish mackerel and tail fish etc. (Jin et al., 2001; Wei and Jiang, 1992). Therefore, Japanese anchovy was listed as one of the key species in terms of the studies on the ecosystem (Tang and Su, 2000). This paper reports the experimental results of the first feeding rate of Japanese anchovy larvae, growth to the PNR, and larval pectoral angle, in order to accumulate basic data for the research on mechanisms and processes of early recruitment of Japanese anchovy. 2 Materials and methods 2.1 Egg collection and experiment preparation An ichthyoplankton survey of the Japanese anchovy spawning ground in the waters south of Shandong Peninsula in China, and the Huanghai Sea, was carried out during the period of 13~26 June 2000 (see Fig. 1). Naturally fertilized eggs of Japanese anchovy were collected with a horizontally towed zooplankton net at N, E on 26 June The net is made of bolting silk, with an opening diameter of 80 cm, a length of 270 cm, and its mesh size is mm. The eggs were collected from 08:00 am to 10:00, and started hatching at 11:00. One hour after hatching, the larvae were carefully collected in a bucket and prepared for the experiment. 2

3 38º N Bohai Sea Huanghai Sea KOREA Shangdong Peninsula Qingdao 36º China N Pen. 34º km 118º 120º 122º 124º 126º E Fig. 1. Sampling stations ( ) of the ichthyoplankton survey on the Japanese anchovy (Engraulis japonicus) spawning ground in the Huanghai Sea during the period of 13~26 June represents the sampling site where the eggs of Japanese anchovy were collected for laboratory experiments. 2.2 Method of cultivation The collected Japanese anchovy larvae were divided into two groups, one control group and one food deprivation group, the larvae were kept in 15 L blue plastic buckets. Each bucket contained about 500 larvae. For the control group, the larvae were fed with Chlorella sp. and Brachionus plicatilis (enriched by Chlorella sp.) from the second day after hatching. The food density was ~ cells/cm 3 of Chlorella sp. and 10~15 ind./cm 3 of B. plicatilis. Culture seawater was filtered by sand. The condition of the larvae was checked every day at 08:00, and one third of the water exchanged. New feed was added at 17:00 every day. The starvation group was not fed, but one third of the water was changed at 08:00 every day. The culture seawater for the starvation group was filtered by absorbent cotton in addition to sand. The temperature of the water during the experiment was maintained at in-situ conditions, i.e. 23.0~24.8 C, and the salinity was 30.00~ Light conditions followed the natural diurnal cycle. 2.3 Initial feeding rate The initial feeding rate of larvae was obtained by a series of feeding experiments. From the second day after hatching, more than 20 larvae were sampled randomly from the starvation group every day at 08:00, and carefully transferred into a 400 ml beaker with water temperature of 23.0~24.8 C during the experimental period. The larvae were fed with Chlorella sp. (about cells/cm 3 ) and Brachionus plicatilis (about 20 ind./cm 3, enriched by Chlorella sp.). After 2 h the larvae, in groups of 5 individuals, were taken out and anaesthetized (200 µg/cm 3 Benzocaine) for immediate microscopic observation of gut contents. Studies on the feeding of Japanese anchovy in the waters south of Shandong Peninsula made by Meng (2001) showed that, in their natural environment, the Japanese anchovy larvae mainly feed on animal food, including protozoa, eggs and nauplii of copepods, etc., and only a few individuals were found to feed on 3

4 pelagic diatoms. The feeding individuals were those with Chlorella sp. or B. plicatilis in their alimentary canals. The initial feeding rate is defined as the percentage of feeding individuals of the observed larvae. 2.4 Determination of PNR The PNR is a threshold point of larvae during progressive starvation. At the PNR stage, though larvae were still alive for a while, they are too weak to regain their feeding ability even if food is available in excess. The PNR has also been called the point of irreversible starvation. The PNR was determined by feeding experiments. First feeding rate of the larvae in the starvation group was measured every day, and the highest feeding rate was determined. When the first feeding rate dropped to half of the highest feeding rate, the time of PNR is determined (Blaxter and Hempel, 1963). The PNR is expressed in days after hatching. 3 Results and discussions 3.1 Mixed feeding stage During the early developmental stages of marine fishes the period during which both endogenous yolk and exogenous food may be utilized is here called the mixed feeding stage. Japanese anchovy larvae started to feed exogenously on the second day after hatching at a water temperature of 23.0~24.8 C. On the third day no remnants of yolk appeared. The duration of the mixed feeding stage of Japanese anchovy larvae was a little more than 1 d. The result is basically unanimous with that of Fukuhara (1983), who found that Japanese anchovy larvae opened their mouths on the second day after hatching, and the yolk was absorbed completely on the third day at 23~25 C. Ruan (1984) reported that at a water temperature below 18 C, Japanese anchovy larvae formed the mouth on the second or the third day after hatching; most of its yolk was absorbed on the fourth day, and on the sixth day its yolk disappeared completely. The mixed feeding stage lasted for 3~4 d, which is 2~3 d longer than what was observed in this experiment. Imai and Tanaka (1996) reported that the yolk of Japanese anchovy was absorbed completely in 1.9 d (25 C), 2.2 d (22 C), 2.7 d (20 C) and 4.3 d (16 C) after hatching, respectively. Obviously, the higher the water temperature is, the shorter is the duration of the mixed feeding stage. Larvae have to establish their exogenous feeding ability within the mixed feeding stage; otherwise, they will suffer progressive starvation (Blaxter and Hempel, 1963). Extending the mixed feeding stage is beneficial for larvae to accumulate sufficient feeding experience to establish their exogenous feeding ability successfully, to avoid starvation, and thus improve survival rate. Therefore, the water temperature of the spawning ground is among the key factors affecting the early survival rate of Japanese anchovy. 3.2 Initial feeding rate and PNR For the starvation group, the first feeding experiments were conducted daily from the second through the seventh day. The initial feeding rates of the starved Japanese anchovy larvae were 9.52 %, %, %, %, % and %, respectively. It is concluded that the PNR of Japanese anchovy is on the sixth day after hatching (see Fig. 2). On the eighth day after hatching, most larvae died. This shows that, at a water temperature of 23.0~24.8 C, the feeding ability of the Japanese anchovy larvae deprived of food can only be maintained for 3~4 d, which is shorter than in many other marine fishes. This suggests that in order to survive the larvae should 4

5 begin exogenous feeding when still having yolk and rapidly switch from endogenous to exogenous feeding as long as suitable food are available. Therefore, the presence of suitable prey organisms at the right time is of vital importance for the survival, hence recruitment success of Japanese anchovy. This supports the match-mismatch theory about survival rates of fish larvae (Cushing, 1990a, b). Initial feeding rate (%) Age/d 47 50% feeding equivalent to half the initial highest percentage Fig. 2. Change in initial feeding rates during period of starvation of early development of Japanese anchovy larvae. Numbers in parenthesis are the observed values. 3.3 Growth and development of larvae Under a temperature regime of 23.0~24.8 C, the mean standard length of the larvae in the control group (fed group) was (2.92 ± 0.18) mm on the fourth day after hatching (see Fig. 3a). An anterior mouth with an upper and a lower jaw was completely formed. The alimentary canal developed into esophagus and intestine, with clear folds and visible peristaltic waves in the intestine. The pectoral fin was already movable, and the larvae had already acquired certain swimming and prey-chasing ability. Through the transparent intestinal wall, ingested Brachionus plicatilis was clearly seen (light green). On the sixth day, the mean standard length of the larvae was (3.80 ± 0.28) mm (Fig. 3c). The dorsal-fin membrane and the anal-fin membrane were clearly concave, and the dorsal-fin membrane above the anus became wider and protrudent. The swimming speed was faster than that of the fourth day, and the feeding movement became more agile. The intestine became thicker, and the peristalsis became stronger. The feeding intensity became higher, in about 60 % of the individuals, the food mass (Brachionus plicatilis) occupied about 1/5~1/4 of the volume of the intestine, and in a few the food contents filled 1/3 of the intestine. Their standard length was 0.88 mm longer than that of the fourth day, with an average growth rate of 0.44 mm/d. Ruan (1984) reported that when the water temperature was below 18 C, the standard lengths of both the fourth day old larvae and the sixth days old larvae after hatching were in the range 3.22~3.69 mm, with no significant increment rate observed. Fukuhara (1983) reported that when the water temperature was 23~25 C, the mean standard length of the fourth day old larvae was (3.76 ± 0.29) mm, and for the eighth day old ones the mean standard length was (5.21 ± 0.39) mm, with a mean increment rate of 0.36 mm/d, which was slightly lower than what was observed in this experiment. In the starvation group, when the larvae s yolk was completely exhausted, the mean standard length of larvae was (2.92 ± 0.20) mm on the sixth day after hatching (Fig. 3d). The size had changed little when compared with the mean standard length [(2.90 ± 0.18) mm] of he fourth day old larvae (Fig. 3b), because they could not get any supplement of nutrition and energy. The larvae 5

6 almost stopped growing. With progressive starvation, the larvae became thinner, their organs atrophied, their intestine became thinner, and the peristaltic waves almost disappeared. a b c d Fig. 3. Pectoral angle comparison of Japanese anchovy larvae. a. Fed larva (fourth day old, 2.92 mm), b. Starved larva (fourth day old, 2.90 mm), c. Fed larva (sixth day old, standard length 3.80 mm) and d. PNR stage larva (sixth day old, standard length 2.92 mm). 3.4 Pectoral angles Observations showed that the pectoral angle appeared both in the fed group and starved group on the fourth day after hatching. However, the pectoral angle of the starved larvae was more noticeable and sharper than that of the fed ones, because the pectoral and abdomen of the starved larvae became thin and sunk (see Fig. 3). Japanese anchovy larvae collected from the spawning ground off the southern Shandong Peninsula in the Huanghai Sea, during 13~16 June 2000, 27~30 June 2003 and 10~16 June 2004, showed that the larvae with yolk nearly exhausted, about 2.70 mm in the standard length, had distinct pectoral angles already (see Fig. 4a). The pectoral angles were larger in the individuals of 3.06 mm in the mean standard length (see Fig. 4b), and much more remarkable in all the individuals ranging from 3.62 mm to 7.44 mm in the mean standard length (see Fig. 4c, d and e), yet became smaller in the individuals of 7.84~8.62 mm in the mean standard length (see Fig, 4f and g). The pectoral angles disappeared completely only when the larvae reached about 9.86 mm in the mean standard length (see Fig. 4h), and at this length, most of the individuals had grown into the juvenile stage. We can conclude that Japanese anchovy larvae have pectoral angles during the larval stages. Literature records showed that the larvae of North Sea herring (Clupea harengus) 6

7 (Yin, 1991a) and Clyde herring (Clupea harengus) (Ehrlich et al., 1976), plaice (Pleuronectes platessa) (Ehrlich et al., 1976) and flounder (Platichthys flesus) (Yin and Blaxter, 1986) all have pectoral angles during their PNR stage. As one of the morphological characteristics of starved larvae (Yin, 1991a), the pectoral angle has been used to distinguish between healthy larvae and starved larvae (Bao and Su, 1998; Xie et al., 1998; Yin and Blaxter, 1986; Ehrlich et al., 1976). a pectoral angle b c d e f 7

8 g h Fig. 4. Photographs showing the pectoral angle of Japanese anchovy larvae collected from the spawning ground off the southern Shandong Peninsula in the Huanghai Sea, during 13~16 June 2000, 27~30 June 2003 and 10~16 June a mm, b mm, c mm, d mm, e mm, f mm, g mm, h mm. The results of this experiment and examination of natural larvae caught from Japanese anchovy spawning grounds showed that the pectoral angles of Japanese anchovy larvae would be not one of the morphological characteristics of the fish during starvation and cannot be used as a criterion to distinguish between healthy and the starved larvae. Therefore, using pectoral angle features to observe the nutritional conditions of fish larvae from the samples of fishery cruise survey should consider the species differences, since the features of pectoral angles standing for starvation may vary with species. When Japanese anchovy larvae starved into the PNR stage, they lost the capability to regain their feeding ability, but they can remain alive for two more days. The larvae which have lost the ability to feed in natural sea areas are often neutral buoyant (Blaxter and Ehrlich, 1974), and can be easily caught by plankton nets (Yin and Blaxter, 1987). The presence of large numbers of larvae past the PNR in the spawning ground could lead to erroneous estimates of viable larval abundance and survival if ichthyoplankton surveys were carried out at this time. When we study the early recruitment processes of Japanese anchovy, the larvae having reached the PNR stage should be excluded from abundance estimates. Thus, differentiating healthy larvae from starved ones is very important and worthwhile of continued studies. 4 Conclusion The mixed feeding in yolk-sac larvae of Japanese anchovy lasted for a little more than 1 d at a water temperature range of 23.0~24.8 C, and the PNR of larvae appeared on the sixth day after hatching. The pectoral angle was used to identify developmental stages of Japanese anchovy larvae, but the pectoral angle could not be a reliable characteristic to distinguish between healthy and starving larvae, which is of ecological significance in observing the health conditions of fish larvae. Acknowledgments The study was financially supported by the National Key Basic Research Program from the Ministry of Science and Technology of China under contract No. 2006CB and G

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