Construction and Application of High Temperature Heat Damage Index for River Crab Culture in Hongze Lake Beach

Xiang CHEN Yuelin WANG Xiaojun ZHAO Miao LU Peng TANG

Abstract Agrometeorology and agricultural statistics methods were used to analyze the cases of high temperature heat damage to crab breeding since 2008 in Hongze Lake Beach， finding that the high temperature heat damage was mainly under the comprehensive effects of the highest temperature， duration of high temperature weather， amount of precipitation and sunshine hours. Based on the function relation between the high temperature heat damage index and the influence factors， the linear model was used to describe the weight coefficient of each heat damage factor by using the synthesis correlation of multiple factors. The historical values of the influencing factors were used to make statistical calculation of the heat damage index model of crab culture in Hongze Lake Beach， which achieved good effects when applied in the meteorological operation of river crab culture.

Key words River crab; High temperature heat damage; Temperature; Index; Beach

Hongze Lake is the fourth largest freshwater lake in China. It has vast lake surface and abundant aquatic resources. Known as the "Top Ten Crabs" and "National Geographical Indications Certification Mark"， the Hongze Lake hairy crab enjoys good fame at home and abroad for its "green back and white belly， golden claw and yellow hair， rich fat， delicious taste and abundant nutrition"， and it is popular all over the world. The beach formed at the riverlake estuary of Hongze Lake from the sand carried by the water in Huaihe River[1]， together with the pleasant climate in Hongze Lake， makes the Hongze Lake Beach enjoy exceptional advantages in river crab breeding. Since the 1990s， fishermen have started river crab farming in the Hongze Lake. In recent years， the crab culture in Hongze Lake Beach has developed rapidly. The annual crab culture area is about 3 000 hm2， with an annual crab output of 2 520 t and annual output value of 450 million Yuan， and river crab breeding has become the main source of income for farmers nearby. Since the crab culture in the Beach is an openair industry， the growth and development of crabs are obviously restricted by meteorological conditions. In other words， the meteorological conditions during the breeding period directly determine the yield and quality of crabs every year. The adverse meteorological conditions that do damage to the crab breeding industry are mainly environmental mutations， which result in the physiological maladjustments of crabs like slow growth or direct death[2]. For example， in 2017， the high temperature weather lasted almost 20 d in Hongze Lake area， which was fairly rare in history， and the river crab breeding suffered serious high temperature heat damage that the crabs delayed their 4th shelling time， some even having no shelling， resulting in severe diseases and large amount of death. Moreover， the survived ones even had their quality decreased. According to the statistics of the local fishery department， a total of 736 crab raising households were affected with the total affected area of 2 415 hm2. There were a total of 319 402 kg of dead crabs， and another 78 810 kg of dead fish， which caused a direct economic loss of 20 265 500 Yuan.

In recent years， in order to ensure the high yield and quality of river crab breeding， many scholars and experts have carried out extensive research on the ecological environment and hazard factors of crab farming[2-18]. Zhang et al.[6]conducted field studies to observe the differences in the development of river crabs between natural and adjusted meteorological conditions for river crab raising around Hongze Lake， and analyzed the survival rate of crab seedlings and the growth of adult crabs under meteorological conditions. Dai et al.[7]detected the amount of dissolved oxygen at different water levels in the crab ecoculture ponds within 24 h， and monitored and analyzed the distribution and changes of dissolved oxygen in different ecological river ponds in the high temperature season caused by different water plants. Based on 5 meteorological factors――air pressure， relative humidity， maximum temperature， minimum temperature， sunshine hours and wind speed， which could well reflect the characteristics of hot weather damage to river crab breeding， Zhu et al.[8]constructed the hot and stuffy weather index model and damage degree classification standards， systematically analyzed the occurrence laws and trends of hot weather damage in the top ten river crab breeding areas in Jiangsu， and evaluated the calculation results to the hot weather damage in Jiangsu. Zhang et al.[2]constructed a high temperature stress model based on high temperature index and weight coefficient according to the main features of high temperature stress during the river crab breeding period. However， these studies construct the meteorological disaster index model based on the growth environment in the ponds raising river crabs， but make no detailed research on the disaster index of openair crab raised in lakes and beaches. Therefore， in this paper， the damage conditions of river crabs raised in the Beach of Hongze Lake were collected and analyzed. The results showed that there were 3 largescale disasters to the river crab breeding in Hongze Lake in 2008-2018， all of which were caused by high temperature heat damage. The high temperature season in Hongze Lake was concentrated from mid or late July to early August， during which time the river crabs were processing their fourth shelling. During the shelling period， the river crabs were weak and had poor resistances， and any changes in the outer growth environment could easily lead to the illnesses and deaths. Therefore， in this paper， based on the characteristics of high temperature heat damage in Hongze Lake Beach， we established a comprehensive meteorological index system for the high temperature heat damage to river crab raising， conducted the comprehensive high temperature heat damage forecasting model， set up the meteorological service system to guide the farmers to scientifically cope with high temperature heat damage， which were of practical significance to improve the yield and quality of river crab breeding as well as the increase of farmers income.

Materials and Methods

Data source

The meteorological elements such as the highest temperature and high temperature duration were derived from the observation data of the Laozi Mountain Regional Meteorological Observatory near Hongze Lake in 2008-2018. The average precipitation and the average sunshine hours in July were obtained by the interpolating of the observatory data of the 2 national meteorological observatories in Hongze and Xiyu， which were adjacent to Hongze Lake. The disaster data of the river crabs in the Hongze Lake Beach were provided by the Hongze District Fisheries Department.

High temperature heat damage grading

The high temperature heat damage grading standard was the same as that of Zhang[2]， which divided the hot temperature damage into 3 grades. After disaster occurs， if the high temperature at which the mortality rate of crabs is 2%-5% （excluding）， and the cumulative number of dead crabs during damage process is 120-450 crabs/hm2， it is the mild degree （grade 1）; if the high temperature at which the mortality rate of crabs is 5%-10% （excluding）， and the cumulative number of dead crabs during damage process is 450-900 crabs/hm2， it is the moderate degree （grade 2）; when the mortality rate of crabs reaches over 10%， and the cumulative number of dead crabs during damage process reaches up to 900 crabs/hm2， the high temperature heat damage reaches the severe degree （grade 3）.

Establishment of high temperature heat damage indices

Generally， the optimum temperature for river crabs is between 25 and 30. When the temperature exceeds 35， the temperature at 0.6 m in the water is above 28， when the river crabs almost do not eat and go to sleep， and some weak crabs with poor constitution will die. If the high temperature weather continues for a long time， the dissolved oxygen in the pond will decrease， which will cause the crab to become sick and die.

Many experts[2-3， 8]have found that the high temperature heat damage to river crab breeding is mainly affected by the highest temperature （TX）， high temperature duration （L）， precipitation （R） and sunshine hours （S）. Assume the comprehensive index of high temperature heat damage is I， the influence index of high temperature degree is ITX， the influence index of high temperature duration is IL， the influence index of precipitation is IR， the influence index of sunshine is IS， and then the functional relation of high temperature heat damage comprehensive index I and influence factors can be expressed in the following equation：

I=f（ITX IL IR IS）

If the relationship between them is described by a linear model， then the equation becomes the following formula （1）

I=b0+b1ITX+b2IL+b3IR+b4IS（1）

Where， bi is the weighting factor of the influence factor. The high temperature heat damage index generally constructs a dimensionless relative number between 0 and 1 by designing membership functions[19]. In this paper， the Level 3 experience was used to construct the relative number， and so came the following formulas （2）-（5）

Highest temperatureOn July 12， 2017， the highest temperature of Hongze Lake Beach broke through 35. In the breeding site， the observed temperature at the water depth of 0.6 m was 29.2， and the water temperature at the bottom of the pond also reached 28.0. At this time， the crabs crouched in the pond and did not eat. Before that， a small amount of crabs had entered the fourth shelling stage， but there was no crab shelling this time. The fourth shelling began only after the ending of the high temperature in early August， which was 15-20 d later than the shelling date in previous years. Up to July 15， when the high temperature had lasted for 3 d， a small quantity of river crabs died. On July 24， the highest temperature reached 41.0， the water temperature at the depth of 0.1 m was 35.7， 33.0 at the depth of 0.6 m， and the water temperature at the bottom of the pond was up to 30.0. According to the onsite observation， the river crabs climbed to the surface of the water and were dead because of the hot weather in less than half an hour. On that day， the number of dead river crabs broke 80 crabs/hm2， and in some ponds， all crabs died on the same day. Moreover， a large number of water plants in crab ponds withered， which also accelerated the deterioration of dissolved oxygen amount in water. Therefore， the highest daily temperature of 35.0 and 40.0 were set as the critical values for the occurrence and aggravation of high temperature heat damage to river crabs in Hongze Lake Beach. After substituting into formula （2）， the function relation of highest temperature was as shown in formula （6）.

High temperature durationAccording to the analysis of river crab growth and meteorological observation data in Hongze Lake Beach in 2008-2017， it was believed that during the fourth shelling period of the river crab in Hongze Lake Beach， when the temperature reached 35.0， the river crabs stopped their growth and feeding， entered the dormancy states， which delayed the shelling time. When the high temperature weather lasted for 3-4 d， the river crabs began to die. The death rate of river crabs became larger with the high temperature weather lasting longer. When the high temperature weather lasted for 14 d， the daily average death toll of river crabs reached 80 crabs/hm2， and the highest was up to 140 crabs/（d·hm2）. Even survived， the quality of the crabs was affected. At this point， the river crab basically stopped growing. Therefore， the lasting time of 3 d and 14 d of the daily highest temperature of ◎35.0 was used as the critical values for the occurrence and aggravation of high temperature heat damage. After substituting into formula （3）， the function relation of high temperature duration was as shown in formula （7）.

PrecipitationAccording to the analysis of years of meteorological observations， the high temperature weather in July was also accompanied with less precipitation in Hongze Lake Beach. The years with high temperature heat damage saw a decrease of over 20% in precipitation in July when compared with that of the normal years， and the precipitation in 2017， when severe high temperature heat damage occurred， was less then onethird of that of normal years. Therefore， the monthly precipitation anomaly -20% and -50% in July was used as the critical values of precipitation influence index for the occurrence and aggravation of high temperature heat damage. After substituting into formula （4）， the function relation of precipitation influence factor was as shown in formula （8）.

Sunshine hoursIn the summer， the long time continuous high temperature must be accompanied with strong sunshine. After analysis， when the high temperature lasted for over 3 d， the average daily sunshine hours in the first 10 d was over 8 h， some even above 10 h. For example， in 2012， the high temperature weather lasted 12 d from July 20 to 31， and the daily average sunshine hours reached 11.7 h from July 22 to 31. In addition， the long hours of sunshine could increase water temperature， accelerated the death and decay of water plants， therefore aggravating the impact of high temperature heat damage. Therefore， the average daily sunshine hours of 8 h and 10 h in 10 d were used as the critical values of sunshine hours for the occurrence and aggravation of high temperature heat damage. After substituting into formula （5）， the function relation of sunshine influence factor was as shown in formula （9）.

Agricultural Biotechnology2019

Weight Coefficients of Influence factors

Determination of the influence factors for high temperature heat damage

There are many ways to determine the weight coefficient of and influence factor. In this paper， the multifactor comprehensive correlation method was used to determine the weight coefficients of the influence factors for high temperature heat damage[20]. If the high temperature heat damage occurred in the year， it was marked as I=1， otherwise， I=0; if the highest temperature reached over 35 in the year， it was marked as ITX=1， otherwise， ITX=0; if the high temperature weather lasted for over 3 d in the year， it was marked as IL=1， otherwise， IL=0; if the monthly precipitation anomaly in July was less than -20%， it was marked as IR=1， otherwise， IR=0; if the daily average sunshine hours over 8 h lasted for 10 d， it was marked as IS=1， otherwise， IS=0.

According to the above rules， the original data of the forecast objects and the forecasting factors were converted into the feature data of （0， 1）， which were listed in Table 1.

Analysis of single factor correlation degree

According to the characteristic data in Table 1， statistics was made separately for the joint frequency contingency tables of influence factors and the high temperature heat damage comprehensive index I.

After substituting into formula （1）， the following model formula （10） was obtained for the high temperature heat damage index：

I=0.04+0.08ITX+0.08IL+0.14IR +0.19IS

Determination of the critical index of high temperature heat damage index

The historical values of the various influence factors in 2008-2017 were then substituted into the high temperature heat damage index model formula （10）， and each obtained individual I value was listed in the 6th column of Table 6. The critical grade index threshold value was determined on the principle of leaving out no high temperature heat damage to the river crab breeding in Hongze Lake Beach. The results showed that when 0.30Ii<0.41， the high temperature heat damage to river crab culture was predicted to be the mild degree （grade 1）; when 0.41Ii<0.51， the high temperature heat damage was predicted to be the moderate degree （grade 2）; when Ii ◎ 0.51， it was predicted that the high temperature heat damage was the serious level （grade 3）.

High temperature heat damage index

Each Ii value was calculated according to the prediction equation， and the historical fit rate was 100%. After the establishment， the high temperature heat damage index model was put into practical trial in 2018. From midJuly to midAugust 2018， there appeared 15 d of high temperature in Laozi Mountain， and the high temperature weather lasted 7 d from July 13 to 19. The extremely highest temperature was 37.3， and the daily average sunshine hours were 10.5 h in the 10 d from July 9 to 19. However， the precipitation in July was 11.4% more than that in the same period in previous years， which made the water level deeper in the crab ponds. In 2018， the water depth in the river crab ponds was generally above 80 cm in the Hongze Lake Beach. The deep water level made the water temperature in the pond bottom low， which was favorable for the river crabs to avoid the heat by diving down to the bottom， thereby cutting down the degree of high temperature heat damage. The continuous high temperature wheatear caused sporadic deaths of river crabs， which began in the middle of July. During the high temperature period， except for 10 ponds， which had severe deaths of river crabs， the deaths caused by the high temperature heat damage were mild in general. During the high temperature heat damage， the cumulative number of dead crabs was 120-150 crabs/hm2 in each pond， and the death rate of river crabs was about 2.5%， falling into the mild level of high temperature heat damage.

The high temperature heat damage index was tested according to the extremely highest temperature （TX）， high temperature duration （L）， precipitation （R） and sunshine hours （S） of Laozi Mountain in 2018. First， the above values were substituted into formulas （6）-（9）， obtaining the values of ITX， IL， IR， IS （0.46， 0.6， 0.0， and 1.0， respectively）， and then the values were then substituted into formula （10）， obtaining I=0.314 8. In other words， the high temperature heat damage in 2018 was of mild degree in Hongze Lake Beach， indicating that the calculated value of the index model was in complete agreement with the actual situation.

Conclusion and Discussion

（1） High temperature heat damage is divided into 3 grades. After disaster occurs， if the high temperature at which the mortality rate of crabs is 2%-5% （excluding）， and the cumulative number of dead crabs during damage process is 120-450 crabs/hm2， it is the mild degree （grade 1）; if the high temperature at which the mortality rate of crabs is 5%-10% （excluding）， and the cumulative number of dead crabs during damage process is 450-900 crabs/hm2， it is the moderate degree （grade 2）; when the mortality rate of crabs reaches over 10%， and the cumulative number of dead crabs during damage process reaches up to 900 crabs/hm2， the high temperature heat damage reaches the severe degree （grade 3）.

（2） The high temperature heat damage to river crab breeding is mainly affected by the highest temperature （TX）， high temperature duration （L）， precipitation （R） and sunshine hours （S）. When the temperature reaches 35.0， the river crabs stop their growth and feeding， enter the dormancy states， which delays the shelling time. When the high temperature weather lasts for 3-4 d， the river crabs begin to die. The degree of high temperature heat damage is also related to the precipitation in July and the intensity of sunshine in Hongze Lake Beach.

（3） The long time strong sunshine weather can increase the water temperature， accelerate the death and decay of water plants， and aggravate the degree of high temperature heat damage. In addition， the water level and the coverage of water plants in the crab ponds also have an impact on the degree of high temperature heat damage， but the relationship between them remains to be further studied.

（4） In this study， only the river crab growth and meteorological observation data in Hongze Lake Beach in 2008-2018 have been studied and analyzed， so the established high temperature heat damage index model needs to be verified and verified in a longer time range. On the other hand， both the history fitting and business trial test shows that the model has accurate results， so it can be used in meteorological services.

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