Article

Prey decline leads to diet shift in the largest population of Indo-Pacific humpback dolphins?

Authors:
  • Cetacea Research Institute, Hong Kong
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Abstract

The Pearl River Delta (PRD) region on the southeast coast of China has long been known as a highly productive fishing ground. Since the late 1980s, fishing pressure in the PRD has been intense, which warrants concerns of potential fishery-related impacts on the food resources and foraging ecology of apex marine predators in this region, such as the Indo-Pacific humpback dolphin (Sousa chinensis). In this study, we examined 54 stomachs with food remains, collected from beached carcasses of humpback dolphins recovered during fifteen years between 2003 and 2017. The 6043 identified prey items represent 62 teleost taxa, primarily small estuarine fish, but also larger reef fish. The dolphins appear to be opportunistic foragers, hunting across the water-column, with preference for shoaling and meaty fishes (e.g. Chrysochir aureus IRI% = 38.6%, Johnius belangerii IRI% = 23.1%, Mugil cephalus IRI% = 14.0%). Our findings suggest a dietary shift in recent years, from primarily demersal (as previously reported) to greater intake of neritic and pelagic fish. Dolphin foraging group size has decreased in recent years, which corresponds with declining size and numbers of prey items retrieved from dolphin stomachs. We suggest that these are indicators of declining food resources. Faced with a shortage of preferred prey, humpback dolphins may have broadened their dietary spectrum to maintain their daily energy intake, while their foraging group size decreased in response to the altered tradeoff between the costs and benefits of group foraging. This article is protected by copyright. All rights reserved

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... Correspondingly, there is a considerable spatial heterogeneity in dolphin prey resources, an apparent result of heavy fishing pressure in the region (e.g., Duan et al., 2009;Lin et al., 2019;Zhou et al., 2019). Prey quantity/availability and size/quality appear to vary spatially across both PRD and LB, likely affecting the energetic costs of foraging Ho, 2021;Lin et al., 2021) which may have implications on individual fitness. Spatial heterogeneity in both resource availability and anthropogenic pressure have been attributed to spatial heterogeneity in demographic process in other taxa elsewhere (e.g., Reid et al., 2008;Harris et al., 2021;Ritterson et al., 2021). ...
... It should not be mistaken, however, with what some earlier studies (e.g. Jefferson, 2000;Jefferson and Hung, 2004) referred to as PRE, as much of the earlier work was based primarily (though not exclusively) in Hong Kong waters, a small section of a much larger LB within still larger and physiographically complex PRD coastal system (e.g., Karczmarski et al., 2016;Lin et al., 2021;Chan et al., 2022a). ...
... The spatially-differing and locally depleted prey resources likely hamper the foraging efficiency of humpback dolphins, especially in the north and west of LB (Lin et al., 2021). This corresponds with the dolphins' decreasing trophic level in the west of LB Ho, 2021), where individuals have to resort to feeding on much smaller prey items (Lin et al., 2021), likely undermining individual fitness. ...
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The Pearl River Delta (PRD) region on the southeast coast of China is among the largest and most populated metropolitan regions of the world, subjecting the Indo-Pacific humpback dolphins (Sousa chinensis) inhabiting coastal waters of this region – apparently the largest population of this species – to intense anthropogenic pressure. In this 5-year study (2011-2015), we applied mark-recapture techniques to quantify population parameters of humpback dolphins in the main body of the extensive estuarine system of the Pearl River, the Lingding Bay (LB), the most urbanized and industrialized part of PRD. The super-population size was estimated at 928 individuals (95% CI=823-1046), which however over-represents the latest number of extant animals. Annual estimates fluctuated between 708 and 750 dolphins, and likely reflect most accurately the latest/current number of humpback dolphins in waters of LB. Both the overall and annual estimates generated by our study are considerably lower than previously published abundance estimates. Apparent survival rates were generally low, estimated at 0.943 (SE = 0.008, 95% CI = 0.929-0.958) and 0.815 (SE = 0.025, 95% CI = 0.766-0.866) for adults and juveniles, respectively. In conjunction with limited recruitment, they may have led to a substantial decline in population numbers over the past two decades. Notably, dolphins exhibited heterogeneous apparent survival rates across the complex anthropogenic seascape of LB. Adult individuals that use Hong Kong waters as their primary habitat exhibited 6.2% higher survival rates than those in other parts of LB. This difference tripled (19.2%) for juveniles, a likely expression of their susceptibility to environmental and/or anthropogenic stresses. We suggest that the difference in survival is largely due to heterogeneous anthropogenic stressors that vary spatially across the estuary, among which habitat loss, pollution, and declining prey resources appear to be the primary threats. Based on our latest findings, we call for a strategic conservation planning with the highest priority given to preserve the remaining key dolphin habitats in Hong Kong waters, and to quantify and mitigate the impacts of major stressor(s) in Guangdong (mainland China) waters. Given the encroachment of ever-more-intense human activities on the coastal habitats of humpback dolphins, understanding their demographic response and the underlying mechanisms of the population processes may prove instrumental in the formulation of a conservation management strategy that can secure their continuous survival.
... In the PRD, they are the only delphinid species seen throughout the year. In Hong Kong, these dolphins have been studied since the mid-1990s (e.g., Parsons 1998;Jefferson 2000;Barros et al. 2004;Jefferson et al. 2012), but only recently was the scope of these studies widened to address various aspects of their population processes and structure (e.g., Chan and Karczmarski 2017;Karczmarski et al. 2017a;Guo et al. 2020;Lin et al. 2021) and expanded geographically across the Lingding Bay and further into the western reaches of the PRD (SCY Chan, W Lin, and L Karczmarski, study in progress; see Methods-Study area for the spatial extent of the region). ...
... Maritime traffic, including cargo ships, tankers, high-speed ferries, and other motorised vessels, is also less intense in the western reaches of the PRD that harbours only one major sea-port, as compared to the economic centre in the eastern PRD where 3 of world's top ten container ports and nearly a dozen other major sea-ports are located (Planning Department HKSAR 2005; General Office of the People's Government of Guangdong Province 2010; Constitutional and Mainland Affairs Bureau HKSAR 2018; World Shipping Council 2022). A newly published study of humpback dolphin diet conducted across the PRD region indicated a dietary shift in recent years, likely due to declining food resources (Lin et al. 2021); and here too the change in the dietary spectrum was notably more pronounced in the eastern than western PRD (Lin et al. 2021). Moreover, the proportion of juvenile dolphins recorded in our study was larger than in the eastern PRD (Chan 2019;Guo et al. 2020;Lin et al., unpublished), which is commonly thought to be indicative of healthier population status (Eberhardt 2002;King et al. 2015;Booth et al. 2020). ...
... Maritime traffic, including cargo ships, tankers, high-speed ferries, and other motorised vessels, is also less intense in the western reaches of the PRD that harbours only one major sea-port, as compared to the economic centre in the eastern PRD where 3 of world's top ten container ports and nearly a dozen other major sea-ports are located (Planning Department HKSAR 2005; General Office of the People's Government of Guangdong Province 2010; Constitutional and Mainland Affairs Bureau HKSAR 2018; World Shipping Council 2022). A newly published study of humpback dolphin diet conducted across the PRD region indicated a dietary shift in recent years, likely due to declining food resources (Lin et al. 2021); and here too the change in the dietary spectrum was notably more pronounced in the eastern than western PRD (Lin et al. 2021). Moreover, the proportion of juvenile dolphins recorded in our study was larger than in the eastern PRD (Chan 2019;Guo et al. 2020;Lin et al., unpublished), which is commonly thought to be indicative of healthier population status (Eberhardt 2002;King et al. 2015;Booth et al. 2020). ...
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Quantifying demographic parameters and patterns of social dynamics is fundamental to understanding the population ecology of group-living animals and carries considerable conservation implications. In the Pearl River Delta (PRD) region, one of the world’s largest, most urbanised and industrialised estuarine systems located on the southeast coast of China, Indo-Pacific humpback dolphins are under a multitude of anthropogenic pressures. While in Hong Kong, at the eastern perimeter of the region, these dolphins have been studied for many years, at the western reaches of the PRD they have received little research and conservation attention. In this study, with the application of mark-recapture and socio-demographic modelling techniques, we quantify population parameters, group dynamics and social structure, establishing socio-demographic baselines for this little known component of the longest-studied population of the genus Sousa. These dolphins live in a fluid fission–fusion society with markedly weak inter-individual affiliations. Individual ranging patterns and spatial preferences (e.g., foraging areas) appear to be among the key factors determining their grouping pattern and socio-spatial structure across the region, with several social clusters which, although discernible, especially at the peripheries of the region, frequently interact socially and overlap spatially. Currently, 914 dolphins inhabit waters of western PRD, but their estimated survival rates are below the previously estimated threshold needed for long-term survival as a viable demographic unit. These findings indicate high levels of environmental stress and raise conservation concerns, especially in the face of manifold and growing anthropogenic pressure. This study calls for comprehensive assessments of cumulative anthropogenic impacts and for a critical revision of present conservation measures.
... The ever-increasing market demand due to the growing human population (The World Bank, 2015) has imposed substantial stress on the marine ecosystems of coastal waters (Wang et al., 2012(Wang et al., , 2015, which is further accelerated by the rapid development of modern fishing techniques (trawler, seiner, gillnetter, line fishing, etc.). It was recently proposed that overfishing has led to prey shortage for an estuarine dolphin (Lin et al., 2021b), and that by-catch might shape the demographic development of an inshore porpoise in the Pearl River Delta region (PRD, also known as the PRD economic zone) which is the largest metropolitan region of the world both in terms of region size and human population (The World Bank, 2015). Though fishing activities and their derived impacts are generally recognized as major threats to coastal cetaceans in Guangdong waters, the intensity and the spatiotemporal pattern of fishing activities have been poorly examined and long overdue. ...
... As an example, fish resources have experienced a >70% decline in the northern Lingding Bay due to habitat destruction (Yu et al., 2016). It was generally recognized that the estuarine ecosystem has been severely destructed, which has not only reduced marine biomass, but also changed the trophic structure of the estuarine ecosystem (e.g., an increase of low trophic level species, Lin et al., 2021b;Yu et al., 2016;Zhang et al., 2019). In line with these findings, the ratio of FR AIS points over the reginal dataset was substantially lower in the PRD region (12.7%) ...
... The present study demonstrated the feasibility of examining the spatiotemporal pattern of overall fishing intensity through AIS data in the coastal waters off Guangdong province, which should be of great interest to fishery resource conservation and coastal management. In particular, the unexpected low level of fishing effort may reflect a severe fishery resource degradation in the PRD region, which will not only compromise the sustainability of the fishing industry but might also reduce the foraging efficiency of marine top predators, and ultimately compromise their population fitness (Lin et al., 2021b). ...
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Fishing activities result in a depletion of fish resources and an increase in bycatch probability of non-target species, which are thought to be the cause of the recent demographic change experienced by coastal cetaceans in waters off Guangdong province, China. However, fishing intensity and its spatiotemporal pattern remain poorly examined in this region. Here, random daily tracks from 100 fishing vessels were used to estimate fishing operations in the coastal waters off Guangdong province. The performance of two methods was tested with Automatic Identification System (AIS) data collected in the Pearl River Delta region (PRD). This assessment found that the Density-Based Spatial Clustering of Applications with Noise (DBSCAN) could falsely identify port regions as fishing grounds, while the speed-based method provided a more accurate estimate of fishing grounds with less computational effort. Subsequently, the spatiotemporal pattern of fishing intensity in the coastal waters off the entire Guangdong province was examined with the speed-based method. Our results showed that 87.75% of the coastal fishing activities occurred in waters <40 m in depth. The method detected two spatially heterogeneous fishing grounds extending from the PRD to either the west or east sides of Guangdong province. In the west fishing ground, 43.4% of fishing vessels operated in the waters off Yangjiang city; while in the east fishing ground, fishing activities were more evenly distributed. The present study suggests that the AIS is able to provide high-resolution data to monitor the development of fishing activities, which may be useful for informing marine resource management and spatial planning.
... Therefore, investigating the body condition of IPHDs could provide us with information about their health status and help us understand the causes of the observed population decline at some locations. Effects of contaminant exposure and changes in the availability of food resources are already of concern for some IPHD populations (Lingding Bay, Lin et al., 2021). In addition, obviously emaciated individuals have been reported in Taiwan (Dungan et al., 2011;Slooten et al., 2013;Wang and Araújo-Wang, 2017). ...
... Most of the fishery data and analyses of IPHDs diet available have been conducted in Lingding Bay and Jiangmen. Dolphins from Lingding Bay have recently gone through a diet shift and reduction of foraging group size that raised concern about prey availability Lin et al., 2021;Zhang et al., 2019). Lingding Bay IPHDs also present a lower foraging group size and a wider range of prey species than what is observed in Jiangmen, which could be linked with strategies adopted to cope with the reduced density of suitable prey . ...
... Analyses of humpback dolphin diet indicate that they feed on several species of soniferous fish (Barros et al., 2004;Lin et al., 2021), including the Sciaenidae such as the lionhead (Collichthys lucida), Belanger's croaker (Johnius belangerii) and anchovies (Coilia and Stolephorus spp.), all of which are likely to generate sounds of 2 kHz frequency range (Ramcharitar et al., 2006;NSYU, 2012). The sound generated by Sciaenids, which are among the dominant prey species of humpback dolphins in the PRD region (Lin et al., 2021), are generally below 5 kHz (Luczkovich et al., 2008). ...
... Analyses of humpback dolphin diet indicate that they feed on several species of soniferous fish (Barros et al., 2004;Lin et al., 2021), including the Sciaenidae such as the lionhead (Collichthys lucida), Belanger's croaker (Johnius belangerii) and anchovies (Coilia and Stolephorus spp.), all of which are likely to generate sounds of 2 kHz frequency range (Ramcharitar et al., 2006;NSYU, 2012). The sound generated by Sciaenids, which are among the dominant prey species of humpback dolphins in the PRD region (Lin et al., 2021), are generally below 5 kHz (Luczkovich et al., 2008). Correlations between humpback dolphin activities and fish chorus were previously seen within the region (Munger et al., 2016;Pine et al., 2017;Pine et al., 2018;Wang et al., 2019) and off western Taiwan (Guan et al., 2015) where the dominant frequency of croaker chorus was at 1.2-2.4 ...
... The Indo-Pacific humpback dolphin appears to be opportunistic feeders, consuming a wide variety of nearshore, estuarine, and reef fishes, with preference for shoaling and meaty fishes, such as the spiny head croaker (Collichthys lucida), the flathead grey mullet (Mugil cephalus), croaker (Johnius spp), and anchovies (Thryssa spp) (Jefferson and Smith, 2016;Lin et al., 2021). Most of the humpback dolphin's prey fish are commercial fish and have been overfished. ...
... Since the pre-mortem movement between sub-regions and post-mortem movement along with the drift is care in the PRD humpback dolphins, the dead dolphins in the study by Lin et al. (2021) represent the ration of the whole population. However, as the signifcantly difference of community structure of potential prey fishes among provinces (Guangdong, Guangxi and Fujian provinces) and stages (2018-2020, 2012-2014 and 2006-2008) (q<0.05, ...
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Introduction Overfishing and climate change have combined to cause fishery stocks to decline and fish community composition to change, further threatening the predation and nutritional health of marine mammals. Methods In this study, we collected potential prey fishes catched by fishermen in six habitats of Indo-Pacific humpback dolphins and analyzed their proximate composition (moisture, water, fat and protein), the fatty acid composition and the amino acid composition to evaluate the possible health effect on humpback dolphins. Results The results showed that the nutritional composition varied significantly with species and locations. Fishes in the families Sciaenidae and Engraulidae displayed richer fatty acid composition, while those in the family Clupeidae had the highest value of amino acid quality index. In Zhuhai, home to the largest Indo-Pacific humpback dolphin population, pelagic/neritic prey fishes possessed lower energy density, PUFA content, PUFA/SFA ratio, DHA content, and EAA content compared to demersal fish, suggesting nutritional stress when there is a dietary switch from demersal to pelagic/neritic fishes in Zhuhai population. Discussion Our study provided a framework, with energy density and fatty acid composition as its most important indicator, for assessment of the marine top predators based on the nutritional composition of their prey fishes and revealed the potential threats. Data here is expected to facilitate the development of scientific programs for successful conservation of not only the Indo-Pacific humpback dolphins, but also other marine top predators, possibly through reconstructing their prey fish’s quantity and quality.
... This shift was consistent with the previous stomach content analysis (SCA) showing that the PRE humpback dolphins shifted their diet from primarily demersal to greater intake of pelagic fish in recent years, 30 and was also in line with the reported decreasing proportion of large carnivorous fishes in the diet of PRE humpback dolphins in recent decades, as revealed by quantitative fatty acid signature analysis (QFASA). 31 Since the 1980s, increased fishing boats, improved fishing technology, and intensified fishing activities in the PRE have largely depleted local fishery resources. 70 Many high TL fish (e.g., Coilia sp., Clupanodon sp., Macrura sp., and Collichthys sp.) have shown signs of overexploitation in the PRE in recent decades. ...
... 29 Collichthys lucidus, which comprised nearly 50% of the estuarine fish stock biomass in the PRE in the late 1980s, has currently declined to below half of this figure. 31 Thus, the temporal dietary changes of the dolphin are presumably related to waning food resources, and especially the depletion of higher TL fishes in the PRE. 3.4. ...
... For instance, up to 75% of licensed bottom trawlers operating in West African waters in 2017 were registered in, or owned by, China (Virdin et al. 2022). Europe also relies heavily on catches by its own distant-water fleets, which operate predominantly in Africa (Belhabib et al. 2015;Virdin et al. 2019;Lin et al. 2021;Fig. 5 ...
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Download pdf (free): http://www.oceancare.org/trawlsupremacy --- Trawling is a type of fishing characterized by the active towing of nets by a moving boat. Trawl nets vary greatly in size and shape, and they target a wide variety of species, including bottom-dwelling fish, crustaceans and molluscs, pelagic and semi-pelagic schooling fish, and deep-water fauna. In this report, we provide a general overview on towed gear, but we focus more specifically on bottom trawling: the towing of nets along the seabed. Bottom trawling has become a cornerstone of global food supplies, accounting for more than one quarter of global fishery landings. In 2016, this equated to over 30 million tonnes of seafood. In several European and African countries, half of fishery landings come from bottom trawling. Bottom trawling, however, has long been known to be detrimental to marine life. It was regarded as a destructive fishing method since the early 14th century, and was often vocally opposed by communities of fishers who saw it as a threat to marine resources and their own livelihoods. The introduction of steam and diesel engines (in the 1830s and 1930s, respectively) marked the modern era of trawling. Engine-powered trawling increased rapidly during the 1960s, and by the 1980s large fleets of trawlers were combing the global oceans. Today’s bottom trawlers can operate virtually anywhere, from shallow inland channels and rivers to deep offshore waters. Countless scientific studies, encompassing decades of fishery research, have documented the harmful nature of bottom trawling, with substantial cumulative evidence of damage to marine species and ecosystems. Bottom trawling reduces the biomass, diversity and complexity of benthic communities, and the action of trawl gear on the seabed causes dramatic mechanical and chemical alterations, compromising the seabed’s functionality and productivity. In addition to the target species, most types of trawl gear take unwanted species, such as threatened elasmobranchs, sea turtles, seabirds and marine mammals. Apart from these biological impacts, recent studies indicate that bottom trawling has a considerable carbon footprint, with high direct and indirect greenhouse gas emissions contributing to climate disruption. Information on the harmful effects of bottom trawling has resulted in public and institutional awareness of environmental damage, and in restrictions that have sometimes included complete bans. Trawling is often prohibited in the most coastal and shallow waters. However, regulations and enforcement levels vary greatly across areas, and environmental protection measures are often ineffective—to the point that the intensity of bottom trawling can be higher inside than outside some Marine Protected Areas. In this report, we review the evidence of how bottom trawling affects marine life and human life. We also summarize some of the primary management approaches that could help mitigate the harmful effects of trawling—consistent with international commitments to protect the marine environment. We conclude that the amount of seafood produced by bottom trawling can no longer justify or excuse the pervasive damage caused to marine ecosystems and communities of small-scale fishers, and we advocate the use of less destructive fishing gear, combined with the creation of areas protected from harmful fishing practices, and more sustainable strategies to “feed the world”.
... In this study, we detected 13 species of concern (12 in the oceanic site, 2 in the estuarine site, and 1 overlapping species) with varying detection probabilities associated with different sampling volumes per replicate. The Bombay-duck H. nehereus, detected only in half of the 10 L samples in the current study, was a dominant prey of the Indo-Pacific Humpback Dolphin population in the PRE in the 2000s (Karczmarski et al. 2016;Lin et al. 2021). Still, due to this species' resource depletion, the dolphin population's dominant prey has changed to the smaller Thryssa spp. ...
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The environmental DNA (eDNA) approach is an emerging tool for monitoring marine biodiversity. However, the sampling effort needs optimization according to the site characteristics and target taxonomic groups. In this study, we optimized the eDNA sampling effort in terms of sample volume and number of replicates to monitor the diversity of marine vertebrates (mainly fish) in Hong Kong's subtropical waters that show a gradient of estuarine to oceanic waters. To maximize detection, we used three pairs of metabarcoding primers (12S‐v5, MiFish‐U, and MiFish‐E). We compared vertebrate diversity in 78 water samples, ranging from 1 to 10 L, collected from oceanic and estuarine sites. Metabarcoding yielded a total of 140 vertebrate species, of which 18 were unique to the estuarine site, 66 unique to the oceanic site, and 56 shared between both sites. The detected species were predominantly ray‐finned fish (136 species), and the three primer pairs exhibited differential sensitivity toward different taxa, especially cartilaginous fish and cetaceans. Increasing sampling volume per replicate generally increased the total detected species, average species per replicate, and species coverage, and sampling 3 or 4 × 4 L represented the most efficient sampling effort for the estuarine and oceanic sites, respectively. The diversity analysis revealed that sampling >2 L per replicate reduced variability and improved diversity analysis. The results also showed that a larger sampling volume per replicate increased the probability of detecting endangered, indicator, invasive, and elusive species, with 4 L representing the most efficient volume. This study recommended sampling 4 L per replicate and 3 replicates for estuarine and 4 for oceanic sites, respectively for effectively monitoring marine fish in subtropical waters using the eDNA approach.
... Our research highlights the tipping points and standard deviation of encounter rates as potential predictors of ecological regime shifts before demographic changes in this species. Moreover, the distribution change of marine cetaceans can serve as a signal for regional species composition and biodiversity because the spatial variation of dolphin distribution is correlated with the availability of prey species (Lin et al. 2021). ...
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Global ocean warming and extreme climate events pose a severe threat to marine biodiversity by inducing species redistribution and ecosystem reorganization. It is important to quantify the impacts of marine heatwaves (MHWs) on marine cetacean habitats to avoid rapid ecosystem shifts. Here we utilized detected breakpoints and early warning indicators derived from sightings data spanning from 2009 to 2021 to assess the distribution change of Indo-Pacific humpback dolphin (Sousa chinensis). We found that (1) during the 2014 MHWs, the encounter rates (number of on-effort sightings per 100 km) of humpback dolphins in Hong Kong waters significantly decreased, with the breakpoint occurring during the autumn of 2014; (2) Since 2014, Hong Kong waters have experienced more prolonged and frequent MHWs, with a significant reduction of core habitat by 26%; according to Granger causality analysis, changes in sea surface temperature drove shifts in dolphin distribution; (3) Our analysis revealed a co-existence of rapid annual increases in MHWs and high habitat usage, with the marine park located in Southwest Lantau being particularly at risk. This study on dolphin distribution shifts and their relationships with marine heatwaves in Southern China made a contribution to our understanding of the action of marine cetaceans' response to climate change. Additionally, it highlights the importance of considering MHWs in dolphin conservation efforts.
... There is a possibility that numerous incidents of interactions between ALDFG and cetaceans in Indian waters are not cited or published in media. Cetacean feeding grounds typically overlaps with active fishing areas, enhancing the interactions with fisheries and ALDFG, subsequently increasing the probability of these organisms becoming entangled (Lin et al., 2021;Zhaolong et al., 2022;Gall and Thompson, 2015). In the literature, cetaceans are commonly reported as entangled in marine litter (Gall and Thompson, 2015;Mghili et al., 2023a). ...
... larval vs. adult insects, seeds vs. foliage of plants). To gain a more complete picture of pastured poultry ecosystem functions and bird feeding behavior, sequencing-based approaches should be combined with other methods, for example, histological, camera recoding, direct observation, GPS-collared tracking, and stable isotope analysis as other authors have suggested (Clark & Gage 1996;Kerley et al. 2015;Murray et al. 2016;Lin et al. 2021). ...
Article
Metabarcoding-based diet analysis is a valuable tool for understanding the feeding behavior of a wide range of species. However, many studies using these methods for wild animals assume accuracy and precision without experimental evaluation with known positive control food items. Here, we conducted a feeding trial experiment with a positive control community in pasture-raised chickens and assessed the efficacy of several commonly used DNA extraction kits and primer sets. We hand-fed 22 known food items, including insects and plants, to six backyard laying hens and collected their excreta for eight h. We evaluated the efficacy of three DNA extraction kits, three primer sets for plant identification (targeting rbcL, trnL, and internal transcribed spacer 2 [ITS2]), and three primer sets for arthropod identification (targeting cytochrome oxidase subunit I [COI]). The detection success rate of our positive control food items was highly variable, ranging from 2.04% to 93.88% for all kit/primer combinations and averaging 37.35% and 43.57% for the most effective kit/primer combination for plants and insects, respectively. Extraction kits using bead-based homogenization positively affected the recovery proportion of plant and insect DNA in excreta samples. The minimum time to detect known food items was 44 min post-feeding. Two COI primer sets significantly outperformed the third, and both recovery proportion and taxonomic resolution from ITS2 were significantly higher than those from rbcL and trnL. Taken together, these results display the potential variability that can be inherently present in DNA-based diet analyses and highlight the utility of experimental feeding trials in validating such approaches, particularly for omnivores with diverse diets.
... However, little is known about the foraging strategies used by IPHDs and their potential social implications. With the intense fishing activities that have taken place for decades along the coast of China, prey depletion could be a factor impacting the social interactions in IPHDs and explaining spatial differences (see Lin et al., 2021). ...
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Tooth rake marks are usually used as an indicator of received aggression in odontocetes whose social interactions are difficult to observe directly. Photographic data from five locations of the northern South China Sea were used to analyze seasonal, demographic, and geographic effects on the density of tooth rake marks and dorsal fin notches in Indo-Pacific humpback dolphins (Sousa chinensis, IPHDs). The analysis revealed that fresh tooth rake marks were most frequent during the dry season. Males exhibited significantly higher rake mark density than females and rake density declined significantly with age. The density of notches was also significantly higher for males than for females, and lower for young dolphins than for mid-aged and adult ones. Geographically, tooth rake mark and notch density were the highest in Sanniang Bay and Southwestern Hainan. These results provide indirect evidence of variation in social interactions across seasons, across the lifespan of an individual, between sexes, and among different locations. This variation may result from changes in biological parameters , differences in social lives, and habitat characteristics. A model to determine the sex of dolphins was tested, and after further validation, such sex assignment could contribute to understanding IPHDs' social structure.
... Many species and populations, including coastal dolphins, are experiencing cumulative anthropogenic pressures such as habitat degradation and modification Kreb et al., 2020), increasing vessel traffic and noise (Bejder et al., 2006;Jensen et al., 2009a;Marley et al., 2017a;Erbe et al., 2019), and pollution (Cagnazzi et al., 2013a;Parra et al., 2017). Direct impacts may come from incidental entanglement in fishing gear (Laist, 1997;Jensen et al., 2009b;Reeves et al., 2013) and vessel strike (Wells and Scott, 1997;Thiele, 2010;Peel et al., 2018;Schoeman et al., 2020), whilst habitat loss, reduced prey from fishing (recreational, commercial and aquaculture activities) and increasing tourism have indirect impacts often resulting in disturbance, avoidance, and displacement from critical habitats (Paiva et al., 2015;Strickland-Munro et al., 2016;Karczmarski et al., 2017;Wang et al., 2017;Clarkson et al., 2020;Kassamali-Fox et al., 2020;Lin et al., 2021). Managers rely on science to better understand these impacts to wildlife populations and, in particular, how to mitigate these. ...
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For long-lived species such as marine mammals, having sufficient data on ranging patterns and space use in a timescale suitable for population management and conservation can be difficult. Yawuru Nagulagun/Roebuck Bay in the northwest of Western Australia supports one of the largest known populations of Australian snubfin dolphins (Orcaella heinsohni)—a species with a limited distribution, vulnerable conservation status, and high cultural value. Understanding the species’ use of this area will inform management for the long-term conservation of this species. We combined 11 years of data collected from a variety of sources between 2007 and 2020 to assess the ranging patterns and site fidelity of this population. Ranging patterns were estimated using minimum convex polygons (MCPs) and fixed kernel densities (weighted to account for survey effort) to estimate core and representative areas of use for both the population and for individuals. We estimated the population to range over a small area within the bay (103.05 km²). The Mean individual representative area of use (95% Kernel density contour) was estimated as 39.88 km² (± 32.65 SD) and the Mean individual core area of use (50% Kernel density contour) was estimated as 21.66 km² (±18.85 SD) with the majority of sightings located in the northern part of the bay less than 10 km from the coastline. Most individuals (56%) showed moderate to high levels of site fidelity (i.e., part-time or long-term residency) when individual re-sight rates were classified using agglomerative hierarchical clustering (AHC). These results emphasize the importance of the area to this vulnerable species, particularly the area within the Port of Broome that has been identified within the population’s core range. The pressures associated with coastal development and exposure to vessel traffic, noise, and humans will need to be considered in ongoing management efforts. Analyzing datasets from multiple studies and across time could be beneficial for threatened species where little is known on their ranging patterns and site fidelity. Combined datasets can provide larger sample sizes over an extended period of time, fill knowledge gaps, highlight data limitations, and identify future research needs to be considered with dedicated studies.
... To simulate the prey library of wild humpback dolphins, we established an extended prey library (containing 19 species of prey, Table 1) which included all 8 species of prey ever fed by the captive dolphin as well as 11 other species of potentially important prey of the wild dolphin population from the Pearl River Estuary based on stomach content information (Barros et al., 2004;Lin et al., 2021). Three prey library subsets (Table 2) were used to test the robustness of the model : (1) the ''actual'' subset, which included only the 5 species of prey that were included in the long-term diet of the captive dolphin, (2) the ''complete'' subset, which included all 8 species of prey that were ever included in the diet of the dolphin, and (3) the ''extended'' subset, which included all the 19 species of prey in the extended prey library. ...
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Accurate diet estimation of marine mammals is fundamental to investigate their ecological roles in marine habitats, but it remains a pervasive challenge to researchers due to the constraints associated with conventional methods based on stomach contents or feces analysis, which are imprecise, limited to last meal and often biased. Quantitative fatty acid signature analysis (QFASA), an emerging method for diet estimations, has been demonstrated to be reliable in reconstructing long-term diets of many marine mammal predators; however, its application hasn’t been seriously evaluated in dolphins. To this end, we compared the QFASA estimates obtained using 8 pre-existing calibration coefficient sets, 3 fatty acid subsets, and 3 prey library subsets to the true diet of a captive Indo-Pacific humpback dolphin (Sousa chinensis) that was fed a long-term diet of mostly gizzard shad (Clupanodon thrissa) with smaller proportions of Japanese seabass (Lateolabrax japonicus), pumpkinseed (Lepomis gibbosus), roughscale tonguesole (Cynoglossus lida), and Chinese herring (Ilisha elongate). The diet of the dolphin was best reconstructed using the calibration coefficients derived from captive feeding trials of American mink (Mustela vison) that were fed herring with the “main” fatty acid subset; this process correctly identified gizzard shad as the main prey source of the dolphin with the highest actual diet proportions and lowest predator signature proportions outside the range of the mean prey proportions. In addition, high accuracy and consistency were acquired in the diet estimations obtained using the calibration coefficients derived from minks fed herring and herring plus seal oil even though pseudo-prey were included in our model, revealing the high robustness and reliability of QFASA in reconstructing the dolphin diet. We recommend caution in selecting fatty acid subsets when using fatty acid signatures derived from different studies. Overall, QFASA offers a promising tool to quantify the long-term diets of humpback dolphins and other odontocetes.
... The continuous decline of fishery resources may lead to a negative impact on food supplies for cetaceans on a global scale (Plagányi & Butterworth 2009). In this volume, Lin et al. (2021b) examined 54 stomachs with food remains, collected from beached carcasses of Indo-Pacific humpback dolphins. The 6043 identified prey items included 62 teleost taxa, primarily small estuarine fish species, but also 13 larger reef fish species. ...
... Their restricted habitat preference to inshore shallow waters renders them susceptible to a wide range of anthropogenic impacts (Jefferson & Smith 2016;Karczmarski et al. 2016;Li 2020), of which, in the long-term, habitat degradation and loss appear to represent the greatest threat Karczmarski et al. 2017a,b;Huang et al. 2020). In China, in the face of the fastest-ever coastal development over the past 2 decades, this species has received considerable conservation and research attention in recent years (Xu et al. 2015;Chen et al. 2016;Karczmarski et al. 2017a;Lin et al. 2021). ...
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Indo‐Pacific humpback dolphins (Sousa chinensis) inhabit shallow coastal waters of the Indo‐Pacific region including southeast China, with at least six putative populations identified to date in Chinese waters. However, the connectivity among these populations has not yet been investigated. In the present study, we compared and cross‐matched photographic catalogs of individual dolphins collected to date in the Pearl River Delta region, Leizhou Bay, Sanniang Bay, and waters southwest of Hainan Island, a total of 3158 individuals, and found no re‐sighting of individual dolphins among the four study areas. Furthermore, there was a notable difference in the pigmentation pattern displayed by individuals from these four regions. We suggest that this may be a phenotypical expression of fine‐scale regional differentiation among humpback dolphin groups, possibly distinct populations. Given the considerable conservation management implications it may carry (e.g. definition of management units), further research is much needed. This article is protected by copyright. All rights reserved
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The Indo-Pacific humpback dolphin is an inshore species occurring in productive estuarine and coastal areas of the Indo-Pacific. Since the end of the 20th century, coastal region of China has experienced tremendous economic expansion, exposing this dolphin species to a wide range of human activities. Despite frequent voices of concern in the past three decades calling for research and conservation attention, protracted debates were not followed by constructive conservation actions due to the lack of or inconsistent use of demographic data. The current study examined 13-year sighting and photo-identification data collected for a small and little known population of humpback dolphins off the coast of Shantou (ST) in the northern South China Sea. Compared to conspecific populations inhabiting large or medium-sized estuarine systems, the ST dolphins have much larger home range (mean = 181.3 km2, S.D. = 87.7) which, however, has been shrinking recently due to coastal reclamations and expanding aquaculture farming. Although at least two dolphin births were recorded between 2010 and 2022, there was no successful recruitment of a non-calf individual. Meanwhile, the population shrank by 29.4%, with the most updated population size of only 12 individuals, estimated in 2022. Based on the current demographic trend, we predict that the ST humpback dolphins will be extirpated within one or two decades. Furthermore, as the dolphins are aging and becoming less reproductively viable, there is little chance to reverse the declining trend even without additional stressors. Moreover, we noticed prevailing vertebral column deformities amongst the ST dolphins, possibly a sign of inbreeding depression of a small population. The present study exemplifies a typical case of a small, isolated and locally endangered dolphin population that is approaching local extinction without efficient monitoring program and conservation action. We emphasize that without solution-based research and efficient conservation planning, other humpback dolphin populations in China might soon follow in the steps of ST humpback dolphins.
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Indo-Pacific humpback dolphins (Sousa chinensis) inhabiting the Chinese coastline of the northern South China Sea are subject to intense human activity. To investigate potential impacts to dolphins, photographs taken from 2010 to 2021 in five different locations were analyzed. Among individual dolphins (N = 1,159), 10.6% presented human-induced injuries. Adults were more frequently injured than young dolphins, and individuals from around Jiangmen, Hainan, and Sanniang Bay presented the highest injury prevalence. More research should be conducted in areas that have been subject to less research, such as Jiangmen and Hainan, and the dolphin watching activities occurring in Sanniang Bay should be monitored and regulated. Forty-seven percent of dolphins presented skin conditions, with orange patches being the most frequent (41.5%). Orange patches and hyper-pigmented pink spots were significantly less prevalent in summer and fall than in winter and spring. Orange patches were less frequent in females than in males, as well as with calves relative to older age classes. Seasonal patterns may be linked to water temperatures, while demographic patterns may be associated with immune defenses. These findings allow for a better understanding of the potential impact of human activities on Indo-Pacific humpback dolphins and should inform further conservation measures.
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It is generally accepted that vessel activity causes various behavioral responses of cetaceans and undermines individual fitness. Whether or how it can lead to a demographic response of populations remains rarely examined. In the northern Beibu Gulf, China, vessel activities have sharply increased in the past two decades, while abnormal demographic dynamics was recently noted for the resident Indo-Pacific humpback dolphins. The present study first examined the humpback dolphins' utilization distribution (UD) from 2003 to 2019. Habitat suitability was then modeled with the sighting data collected before the most recent population reduction. Finally, we tried to disentangle the anthropogenic driver of dolphin demography by cross-referring the spatiotemporal development of dolphins' UD, vessel activities, and habitat suitability. Our results showed that the dolphins' UD shrank substantially during the port expansion in the early 2010s, and we suggest that the consequential increase in vessel activities might impose extra marine stressors on the resident humpback dolphins. To reduce the boat interaction , the dolphins steadily shifted their core area to a less suitable area in the east during 2015-2017, when unnaturally low survivals were recorded. Afterward, the dolphin core area partially shifted back to the more suitable area in the west, which corresponded to the improving dolphin survival in 2018. Our finding suggested that the vessel activity may be responsible for the dolphin displacement, while staying in the less suitable area may further lead to a more severe and acute demographic consequence on the population. The underlying and indirect impact of vessel activities as disclosed by the present study is particularly important for port management, marine planning, and conservation practice regarding coastal cetaceans, especially for those resident and endangered populations inhabiting the urbanized coastal areas.
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Reserve planning based on the investigation of the core habitat distribution of wild cetaceans is a powerful conservation strategy for protecting target species. However, studies on core habitats and their variations at the large-scale distribution of cetaceans are limited. In this study, we conducted a seven years (2015-2021) boat-based field observation surveys with highly applicable and generalized methods to analyze the habitat changes and how these changes influenced the largest known Indo-Pacific humpback dolphin (Sousa chinensis) population in the Pearl River Estuary (PRE), China. Our findings revealed that there is 3204 km² of dolphin habitat in the PRE, and dolphin distribution preferences are related to nutrients and biological resources. The decreasing trend of dolphin sighting rates in most areas (73.2%) indicated that the dolphin habitat had declined, and the annual habitat decline rate (2.83%; 95% confidence interval: 0.58%-5.08%) was consistent with the population decline rate. The annual habitat decline rate in the hypoxic zone revealed that low dolphin sighting rates from March to October in the hypoxic zone was 0.43% faster than that in the non-hypoxic zone, indicating long-term sustained effects of summer hypoxia on the dolphin population. Existing reserves have partially prevented habitat decline. However, more than 82.3% of the habitat has not been effectively protected and encounters the threats of pollution and hypoxic stress. This study offers new scientific evidence for developing effective monitoring strategies for humpback dolphin populations in the PRE, and also help establish spatial planning and management measures for reserve habitats.
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Morphological abnormalities in wild animals can be indicators of the underlying health of a population and may be determined through routine photographic surveys. Here we assess unusual rostrum conditions in Indian Ocean humpback dolphins (Sousa plumbea) inhabiting South African coastal waters to understand the rate of prevalence of abnormal rostrums and formulate hypotheses on potential causes. Photographic data were collated from systematic boat surveys and opportunistic sightings, obtained between April 1998 and March 2021 in various regions along the South African coast. Overall, 31 unique individuals were found with abnormal rostrum conditions, varying from slight misalignments to severe wounds and/or aberrant morphologies. In most cases, injuries were likely caused by natural events during the animal's life history such as interactions with sharks and/or reef-associated hunting strategies. Mark-recapture data indicated that individuals had survived with these injuries for up to ten years. This study reports the highest incidence of rostrum abnormalities in the species. As numbers reflect only those that have survived their injuries, they are considered a minimum estimate. Better understanding of the cause(s) of these injuries is important given the endangered status of this species. This article is protected by copyright. All rights reserved.
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Several populations of odontocete cetaceans, including at least 19 species, have modified their behavior and adapted to foraging in association with trawlers. We review information on odontocete interactions with different types of trawlers across 13 Food and Agriculture Organization fishing areas around the world. We also review knowledge gaps, the effects on odontocete ecology, distribution, behavior and social organization, the main mitigation options, and some management avenues that could help reduce incidental mortality. Trawlers involved in the interactions varied greatly in gear and target species, implying odontocetes have developed behavioral specializations to forage under a variety of conditions. Specialized behavior included venturing into a moving trawl net to feed on the organisms trapped in the net, feeding on fish stirred up by the net, extracting fish from the outer mesh, feeding on catch lost during hauling, and scavenging on discarded catch. Foraging behind trawlers facilitates access to prey, and in some instances may compensate for scarcity of natural prey within areas exposed to intensive fishing or environmental degradation. This opportunistic foraging strategy, however, exposes the animals to potential harm and mortality in trawl gear. The combined effect of facilitated foraging and bycatch on the status and trends of odontocete populations is unknown. The economic damage caused by odontocetes, e.g. in terms of loss of marketable catch and gear damage, remains largely conjectural. Attempts to reduce depredation and/or bycatch in trawl gear have included acoustic deterrents and exclusion devices installed in nets, although neither technique has proven to be consistently effective. --- https://doi.org/10.1007/s11160-022-09712-z
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Accurate diet identification of top predators is crucial to fully understand their ecological roles. Compared to terrestrial animals, gathering dietary information from cetaceans is notoriously difficult. Here, we applied a multilocus metabarcoding approach to investigate the diet of vulnerable Indo‐Pacific humpback dolphins and Indo‐Pacific finless porpoises from the Pearl River Estuary (PRE), China. Our analyses identified 21 prey fish species from the 42 humpback dolphin stomachs, as well as 10 species of fish and one species of cephalopod from the 13 finless porpoise stomachs. All of the taxa were assigned to the species level, highlighting that the multimarker approach could facilitate species identification. Most of the prey species were small and medium‐sized fishes that primarily fed on zooplankton. The calculated similarity index revealed a moderated dietary overlap between the two cetaceans, presumably due to the feeding of the two predators in association with fishing vessels in the PRE. A more diverse diet was observed in humpback dolphins in the closed fishing season compared to the fishing season, implying the influence on the dolphin diet due to the availability of commercial fishery resources. However, according to the results of species rarefaction curves, our findings on the feeding habits of the two cetaceans are still limited by insufficient sample size and therefore should be interpreted with caution. This study represents a first attempt to apply the multilocus DNA metabarcoding technique in the diet analysis of small cetaceans, although more efforts are needed to improve this type of analysis. This article is protected by copyright. All rights reserved
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Age composition is an important population parameter for understanding and managing endangered species. There are very few studies involving cetaceans in estimating the age of individuals in the wild. In this study, we rigorously quantified the body color pattern of 37 stranded Indo-Pacific humpback dolphins (Sousa chinensis) of the Pearl River Estuary (PRE), which showed a significant, albeit with variability between individuals, correlation with age for both the males (between age 1 and 35, r2 = 0.84) and females (between age 1 and 25, r2 = 0.85). The population-averaged correlation (r2 = 0.85) was then applied to a large volume of photo-identification data (2011–2015) to estimate the age composition of the population occurring in the greater deltaic region, which further suggested a spatial difference in age composition and, therefore, a complex demographic process of the humpback dolphin across the region. In particular, the age composition of humpback dolphins in the east PRE (Lingding Bay) is severely aged. Finally, a population viability analysis with the consideration of observed age structure and the simulated age-specific fecundity suggested that 95.75% of the Lingding Bay humpback dolphins are projected to be lost after three generations. Given the sophisticated anthropogenic landscape in the PRE, we suggest that management units of the humpback dolphins should be clearly defined and the regional-specific conservation measures are much needed.
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Humpback dolphins (genus Sousa), obligatory inshore delphinids, are frequently exposed to adverse effects of many human activities. In Hong Kong, one of the world’s most urbanised coastal regions, ~ 50% of the dolphins suffer from at least one type of epidermal lesions, likely related to anthropogenically degraded habitat. Furthermore, one in every ten dolphins has physical injuries indicative of vessel collisions, propeller cuts and fishing-gear entanglements. As top predators with long lifespan, dolphins are good “barometers” of marine environment and their compromised health conditions are symptomatic of increasingly degraded ecological conditions of coastal seas, especially in rapidly developing regions of fast-growing economies.
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Early studies of Chinese white dolphins applied primarily line-transect survey techniques to acquire distribution data and generate abundance estimates, while many recent studies rely on mark-recapture techniques to gain insights into population dynamics and demography. The application of an individual photo-identification approach provides the means to estimate vital population parameters with mark-recapture models which in turn facilitate population viability analysis. Here, we review the application of the photographic mark recapture technique in demographic studies of Chinese white dolphins in China waters; we summarize the quality assessment of an individual mark recapture databases; ran simulations to evaluate how survey effort may impact the accuracy and precision of population size estimates; discuss how the survey design, selection of individual-ID marks and data filtering may impact the mark-recapture analysis; and we point out how to minimize the bias associated with data over-dispersion through model selection according to goodness-of-fit testing. We discuss also conceptual challenges that may lead to misinterpretation of data when comparing results from different studies that used different field techniques. With improved understanding of analytical principles behind the data gathering, we hope that demographic studies of Chinese white dolphins in the region will expand and improve in the near future.
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The world's largest population of Indo-Pacific humpback dolphins (Sousa chinensis) in the Pearl River Estuary is exposed to some of the world's greatest concentration of anthropogenic threats, much of it due to the region's rapid urbanisation and industrialisation in recent decades. By assessing behavioural responses of humpback dolphins to long-term and large-scale environmental change in Hong Kong, and short-term but frequent behavioural disturbance due to unregulated dolphin-watch tourism, this study investigates humpback dolphins as a model system of a coastal delphinid in the coastal seas of the Anthropocene. Field surveys were conducted between 2011 and 2014 to identify humpback dolphins’ distribution pattern. Resource selection function was applied to model the dolphin's habitat selection process and quantify their long-term habitat preference. The results indicate that humpback dolphins display a heterogeneous pattern in habitat selectivity that is conditional to distinctive sets of key factors at different spatial scales. By using Landsat data and historic archives, and by quantifying a cumulative habitat loss over the past 40 years, this study reveals that during that time the natural shoreline providing critical ecological functions to humpback dolphins in Hong Kong has been drastically reduced. Occurrence probabilities reconstructed for the time-periods prior to major habitat alterations, and occurrence probabilities projected into future years (when all currently planned projects are to be completed) indicate a major drop of habitat use by the dolphins in areas neighbouring reclamation sites. All current Marine Protected Areas (MPAs) for humpback dolphin conservation are already affected by the ongoing environmental change and this will intensify in the future. Shore-based scan-sampling surveys were conducted between 2013 and 2014 to identify a distribution pattern of the dolphins at one of their main feeding areas in Hong Kong. With year-round sightings, the dolphins used this area selectively at different seasons and time of day. Only a small portion of the area was used as feeding location, within a narrow range of distances from shore, demonstrating a clear pattern of spatio-temporal fine-scale habitat selection that has not been accounted for in all current conservation actions in Hong Kong. Behavioural responses of humpback dolphins to small-scale dolphin-watch operations were investigated by tracking the dolphins' movement and recording fine-scale changes in the movement parameters. Analyses show that movement patterns of the dolphins, distinctive between behavioural states, were significantly altered by the presence of dolphin-watch boats. Multivariate models reveal that the disturbance by dolphin-watch operations can be seen as an external factor affecting the dolphins’ spatial decisions. Consequently, an appropriate regulatory mechanism is needed to manage the dolphin-watch tourism and minimise its impact on the target species. In summary, this study quantifies humpback dolphins’ response to large long-term and short-term chronic anthropogenic impacts, and indicates that various human activities affect the dolphins' daily behaviour and habitat selection. The conservation actions that are currently in place are ineffective in protecting humpback dolphins from man-made threats and will not be effective in preserving their habitat in the long-term. This implies serious ecological consequences and an urgent need for science-based and habitat-oriented conservation strategy.
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Predator diet is expected to respond to changes in the abundance of important prey items. We investigated whether common dolphin diet has changed with changes in the pelagic fish community off the Iberian Peninsula in recent years, in particular with reference to the decline of sardine (Sardina pilchardus) and the increase of Atlantic chub mackerel (Scomber colias) and scads (Trachurus spp.), and estimated how much fish was removed by common dolphins in coastal waters of mainland Portugal. Based on analysis of stomach contents of stranded animals from 2010 to 2013 (n = 150), we related diet composition to sardine abundance, dolphin size and sex, season and region. Despite the decline in sardine stocks in recent years, sardines were the most important prey of common dolphins, as was previously reported in the 1990s, followed by chub mackerel and scads. However, small sardines have disappeared from the diet, consistent with recent poor recruitment to the stock. The relative importance of these pelagic species in the diet increased significantly with increasing dolphin length. The dietary importance of chub mackerel also varied regionally, with higher intake on the southern coast. Finally, we compared the estimated biomass of fish removed by dolphins with that taken by fisheries. We found that even with the severe quota restrictions on sardine catches, total biomass removed by fisheries was over five times greater than the estimated removal by common dolphins. Our results confirm the previously reported preference for energy-rich prey, especially sardine, suggesting that common dolphins in Portuguese waters specialise in feeding on this species.
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The ecological role of species can vary among populations depending on local and regional differences in diet. This is particularly true for top predators such as the bottlenose dolphin (Tursiops truncatus), which exhibits a highly varied diet throughout its distribution range. Local dietary assessments are therefore critical to fully understand the role of this species within marine ecosystems, as well as its interaction with important ecosystem services such as fisheries. Here, we combined stomach content analyses (SCA) and stable isotope analyses (SIA) to describe bottlenose dolphins diet in the Gulf of Cadiz (North Atlantic Ocean). Prey items identified using SCA included European conger (Conger conger) and European hake (Merluccius merluccius) as the most important ingested prey. However, mass-balance isotopic mixing model (MixSIAR), using δ¹³C and δ¹⁵N, indicated that the assimilated diet consisted mainly on Sparidae species (e.g. seabream, Diplodus annularis and D. bellottii, rubberlip grunt, Plectorhinchus mediterraneus, and common pandora, Pagellus erythrinus) and a mixture of other species including European hake, mackerels (Scomber colias, S. japonicus and S. scombrus), European conger, red bandfish (Cepola macrophthalma) and European pilchard (Sardina pilchardus). These contrasting results highlight differences in the temporal and taxonomic resolution of each approach, but also point to potential differences between ingested (SCA) and assimilated (SIA) diets. Both approaches provide different insights, e.g. determination of consumed fish biomass for the management of fish stocks (SCA) or identification of important assimilated prey species to the consumer (SIA).
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Indo-Pacific humpback dolphins (Sousa chinensis) inhabiting Hong Kong waters are thought to be among the world's most anthropogenically impacted coastal delphinids. We have conducted a 5-year (2010–2014) photo-ID study and performed the first in this region comprehensive mark-recapture analysis applying a suite of open population models and robust design models. Cormack-Jolly-Seber (CJS) models suggested a significant transient effect and seasonal variation in apparent survival probabilities as result of a fluid movement beyond the study area. Given the spatial restrictions of our study, limited by an administrative border, if emigration was to be considered negligible the estimated survival rate of adults was 0.980. Super-population estimates indicated that at least 368 dolphins used Hong Kong waters as part of their range. Closed robust design models suggested an influx of dolphins from winter to summer and increased site fidelity in summer; and outflux, although less prominent, during summer-winter intervals. Abundance estimates in summer (N = 144–231) were higher than that in winter (N = 87–111), corresponding to the availability of prey resources which in Hong Kong waters peaks during summer months. We point out that the current population monitoring strategy used by the Hong Kong authorities is ill-suited for a timely detection of a population change and should be revised.
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Intensive fishing can strongly impact marine ecosystems; among other things, it usually causes the mean trophic level of the catches to decline, an indicator of the occurrence of the ‘fishing down’ (FD) phenomenon. Although FD occurs throughout the world oceans, it can easily be masked by diverse factors, which has misled authors as to its generality. In this contribution, which uses the East China Sea as an example, we explore the masking effect on FD of the taxonomic coarseness of catch data, of assuming that individual sizes remain constant after intensive fishing, and the geographic expansion of fisheries. The result showed that all of these masking factors occur in the East China Sea, where only a few species are reported separately and the bulk of the catch is pooled into non-informative ‘mixed fishes’. Also, the small mesh sizes and intensive fishing have reduced the sizes of fish and their trophic levels, while the fisheries have expanded offshore. Overall, taking the masking factors into account, the fishing down effect, i.e., the decline of the mean trophic level of the catch between 1979 and 2014 is in the order of 0.15 TL per decade, i.e., one of the highest estimates of FD in the world. Some ecological implications are presented.
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Defining demographic and ecological threshold of population persistence can assist in informing conservation management. We undertook such analyses for the Indo-Pacific humpback dolphin (Sousa chinensis) in the Pearl River Delta (PRD) region, southeast China. We use adult survival estimates for assessments of population status and annual rate of change. Our estimates indicate that, given a stationary population structure and minimal risk scenario, ~2000 individuals (minimum viable population in carrying capacity, MVP k) can maintain the population persistence across 40 generations. However, under the current population trend (~2.5% decline/annum), the population is fast approaching its viability threshold and may soon face effects of demographic stochasticity. The population demographic trajectory and the minimum area of critical habitat (MACH) that could prevent stochastic extinction are both highly sensitive to fluctuations in adult survival. For a hypothetical stationary population, MACH should approximate 3000-km 2. However, this estimate increases four-fold with a 5% increase of adult mortality and exceeds the size of PRD when calculated for the current population status. On the other hand, cumulatively all current MPAs within PRD fail to secure the minimum habitat requirement to accommodate sufficiently viable population size. Our findings indicate that the PRD population is deemed to become extinct unless effective conservation measures can rapidly reverse the current population trend.
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For the Indo-Pacific humpback dolphin, Sousa chinensis, an obligatory shallow-water inshore species, the degradation of coastal habitats can have major consequences for population persistence and distribution. Off Taiwan's west coast (TWC), these animals are predominantly seen in two areas separated by a stretch of coast with only sporadic sightings; suggesting that either (a) only two sectors of TWC offer sufficiently suitable habitat for the dolphins, or (b) a recent environmental change limits the population connectivity. We measured the extent of habitat destruction due to land reclamation off TWC since 1972 using a habitat integrity index (HII) and applied general linear models (GLMs) to compare HII with sightings of dolphins per unit effort (SPUE). While early Landsat data reveal extensive continuity and diversity of coastal habitats, by 2013 a total area of over 222 km2 was lost to land reclamation (23% of dolphin habitat and 40% of dolphin foraging habitat). GLM analysis showed a significant relationship between HII and SPUE; the lower HII the lower SPUE, indicating that off TWC the current discontinuous distribution of humpback dolphins is likely due to a different extent of habitat degradation rather than natural patchiness of their environment. We emphasize that the history of coastal habitat alteration must be considered when interpreting cetacean distribution from survey data and formulating habitat management decisions; especially in areas experiencing extensive anthropogenic coastal change.
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We assessed the spatio-temporal trends of the concentrations of 11 heavy metals (HMs) in the liver and kidney of Indo-Pacific humpback dolphins (Sousa chinensis) from western Pearl River Estuary (PRE) during 2004-2015. The hepatic levels of Cr, As and Cu in these dolphins were among the highest reported for cetaceans globally, and the levels of Zn, Cu and Hg were sufficiently high to cause toxicological effects in some of the animals. Between same age-sex groups, dolphins from Lingdingyang were significantly more contaminated with Hg, Se and V than those from the West-four region, while the opposite was true for Cd. Generalized additive mixed models showed that most metals had significant but dissimilar temporal trends over a 10-year period. The concentrations of Cu and Zn increased significantly in recent years, corresponding to the high input of these metals in the region. Body-length-adjusted Cd levels peaked in 2012, accompanied by the highest annual number of dolphin stranding events. In contrast to the significant decrease in HM levels in the dolphins in Hong Kong waters (the eastern reaches of the PRE), the elevated metal exposure in the western PRE raises serious concerns.
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The stomach contents of 21 stranded bottlenose dolphins were collected by a marine mammal research team (Grupo de Estudos de Mamíferos Aquáticos do Rio Grande do Sul - GEMARS) during systematic beach surveys (mostly monthly) along the central-northern coast of Rio Grande do Sul, southern Brazil, between November 1991 and October 2008. The area surveyed covered about 250 km of exposed sandy beaches, from Torres (29º19´S, 49º43'W) to the Lagoa do Peixe National Park (31º15'S, 50º54'W). The diversity of prey ingested represents a total of 15 fish species belonging to eight families. Cephalopods were represented by three species (Argonauta nodosa, Doryteuthis plei and D. sanpaulensis). The most important family of fish found was Sciaenidae, accounting for eight species. All the other families were represented by one member only. The diversity of prey items ingested by each dolphin varied from one to 12 species (x = 4.80; SD = 2.87).
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The environmental consequences of bitumen extraction from oil sands deposits are at the center of North American natural resource and energy policy debate, yet impacts on ocean environments have received little attention. Using a quantitative framework, we identify knowledge gaps and research needs related to the effects of oil sands development on marine biota. Fifteen sources of stress and disturbance – varying greatly in spatial and temporal scale – are generated via two pathways: (1) the coastal storage and oceanic transport of bitumen products, and (2) the contribution of industry-derived greenhouse gases to climate change in the ocean. Of highest research priority are the fate, behavior, and biological effects of bitumen in the ocean. By contrast, climate-change impacts are scientifically well established but not considered in key regulatory processes. Most stressors co-occur and are generated by other industries, yet cumulative effects are so far unaccounted for in decision making associated with new projects. Our synthesis highlights priority research needed to inform future energy development decisions, and opportunities for policy processes to acknowledge the full scope of potential and realized environmental consequences.
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Indo-Pacific humpback dolphins (Sousa chinensis) have attracted considerable attention due to their critically endangered status and related conservation issues, but their trophic relationships and ecological significance in coastal ecosystems are poorly understood. For instance, this species is noticeably more abundant in the Xin-Huwei River Estuary (Ex) of Western Taiwan than in the nearby Zhuoshui River Estuary (Ez), though it is unclear why the distribution shows such partitioning. To explore this topic, we conducted field surveys seasonally for two years from 2012 to 2013 and constructed Ecopath models of Ex, Ez, and an offshore site (Dm) to compare energy flow within the food webs. Model comparisons showed that the availability of food resources was the main factor influencing the biomass of Indo-Pacific humpback dolphins. Specifically, its more frequent occurrence in Ex can be attributed to greater phytoplankton production and greater biomasses of macroinvertebrates and prey fish than in the other two areas. An increase in fishing activity might decrease the food availability and, consequently, the biomass of the dolphins. Although the decline in the dolphin population would increase the biomass of some prey fish species, local fishermen might not necessarily benefit from the decline due to the concurrent decrease of highly valued crabs and shrimp. Collectively, our work suggests that the Indo-Pacific humpback dolphin is a keystone species in tropical coastal waters of Taiwan, and thereby exhibit a disproportional large ecological impact given their relatively low abundance.
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Over the past few thousand years, human development and population expansion in southern China have led to local extirpation and population contraction of many terrestrial animals. At what extent this early human-induced environmental change has also affected coastal marine species remains poorly known. We investigated the demographic history of the Indo-Pacific humpback dolphin (Sousa chinensis) in the Pearl River Delta (PRD); an obligatory inshore species known for its susceptibility to anthropogenic impacts in one of China's most developed coastal regions. Although the deltaic evolution of PRD has been influenced by climate since the Holocene, ~74% reduction of the dolphin's effective population size occurred within the last 2000 years, consistent with ~61% habitat contraction during this period. This considerable and recent population contraction may have been due to land use practices and deforestation in the upper/middle Pearl River region, all leading to increasing sedimentation rate in the estuarine area. As anthropogenic impacts within the drainage of Pearl River affected a vast area, coastal dolphins and large terrestrial mammals in southern China may share a similar demographic history, whilst the demographic and biogeographic history of the PRD humpback dolphins may be symptomatic of similar processes that this species may have undergone elsewhere in the region.
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Indo-Pacific humpback dolphins inhabit shallow coastal waters within the tropics and subtropics of the Indian and Western Pacific Oceans. Their taxonomy remains unresolved, between a single widespread and highly variable species, two species, and three species being currently proposed. Their inshore distribution renders them highly susceptible to the adverse effects of many human activities; for most of the known remaining populations their continuous survival is a subject of major conservation concern. In this chapter, we describe the use of demographic analysis to quantify population trend and, more informatively, predict the risk (probabilities) of extinction. The results of demographic analyses provide valuable means of assessing conservation status. Using the population of humpback dolphins from the Pearl River Estuary as an example, we show the power of demographic analyses, predicting a significant population decline before it is directly documented by other standard techniques. Comparing our findings with known, albeit limited data from southeast Africa, and considering the current ambiguity of the taxonomic classification adopted by IUCN, we question the current listing of humpback dolphins under the IUCN Red List of Threatened Species. We urge that their conservation status classification be reconsidered as it likely understates, perhaps severely, the threats faced by many fragmented populations off Southeast Asia and the western Indian Ocean.
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Unpredictable inter-annual variations in the timing, spatial extent and intensity of the sardine run (Sardinops sagax) have been documented in recent years along the coastline of KwaZulu-Natal, South Africa. This study aimed to determine whether variations in the availability of sardine were reflected in the diet and condition of common dolphins (Delphinus capensis) over the past three decades. Stomach contents from 95 common dolphins incidentally caught between 2000 and 2009 were analysed and compared to historical data collected between 1972 and 1992. A shift in the principal prey species consumed by the dolphins was observed over the past 30 years. Prior to 1992, sardine comprised up to 49 % of the total stomach contents by mass, whilst chub mackerel (Scomber japonicus) was the dominant prey recorded in the stomach contents between 2000 and 2009 (66 %). As common dolphins prey on locally abundant fish species, this dietary shift may indicate changes in the availability of the most abundant fish prey. Stable isotope analyses of tooth tissue revealed no significant changes in the δ15N and δ13C isotopic ratios over the past three decades (P = 0.283 for N and P = 0.922 for C). Blubber thickness as a percentage of body length and blubber weight as a percentage of total body weight were assessed as indicators of animal condition. No significant changes in proportional blubber weight or blubber thickness were seen over the last 40 years (1970–2009) for all age cohorts. This species appears to be well adapted to cope with changes in prey species availability, without impacting on body condition.
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Behavioral scientists have developed methods for sampling behavior in order to reduce observational biases and to facilitate comparisons between studies. A review of 74 cetacean behavioral field studies published from 1989 to 1995 in Marine Mammal Science and The Canadian Journal of Zoology suggests that cetacean researchers have not made optimal use of available methodology. The survey revealed that a large proportion of studies did not use reliable sampling methods. Ad libitum sampling was used most often (59%). When anecdotal studies were excluded, 45% of 53 behavioral studies used ad libitum as the predominant method. Other sampling methods were continuous, one-zero, incident, point, sequence, or scan sampling. Recommendations for sampling methods are made, depending on identifiability of animals, group sizes, dive durations, and change in group membership.
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Sixteen per- and polyfluoroalkyl substances (PFASs) were measured in liver (n = 52) and kidney (n = 18) tissues of Indo-Pacific humpback dolphins (Sousa chinensis) stranded in the Pearl River Estuary (PRE) of China between 2004 and 2016. The average concentrations of perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA) and most of other PFASs in the liver samples were respectively greater than any records previously reported in cetaceans globally. PFOS levels in 46% of dolphin liver samples exceeded the hepatic toxicity threshold in cetaceans. For the first time, we found a U-shaped trend for the distribution pattern of perfluorinated carboxylic acids (PFCAs) between liver and kidney with increasing carbon chain lengths (C5-C16), whereas a descending trend (C4-C10) was found for perfluoroalkane sulfonic acids (PFASs), which may be explained by binding efficiencies of PFAS analogues to proteins. Dolphins with the highest levels of ∑PFASs (age-corrected) were clustered near the river outlets in Lingdingyang area, which agrees with the spatial distribution of PFASs in the environment. Significant temporal trends were observed for many PFASs. Concentrations of PFOA, perfluorononanoic acid (PFNA), perfluorodecanoic acid (PFDA) and perfluoroheptanoic acid (PFHpA) all peaked in year 2011, followed by a decreasing trend, while a consistently descending trend was shown for perfluoroundecanoic acid (PFUdA) and perfluorodecane sulfonate (PFDS). Our findings contribute to the knowledge of tissue distribution and spatiotemporal trends of PFASs in the PRE dolphins, which are valuable for us to understand the PFASs exposure risk and their industrial emission in Southern China.
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As long-lived apex predators in the Pearl River Estuary (PRE) of China, Indo-Pacific humpback dolphins (Sousa chinensis) are particularly vulnerable to anthropogenic impact and may undergo considerable ecological trait changes. The variability of traits, however, is often difficult to trace back in nature. Here, we analyzed stable isotope ratios of carbon and nitrogen in muscle samples of 88 S. chinensis stranded in the PRE from 2004 to 2016 to investigate the ecological changes occurring in the dolphins. Stable isotope analysis revealed the existence of two sub-aggregations of S. chinensis in the PRE. Generalized additive models showed significant decreasing trends in both carbon and nitrogen isotopic signatures over time, indicating the habitat changes and dietary shifts, possibly due to the influence of increased coastal developments and fishing activities in the PRE. Diet modeling suggests that the proportional contribution of higher trophic-level prey decreased in the S. chinensis diet over time, while increased consumption of lower trophic-level prey was observed. This shift was related to depletion of higher trophic-level prey caused by overfishing. Although S. chinensis could temporarily compensate for the lost energy supply through feeding plasticity (revealed by the negligible differences of isotope niche width among different stranding periods), long-term depletion in prey availability may cause long-lasting negative effects on this dolphin population. This study highlights the crucial relationships between fishery management and dolphin conservation, providing scientific evidence for the long-term protection of this threatened species in the PRE region.
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Harmonizing biodiversity conservation and natural resource management often requires anticipating the potential impact that certain species can cause through their exploitation of specific resources. During the winter 2008/2009 we studied the feeding ecology of the Great Cormorant Phalacrocorax carbo sinensis in SE Spain, in order to determine its dependence on different food sources available along an oceanic-continental gradient (open sea-freshwater bodies). Trophic niche was assessed using C and N stable isotopes, applying up-to date metrics and analytical tools. Isotopic signatures revealed high mobility patterns for cormorants, illustrating differences between roosting and feeding areas (spatially well separated). Cormorants roosting in open sea move to inland freshwater bodies, their trophic niche overlapping that of individuals settled there. Similarly, individuals settled around transitional waters (coastal lagoon), seem to divide the predation pressure between different food sources along the marine-continental gradient. Recently in Spain, the Great Cormorant has been devoid of legal protection due to its supposed impact on fisheries. Our methods provide a useful and non-invasive tool that can be used as a first step for assessing such interaction. On the basis of its dispersed feeding distribution and expected predation pressure the cormorant does not seem to represent a serious threat to recreational or commercial fisheries which would justify downgrading its protection.
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Energy is a variable of key importance to a wide range of research in primate behavioral ecology, life history, and conservation. However, obtaining detailed data on variation in energetic condition, and its biological consequences, has been a considerable challenge. In the past 20years, tremendous strides have been made towards non-invasive methods for monitoring the physiology of animals in their natural environment. These methods provide detailed, individualized data about energetic condition, as well as energy allocations to growth, reproduction, and somatic health. In doing so, they add much-needed resolution by which to move beyond correlative studies to research programs that can discriminate causes from effects and disaggregate multiple correlated features of the social and physical environment. In this review, I describe the conceptual and methodological approaches for studying primate energetics. I then discuss the core questions about primate feeding ecology, social behavior, and life history that can benefit from physiological studies, highlighting the ways in which recent research has done so. Among these are studies that test, and often refute, common assumptions about how feeding ecology shapes primate biology, and those that reveal proximate associations between energetics and reproductive strategies.
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There has been very little previous research on Indo-Pacific humpback dolphins (Sousa chinensis) in the Beibu Gulf of southern China. Here, we report on the population size, habitat and ecology, threats, and overall conservation status of this putative population. 'Population size' was estimated based on photo-identification mark/recapture analysis. It was estimated to number a total of 398-444 individuals (95% CI: 393-506), with two apparently distinct groups in the Dafengjiang-Nanliujiang Estuary and at Shatian-Caotan. Movements of dolphins in the Beibu Gulf appear to be limited, with high site fidelity. These dolphins were found to occur mainly in shallow coastal waters near estuaries. The main threats are fisheries interactions (including by-catch), vessel traffic, mariculture operations, dolphin-watching tourism, and habitat degradation (including marine construction activities and large-scale land reclamation). Although the conservation status of this putative population has been considered to be better than that of other populations of the species in more northern areas of China, there is still reason for strong concern about its future, and several management recommendations are made.
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Based on data of demersal trawl samples from 1987 to 1988 in the Zhujiang Estuary and its adjacent waters, relationships between fish community and environmental factors are studied by using classification, MDS ordination and several community parameters. The results show that the distribution of fish species is zoned by salinity and divided into three types. The freshwater community which is restricted in the Lianhua Hill - Humen waterway is simple and stable, the coastal community which lives in the shallow waters is complex, and the estuary community which is identified in the Lingdingyang area and affected by physical and ecological processes more greatly has obvious seasonal variations.
Article
The IUCN Red List designation of the Indo-Pacific humpback dolphin (Sousa chinensis) is re-assessed in light of its newly recognized taxonomic status (it has recently been separated into three species) and findings that humpback dolphins along the coast of Bangladesh, and possibly eastern India, are phylogenetically distinct from other members of the Sousa genus. Sousa chinensis is found in Southeast/South Asia (in both the Indian and Pacific oceans), from at least the southeastern Bay of Bengal east to central China, and then south to the Indo-Malay Archipelago. There are no global population estimates, and the sum of available abundance estimates add up to about 5700 individuals, although only a portion of the range has been covered by surveys. This species occurs in shallow (<30m deep), coastal waters of the tropics and subtropics, and feeds mainly on small fishes. It has a similar reproductive biology to other large dolphins, occurs mostly in small groups, and generally has individual movements of about 50-200km2. Major threats throughout the range include entanglement in fishing nets (primarily gillnets) and habitat destruction/degradation, although in some more industrialized areas, vessel traffic, and environmental contamination from organochlorines are also serious issues. Conservation management is largely lacking in most parts of the species' range, although there has been significant (though still inadequate) attention in some parts of China (e.g. Hong Kong and adjacent areas, and Taiwan). Much greater efforts are needed toward conservation of Indo-Pacific humpback dolphins to stop apparent declines, and to lower the species' extinction risk. Sousa chinensis meets the IUCN Red List requirements for Vulnerable (under criteria A4cd), with fisheries bycatch and habitat loss/degradation being the main pervasive threats.
Article
Length-weight relationships (LWRs) were estimated for eight native fish species collected in the lower reaches of the Pearl River, Guangdong Province, South China. Fish were sampled in March and September 2014 and January 2015, using gillnets with various mesh sizes (5-8 cm) and trawl nets with a 3 cm mesh size. A total of 568 specimens of the eight species were analyzed. The r2 values for these species ranged from 0.951 to 0.991. The b values ranged from 3.00 for Takifugu ocellatus and Moolgarda cunnesius, to 3.57 for Clupanodon thrissa. New maximum total lengths are recorded for Takifugu ocellatus (17.6 cm) and Liza carinata (24.4 cm). For five species (Clupanodon thrissa, Megalobrama terminalis, Collichthys lucidus, Coilia grayii, Takifugu ocellatus), the LWRs are presented to fishbase for the first time.
Article
Understanding of the distributional ecology of the Atlantic humpback dolphin (Sousa teuszii) has been hampered by a lack of systematic and consistent sampling effort. The only comprehensive species distribution review was published in 2004; since then a considerable amount of novel information has emerged. We compiled 853 sighting, capture and specimen records of the species, and produced global and regional distribution maps. Of the 830 records where year was available, 63.1% dated from ≥2005 and confirm a contemporary occurrence in six marine ecoregions and 11 countries: Western Sahara, Mauritania, Senegal, Gambia, Guinea-Bissau, Guinea, Benin, Cameroon, Gabon, Congo Republic and Angola. Additionally, Togo is a recently confirmed range state. Group sizes ranged from 1 to 45 animals, with small groups of 1 to 10 animals comprising 65% of the sightings. Similarities were noted in the regions inhabited by Atlantic humpback dolphins across their range, particularly an occurrence in relatively shallow (predominantly ≤20m) depths, in warm waters (average SSTs of 15.8-31.8°C) and in dynamic habitat strongly influenced by tidal patterns. These conditions occur in various habitats occupied by the species, including estuarine systems, open coasts, archipelagos, tidal mud-flats and sheltered bays. Sightings were recorded at distances of 13m to 12.8km (mean of 573m) from land, indicating that the species occurs several kilometres from shore when suitable shallow habitat is present. The Atlantic humpback dolphin may be a 'nearshore' species based on oceanographic definitions incorporating water depth, wave action and sedimentation rather than on spatial distance from the coast.
Article
The delphinid genus Sousa has recently undergone a major revision, and currently contains four species, the Atlantic humpback (Sousa teuszii), Indian Ocean humpback (Sousa plumbea), Indo-Pacific humpback (Sousa chinensis), and Australian humpback (Sousa sahulensis) dolphins. Recent molecular evidence suggests that humpback dolphins in the Bay of Bengal may comprise a fifth species. These moderate-sized dolphin species are found in shallow (<30m), coastal waters of the eastern Atlantic, Indian, and western Pacific oceans. Abundance and trends have only been studied in a few areas, mostly in eastern Africa, China, and northern Australia. No global, empirically derived abundance estimates exist for any of the four species, but none appear to number more than about 20,000 individuals. Humpback dolphins feed mostly on small fishes, and sometimes shrimps; occur for the most part in small groups (mostly 12 or less); have limited nearshore movements; and in most parts of their range exhibit a fission/fusion type of social organization. Major threats that affect all the species are entanglement in fishing gear, and habitat degradation/destruction from various forms of coastal development. Impacts from vessel traffic (including behavioural disturbance and displacement, as well as mortality and morbidity from collisions with vessels) appear to be significant in most areas. Several other threats are apparently significant only in particular parts of the range of some species (e.g. high levels of organochlorine contaminants affecting Indo-Pacific humpback dolphins in Hong Kong). Direct hunting only occurs in limited areas and primarily on a small scale. Conservation actions so far have been limited, with most populations receiving little study and almost no management attention. Much more work is needed on humpback dolphin population status, threats, and how the major threats can be reduced or eliminated. Extinction risks for the four species and some populations are preliminarily re-assessed using the IUCN Red List criteria in the current volume. The results suggest that all four species in the genus are threatened at some level (suggested Red List status ranges from Vulnerable for S. chinensis and S. sahulensis to Critically Endangered for S. teuszii).
Article
The Indo-Pacific hump-backed dolphin (Sousa chinensis Osbeck, 1765) is found throughout the western Pacific and Indian oceans, from southern China and northern Australia in the east to South Africa in the west. Throughout most of its range it has not been well studied, and in southern China very little is known of its biology. The goal of the present study was to provide scientific information needed for the long-term conservation and management of the population that occurs in Hong Kong waters. From September 1995 to November 1998, 38,105 km of systematic line transect surveys were conducted throughout marine waters of the Hong Kong Special Administrative Region (SAR) of the People's Republic of China (PRC) and adjacent waters to provide data on distribution and abundance. Photo-identification of individual dolphins allowed for examination of movement patterns, home ranges, and social organization. Collection of stranded dolphin carcasses and detailed necropsies provided information on causes of death as well as samples for life history studies, such as feeding habits, growth and reproduction, ecotoxicology, and stock structure. The dolphin population appears to be centered around the Pearl River Estuary, and Hong Kong waters represent the eastern portion of the range, which extends far into mainland Chinese waters (Lingding Bay) and covers at least 1,800 km2. Within Hong Kong, dolphins only occur in western waters around Lantau Island. The area north of Lantau Island is heavily used throughout the year and represents by far the most important habitat in Hong Kong. Line transect analyses indicate that between 88 (spring) and 145 (summer). dolphins occur in Hong Kong. Based on 27,600 photographs taken, 213 individual dolphins were identified. The total size of the Pearl River breeding population is unknown, but is estimated to consist of at least 1,028 dolphins, based on line transect analysis. An apparent decline in the number of dolphins in the North Lantau area over the period from 1996-1998 was not statistically significant. Individual dolphins have overlapping home ranges of about 30-400 km2 in different sections of the population's overall range. Groups of dolphins in Hong Kong number up to 23 animals, with an average group size of 3.8 ± s.d. 3.63 animals. There is no significant seasonal variation in group size, but groups feeding behind pair trawl fishing vessels (mean = 9.6 ± s.d. 5.37) are significantly larger thane other groups, and groups in Lingding Bay, in Chinese waters west of Hong Kong (mean = 8.3 ± s.d. 7.84) are significantly larger than those in Hong Kong. Groups are very fluid and change composition frequently, with association indices ranging from 0-0.333. Behavioral patterns are similar to those of other coastal dolphins, but Hong Kong hump-backed dolphins only rarely ride bow waves. Following pair trawlers represents an important feeding strategy for some individuals. Construction work on an airport fuel facility in the dolphins' main habitat appears to have caused some disturbance (indicated by increased swimming speeds) and possibly temporary evacuation of the surrounding area. There is a great deal of developmental variation in the color pattern of southern Chinese hump-backed dolphins, with a general lightening from newborn to adult stages. Males appear to retain more spots in adulthood than females. Length at birth appears to be about 100 cm, and postnatal development is characterized by rapid growth in the first year and a levelling-off of the growth curve after reaching adulthood. Asymptotic length is reached at around 243 cm. Length and weight are related exponentially, with the maximum weight about 250 kg. Calving occurs throughout the year but most young are born from January through August, with a peak in spring/summer. Scant evidence suggests that sexual maturity in females is reached at about 9-10 years of age. Dolphins feed mainly on several demersal and pelagic fish species that are often associated with estuaries. There is a lack of evidence for long-distance movements (on the order of hundreds of linear kilometers), and this presumably results in isolation of groups around major Chinese fiver mouths. Thus it appears that there may be at least 8 separate populations of hump-backed dolphins along the coast of southern China. The Pearl River Estuary, including Hong Kong, is apparently inhabited by one of these populations, although preliminary genetic work has shown only equivocal evidence of population separation from dolphins in the Xiamen area. Human-related causes of mortality include entanglement in fishing nets and vessel collisions. Some environmental contaminants (especially the heavy metal mercury and the pesticide DDT) were found in high levels in some dolphins, and preliminary evidence suggests that these may be affecting the health of the animals. A series of recommendations for management and for further research have been made to aid in the conservation of these animals. Principles for the conservation of wild living resources should be followed, and information from both the natural and social sciences are needed for proper management. Research and long-term monitoring of the population must continue for management strategies to be evaluated and refined. Overall, the population of hump-backed dolphins that occurs in Hong Kong waters appears to be viable and should be able to survive with appropriate conservation efforts.
Chapter
Tigers are increasingly disappearing from the ecosystems where they evolved and the nation states in which they live. Their vast range in Asia has been reduced to a small number of isolated populations, they are hunted intensively for the trade in tiger parts, and the prey on which they depend is reduced throughout much of their range. Many different people and organizations are striving to reverse these trends. Species conservation planning is the science and art of allocating conservation efforts to those priority places and actions that will provide the greatest returns for species survival and ecological function in the wild. It requires clearly stated goals, an assessment of the current status of the species, a directed process for selecting where to work, and a mechanism to measure success. The field of species conservation planning as a whole has changed over the past decade. Species conservation planning has also changed in terms of the data and methods available. This chapter outlines the datasets and methods used, presents the essential results, and sets measurable conservation goals against which future efforts—successful or otherwise—can be measured.
Article
The length–weight relations (LWR) were estimated for 20 fish species from the Pearl River, South China. A total of 3610 specimens representing 10 families were used to estimate the relation parameters. The b values in the LWR (W = aLb ) ranged from 2.068 for Odontamblyopus lacepedii (Temminck et Schlegel, 1845) to 3.423 for Pseudogobius javanicus (Bleeker, 1856). The LWR with high coefficient of determination (r 2) is significant for all the species. The r 2 value ranged from 0.919 to 0.993. This study presents the first reference on length–weight relations for 7 species and new records of maximum total length for 6 species. The results may be helpful in future fisheries studies in this areas.
Article
In recent years, fisheries management has succeeded in stabilizing and even improving the state of many global fisheries resources [1–5]. This is particularly evident in areas where stocks are exploited in compliance with scientific advice and strong institutional structures are in place [1, 5]. In Europe, the well-managed northeast (NE) Atlantic fish stocks have been recovering in response to decreasing fishing pressure over the past decade [3–6], albeit with a long way to go for a universal stock rebuild [3, 7]. Mean-while, little is known about the temporal development of the European Mediterranean stocks, whose management relies on input controls that are often poorly enforced. Here, we perform a meta-analysis of 42 European Medite-rranean stocks of nine species in 1990–2010, showing that exploitation rate has been steadily increasing, selec-tivity (proportional exploitation of juveniles) has been dete-riorating, and stocks have been shrinking. We implement species-specific simulation models to quantify changes in exploitation rate and selectivity that would maximize long-term yields and halt stock depletion. We show that stocks would be more resilient to fishing and produce higher long-term yields if harvested a few years after maturation because current selectivity is far from optimal, especially for demersal stocks. The European Common Fisheries Policy that has assisted in improving the state of NE Atlantic fish stocks in the past 10 years has failed to deliver similar results for Mediterranean stocks managed under the same policy. Limiting juvenile exploitation, advancing manage-ment plans, and strengthening compliance, control, and enforcement could promote fisheries sustainability in the Mediterranean.
Article
Summary Sixty-nine fish species belonging to 41 genera from 29 families and seven orders were collected during 2006–2007 by otter trawl in Beibu Gulf, northern South China Sea, for analysis of their length-weight relationships (WLRs). The analysis included 19 species for which no previous WLR estimates were available in FISHBASE.
Article
This study examined the stomach contents of 11 bottlenose dolphins (Tursiops sp.), five Indo-Pacific humpback dolphins (Sousa chinensis) and two spinner dolphins (Stenella longirostris) that were found stranded along the Omani coastline. Across the three species examined, a total of 4796 fish otoliths and 214 cephalopod beaks were found, representing at least 33 species in 22 families. Prey item importance was calculated using the percentage by number and percentage by frequency of occurrence methods, and a modified index of relative importance. The fish families Apogonidae, Carangidae and Scombridae were the most numerically important prey of the bottlenose dolphins. Sciaenidae was the most numerically important fish family for the Indo-Pacific humpback dolphins. The myctophid Benthosema pterotum formed the majority of the prey items of spinner dolphins. Cephalopod remains found in the stomach samples were represented by the families Sepiidae, Loliginidae and Onychoteuthidae. The known depth distribution of prey items of bottlenose dolphins indicated that the animals fed in a wide variety of habitats. Indo-Pacific humpback dolphin prey items indicated feeding in shallow coastal areas. Spinner dolphins appear to have exploited the upper 200 m of the water column for food, where their vertically migrating mesopelagic prey are found at night. Most prey species found in the stomach contents do not appear to be of current commercial importance in Oman. However, the findings here indicated that all three species of dolphins were feeding in areas where artisanal and/or commercial fishing occurs and has conservation implications.