History and Ecology of Seals

History and Ecological Role of Seals in the US NW Atlantic

With the arrival of Europeans to the region, Cape Cod and the Gulf of Maine ecosystems have been dramatically reshaped through human exploitation of natural resources. The story of the Gulf of Maine marine ecosystem that is emerging today, however, is one of resilience and recovery.

Gray seals and harbor seals were hunted and extirpated from US waters during the 20th century, until roughly the late 1960’s. They only returned after they were given protection with the passage of the Marine Mammal Protection Act (MMPA) in 1972. Until their populations recently began to recover, no one within living memory had seen a healthy marine ecosystem with seals interacting in their natural role.

The MMPA is one of the most effective environmental laws ever passed in the United States. As some populations of marine mammals like gray seals and humpback whales recover and repopulate their former ranges, humans are learning to live with them again. While we re-adjust to each other and our roles in a shared ecosystem, we can look to science for some of the information that we need to coexist.  

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Ecological relationships are extremely complex. They are almost never the simple one-to-one cause-and-effect interactions that people would like them to be. An ecosystem is like an intricate engine that keeps the world’s life-support running. Changing part of it, or removing a species, is like throwing parts of the engine away while wearing a blindfold. We can’t see how alterations we make might change the way the engine works, or how it might break.

Healthy populations of large predators are often a sign of an ecosystem’s overall health. Ecologists know that healthy predator populations in general, tend to promote greater biodiversity within an ecosystem (also an indicator of health) and healthier prey populations. In many cases, big populations of large-bodied animals like whales in the Gulf of Maine, or wildebeest in the African Savannah, have unintended benefits. They create positive feedback loops within their ecosystems that indirectly promote growth and greater abundance of other organisms.

Ongoing work around the world on rebounding pinniped species is giving us a perspective on what impacts are of seals on other species, and the impact of what removing seals (and other marine mammals) did to the system. The results of studies reflect some direct prey-predator relationships, and a growing body of scientific studies indicate that there are much deeper ecological interactions, and many benefits to re-stabilizing populations of predators and meso-predators (gray seals being an important one) (Bowen et al. 2012).

Marine mammals, including seals, have been shown to be important contributors to necessary nutrient circulation in our oceans. Some of the fundamental work that has been done into understanding this phenomenon, has been done in the Gulf of Maine (Roman et al. 2010, Roman et al. 2014, Doughty et al. 2016).

Seals also have the unique ability to contribute nutrients to the edge of the ocean, and have been recognized as important contributors to restore vegetation on nutrient limited islands such as Sable Island (Lysak 2013). The local grey seal population appears to be enriching the local vegetation and is having widespread positive effects on plant productivity and thereby promoting ecosystem stability (Lysak 2013, McLoughlin et al. 2016).

We are only recently learning what our rebounding marine mammal species contribute to global ocean health and ecosystem function. We know that marine mammals can act as ecosystem engineers, beneficially manipulating their environment:  Whales are able to cycle nutrients from the bottom to the top of the ocean and when they die, provide nutrients and community basis of unique ecosystems called whale falls.  We know that the Australian fur seal gut microbiome is essential to provide useful readily accessible nutrients to ocean systems, and, we know that harbor seals may actually be increasing  herring stocks by consuming their preferred diet of hake in the Straits of Georgia. 

Current ecosystem based management understands that the relationships between organisms and ecosystems are complex and the consequences of simplifying these relationships can have deleterious and unforeseen effects on ecosystem function (DeFries and Nagendra 2010).  The understanding that marine top predators can play important roles as keystone species, and the consequences of removing them, has been established through many studies and species, from sea otters to sleeper sharks (Heithaus et al. 2008, Estes et al. 2011). Harbor seals for example consume the predators of commercially important fish, and their presence can enable the recovery of important commercial fish stocks (Li et al. 2010).

When fisheries management failed, and groundfish like cod, were removed, the effect was the opening of a trophic niche to voracious predators like dogfish. Without a predator or meso-predator (seals), these cartilagenous predators flourished (Morgan and Sulikowsky 2015, Bundy et al. 2010). There are many accounts of depredation of fisheries being attributed to pinnipeds, when in fact, spiny dogfish are the main culprits. This includes here on the Cape as well (Rafferty et al. 2012). Attributing impacts to the proximal cause of these effects and understanding the impacts, is essential to reducing them.

Many of the consequences  of removing seals, relates not just to biomass or predatory interactions. For example, with the return of Australian Fur seals, it has been shown that their gut microbiome (and theirs alone) contributes to greater nutrient availability for primary production- which is essential for producing food for the entire ecosystem base (Lavery et al. 2012).  

This is all not to say, that there aren’t detrimental impacts to fisheries or inconveniences that occur when a rebounding species makes its way back into a system. We have lived for three generations without these animals in our backyard. There are real, and perceived, conflicts for sure. One urgent way to address these issues is to do the science to address these particular concerns. In our region, researchers are working to address the basic understanding gray seals- from what they eat, to where they go, how they interact with fisheries, how human behavior alters seal behavior, and how they contribute to ecosystem function. We can also look to other areas where pinnipeds have already returned, or have long been a part of the system.

In order to assess the impacts of prey consumption or impacts of harbor and gray seals here, we need to have adequate information on diet. Similarly, this is needed to understand the effects of fisheries on seal populations. The impacts on these rebounding populations is also significant- as over 1,000 seals die from fishery interactions, and hundreds more are living with sub-lethal to lethal entanglements- it’s a number close to what was taken from the system during the bounties. 

There is still a lot that we don’t know about the role of seals in the Gulf of Maine, but studying it as their population recovers gives an unprecedented look into the inner-workings of one of the most historically productive marine systems on Earth.

Last updated: October 27, 2018