Marine food web structure
Everything must consume something else to survive. Essentially, marine food webs are all about energy and nutrient exchange. A food chain is a single pathway depicting a simple relationship between a primary producer, primary consumer, secondary consumer and tertiary consumer.
A food web shows how a series of food chains in an ecosystem are inter-related, depicted by multiple pathways and connecting species in an intricate relationship of interdependence. Primary producers create the base of the food web, are eaten by more than one primary consumer, which are eaten by more than one secondary and tertiary consumer. Marine food webs are incredibly complex, and many consumers feed at more than one trophic level.
Phytoplankton are marine plants and algae and the ocean’s primary producers that ‘consume’ sunlight via the photosynthetic process, by taking in carbon dioxide and producing most of the oxygen on the planet, providing the foundation for the marine food web.
Phytoplankton are in turn, eaten by the primary consumers, including zooplankton, young forage fish, herbivorous fish and crustaceans. Zooplankton consists of microscopic animals, shrimp, krill, copepods, jellyfish and the immature or larval stages of larger animals, including crustaceans. Herbivorous parrotfish feed on the epilithic algae on corals and the Sirenians (manatees and dugongs), the herbivorous marine mammals, feed on seagrass.
Zooplankton provide the link between primary producers and secondary consumers. Primary consumers are eaten by secondary consumers, including carnivorous fish, seals, small sharks, corals, squid, crustaceans and baleen whales – think Balaenoptera musculus (Blue Whale) and Euphausia superba (Antarctic Krill).
Secondary consumers are eaten by tertiary consumers, including dolphins, tuna, squid, sharks and seabirds. Some tertiary consumers are also apex predators, including Orcinus orca (Killer Whale). Physeter macrocephalus (Sperm Whale) are both quaternary consumer and apex predator. Killer whales are natural predators of sperm whales.
Marine food webs and cetaceans
Cetaceans are carnivorous marine mammals that feed on other marine species. Mysticetes (baleen whales) feed on zooplankton (krill) while odontocetes (toothed whales) feed on finfish (tuna, mackerel and herring) and shellfish (crustaceans, cephalopods and bivalves).
Eubalaena glacialis (North Atlantic Right Whale) filter feed on the primary consumer Calanus finmarchicus, a planktonic copepod and key pelagic species in the North Atlantic food web. Populations of C. finmarchicus have declined significantly in right whale habitat, due to extremely high, above-average temperatures, which is driving these baleen whales further north in search of new feeding grounds, where their survival as a species is threatened by entanglement in fishing lines cast by the lobster industry (see Entanglements & North Atlantic Right Whales under Impacts).
The most recent population estimate in 2020 found only 74 Southern Resident Killer Whales (Orcinus orca) remaining in the coastal waters of the Pacific Northwest, separated into three pods J (24 whales), K (17 whales) and L (33 whales) (CWR 2020).
The SRKWs are an endangered population, with their decline attributed to three main causes – lack of a sufficient and consistent prey base being the most important driver (toxins and vessel noise are the others) and a limiting factor in their recovery. The SRKWs primary food is Oncorhynchus tshawytscha (Chinook Salmon), whose populations have also declined drastically due to human activity, with overfishing a key threat.
A study by Hanson et al (2021) showed that although SRKWs favour Chinook salmon as their primary food source, they will vary their diet when this species is unavailable. Between 2004 and 2017, prey and faecal samples from 150 SRKWs and Northern Resident Killer Whales (NRKWs) were analysed and found O. tshawytscha constituted 100% of their diet in spring (historical studies showed O. tshawytscha was the primary food source in summer).
Chinook salmon comprised 70 – 80% of their diet in mid-winter and early spring, while in autumn when O. tshawytscha was scarce, this species constituted 50% of their diet, with necessary supplementation provided by other salmonid and non-salmonid species, including Oncorhynchus mykiss (Steelhead Trout) and Hippoglossus stenolepis (Pacific Halibut).
S.O.S. (Save Our Sharks)
Shark finning is a cruel and unsustainable practice. The dorsal fin is severed from a live shark and the body is dumped back into the ocean. If the shark is alive, it cannot swim properly or pass water over its gills and will drown, bleed to death, or be eaten. The fin is commercially valuable, so it is more economical to take the fin and discard the body.
About 100 million sharks are killed every year, with the shark finning trade a key threat to shark populations (sharks are also caught for their cartilage, meat and oil). In a study of 1,041 chondrichthyan fish species (sharks, rays and chimaeras), it was estimated that one in four species are threatened with extinction (Dulvy et al. 2014), mainly because of overfishing.
Sharks are important elements in marine food webs. Longer-lived and slower-growing, with low reproductive rates, sharks reach reproductive maturity much later than smaller species, so their populations take longer to recover, making them highly vulnerable to overfishing.
Trophic cascades in the marine environment
Cetaceans occupy the highest levels of the marine food web with other predatory carnivorous species (polar bears, sharks, tuna), performing multiple functions as predator and prey, and regulating the abundance and distribution of species above and below them in the food chain.
Changes to food web dynamics may cause top-down and bottom-up trophic cascades and exert significant pressure on, and control over, ecosystems, affecting predator-prey relationships and creating ecological imbalances in environmental systems.
A simplistic model of a trophic cascade in a marine food chain might look like this – if a large predator (shark) was removed from an ecosystem, the prey species (tuna) would increase and eat more of their prey (herring), causing a decrease in that species. This may cause their prey species (sardine) to proliferate, that would in turn eat more of their ‘prey’ (plankton).
Although 90% of large predatory fish (sharks, tuna, swordfish) have been removed from the ocean due to overfishing, the existence of trophic cascades and their impact on marine ecosystems from removal of large predators is highly contentious, heavily debated, and far from resolved.
A well-known example of a marine trophic cascade occurs in ‘urchin barrens’ where herbivorous sea urchins proliferate (in the absence of predatory sea otters), cover the seafloor and feast on kelp forests, often until there is nothing left. Habitats with healthy sea otter populations control sea urchin populations, allowing the kelp forests to flourish.
Biodiversity and ocean resilience
The higher the biodiversity (the greater number of species) in the ocean, the more resilient marine organisms, and the more robust marine ecosystems, are against impacts from human activity and environmental variability. Biodiversity strengthens the resilience of organisms and marine systems, especially fisheries. Removing too many marine species, and especially keystone species, from their natural habitat can impact ecosystem health, and the biodiversity that habitat supports.
Forage fish and African penguins
Fish are critical elements in marine food webs, producing more offspring than are expected to survive to adulthood, with most progeny being eaten by other organisms, providing a valuable source of energy and nutrients in marine (and terrestrial) environments.
Sardine and anchovy stocks in South Africa are depleted due to overfishing and climate change and are no longer available where they have historically been found, placing the endangered Spheniscus demersus (African Penguin), whose favoured prey are these two fish, at risk.
S. demersus (African Penguin) is listed as Endangered on the IUCN Red List. The population of African penguins across their entire range in South Africa and Namibia have declined by 80%, with only 23,000 breeding pairs left in the wild (Birdlife International 2020).
A study tracked 54 juvenile penguins from 8 colonies for 3000 days between 2011 and 2013, using satellite transmitters along their initial solo journey out of the parental nest, a time when they must rely on their instincts and follow the environmental cues that have always led their ancestors to traditional feeding grounds, where they feed so they can survive their first year at sea (Sherley et al. 2017).
But the penguins are falling into an ‘ecological trap’ because their prey is no longer there. Historically, African penguin feeding grounds were in areas of low sea temperatures, high phytoplankton productivity and high abundance of sardines and anchovy. But overfishing and climate change (see Overfishing & Climate Change under Impacts) have depleted stocks.
The penguins are responding to environmental cues that usually signal high prey abundance, including the presence of the phytoplankton and zooplankton. But the secondary consumers, the forage fish that normally feed on these primary producers and primary consumers, are scarce, leading to high African penguin mortality, particularly during that first lone voyage.
Sherley et al. (2017) demonstrate how the collapse of forage fish stocks from overfishing (and climate change) creates an ecological trap that juvenile penguins are unable to behaviourally adapt to, causing a decline in their species, with potentially severe impacts on their predators.
The researchers also recommend conservation measures including reduction of current fish quotas and suspension of fishing when prey species fall below critical ecological thresholds.
© 2016 – 2021 Seafood Free September
BirdLife International. 2020. Spheniscus demersus. The IUCN Red List of Threatened Species 2020: e.T22697810A157423361. https://dx.doi.org/10.2305/IUCN.UK.2020-3.RLTS.T22697810A157423361.en. Downloaded on 06 August 2021.
Centre for Whale Research (CWR) (2021), https://www.whaleresearch.com/orca-population
Nicholas K Dulvy, Sarah L Fowler, John A Musick, Rachel D Cavanagh, Peter M Kyne, Lucy R Harrison, John K Carlson, Lindsay NK Davidson, Sonja V Fordham, Malcolm P Francis, Caroline M Pollock, Colin A Simpfendorfer, George H Burgess, Kent E Carpenter, Leonard JV Compagno, David A Ebert, Claudine Gibson, Michelle R Heupel, Suzanne R Livingstone, Jonnell C Sanciangco, John D Stevens, Sarah Valenti, William T White (2014), Extinction risk and conservation of the world’s sharks and rays, eLife 2014;3:e00590, https://doi.org/10.7554/eLife.00590
Gavrilchuk K, Lesage V, Fortune SME, Trites AW, Plourde S (2021) Foraging habitat of North Atlantic right whales has declined in the Gulf of St. Lawrence, Canada, and may be insufficient for successful reproduction. Endang Species Res 44:113-136. https://doi.org/10.3354/esr01097
Hanson MB, Emmons CK, Ford MJ, Everett M, Parsons K, et al. (2021) Endangered predators and endangered prey: Seasonal diet of Southern Resident killer whales. PLOS ONE 16(3): e0247031. https://doi.org/10.1371/journal.pone.0247031
Protecting Orca by Restoring Salmon Project (2021), Save our Wild Salmon, https://www.wildsalmon.org/projects/protecting-orca/about-protecting-orca.html
Sherley et al., 2017, Current Biology 27, 563–568, February 20, 2017 ª 2017 The Authors. Published by Elsevier Ltd. https://dx.doi.org/10.1016/j.cub.2016.12.054