Population Dynamics And Predation Strategies Of Bryde's Whales In Guangxi

Balaenoptera edeni is one of the most mysterious cetaceans.The IUCN lists Bryde's whale as"Least Concern"in 2018.However,this assessment was limited to common Bryde's whale,the offshore subspecies.And the inshore Eden's whale was not taken into account,for which baseline information was extremely scarce.In April 2018,we discovered an inshore population of Balaenoptera edeni edeni in the waters of Weizhou Island,Guangxi.It was a new record of Eden's whale population in the world.It was the third population in addition to Thailand and the East China Sea(including inshore Japan).It was also the first large whale population found in Chinese inshore waters since China banned whaling in 1980,and was of great research value.The newly revised List of State Key Protected Wildlife upgrades it to wildlife under first class protection on Feb 5,2021.The baseline information of Eden's whales in Guangxi is blank at present.This study was performed based on a 3-year vessel-based survey in Weizhou Island,Guangxi.The population of Eden's whale in Weizhou Island was estimated by the capture-mark-recapture CMR method.Through the multi-angle analysis of the horizontal camera from the ship and aerial drone video,investigate the feeding behaviors and strategies of Eden's whale;Based on seven environmental factors,including chlorophyll a,surface water temperature and others,MaxEnt model was used to predict the suitable habitat of Eden's whale around the world.Suggestions on the future research direction of Eden's whale were put forward.The results were as follows:(1)Population size of Eden's whale in the waters around Weizhou Island,Guangxi.The vessel-based surveys conducted in the waters around Weizhou-Xieyang Island in Guangxi for 122 days between April 2018 and April 2021,and 404 Eden's whales were found.Based on the stability of the dorsal fin identification features,55 and 68individuals of Eden's whale were identified from the left and right dorsal fins,respectively.And a database of Eden's whale individual identification in Guangxi was established.The results of Joly-Seber model showed that the population size calculated using the left dorsal fin data was N=66(95%CI=58-93)and the right side was N=80(95%CI=71-105).The cumulative number of identified individuals of Eden's whale in Guangxi is still increasing year by year,so the study on population size needs to be improved by continuous investigation.The population size in 2019,2020 and 2021were 69-73,73-85 and 65-67,respectively,with the largest number in 2020 and the smallest number in 2019.There was no significant difference in the number of left and right dorsal fins in the same year,and there was no significant difference in the number of left and right dorsal fins in the three years.The abundance of Eden's whale in Guangxi was more than that in Thailand,but less than that in East China Sea.This study was only to estimate the population size of recognizable Eden's whale during the study period,without the application of the identification rate correction.Moreover,new individuals of Eden's whales were still identified every year.Therefore,the population size of Eden's whales in Guangxi might be larger than the current estimate,and continuous monitoring is needed.(2)The feeding behaviors of Eden's whale in the waters around Weizhou Island,Guangxi.The feeding behaviors of Eden's whale in Guangxi were found to be various.Eight different feeding behaviors have been identified,and two new forms have been defined:cooperative lunge feeding(COL,N=435,42.81%)and pirouette feeding(PIF,N=163,16.04%).The other six behaviors included upright lunge feeding(UPL,n=67,6.59%),right lateral lunge feeding(RLL,n=116,11.42%),left lateral lunge feeding(LLL,n=2,0.20%),vertical lunge feeding(VTL,n=197,19.39%),inverted lunge-feeding(INL,n=3,0.30%),and tread-water feeding(TWF,n=33,3.25%).There were significant differences among the feeding events with different behaviors.Laterality is a typical characteristic of the feeding behavior of cetaceans.The behavior of Eden's whale in Guangxi had strong laterality,and it was mainly right-biased.Through the analysis of the feeding behavior of Eden's whale,it was confirmed that the cooperative lunge feeding behavior is the first discovery of Balaenoptera.Cooperative feeding required a high level of temporal and spatial coordination between two or more Eden's whales,suggesting that Eden's is an efficient and adaptable predator capable of employing a wide variety of behaviors,perhaps more diverse than that of other baleen whales.(3)Suitable habitat prediction of Eden's whales in global watersChlorophyll a concentration(dry and wet season),bathymetry,sea surface temperature(dry and wet season),primary productivity,phosphate,silicate and diffuse attenuation were selected as environmental factors and distribution sites were downloaded and screened.The MaxEnt model was used to predict the suitable zones of the dry season(November to April)and wet season(May to October)in the global warm waters of Eden's whales.The results showed that the model had good prediction ability(AUC_training=0.968 in the dry season,AUC_test=0.966;Wet season AUC_training=0.986,AUC_test=0.985),suggesting that the results were reliable.According to the prediction results of the model,there were 15 suitable habitats for Eden's whales in the dry season,which were scattered in the coastal areas of the western Pacific Ocean,the northern Indian Ocean and the western Atlantic Ocean.Among them,seven areas were predicted as suitable areas but did not have actual distribution sites of Eden's whales,two areas had actual distribution sites but the predicted suitability was low,and six areas had high suitability as well as actual distribution.In the wet season,the suitable habitat of Eden's whales was mainly distributed in the western Pacific Ocean,from the East China Sea and South China Sea to the southwest of Korea and Japan.While the suitable habitat was reduced in other areas,and there is almost no suitable habitat in the coastal areas of North America,South America and southern Asia.The total area in dry and wet season were about 9316,000 km² and 1185,000 km²,respectively.Bathymetry had the most contribution rate of suitability in either dry or wet season.The prediction results were beneficial to identify the potential distribution and seasonal migration routes of this subspecies in global waters.Compared with the dry season,the predicted suitable habitats'latitude was higher in wet season.This paper estimated the population size of Eden's whales in Guangxi,grasped the abundance and inter-annual fluctuation,increased the population baseline information of Eden's whales in Guangxi.Eight feeding behaviors were identified and two new feeding strategies were defined,which laid a foundation for behavioral research of Eden's whales.The suitable habitat distribution and potential habitat of Eden's whales are predicted,which could provide scientific basis for the short-distance migration of Eden's whales,and have a certain guiding role for the protection of Eden's whales in global waters.Overall,we suggested that:on the one hand,continued to perform vessel-based surveys in Guangxi,for further population baseline information,use the international methods for satellite positioning,grasp the short distance migration route of Eden's whales,and improve the protection and management of Eden's whales;on the other hand,it was supposed to increase the investigation and research on the Eden's whales in the northern Indian Ocean and the western Pacific Ocean,analyzing the genetic diversity between different regions,so as to grasp the actual habitat distribution of Eden's whales and protect them.