Do cetaceans eat mesopelagic organisms?

Mesopelagic organisms are important prey for cetacean species, that are known to feed heavily on them, diving to great depth or relying on the diel vertical migration of mesopelagic prey to surface waters [Gimenez et al., 2018]. However, limited information is available on how much mesopelagic prey is consumed by cetaceans. Consumption estimates for cetaceans are often lacking due to the data deficiencies on diets and abundance estimates. One of the objectives of project SUMMER is to quantify consumption estimate for cetaceans and investigate the importance of mesopelagic organisms on their diet. To achieve this objective, the IMAR – Institute of Marine Research implemented a bioenergetic modelling approach developed by Spitz et al., (2018) using a new diet composition and energy densities database, compiled within the project SUMMER, combined with the first population estimates of cetaceans in territorial waters of the central islands of the Azores region and the energetic requirements of the cetacean species inhabiting this region.

Figure 1. The most important prey species consumed by cetaceans inhabiting the central group of the Azores region

Diet information from stomach contents of stranded and bycaught cetaceans from the North Atlantic was retrieved from scientific peer-reviewed articles and technical reports and compiled into a database, [Silva et al 2021]. The database included individual information on the area of each study, on the cetacean species (ie number of individuals, length) and on their prey (ie % of total weight composition) and on the energy density of each prey species. Prey species were grouped into functional prey groups (mesopelagic cephalopods, non-mesopelagic cephalopods, crustaceans, demersal fish, epipelagic fish, mesopelagic fish and zooplankton).

Abundance estimates for cetaceans were obtained from the line transect surveys carried out during July 2018 around the central group of islands (Faial, Pico, São Jorge, Graciosa and Terceira) of the Azores region and the offshore banks area south of Pico and Faial Islands (Saavedra et al., 2018). The study estimated abundances for 11 cetacean species that were included on this analysis to estimate their consumptions (common dolphin, striped dolphin, spotted dolphin, bottlenose dolphin, risso’s dolphin, short-finned pilot whale, cuvier’s beaked whale, sowerby’s beaked whale, sperm whale, northern bottlenose whale and sei whales)

The energetic requirements of each cetacean species were calculated using the basal metabolic rate (BMR; kilojoules per day), which is the amount of energy that an individual uses to simply stay alive in the absence of any physical activity, which varies with body mass (kilograms) (Spitz et al., 2018). As the energetic costs of both physical and physiological activities (ie., thermoregulation, foraging and reproduction) vary between cetacean species, a different activity cost value specific for each cetacean species was added to the BMR to estimate the average daily metabolic rate (ADMR). To estimate the amount of daily prey required by cetacean species (average daily ration; kilograms), the ADMR was converted to wet mass of ingested food by using the energy density of each prey species combined with the proportion biomass of prey species in the diet of a cetacean, and corrected by the assimilation efficiency.

Estimates of cetacean consumption for each functional prey groups were calculated based on the mean estimate of abundance for cetacean species, the average daily ration, the proportion of biomass of prey group in the diet of cetacean species and on the number of days that cetaceans’ species spend in the study area.
The literature survey showed that the cetaceans inhabiting the Azores region feed on a wide variety of prey, but Cranchiidae (Megalocranchia sp., Teuthowenia megalops), Histioteuthidae (Histioteuthis arcturi, Histioteuthis bonnellii), Myctophidae (Diaphus sp., Notoscopelus kroeyeri), Octopodidae (Octopus vulgaris, Eledone cirrhosa) and Ommastrephidae (Todarodes sagittatus, Illex coindetii) represented 50% of all prey consumed (Figure 1; 2).

Figure 2. Percentage of prey weight on the diet composition of cetacean species by prey families.

Overall, these cetacean species consumed more mesopelagic organisms than non-mesopelagic organisms, with mesopelagic cephalopods being the most important prey group, followed by mesopelagic fish, non-mesopelagic cephalopods and epipelagic fish (Figure 3). The diet of the deep diving species, such as sperm and beaked whales, was mainly represented by mesopelagic cephalopods (> 90% of this functional group), which contrasts with the more diverse diet of small delphinids, such as common and striped dolphin, that include mesopelagic cephalopods, demersal and epipelagic fish.

Figure 3. Percentage of prey weight on the diet composition by individual cetacean species

Regarding the average daily consumption of the cetacean species, large differences were found between species, from only 7.6 kg per day for a common dolphin to more than 350 kg for a sei whale (Figure 4). In the study area, biomass removals during the summer months by the 11 species of cetaceans equated to 11,940 tons (80% CI 9,054 – 15,194 ) across all prey groups. The bulk of the biomass consumed by all cetaceans were 90% of mesopelagic species (10,706 tons/summer; 3.6 kg.km²/day) and only 10% of non-mesopelagic species (1227 tons/summer; 0.4 kg.km²/day).

Figure 4. Average daily ration (Kg) per cetacean species

At the species level, sperm whale was the highest consumer of mesopelagics (35% of mesopelagic biomass consumption at 3766 t/summer), followed by spotted dolphin (21% mesopelagic biomass at 2235 t/summer), sei whale (13% at 1376 t/summer) and northern bottlenose whale (12% mesopelagic biomass 1248 t/summer)(Figure 5). These four species comprised 81% of all mesopelagic species consumed.

Figure 5. Consumption estimates of prey groups by cetacean species

References:

Gimenez J et al. (2018) Feeding ecology of Mediterranean common dolphins: The importance of mesopelagic fish in the diet of an endangered subpopulation. Mar Mammal Sci 34:136–54.

Olafsdottir D et al. (2016) Dietary evidence of mesopelagic and pelagic foraging by Atlantic bluefin tuna (Thunnus thynnus L.) during autumn migrations to the Iceland Basin. Front Mar Sci 3:108

Saavedra C, Santos M. B, Valcarce P, et al. (2018) Macaronesian Roof Report. Technical Report. Project MISTIC SEAS II.

Silva MA, et al. (2021) SUMMER Project. Deliverable 3.1: Database of Existing Diet/Trophic Level Data of Key Mesopelagic Species and Predators and of Nutritional Quality of Mesopelagic Organisms.

Spitz J, et al. (2018) Prey consumption by cetaceans reveals the importance of energy-rich food webs in the Bay of Biscay. Progress in Oceanography. 166: 148-158.

Text and pictures by:

Sergi Pérez-Jorge^1,2 and Mónica A. Silva^1,2