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Inter-Annual Variability in Blue Whale Distribution off Southern Sri Lanka between 2011 and 2012
Journal of Marine Science and Engineering (JMSE)
Abstract and Figures
Blue whale (Balaenoptera musculus) movements are often driven by the availability of their prey in space and time. While globally blue whale populations undertake long-range migrations between feeding and breeding grounds, those in the northern Indian Ocean remain in low latitude waters throughout the year with the implication that the productivity of these waters is sufficient to support their energy needs. A part of this population remains around Sri Lanka where they are usually recorded close to the southern coast during the Northeast Monsoon. To investigate inter-annual variability in sighting locations, we conducted systematic Conductivity-Temperature-Depth (CTD) and visual surveys between January–March 2011 and January–March 2012. In 2011, there was a notable decrease in inshore sightings compared to 2009 and 2012 (p < 0.001). CTD data revealed that in 2011 there was increased freshwater in the upper water column accompanied by deeper upwelling than in 2012. We hypothesise that anomalous rainfall, OPEN ACCESS J. Mar. Sci. Eng. 2014, 2 535 along with higher turbidity resulting from river discharge, affected the productivity of the inshore waters and caused a shift in blue whale prey and, consequently, the distribution of the whales themselves. An understanding of how predators and their prey respond to environmental variability is important for predicting how these species will respond to long-term changes. This is especially important given the rapid temperature increases predicted for the semi-enclosed northern Indian Ocean.
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... The only region in the Indian Ocean where blue whales have been the subject of frequent and relatively long-term studies is Sri Lanka (e.g. Alling et al. 1991, Ilangakoon & Sathasivam 2012, De Vos et al. 2014, Kirumbara et al. 2022, Liyanage et al. 2023. ...
... Further, the relatively high detection rate of songs at this shallow location could be indicative of higher densities of blue whales than would be indicated by the visual data alone. The acoustic seasonality from the Seychelles is similar to the seasonality of sighting events from Sri Lanka, which are highest from December to April (Ilangakoon & Sathasivam 2012, De Vos et al. 2014. Off Mirissa, Sri Lanka, November was the month with the fewest sightings overall, although blue whales are seen there nearly year-round (Russell et al. 2020, Liyanage et al. 2023. ...
Historically, the Seychelles archipelago was an opportunistic whaling ground for fleets en route to and from the Antarctic. Soviet whalers illegally killed 500 blue whales near the Seychelles in the 1960s. Since then, no dedicated research has occurred to understand the ecological importance of this region for blue whales. Based on opportunistic sightings, we undertook 2 expeditions to assess the occurrence of blue whales. The overall goals were to determine blue whale distribution, obtain photo-identification data and collect the first acoustic data on this species in this region using a hydrophone deployed for a year. The expeditions consisted of vessel-based visual surveys that focused on the slope habitat (500-2000 m) off the northern portion of the Mahé Plateau. Over the 2 expeditions, a total of 5 sightings of up to 10 animals were seen. The results of our acoustic monitoring off Seychelles demonstrate that blue whales occur there regularly, primarily from December to April, and that the acoustic population identity matches that from near Sri Lanka. Published records and the results of our work suggest that blue whales from the northwestern Indian Ocean are seasonally present in the western equatorial Indian Ocean.
... Moreover, Sri Lanka is located at the centre of the Indian Ocean Marine Mammal Sanctuary (IOMMS) between the latitudes 5 • 55 and 9 • 51 N and longitudes 79 • 41 and 81 • 53 E, south of the Indian subcontinent, separated by the narrow Palk Strait. Oceanic processes around the island are driven by two major monsoonal winds in combination with bathymetric features (i.e., submarine canyons), all of which produce intense upwelling [5], which, in turn, provide ideal foraging conditions for aggregations of BWs [6]. An increasing number of previous studies, e.g., [7][8][9][10], have shown that BWs are a group of stenophagous predators and exclusively prefer to make feeding aggregation on productive upwelling regions, where patchy and dense krill aggregations occur. ...
... Sighting of pregnant cow (1) and mother-calf pairs(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) off Mirissa, south coast of Sri Lanka. TSS is the traffic separation scheme. ...
This paper describes the distribution, abundance and seasonal variation in blue whales (BWs) on the south coast of Sri Lanka, off Mirissa, where they are faced with anthropogenic threats. Data collection encompassed opportunistic sightings by whale-watching (WW) operations. This study revealed that large aggregations of BWs consistently overlapped with busy shipping lanes located between Dondra Head and Galle within important foraging and breeding regions on the south coast. Throughout 2015 (except June and July), 729 BWs were sighted over 177 sighting days. The sighting frequency was higher during the northeast monsoon and the first intermonsoon and lower throughout the second intermonsoon (n = 9) and the latter part of the southwest monsoon. The highest frequencies of BW encounters per day occurred in April (n = 15) and December (n = 20), while the mean annual group size per sighting was 3.07 =/- 0.24. From the January-to-April season, 13 mother–calf combinations and 1 pregnant cow were sighted off the southern coastline of Mirissa, suggesting the calving season peaks between the months of March and April in Sri Lanka. As
this important habitat overlapped with the busiest shipping lanes, fishing and commercial whale watching activities, the authorities have to take action toward the conservation of this ecosystem and whales as well as their safe navigation.
... The south and southwest coastal upwelling areas off of Sri Lanka are rich in phytoplankton and zooplankton biomass [28]. One of the prominent whale aggregations occurs along the continental shelf break in the Sri Lankan southern upwelling region, which is rich in their main food sources, sergestid shrimp and euphausiids (krill) [29,30]. These aggregations, at the narrowest continental shelf off Dondra to Galle, coincide with one of the most trafficked shipping lanes in South Asia, and consequently, the whales are prone to ship strikes [5,6,12,31]. ...
... Surveys of the full range of blue whales are logistically and technically difficult due to their wide range, seasonal movements, pelagic lifestyle and occurrence in the open ocean. Therefore, many surveys and research programs on blue whales undertaken in Sri Lanka have been limited in duration and confined to a small proportion of the overall distribution area, e.g., [3,5,6,14,29,31,36]. The survey results presented here covered the coastal and offshore waters (20 to 1000 m depth) of Sri Lanka, except for the shallow northern waters. ...
... The south and southwest coastal upwelling areas off of Sri Lanka are rich in phytoplankton and zooplankton biomass [28]. One of the prominent whale aggregations occurs along the continental shelf break in the Sri Lankan southern upwelling region, which is rich in their main food sources, sergestid shrimp and euphausiids (krill) [29,30]. These aggregations, at the narrowest continental shelf off Dondra to Galle, coincide with one of the most trafficked shipping lanes in South Asia, and consequently, the whales are prone to ship strikes [5,6,12,31]. ...
... Surveys of the full range of blue whales are logistically and technically difficult due to their wide range, seasonal movements, pelagic lifestyle and occurrence in the open ocean. Therefore, many surveys and research programs on blue whales undertaken in Sri Lanka have been limited in duration and confined to a small proportion of the overall distribution area, e.g., [3,5,6,14,29,31,36]. The survey results presented here covered the coastal and offshore waters (20 to 1000 m depth) of Sri Lanka, except for the shallow northern waters. ...
Blue whales Balaenoptera musculus are the most abundant and widely distributed cetacean species in Sri Lankan waters. A vessel-based opportunistic line transect survey focusing on marine mammals was conducted in Sri Lankan waters between 24 June to 12 July 2018, while an ecosystem survey was performed by the R/V Dr. Fridtjof Nansen. The entire transect length was 2090 km within an area of 3895 km2. Overall, 72% (n = 57) of blue whale observations were group assemblages ranging between 1 and 5 individuals out of a total of 79 sightings. The largest aggregations of blue whales were recorded at the intersection of the submarine canyon off the coastline of Mirissa and busy shipping lanes between Dondra Head and Galle, where previous ship strikes were recorded. Overall, the average observed group size was 2.64 (CV, 12.34%; 95% CI, 2.07–3.38), the average group density was 0.0029 km−2 (CV, 35.96%; 95% CI, 0.00145–0.00610), and the total blue whale abundance within the survey area was 513 individuals (CV, 38.02%; 95% CI, 243–1083). These survey results fill vital knowledge gaps regarding the abundance and distribution of blue whales in Sri Lanka, which is essential for the establishment of management and conservation strategies.
... This relationship occurs because ocean current flows associated with these structures enhance primary productivity and food availability for marine megafauna (White et al. 2007). For example, longshore current flow can entrain plankton within a canyon, accumulating highest densities at the canyon head where it can be readily accessed by large filter feeders such as baleen whales (Rennie et al. 2009;De Vos, Pattiaratchi, and Harcourt 2014). Similarly, current flows around and over seamounts and pinnacles can create eddy systems that entrap plankton (the "island mass effect"; Lavelle and Mohn 2010;Su, Wijeratne, and Pattiaratchi 2021;De Falco et al. 2022) and aggregate marine predators up through the food chain (Worm, Lotze, and Myers 2003;Morato et al. 2010). ...
Aim
To understand how natural geomorphological features and oil and gas platforms (OG platforms) influence the habitat use and seascape connectivity of the whale shark (Rhincodon typus).
Location
East‐Indian Ocean and North‐West Australia.
Methods
We compiled a satellite tracking dataset of 78 whale sharks tagged across a 14‐year period at Ningaloo Reef and Shark Bay World Heritage Areas in Western Australia to develop spatial networks for the regions of the East‐Indian Ocean and North‐West Australia. We then applied a Bayesian modelling framework to assess the effects of natural features and OG platforms on spatial patterns and habitat connectivity.
Results
Geomorphological features such as pinnacles, canyons, and seamounts promoted habitat connectivity and strongly influenced the habitat use of whale sharks across both regional (1000's km; East‐Indian Ocean) and local (100's km; North‐West Australia) spatial scales. In the North‐West of Australia, OG platforms had similar effects on habitat use as natural feature types and also enhanced habitat connectivity. The OG platforms most visited by whale sharks were situated close to the edge of the continental shelf and near natural geomorphological features that likely enhance productivity.
Main Conclusion
Our work identified natural geomorphological features that promoted habitat use and connectivity for whale sharks across oceanic and coastal seascapes. Sharks routinely visited OG platforms, which acted as migratory stepping stones that further enhanced habitat connectivity. Protection of natural feature types that promote habitat use and connectivity could assist conservation management of whale sharks. We suggest that the influence of OG platforms on their movement and habitat use beyond individual structures, should be considered in environmental impact assessments during operation and decommissioning phases.
... oceanographic features. De Vos et al. (2014) found that blue whale sightings declined in nearshore waters when there was increased freshwater input, subsequently leading to lower salinity surface waters and deeper upwelling. Similarly, Benson et al. (2002) documented an increase in baleen whales nearshore when an El Nino event caused higher sea surface temperatures, declines in upwelling, and reduced productivity in offshore waters. ...
... Due to this difference in directionality and intensity of wind forcing and currents, the type of circulation features and nature of the IME developed are unique to each monsoon period. The effects of freshwater input rainfall/runoff in the study region are limited to a narrow coastal band along the coast (de Vos et al., 2014a). ...
... Due to this difference in directionality and intensity of wind forcing and currents, the type of circulation features and nature of the IME developed are unique to each monsoon period. The effects of freshwater input rainfall/runoff in the study region are limited to a narrow coastal band along the coast (de Vos et al., 2014a). ...
The monsoon circulation in the Northern Indian Ocean (NIO) is unique since it develops in response to the bi-annual reversing monsoonal winds, with the ocean currents mirroring this change through directionality and intensity. The interaction between the reversing currents and topographic features have implications for the development of the Island Mass Effect (IME) in the NIO. The IME in the NIO is characterized by areas of high chlorophyll concentrations identified through remote sensing to be located around the Maldives and Sri Lanka in the NIO. The IME around the Maldives was observed to reverse between the monsoons to downstream of the incoming monsoonal current whilst a recirculation feature known as the Sri Lanka Dome (SLD) developed off the east coast of Sri Lanka during the Southwest Monsoon (SWM). To understand the physical mechanisms underlying this monsoonal variability of the IME, a numerical model based on the Regional Ocean Modeling System (ROMS) was implemented and validated. The model was able to simulate the regional circulation and was used to investigate the three-dimensional structure of the IME around the Maldives and Sri Lanka in terms of its temperature and velocity. Results revealed that downwelling processes were prevalent along the Maldives for both monsoon periods but was applicable only to latitudes above 4°N since that was the extent of the monsoon current influence. For the Maldives, atolls located south of 4°N, were influenced by the equatorial currents. Around Sri Lanka, upwelling processes were responsible for the IME during the SWM but with strong downwelling during the NEM. In addition, there were also regional differences in intra-seasonal variability for these processes. Overall, the strength of the IME processes was closely tied to the monsoon current intensity and was found to reach its peak when the monsoon currents were at the maximum.
Microparticles, such as microplastics and microfibers, are ubiquitous in marine food webs. Filter-feeding megafauna may be at extreme risk of exposure to microplastics, but neither the amount nor pathway of microplastic ingestion are well understood. Here, we combine depth-integrated microplastic data from the California Current Ecosystem with high-resolution foraging measurements from 191 tag deployments on blue, fin, and humpback whales to quantify plastic ingestion rates and routes of exposure. We find that baleen whales predominantly feed at depths of 50–250 m, coinciding with the highest measured microplastic concentrations in the pelagic ecosystem. Nearly all (99%) microplastic ingestion is predicted to occur via trophic transfer. We predict that fish-feeding whales are less exposed to microplastic ingestion than krill-feeding whales. Per day, a krill-obligate blue whale may ingest 10 million pieces of microplastic, while a fish-feeding humpback whale likely ingests 200,000 pieces of microplastic. For species struggling to recover from historical whaling alongside other anthropogenic pressures, our findings suggest that the cumulative impacts of multiple stressors require further attention.
Benthic and pelagic food webs are dominated numerically and functionally by microbes. Benthic food webs are a mixture of small and large invertebrates known collectively as meiofauna and macrofauna with smaller microbial groups such as protozoans, bacteria, archaea, viruses, and fungi making up the microbenthos. Reflective sandy beaches are open ecosystems in which food webs receive most of their energy input from either the land or sea, or both. The dominant space‐occupying primary producers on most tropical rocky shores are low‐lying, encrusting algae and turf algae. Predatory fish are common in seagrass meadows, feeding on other fish, shrimps, amphipods, worms, and other small invertebrates. Direct grazing of seagrasses by herbivores can represent a significant transfer of carbon and energy to food webs. The food webs of coral reefs are extraordinarily complex, with different reef zones dominated by a menagerie of food webs.
The distribution of whales and krill in two survey boxes north of South Georgia was examined by comparing sightings and underway acoustic data collected as part of a multi-disciplinary research cruise carried out during January/February 1998. A total of 222 cetaceans of 10 species was recorded with the southern right whale (Eubalaena glacialis) and humpback whale (Megaptera novaeangliae) the two most frequent. The largest aggregation of cetaceans (21 southern right whales, 16 fin whales (Balaenoptera physalus), 4 sei whales (B. borealis), 1 humpback whale and 8 hourglass dolphins (Lagenorhynchus cruciger) occurred close to the largest single aggregation of krill. The level of association between baleen whales and krill was examined at a number of spatial scales. There was a positive relationship between whale abundance and mean krill density at the largest spatial scale examined (803100km). At progressively smaller scales the relationship weakened, due mainly to the increased frequency of areas of high krill density where whales were not recorded. In particular, whales were absent from inshore areas (up to 300m depth) that had higher mean krill densities compared with areas where whales were recorded. To thoroughly compare krill and whale distribution, particularly at smaller scales, will require information on krill swarm structure and density, as well as more information on the behaviour and feeding requirements of whales. Such information may also be crucial to understanding the role of scale-dependence in potential interspecies competition among krill-feeding marine predators.
The waters off Sri Lanka, an island in the northern Indian Ocean, are an important habitat for blue whales, a species listed as Endangered in the IUCN Red List of Threatened Species. The subspecies status, genetic affinities and movement patterns of this northern Indian Ocean population of blue whales is not yet clearly understood but off Sri Lanka they can be found throughout the year. Sri Lanka also lies on a busy international shipping lane. Since 2009 a boat-based commercial whale-watching industry targeting the blue whale, has been developing off the south coast, with annual growth in numbers of boats and visitors. This industry however remains unregulated in any formal or informal manner due to the absence of regulatory mechanisms in this small, developing, island nation. Consequently unethical practices are leading to harassment resulting in direct and indirect impacts on the targeted whales. Sighting and stranding data from before and after the inception of whale watching activities indicate a spatial shift in the area of occurrence and concentration of whales and a corresponding increase in fatal vessel collisions along the southwest coastline of the island. There are clear indications of displacement from preferred near-shore feeding areas to the vicinity of the adjacent shipping lane, causing an increase in collision risk. Although these findings are based on limited data, it is recommended that a precautionary principal is followed and anthropogenic activities are quickly regulated to mitigate adverse impacts on these endangered blue whales. [JMATE. 2012;5(1):3-7]
We evaluated copepod taxonomic diversity as a potential biological indicator of ocean conditions in the northeast Pacific Ocean using data collected biweekly between May 1996 and December 2004 and from 1969 to 1973 and 1983 off Newport, Oregon. During the summer, low copepod biodiversity is accompanied by high biomass, with the opposite patterns prevailing in the winter. High biodiversity, and associated low biomass, is also observed during the summers of major El Niño events (1983, 1997-1998) and during years when the Pacific Decadal Oscillation is in a positive phase. From 1996 to the present, seasonally adjusted monthly anomalies of taxa richness and the Shannon-Weiner diversity index were compared to hydrographic and meteorological variables and to basin-wide climatological indices. Correlations of biodiversity with physical variables were strongest during the summer months for the Multivariate El Niño/Southern Oscillation Index, temperatures at a water depth of 50 m, and the Pacific Decadal Oscillation. During a 4-year cool period (1999-2002), biodiversity was low, likely a result of the increased transport of coastal subarctic waters into the northern California current. In recent years (2002-2005), however, there has been a dramatic increase in biodiversity. These increases may be attributable to the influence of a weak El Niño event in 2003; yet, high biodiversity persisted through the summer of 2005 and at times was higher than during the strongest El Niño events of the 20th century (1983 and 1997-1998). Our analyses suggest that changes in source waters, driven by remote basin scale forcing and not local environmental events, cause interannual-to-decadal variations in copepod biodiversity in the northern California current. © 2006, by the American Society of Limnology and Oceanography, Inc.
Two pygmy blue whales were struck and killed in Sri Lankan waters within a 12-day period in early 2012. The first was found draped on the bow of a container ship in Colombo Harbour on 20 March 2012 and following its disposal 25 km offshore, washed up on a beach 10 km south two days later. The second whale was found floating dead at sea by one of us (TW) on 2 April 2012. As this individual did not strand, no details apart from species and sex are available. The southern coast of Sri Lanka is one of the busiest shipping routes in the world (Kaluza et al., 2010) and overlaps with an area of high whale sightings. Because there is no abundance estimate for the local population of blue whales, we do not know what impact these deaths might have on the population. However, the reported deaths can only be considered minimum values. These deaths and the unknown population size highlight the urgent need for long-term monitoring of the blue whale population in Sri Lankan waters and elsewhere in the northern Indian Ocean.
With increasing interest in the field and its relevance in global environmental issues, Oceanography and Marine Biology: An Annual Review provides authoritative reviews that summarize results of recent research in basic areas of marine research, exploring topics of special and topical importance while adding to new areas as they arise. This volume, part of a series that regards the all marine sciences as a complete unit, features contributions from experts involved in biological, chemical, geological, and physical aspects of marine science. Including a full color insert and an extensive reference list, the text is an essential reference for researchers and students in all fields of marine science.
Selected case studies are presented to illustrate how seasonality of freshwater discharge, in conjunction with hydrography/topography of the receiving environment, can influence both production and transfer of material as well as species composition of the plankton. Spatial and temporal environmental patterns (zones or phases with specific combinations of physicochemical properties) that arise along the salinity gradient are colonized by characteristic organism types. It is argued that these organisms possess survival strategies (benthic or pelagic resting stages) that enable maintenance of resident populations within such regions. Some pelagic “life history types” are presented and it is shown that behavioural features of these organisms can have considerable impact on production as well as vertical and horizontal transfer of material introduced by freshwater discharge.
The coastal upwelling ecosystem near Monterey Bay, California, is a productive yet variable ecosystem and an important foraging area for many mobile apex predators, such as marine mammals. Long-term studies are necessary to better understand how wide-ranging predators respond to temporal environmental variability; however, few of these studies exist. We conducted monthly shipboard line-transect surveys in Monterey Bay from 1997 to 2007. We identified 22 species of marine mammals, and calculated monthly and annual densities for the 12 most commonly sighted (focal) species. Species richness remained relatively constant (mean richness +/- SE: 13.7 +/- 0.396 species yr(-1)) from 1997 to 2006. Focal species were most evenly distributed (Shannon's equitability, E-H = 0.820) but least dense (mean density +/- SE: 0.0598 +/- 0.0141) during the anomalous upwelling conditions of 2005, and least even (1997 E-H = 0.413; 1998 E-H = 0.407) but dense (mean density +/- SE: 1997: 0.433 +/- 0.177; 1998: 0.438 +/- 0.169 ind. km(-2)) during the 1997/1998 El Nino event. There were no statistically significant differences in the densities of marine mammal species between warmer and cooler years. The community and species-specific responses of marine mammals to warm-water years differed depending on the mechanism of oceanographic variability. During the 1997/1998 El Nino (a basin-wide event), marine mammals aggregated in nearshore areas, such as Monterey Bay, with relatively greater productivity than offshore regions, whereas during anomalous upwelling conditions of 2005 (a more localized oceanographic event), marine mammals redistributed away from Monterey Bay to areas less affected by the anomaly.
Standard metabolism is estimated for the fin whale, Balaenoptera physalus, from the energy yield of lipid stores consumed while in regions of limited prey availability. The metabolic rate appears better described by a surface rather than a volumetric rule. The larger body size of some Antarctic Balaenoptera compared with those of the Northern Hemisphere is attributed to selection for body proportions that reduce the specific metabolic rate and establish an optimal surface area for deposition of a lipid cache. Such proportions allow short-term forays into areas where prey is extremely dense but of restricted seasonal availability, and permit extended use of lipid stores during exclusion to warmer but less productive waters. Thus, adult body size within species is inversely correlated with the length of the feeding season and directly correlated with prey availability during that period. As differences in diversity of prey and/or their year-round availability become less marked between hemispheres, so do the body-size differences of the lesser rorquals. This is also applied to odontocetes. Thermal homeostasis is considered to be equally dependent upon morphology and behavior. Minimal densities of prey necessary for the maintenance of rorquals could be estimated from certain known parameters.