Deep Sea Research Part II: Topical Studies in Oceanography
An integrated approch to the foraging ecology of marine birds and mammals
Abstract
Birds and mammals are important components of pelagic marine ecosystems, but our knowledge of their foraging ecology is limited. We distinguish six distinct types of data that can be used in various combinations to understand their foraging behavior and ecology. We describe methods that combine concurrent dive recorder deployment, oceanographic sampling, and hydroacoustic surveys to generate hypotheses about interactions between the physical environment and the distribution, abundance, and behavior of pelagic predators and their prey. Our approach is to (1) map the distribution of whales in relation to the distribution of their prey and the physical features of the study area (bottom topography, temperature, and salinity); and (2) measure the foraging behavior and diet of instrumented whales in the context of the fine-scale distribution and composition of their prey and the physical environment. We use this approach to demonstrate a relationship between blue whale distribution, sea surface temperature, and concentrations of their euphausiid prey at different spatial scales offshore of the Channel Islands, California. Blue whale horizontal spatial distribution was correlated with regions of high acoustic backscatter. Blue whale dive depths closely tracked the depth distribution of krill. Net sampling and whale diet revealed that whales fed exclusively upon dense schools of Euphausia pacifica (between 100 and 200 m) and Thysanoessa spinifera (from the surface to 100 m). Whales concentrated foraging efforts upon those dense euphausiid schools that form downstream from an upwelling center in close proximity to regions of steep topographic relief. We propose that (1) the distribution of Balaenoptera whales in the coastal California Current region is defined by their attraction to areas of predictably high prey density; (2) the preferred prey of these whales are several species of euphausiids (E. pacifica, T. spinifera, and N. simplex) that are abundant in the California Current region; (3) blue whales concentrate their foraging efforts on dense aggregations of euphausiids found at discrete depths in the water column; (4) these localized areas of high euphausiid densities are predictable and sustained by enhanced levels of primary productivity in regions which are located downstream from coastal upwelling centers (indicated by sea surface temperature); (5) topographic breaks in the continental shelf located downstream from these upwelling centers work in concert with euphausiid behavior to collect and maintain large concentrations of euphausiids swarms, and (6) despite seasonal and inter-annual variability, these processes are sufficiently consistent that the distribution of Balaenoptera whales can be predicted.
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Multiple stable isotopes (C, N & S) provide evidence for fin whale (Balaenoptera physalus) trophic ecology and movements in the Humboldt Current System of northern Chile
2023, Marine Environmental ResearchReflecting the intense coastal upwelling and high primary productivity characteristic of the Humboldt Current System (HCS), the northern coast of Chile supports a diverse and productive community of marine consumers, including worldwide important pelagic fisheries resources. Although marine mammals are relatively understudied in the region, recent studies have demonstrated that fin whale (Balaenoptera physalus) is the most frequently encountered whale species, and forages in these waters year-round. However, a current lack of information limits our understanding of whether fin whales actively feed and/or remain resident in these waters or whether whales are observed feeding as they migrate along this part of the Pacific. Here, we use stable isotope ratios of carbon, nitrogen and sulphur of fin whale skin samples collected in early summer 2020 (n = 18) and in late winter 2021 (n = 22) to examine evidence of temporal isotopic shifts that could provide information on potential migratory movements and to estimate likely consumption patterns of putative prey (i.e. zooplankton, krill, pelagic fishes and Pleuroncodes sp.). We also analysed prey items in fin whale faecal plumes (n = 8) collected during the study period. Stable isotope data showed significant differences in the isotopic values of fin whales from summer and winter. On average, summer individuals were depleted in 15N and 34S relative to those sampled during winter. Whales sampled in summer showed greater isotopic variance than winter individuals, with several showing values that were atypical for consumers from the HCS. During winter, fin whales showed far less inter-individual variation in stable isotope values, and all individuals had values indicative of prey consumption in the region. Analysis of both stable isotopes and faeces indicated that fin whales sighted off the Mejillones Peninsula fed primarily on krill (SIA median contribution = 32%; IRI = 65%) and, to a lesser extent, zooplankton (SIA zooplankton = 29%; IRI copepod = 33%). These are the first isotopic-based data regarding the trophic ecology of fin whales in the north of Chile. They provide evidence that fin whales are seasonally resident in the area, including individuals with values that likely originated outside the study area. The information presented here serves as a baseline for future work. It highlights that many aspects of the ecology of fin whales in the Humboldt Current and wider SE Pacific still need to be clarified.
Vaquita's habitat suitability in the Upper Gulf of California between two contrasting environmental years: 1997 and 2008
2023, Regional Studies in Marine ScienceThe vaquita (Phocoena sinus) is the most endangered marine mammal in the world, with an estimated population of around ten individuals in 2018 and with the smallest geographic range. We aim to describe the differences in the preferred vaquita habitat in the Upper Gulf of California between 1997, which was classified as a warm phase of the El Niño-Southern Oscillation (ENSO), and the year 2008 classified as a neutral phase of ENSO. We used vaquita’s acoustic and visual records in 1997 and 2008; environmental factors were derived from satellite imagery (sea surface temperature and depth) and measured during oceanographic cruises (salinity, sediment) through the Maximum Entropy algorithm (MaxEnt). The MaxEnt model estimated an area of 649 km2 in 1997 and 546 km2 in 2008, where the probability of vaquita’s occurrence was equal to or higher than 50%. Thus, suggesting an essential reduction in the preferred habitat (16% approx.) between both periods. In both years, the area with the highest probability of occurrence was defined by silty sediment. The habitat characteristics were coastal and shallow water. The temperature and salinity exhibit significant differences (p < 0.05) by year, with higher temperatures in 1997 (24–25 °C) and lower salinities in 1999 (35.5–36.4 psu) compared with 2008 (23–24 °C; 35.6–36.2 psu). We consider that the reduction of the vaquita’s habitat demonstrates the urgent interannual monitoring to assess the environmental changes that occur in the region since any change would affect the Vaquita habitat and its distribution.
National Marine Sanctuaries capture enhanced abundance and diversity of the California Current Ecosystem avifauna
2023, Journal of Marine SystemsQuantifying the spatial patterns of marine predators, such as seabirds, reveals areas of ecological importance and associated food web characteristics, upon which marine conservation and management plans can be based. Owing to high productivity, the California Current Ecosystem (CCE) harbors an abundant and diverse avifauna comprised of resident and migratory species. With a goal of protecting habitats in three biogeographic regions — North, Central, and South CCE — a network of marine protected areas have been designated, including five National Marine Sanctuaries (NMS). Using an extensive at-sea survey dataset (1980 to 2017), we characterize spatial patterns of seabirds, and compare mean relative abundance, diversity, and community composition both within and outside the NMSs. We found that within the CCE, seabird abundance and diversity increase linearly from south to north (R2 = 0.54 and R2 = 0.55, respectively) and decrease linearly with distance from the coast (R2 = 0.16 and R2 = 0.23, respectively). The avifauna of the North and South regions are the most distinct, with the Central region being transitional between the two. The CCE avifauna shows limited overall diversity, with just 10 species contributing >93% of abundance totals. In addition, certain foraging guilds and prey preference groups were dominant: pursuit-feeding and piscivorous species in the North and surface feeding generalists in the South. Overall, seabird relative abundance and diversity are higher within NMS boundaries compared to outside. Although relative abundance and diversity of seabirds within NMSs were broadly representative of corresponding biogeographic regions, the overall NMS network captures a range of distinct seabird communities. The analysis of this extensive dataset provides a better understanding of seabird spatial patterns and their ecological roles within different regions, thus facilitating more effective, adaptive management of CCE biotic resources.
On the rise: Climate change in New Zealand will cause sperm and blue whales to seek higher latitudes
2022, Ecological IndicatorsClimate impacts affect marine ecosystems worldwide with island nations such as New Zealand being extremely vulnerable because of their socio-economic and cultural dependence on the marine and costal environment. Cetaceans are ideal indicator species of ecosystem change and ocean health given their extended life span and cosmopolitan distribution, but limited data availability prevents anticipating change in distribution under future climate changes. We projected the range shifts of a key odontocete and mysticete species (Physeter macrocephalus and Balaenoptera musculus) in 2100 relative to present day in New Zealand waters, using an ensemble modelling approach, under three climate change scenarios of different severity.
The results show a latitudinal shift in suitable habitat for both whale species, increasing in magnitude with severity of sea surface temperature warming. The most severe climate change scenario tested generated 61% and 42% loss and decrease of currently suitable habitat for sperm and blue whales, respectively, mostly in New Zealand’s northern waters. These predicted changes will have a strong impact on the ecosystem functioning and services in New Zealand’s northern waters but also in coastal areas (critical for the species’ foraging and survival). Not only do these simulated range shifts help to identify future potential climate refugia to mitigate a global warming, they also generate a range of socioeconomic consequences for island nations relying on wildlife tourism, industry, and environmental protection.
Euphausiid spatial displacements and habitat shifts in the southern California Current System in response to El Niño variability
2021, Progress in OceanographyWe analyzed spatial distributions of 10 euphausiid species in the southern California Current System across seven El Niño events (1951–2018) and the 2014–15 Warm Anomaly to determine variations in habitat utilization and reproduction during Eastern Pacific (EP) and Central Pacific (CP) Niños. Our goal was to characterize the main forcing mechanisms by which El Niño events influence these dominant species. Our findings suggest cool-water euphausiids respond predominantly to changing in situ habitat conditions during El Niño, while subtropical species require initial advection to increase in the southern CCS. Cool-water coastally-associated species (Euphausia pacifica, Thysanoessa spinifera) compress shoreward and retract poleward to upwelling waters during EP Niños, likely in response to offshore warming. A subtropical coastal species (Nyctiphanes simplex) extends poleward nearshore during EP Niños, suggesting anomalous advection, but increases only moderately and variably off southern California during CP Niños. A Tropical Pacific-Baja California species (Euphausia eximia) only appears off southern California in spring during El Niño years (EP, some CP), suggesting direct advection and low tolerance for cooler, fresher conditions. Subtropical offshore species (Euphausia gibboides, Euphausia recurva, Stylocheiron affine, Euphausia hemigibba) expand shoreward during most Niños (strongest during 2014–15 Warm Anomaly) and show moderate in situ post-event persistence, suggesting combined influence of advection and temporarily favorable habitat nearshore. Regionwide temperate species (Nematoscelis difficilis, Thysanoessa gregaria) contract only moderately shoreward during some Niños. Predictions of Year 2100 distributions using generalized additive models suggest future non-Niño conditions and CP Niños will produce regionwide in situ increases in subtropical species and moderate poleward and onshore expansions, while EP Niños will produce continued nearshore habitat compression and reduced abundance of coastal species. Understanding zooplankton spatial responses to El Niño can help predict community compositional shifts under other ocean changes (e.g., long-term trends, basin-wide warm anomalies).
Comparison of condition metrics and lipid content between Euphausia pacifica and Thysanoessa spinifera in the northern California Current, USA
2020, Progress in OceanographyKrill are a key component of pelagic food webs where they are vital to the transfer of energy from phytoplankton to higher trophic levels. Krill have a high lipid content compared to other zooplankton and form dense aggregations, making them an important prey source for higher trophic level fish, seabirds, and marine mammals. The two dominant euphausiid species in the northern California Current (NCC) are Euphausia pacifica and Thysanoessa spinifera. E. pacifica is the most abundant species of euphausiid in the NCC, but T. spinifera has a higher potential energetic content due to its larger body size and higher lipid density. Most studies have inferred differences in lipid content and body condition between the two species, but few studies have quantified these differences in the NCC. Here, we report on the body condition, carbon and nitrogen content, as well as lipid and fatty acid composition of these two krill species, and the extent to which these metrics vary across season, year, and reproductive status. Body condition, elemental composition, and total lipids strongly differed between the species. T. spinifera had higher length-weight, Fulton’s K, hepato-somatic index, carbon to nitrogen ratio, total lipid per wet weight, and storage lipid compared to E. pacifica, indicating that T. spinifera has a higher energetic value for predators. However, there were strong seasonal differences in the energetics of T. spinifera. Carbon and lipids were highest in non-reproductive T. spinifera from August through October. Although there were strong ontogenetic and inter-specific differences, the lipid and fatty acid compositions of both species followed a seasonal progression characterized by low lipids during the pre-upwelling period, an increase in lipids, triacylglycerols, and diatom markers during upwelling, and increased proportions of dinoflagellate and bacterial diet markers during the fall post-upwelling period.