Introduction

The gray (or grey) seal appears remarkably similar to harbor seals, but key features exist to differentiate the two with overlapping ranges. Found in coastal to near coastal waters of the Northern Atlantic – that is, northeastern US and Canada, northwestern Europe and the Baltic Sea – the gray seal sports similar spotted patterns on their bodies as harbor seals (though these spots tend to larger). However, when looking at the facial features of this species, the differences become clearer. Gray seals have a more elongated and distinctively straight head, ending with a large snout and longer nose in the males (but not as long as an elephant seal’s!). Gray seals are actually quite large as well: those found in the Eastern Atlantic can grow up to 7.5 feet and 680 feet, while those in the Western Atlantic (here in the US) can grow even larger to just shy of 9 feet and 880lbs, with the record at 10 feet, 10 inches long!

Diet

Gray seals are opportunistic predators, meaning they are not picky and will eat what is most abundant! They are benthic (on the seafloor) and demersal (near the seafloor) feeders eating mainly fish (sand eels, hake, whiting, cod, haddock, pollock, flatfish), crustaceans, squid and octopuses. They have even been known to take seabirds sometimes! They may also eat other mammals – more on this later! Gray seals feed in the open ocean, usually within about 100km of the haul-out sites, and sometimes can spend up to 30 days at sea on foraging trips. Though they do not feed during molting or the mating/pupping season. They can dive up to 1,560ft for up to an hour and eat 4-6% of their body weight per day. The key word for gray seals is variation. There is variation in diet between age, sex, season and geographic region. There is even substantial individual variation in habitat usage. 

Behavior

Gray seals can be found alone, in small groups or in large aggregations. They are gregarious but not sociable, which may seem counterintuitive. What this means is that they will congregate in large numbers (sometimes hundreds of animals) at haul-out locations, but they keep a distance from one another. So they are all on the breach together, but not really interacting together socially. They haul-out for rest, molting (shedding their fur) and mating/pupping on rocks and beaches, usually of uninhabited offshore islands, but some will haul-out on secluded mainland beaches. They can be very vocal, and aggressive. They have good hearing and vision, making them excellent hunters.

Living 25-35 years (with females generally living longer), females become mature at 3-5 years of age, and males around 6, though they are probably not socially mature until about 8. This is common in many species, as they can physically mate earlier but it takes a little longer to know the social rules that govern how it is done. The seals will aggregate in large groups for mating/pupping season. On land, males will mate with many different females, and may also fight other males for position within a group of females causing deep scars on their necks. Although they don’t normally feed during this time, some males have taken up a different strategy and have been shown to make short foraging trips between bouts of copulation events (more on this later!). If how long you can stay and mate is limited by your energy reserves, this strategy might be good to increase the amount of time you can mate, and ultimately how many pups you can sire!

Gestation is about 11 months (including ~3 months of delayed implantation) and pups are born on land or sea ice at different times of the year depending on location: Western Atlantic = December to February; Eastern Atlantic = September-November; Baltic Sea = March. Pups are 35lbs at birth and have white fur called “laguno” that absorbs sunlight and traps heat, keeping them warm. They lose their laguno coat at the end of lactation They nurse for about 2-3 weeks on high fat milk (60%), gaining about 3lbs per day and building a thick blubber layer. This is important as they will live off these blubber stores for about 36 days after mom leaves – this is the time they learn how to forage. Research (using tags) has shown that pups stay close to their birth site for 1 week and stay in waters shallower than 40 meters until about 4 weeks. They increase the frequency of foraging until 7 weeks, after which it decreases as they become more efficient- you have to hunt more when you aren’t as good at it! How far they travel from the birth site increases with age, likely due to their increased swimming ability as they get older. This just shows how important practice is!

Status

There are thought to be approximately 650,000 gray (or grey!) seals worldwide, with the largest concentrations found on the east coast of North America and in the UK. In fact, up to 95% of Europe’s gray seals are in the UK!

Gray seals are protected by the Marine Mammal Protection Act (MMPA) throughout their range, and in Europe are legally protected through the EC Habitats Directive.

Threats

• Entanglement: Gray seals are susceptible to entanglement in active fishing gear including nets, pots, lines, etc. They often forage in the same areas as fishermen are working, and as curious predators can become entangled in lines or nets while exploring potential prey sources. Gray seals are also at risk of becoming entangled in discarded fishing gear (either on land or at sea), which can be equally dangerous.
• Intentional Killing: Unfortunately, illegal killing of gray seals is not uncommon. As conspicuous coastal marine mammals, they are easily spotted which means they are also easy targets for harassment. Frustrated fishermen or boaters have been known to club and/or shoot seals, blaming them for stealing their fish or assuming that low catches are a direct result of gray seal presence in an area. Illegal feeding of gray seals is also a problem, and as we’ve discussed before this can lead to changes in their behavior, familiarization with humans (which isn’t a good thing!), and ramifications for their survival in the wild.
• Contaminants: As coastal dwellers, gray seals are more at risk of exposure to contaminants due to their proximity to industrial and residential run-off. Seals are also exposed to many contaminants on beaches during haul out periods, and are therefore at a higher risk of contamination with toxins, diseases, and other sources. Like most top predators, they can also consume contaminants through their prey, resulting in bio-accumulation of contaminants within their bodies (i.e. the more contaminated prey they eat, the more contaminated the seal becomes).
• Oil Spills & Renewable Energy: If oil comes into contact with the seal’s fur, the water-resistant qualities are reduced resulting in risk of hypothermia and even death for seals contaminated by oil spills. These can also result in massive depletions in prey stocks, which could impact seals through malnutrition. In the continuing source for “cleaner” energy sources many are now turning to tidal turbines and wind farms for renewable energy. However the construction phase to install these devices are very loud and involve a lot of equipment; for gray seals, who live in the coastal waters where many renewable energy plants are located, this can be particularly disruptive and/or dangerous. Once installed, tidal turbine blades can also be hazardous to seals swimming by, and so many operations simply shut down turbines when seals are detected close by to reduce risk of injuring the seals.
• Vessel Interactions: Direct boat strikes, or harassment by boats near haul out sites (resulting in seals being flushed back into the water) can either be directly harmful (i.e. directly injuring the animal), or indirectly harmful, as the constant fleeing can wear animals out to the point of exhaustion. Seals require time on land to warm up, dry out, rest, and raise their young, so please be sure to give any beaches with seals present, or known haul out/pupping sites, a wide berth if you’re on your boat!

Fun Facts

• Gray seals have one of the best Latin names we’ve come across so far! Their Latin name, Halichoerus grypus, literally translates to: “hook-nosed pig of the sea”! How cool is that?!
• Although all gray seals have spots, males actually have a dark coat with light spots while females have a light coat with dark spots.
• Gray seals are the largest breeding mammal in the UK.
• They utilize extensive vocalizations, and have even been found to “clap” underwater! This is thought to be a mating display, either to attract potential mates and/or ward off competition. Watch some incredible footage of these underwater claps here. Gray seals also make a wide range of other vocalizations, and have even been taught to mimic notes on a scale to replicate “singing”! Researchers at the University of St Andrews have explored this ability in-depth, and while it might not be the most melodious sound ever, you can watch some adorable footage here!
• Gray seals can swim up to 35km/hour (22mph) when being chased: that’s pretty fast!

Bonus Feature: Gray Seal Murder Mystery

Perhaps the most interesting thing about gray seals is their role in a murder mystery in the marine mammal world.

Stranded seals began showing up with strange corkscrew lesions in many locations throughout the US, Canada and Europe. At first it was thought that these marks might be caused by propellers (which makes sense due to the corkscrew nature of the lesions), or even Greenland sharks, but while these two causes were possible, neither seemed to fully fit the bill.

In 2014, however, the mystery was finally solved. On the Isle of May, Scotland, a doctoral student studying the gray seal breeding colony observed an adult male gray seal killing and eating 5 weaned gray seal pups (cannibalism!). When the pups were examined, the same characteristic corkscrew/spiral lesions were present: the killer had been found! These corkscrew lesions have been documented on stranded harbor, gray, hooded and harp seals in various locations, all of which have gray seals present in the area. Even more interesting is that the seasonality of this behavior follows the breeding seasons of the different species.

So why target pups? Well, weaned seal pups have lowered metabolism and movement, making them easier to capture. Even though male gray seals usually don’t eat during mating season, this behavior has occurred during that time. Once again, if they are limited by their energy reserves, having some high energy snacks (e.g. seal pups) may allow the males to stay longer and thus have more mating opportunities. In fact, they may not even need to eat a lot of the pup in order to gain the benefits – it has been shown that one male can kill 8-14 pups in a 10 day period, often not eating much of the carcass before killing the next.

But seals are not the only mammal gray seals have been known to eat – they have also been observed killing and eating harbor porpoises! Evidence shows that they may drown them, utilizing an ambush or sit-and-wait strategy, and primarily eat the skin and blubber (not muscle). This isn’t surprising, as the fat has the highest energy and is easily digested – more bang for your buck!

It is important to note that although not all corkscrew lesions may be caused by gray seals (others could be caused by propellers or other sources), the evidence points to gray seals as the cause for most of them.

It is obvious that we don’t know everything about the gray seal diet and behavior. Remember that individual variation discussed before? It may also be that only certain individuals show this behavior. Has this always happened, and we are only seeing it more recently, or is this a new behavior? And if so, why is it happening now? There are so many more questions to answer about this intriguing species!