Brandt's Cormorant Diet Studies

My internship at the Farallones involved many different fascinating studies, but one of my favorite studies were the seabird diets, as they really tie in the oceanographic aspect of marine ornithology. We are lucky to be able to live on this incredible, rugged island surrounded by the Pacific ocean and work with the birds that call it home, but sometimes it can be easy to take for granted just how strong the connection is that these birds have to the marine environment. By incorporating the feeding ecology of the seabirds, we are also considering vast topics like oceanic health, fisheries ecology, and climate change, much of which is still poorly understood. Taking part in studies that delve into this mysterious, watery realm is pretty exciting. Brandt’s cormorants also happen to be one of my favorite birds on the island - how could you not love the silky black birds that look like Muppets with long necks and giant feet, and whose eyes are a vibrant, deep turquoise?

One of the last seabird studies to take place on the Farallones is Brandt’s cormorant diet sampling.  The diet study is based on the contents of cormorant pellets, which are regurgitated by the birds at the colony. This is one of the longest running diet studies in the world, with data ranging back 40 years.  And it has proven incredibly valuable in disclosing what we can’t see that goes on beneath the surface of the ocean. The cormorants, through their foraging activity, can be used as a method of sampling the most abundant prey species available to Farallon seabirds, which can in turn be used as a measure for the health of the surrounding marine ecosystem and the changes that it goes through over time. 

gawky Brandt's cormorant chick

Seabird diet studies are conducted in many different ways. Some diet studies are strictly observational, as in the case of Common Murre and Pigeon Guillemot whose prey items are observed from a blind as they bring them back in to feed their chicks at their nest sites. Others, such as the nocturnal Rhinocerous Auklet, involve capturing the birds at night as they bring fish back to the nest, and when captured drop their prey whole. Whole prey items are very useful as the weight, exact size, and species (and therefore energetic value) can be accurately determined. Rhinocerous Auklet diet information is also used to supplement information from cormorant diet studies, especially in terms of rockfish diversity.

Brandt's Cormorant chick reaching deep down a parent's throat for food

Brandt’s Cormorant feed their chicks straight from the gullet, therefore observational diet watches would be in vain. Instead, we rely on collecting regurgitated pellets from which prey parts can be separated and identified. These pellets can only be collected post-breeding season, after the cormorant chicks have fledged and the colony is abandoned. 

searching for pellets at the colony

 The pellets are constituted of mucous membrane encasing indigestible fish bones, which the birds will regurgitate after consuming a meal. They can be hard to find among the deserted colony, and it takes some practice figuring out the pellets from dried clods of dirt and guano! 

After collection, the pellets are sent off the island to the Point Blue lab, where lab technicians treat the pellets with a detergent (to stop enzyme activity, which could destroy the contents), soak them in water, and pass them through a fine sieve to pick out the relevant bones. Specifically, lab techs are looking for otoliths – tiny bones found in the inner ear of all vertebrates that can be used to accurately identify fish prey down to the species level. Invertebrate prey items can also be identified by their hard parts – cephalopod beaks and decapod carapaces.

Rockfish sp. otolith

Overall, population data collected on Southeast Farallon show that the number of breeding adults have declined through the 1970s and 1980s, fluctuated in the 1990s, increased in the early 2000s, and dropped precipitously in 2007. The highest count on the island was in 1974, when 23,800 adults were counted, while 2009 held the lowest population at 1,248 adults. Since 2008, the numbers of adult cormorants on the island have not exceeded 5,000. While it is not known why the population declined so dramatically in 2007, years of abnormally low productivity correspond to years with El Nino or other warm-water events. Other factors that play into the variability of available prey are related to the pressure exerted upon prey populations by other seabirds, many of which are competing for the same resources. While the diversity of seabird species present on the Farallones is supported in part by resource partitioning (such as foraging depth, habitat, etc), in years with lower forage fish diversity, many of the breeding seabirds may be left with no choice but to target the same prey species, creating much more difficult conditions for finding enough food for themselves and their chicks, leading to low productivity.

Southeast Farallon Brandt's Cormorant Diet summary

Brandt’s are pursuit-diving seabirds that propel themselves underwater using their large webbed feet. Although little direct information is available on the actual diving limit, it is thought that they can forage at up to 150 feet deep, with a max diving time of 95 seconds and a mean diving time of 51 seconds. Cormorants forage in offshore pelagic zones as well as on nearshore rocky benthic reefs, depending on where fish are available. Pellets have revealed that the diet of cormorants on SEFI is composed of mainly three groups of fish: northern anchovy (Engraulis mordax), rockfish species (genus Sebastes, family Scorpaenidae), and flatfish species (order Pleuronectiformes). Other prey items commonly found in the cormorant diet include plainfin midshipman (Porichthys notatus), a variety of sculpin species (family Cottidae), cod species (family Gadidae), and spotted cusk-eel (Chilara taylori). Trends in the fish present in cormorant diet have changed over the years, with the 1970s showing a majority of rockfish in the diet. In 1994, however, northern anchovy became more frequent, and then by 2008 this species was decreasing in the diet, being replaced by rockfish again. In the past few years, the diet has fluctuated much more. The diet switched from anchovy to rockfish from 2009-2010, then another switch to flatfish occurred from 2011-2012, and then another switch back to rockfish in the last couple of years. The shift from anchovy and offshore rockfish species to nearshore-settling rockfish and neritic flatfish is most likely one based on abundance and availability rather than preference.  Changes in oceanic conditions could be driving this change in community structure, which have become favorable to nearshore species like flatfish and certain rockfish species. This further seems to be supported by the appearance and growth of several Brandt’s cormorant colonies on the mainland, which are located closer to nearshore shallow foraging habitat.

 

By studying the feeding ecology of seabirds, attempts are made to elucidate the factors that limit population size and breeding success, and to monitor the health of the surrounding marine ecosystem. Marine systems are constantly in flux, yet we are likely to see more anomalies and fluctuations as the climate changes. By monitoring what the seabirds are foraging to feed themselves and their chicks, we gain a glimpse into what is happening with forage fish species in the ocean in relation to ocean health. This is very important information as forage fish are near the base of the marine food web and support not only other seabird species, but a great many other marine species as well. Through pellet collection, we have formed a better understanding of why in certain years Brandt’s cormorant productivity dropped precipitously, and thus, how they might respond to changes in the oceanic climate in the future. This kind of information can have profound conservation implications, and is vital in the adaptation of climate-smart, ecosystem-based management policies. 

Posted by Eva D. Gruber, SEFI Seabird Intern