Confronting assumptions about prey selection by lunge‐feeding whales using a process‐based model

Abstract

The relative energetic benefits of foraging on one type of prey rather than another are not easily measured, particularly for large free‐ranging predators. Nonetheless, assumptions about preferred and alternative prey are frequently made when predicting how a predator may impact its environment, adapt to environmental change or interact with human activities. We developed and implemented a process‐based model to investigate the potential energetic benefit (PEB) of in situ foraging opportunities in rorqual whales. The model integrates and evaluates the energetic importance of measured prey patch characteristics (prey distribution, energy content and predator avoidance) and predator characteristics (morphometrics, foraging tactics and feeding rates). We applied the model to test the assumption that hatchery‐released juvenile salmon are an ‘easy meal’ for humpback whales compared to more common prey: herring and krill. In 11 out of the 13 foraging situations considered, whales were found to be feeding in a manner where net energy gain was greater than the energetic costs of non‐foraging swimming. Humpback whale PEB for hatchery‐released juvenile salmon fell within the range of the PEB for krill and herring but varied by species, from relatively high PEB for chum salmon to relatively low for coho salmon. Our model provides behavioural insight as well, indicating that shallow feeding may be more important for reducing energy expenditure through slower lunge speeds than for increasing prey capture. The model also provides a means of identifying prey patch characteristics, with prey aggregation playing the largest role in determining PEB despite being a poor overall proxy for PEB, supporting the use of the complex model framework. Modelling approaches are especially valuable where they can use reasonable assumptions to substitute for lack of reliable observations, thereby integrating a range of interacting factors into a single framework. Additionally, because process‐based models can be used to make predictions outside the range of previously observed conditions, they will be increasingly useful in a changing climate. A free Plain Language Summary can be found within the Supporting Information of this article.

Cheap gulp foraging of a giga-predator enables efficient exploitation of sparse prey

“…For a common Antarctic krill type, Euphasia superba , the energetic value has been measured at 3.8 to 5.4 MJ kg −1 ww ( 9 , 74 ). Similar values were found for krill species in Greenland and Iceland ( Thysanoessa raschii and Meganyctiphanes norvegica ) 3.9 to 6.4 MJ kg −1 ww ( 75 , 76 ) and for two other species of krill ( Thysanoessa spinifera and Euphausia pacifica ) 2.94 to 3.8 MJ kg −1 ww ( 21 ). For capelin ( Mallotus villosus, Osmeridae) in Greenland, the energy density is, on average, 4.2 MJ kg −1 ww ( 77 ) but can also vary between sex and season ( 78 ).…”

“…Thus a 30-t humpback whale expends net zero energy in a lunge that yields merely 225 g of prey in the approximately 21 m 3 of engulfed water (11), equivalent to one capelin m −3 or about 10 krill m −3 . Despite the persistent narrative that lunges are highly energetic (4,(6)(7)(8)20), our median lunge cost estimate of 0.77 MJ is within the range of estimates from biomechanical models that report costs between 0.5 and 2.6 MJ per lunge (6,18,(20)(21)(22). Thus, both our field respirometry estimates and biomechanical modeling of acceleration and speed data from biologging show that foraging costs are low in humpback whales despite a high-drag feeding mechanism.…”

“…The engulfment capacity of humpback whales is 15 to 27 m 3 with a mean of 21 m 3 per lunge ( 11 , 27 ). Assuming a 50% catch rate of prey ( 22 ), an average prey density of 0.37 kg m −3 is needed to obtain an average of 3.8 kg of prey per lunge, which is within the observed krill and fish densities in feeding areas ( 11 , 21 , 28 , 29 ). Remarkable footage of feeding humpback whales show that they sometimes engulf particularly dense prey patches [movie S4; ( 22 , 29 )] to achieve extraordinarily high foraging efficiencies.…”

“…Remarkable footage of feeding humpback whales show that they sometimes engulf particularly dense prey patches [movie S4; ( 22 , 29 )] to achieve extraordinarily high foraging efficiencies. In these extreme cases, peak densities from echo sounder surveys ( 11 , 21 , 22 , 29 ) suggest that humpback whales may ingest tens of kilograms of prey per lunge, but our estimated average intake show that such very successful lunges are rare (movies S1 to S3). Instead, food returns per lunge are likely to be log-normally distributed ( 29 ) over a season with fewer rich patches interspersed with many more poor ones.…”

Critically endangered Rice’s whales (Balaenoptera ricei) selectively feed on high-quality prey in the Gulf of Mexico

“…Understanding predator-prey interactions is critical in ecology, but it remains challenging when investigating highly mobile and elusive species foraging at depth in marine ecosystems [1][2][3][4] . This is particularly the case for cetaceans, for which dietary data are often lacking due to the extent of their movements, both horizontally and vertically in the water column, and the lack of stomach content samples or observational evidence of their diets and foraging behavior [5][6][7][8] . Obtaining dietary data is, however, crucial to predicting how changes in predator-prey interactions may influence individual and population-level fitness in light of impacts from human activities, including overfishing and climate change [9][10][11][12] .…”