EFFECTS OF RELEASE SIZE ON SURVIVAL, GROWTH AND YIELD OF HATCHERYREARED MANDARIN FISH STOCKED IN A SHALLOW YANGTZE LAKE

LI Wei， CAI Xing-Wei， ， GUO Chuan-Bo， ZHANG Tang-Lin， LIU Jia-Shou and LI Zhong-Jie

(1. State Key Laboratory of Freshwater Ecology and Biotechnology， Institute of Hydrobiology， Chinese Academy of Sciences，Wuhan 430072， China; 2. University of Chinese Academy of Sciences， Beijing 100049， China)

Abstract: A tag-release-recapture study was conducted to evaluate effects of size at release on survival，growth and yield of hatchery-reared mandarin fish Siniperca chautsi fingerlings stocked in a shallow Yangtze lake. In May and June 2012， a total of 2400 juvenile mandarin fish were graded into three size groups， identified with coded wire tags and tail fin clipping， and released into Xiaosai Lake. Mandarin fish were collected after the lake drained in December 2012. Overall， a total of 973 mandarin fish individuals were captured， with an average survival rate of 40.5%. Survival rate of large sized group (63%) is more than three times higher than that of small sized group (19.5%) and 1.5 times higher than that of middle sized group (39.1%). The mean final length and weight of mandarin fish increased with the increase of release size and showed significant difference among groups. Yield was significantly affected by release size， which was the highest in large sized group and the lowest in small sized group. The net income of stocked mandarin fish increased with the increase of fish size at release. Our results indicated that mandarin fish size at release can have a major impact on success of hatchery releases， and it is suggested that the optimum release size of mandarin fish should not be less than 50 mm in Xiaosai Lake.

Key words: Release size; Mandarin fish; Survival; Growth; Shallow lake

Mandarin fishSiniperca chuatsi(Basilewsky) is one of the most important commercial freshwater fishes in China， where it is distributed in the waters from the south in the Zhujiang River system to the north in the Amur River system[1]. The fish is a traditional high-valued food fish， with delicious taste， high nutritional value and little bone among muscle， and cherished by Chinese people. Nowadays， mandarin fish is more widely cultured throughout the country，and is also important in stock enhanced fisheries in lakes and reservoirs due to its large size， rapid growth， short culture period， ready availability of seed， high market demand and value[1—3].

In China， mandarin fish is considered to be one of the important predators in lakes. However， a major conventional fishery practice in many lakes of China has been to overfish piscivorous fish and to stock herbivorous species (e.g. grass carpCtenopharyngodon idellus) and planktivorous species (e.g. bighead carpAristichthys nobilisand silver carpHypophthalmichehys molitrix) in the past decades[4，5]. Such fishery practices cause the sharp decline of the natural resources of mandarin fish and resulted in a series of ecological problems. Overfishing of commercially important piscivorous fishes induced dramatic decline of their own resources and the flourishing of small-sized fishes[6，7]. Overstocking of grass carp results in drastic reduction or elimination of submersed macrophytes and consequently， increase in algae and decline in fish and shellfish dependent on macro-phytes[8]. To increase bighead and silver carp production， sewage and fertilizers are used in some lakes，which accelerate eutrophication[9]. Piscivorous fish stocking can be used as a lake restoration tool to improve water quality by creating a trophic cascade[10，11]，also can be profitable at relatively low yields[3，12].Therefore， there has been a shift in fish stocking from common carps to piscivorous fishes， especially mandarin fish in the past twenty years.

In general， stocking strategies that maximize survival of stocked individuals are necessary to fully evaluate the prospect for stock enhancement of a given species. A suite of factors， including stocking size，stocking season， stocking habitat， and stocking density， have been identified as important to post-release survival and growth of stocked animals. Release size is widely recognized as an important factor impacting post-release survival and has been relatively well studied[13—16].

Mandarin fish has been stocked in many lakes totaling at least 133000 ha in the middle and low reaches of the Yangtze River[1]. A total of about 10 million hatchery-reared fry ranging from 25 to 100 mm TL were stocking in lakes each year since 2006. The economic benefits of mandarin fish stocking in several lakes is well documented[12]， and some studies focused on survival， growth， recaptured rate and foraging success of stocked mandarin fish have been conducted[17—19]. However， restricted data on the relationship of survival and growth to size of release were collected due to limitations of the tagging procedure，different living environments and overlap in size groups， and there is still little known about the most appropriate fish size for stocking. Stocking multiple sizes of a given species in a single lake is one method used to optimize stocking success[20]. The objective was to quantify survival， growth and yield when three sizes of mandarin fish were stocked concurrently into a shallow Yangtze lake in 2012， and assess the direct and interactive effects of release size on stock enhancement success for the mandarin fish in the Yangtze lakes.

1 Materials and Methods

1.1 Study area

Xiaosai Lake (30°15′N， 114°43′E) is located on the south bank of the middle reach of the Yangtze River， Hubei Province， Central China. The lake， with an area of 53.3 ha and a mean water depth of 2.90 m，is mostly used for culture-based fisheries. The major form of fishery had been the stocking of the bighead carp and silver carp. During December every year， the lake is nearly drained and all large-sized economic fish are captured， which is beneficial for us to accurately evaluate the survival and growth of stocked fish species. This was the main reason to choose this lake for conducting the experiment. Before juvenile mandarin fish stocking， prey fish in Xiaosai Lake was evaluated by multi-sized gillnets and physicochemical parameters of water quality were measured in May， 2012 (Tab. 1). The total density and total biomass of prey fish were (0.326±0.095) ind./(m2·h) and(4.66±1.03) g/(m2·h)， respectively. According to the trophic state index (TSI)， Xiaosai Lake was mesotrophic.

1.2 Mandarin fish stocking

Tab. 1 Physicochemical parameters of water quality in Xiaosai Lake in May， 2012

Tab. 2 Stocking information of three sized hatchery-reared mandarin fish (MS) in Xiaosai Lake in 2012

During May to June in 2012， a total of 2400 juveniles of hatchery-reared mandarin fish were stocked in Xiaosai Lake with three different sizes (Tab. 2). To distinguish the three sized individuals， the middle sized juveniles were tagged using tail fin clipping and large sized juveniles were tagged with coded wire tags (CWTs) before release. CWTs and tail fin clipping marking have been used successfully in previous studies， and have negligible effects on mortality and growth[1，21]. Hatchery-reared juvenile mandarin fish for field release experiments were obtained from a state-owned hatchery. Hatchery mandarin fish were spawned in the laboratory from mated mature stocks collected from the Yangtze River. Mandarin fish juveniles were reared on live silver carp fry until they were stocked. Stocking dates were dependent on availability of mandarin fish of desired size. The mandarin fish juveniles of each size were transported by air-conditioned car in eight oxygen-filled plastic bags(Volume of 20 litres) from hatchery to Xiaosai Lake，and then released in the similar nearshore area.

1.3 Sample collection

Mandarin fish were sampled after the lake drained in December， 2012. All mandarin fish were collected， and then the total length (TL， to the nearest mm) and weight (BW， to the nearest 0.1 g) were individually measured. A NMT hand-held tester (Made by Northwest Marine Technology， Inc， USA) was used to detect the coded wire tags to distinguish the large sized group from the other groups. The small and middle sized groups were identified by tail fin.Survival was calculated as the number of hatcheryreared mandarin fish collected by the total number released of each sized group. Growth performance of the fish was evaluated using final length and weight，specific growth rate (SGR， %/d)， absolute growth rate(AGR， g/d) and total yield (kg).

The economic analysis followed that of Gomes，et al.[22]Total net income equaled total revenue minus total cost. The total revenue was dependent on the harvested mandarin fish that sold at 50 CNY/kg in the local market. The total costs consisted of the production costs of fish fingerlings and labor for fish transporting， stocking and harvesting. The production cost is 0.9， 1.8 and 3 CNY/individual for small， middle and large sized mandarin fish， respectively.

1.4 Statistical analysis

All data were expressed as mean±SD. One-way analysis of variance (ANOVA) was used to determine the differences among treatments followed by a Tukey test to evaluate differences among treatment means for post hoc comparisons. Differences were considered significant atP＜0.05. All analyses were performed using the SPSS 16.0 statistical package(SPSS Inc.， Chicago， USA).

2 Results

2.1 Survival and growth

A total of 973 individuals mandarin fish were captured， with an average survival rate of 40.5%.Among them， the number of small， middle and large sized stocking mandarin fish is 156， 313 and 504， respectively. The survival rate of large sized group is more than three times higher than that of small sized group and 1.5 times higher than that of middle sized group (Tab. 3). The mean final length and weight of mandarin fish increased with the increase of release size and showed significant difference among groups released as small， middle and large fingerlings(length:F=5.56，df=2 972，P=0.026; weight:F=7.01，df=2972，P=0.009). The final length was not significant difference between large and middle sized groups(Tukey test:P=0.176)， but significant difference existed between large and small sized groups (Tukey test:P=0.021). A similar pattern was observed in the mean final weight. TheAGRweightshowed an increased trend with the increase of release size， while the opposite trend was found in theSGRweight. Yield was significantly affected by release size. It was the highest in large sized group and the lowest in small sized group (Tab. 3).

2.2 Economic benefit

Tab. 3 Growth performance of mandarin fish stocked at 3 different release sizes in Xiaosai Lake

Tab. 4 Cost and return analysis of mandarin fish stocked at three release sizes in Xiaosai Lake

The comparative cost and return analysis for mandarin fish stocking at three release size groups in Xiaosai Lake is shown in Tab. 4. The total revenue and total costs were highest in large size group and lowest in small size group at release. The total revenue was directly determined by mandarin fish yield because captured mandarin fish of three release size groups were sold with a same market price. The major components of total costs were mandarin fish juveniles and cost for capturing mandarin fish， representing 45.13% and 29.79%， 50.51% and 35.46%，53.57% and 37.50% of total costs for the three size groups， respectively. The net income of stocked mandarin fish increased with the increasing of fish size at release， but the net income per kg of fish is similar between middle and large groups.

3 Discussion

Release size is clearly an important factor mediating post-release survival in many species[13—15]. In this study， the survival of stocked mandarin fish was obviously affected by release size. Survival rate showed an increased trend with increasing release size. This is consistent with previous studies that indicated stocking larger individuals generally survived at higher rates than small individuals[13，23—25]， but some results have not always supported the size-dependent survival[20，26，27]. The inconsistency is primarily attributable to the differences such as environmental conditions of stocked water bodies (water temperatures， dissolved oxygen， forage abundance， predatory pressure) and behavioral characteristics of stocked species. In this study， the factor of environmental conditions is excluded because the three size groups of mandarin fish were stocked in the same lake in the same year. The behavioral characteristics of mandarin fish were probably the main reason that explained the differences in survival. Mandarin fish is an absolute carnivore， and was found to capture live fry of other fish species from the first feeding stages[2，28，29]. The abundance and availability of prey fish is crucial for determining survival of stocked mandarin fish in the early stage of their life history.Mandarin fish is also a mouth gape-limited predator which possesses a “predation window” defined by the minimum and maximum size of prey consumed[30，31].So the larger sized mandarin fish at release， with larger mouth gape， have much more availability of prey fish than middle and small sized groups at release at the same abundance of prey fish in Xiaosai Lake， and consequently increase its foraging success. In addition， predation is also largely responsible for discrepancies in the survival of stocked fish. Generally， the larger individuals have better anti-predator ability，which can effectively reduce its mortality induced by other predators. Specially， larger individuals may attain a size refuge from gape-limited predators， or they may be more able to fight off or escape predatory attacks[14]. Also， the larger individuals have stronger resistance to disease. Thus， larger sized mandarin fish at stocking would probably show a higher survival rate than the smaller sizes.

The growth of stocked fingerlings can influence recruitment and the economic returns to lake managers and thus is an important performance measure for stocking programs[13]. In this study， we found that the mean final length and weight of mandarin fish increased with the increase of release size; the absolute growth rate of large and middle sized groups at release at end of experiment was greater than that of small size group， which is also consistent with some similar studies in different stocking species[13，23，25]. In contrast， Pratt and Fox[32]and Olson，et al.[33]found that small stocked walleyes fingerlings had become larger than large ones at the end of the first growing season. The discrepancy in growth rates were generated by the amount of forage[20]， temperature[20，34]，and size of fish[35，36]. In this study， the three sized mandarin fish underwent the same abundance of potential prey fish and the same temperature， but the larger sized group has much more available prey fish from the beginning of stocking compared with smaller size group， which is possibly the primarily reason for the large sized mandarin fish at release obtaining the greatest average growth rate.

In general， the economic benefit is primarily determined by yield and production costs. In our study，the yield and net income of stocked mandarin fish increased with the increasing of fish size at release，which is consistent with the results of some similar studies in Illinois and Wisconsin lakes in USA[13，25].The yield is mainly dependent on survival and growth of stocked species. Production costs increase as the size at stocking increases because of extended periods of feeding， maintenance and handling[20，37].

Based on current stocking rates and costs to produce the various fish sizes， our results indicate that stocking large sized mandarin fish would be most productive. However， fewer mandarin fish can be reared to successively larger sizes because of limited pond space and increased mortality in the process of seedling production of mandarin fish. Given the similar net income per kg of fish between large and middle sized groups， it is suggested that the optimum release size of mandarin fish should not be less than 50 mm in Xiaosai Lake. This study shows that fish size-at-release can have a major impact on success of hatchery releases in Yangtze lakes. Therefore， pilot studies to identify optimal fish size at release should be conducted at all sites targeted for full-scale Yangtze lake hatchery releases.