Disclaimer. Ultimately you need to make your own decisions on how to protect your food in bear country. And you are fully responsible for the outcomes of those decisions. In this article I have tried to be candid and realistic, and to cut through some of the BS, red tape, and conventional wisdom that surrounds this issue. To be clear, I recommend that you listen to the land management agencies -- their rules and their advice. If you feel sufficiently educated, informed, experienced, and comfortable to stray from those guidelines, that is your call.
Bear canisters are hard-sided plastic or carbon fiber cylinders with a removable lid that is designed to protect its contents (namely, food and other scented items) from bears. The canister's shape, hardness, and lid seal mechanism (which require opposable fingers to open/close) make it extremely difficult, if not impossible, for bears to access the canister's contents.
Canisters have gained popularity in the last decade. In order to reduce and/or minimize bear-human conflicts, some land management agencies now require that backcountry users carry them. And, required or not, a few backcountry users carry them for the peace of mind that they provide -- no bear, raccoon, or mouse can ruin their trip or tomorrow night's dinner.
All canisters currently available are cylindrically shaped. But they do differ in volume, which allows you to choose one that is most compatible with the volume of food that you normally carry. (To be clear, the canisters have a fixed volume; no current model has an adjustable volume.) The smallest volume canister is about the size of an industrial-sized roll of paper towels, like one that you'd see inside of a dispenser at an airport. Bigger models are larger both in girth and height. The lightest weight canister weighs about 2 lbs; the heaviest model is about 4 lbs. The cost ranges from $75 to $275, depending on the manufacturer, size, and raw materials.
The following land management agencies require that backcountry users carry an "approved" canister: Yosemite National Park (High Sierra, California), Sequoia-Kings Canyon National Park (High Sierra, California), Inyo National Forest (High Sierra, California), Olympic National Park (Washington state), and New York Department of Environmental Conservation (Adirondack State Park).
Each agency has different rules regarding their canister requirements. Canisters may be required in some specific areas (e.g. High Peaks, Dusy Basin, Elwah Valley, etc.) but not in others. They may be required during certain seasons but not during others. And/or they may be required if the backcountry user does not have another approved technique of protecting their food. See the agency links at the bottom of this page to read the exact rules.
The Sierra Interagency Black Bear Group performs "tests" on bear canisters and, if the canister passes, it is given their seal of approval ("approved"). Their tests are not particularly scientific, but they are quite realistic: they fill a canister with heavily scented items, throw it in a cage with a "problem" black bear (i.e. one that has gotten into too many trash cans, too many campsites, and too many station wagons to leave alone in the wilds), and watch what happens. In order to pass, the canister must prevent the bear from being "rewarded" - i.e. from tasting the contents.
A list of approved canister models is available at the Sierra Interagency website.
The land management agencies that require canisters -- and the canister manufacturers themselves -- tell us that bear canisters are the only failsafe way to protect food. Protecting your food, of course, keeps your hike on track and prevents a few days of starvation. But it also helps protect the bear, since a problem bear is eventually a dead bear; and to protect the next backcountry user, since a bear that obtains your food is more likely to try to obtain another person's too.
But, I ask, are canisters really necessary in order to protect your food, protect the bear, and protect the next backcountry user? I would argue, "No." I think they'd be unnecessary if everyone practiced "bear avoidance" techniques and mastered the PCT Bear Hang Method, both described in Part I of this article. But that's wishful thinking, and the canister polices were implemented with the lowest common denominators (i.e. the careless, irresponsible backcountry users) in mind.
I never carry a canister when it is NOT required. I have enough confidence in my "bear avoidance" and bear hang technique that I do not think a canister is worth carrying. (See Part I of this article.)
I consider carrying a canister when it IS required, and usually I do. I do not carry a canister to protect my food, the bears, or my fellow backcountry user -- I can do those things without a one. Instead, I carry one in order to protect myself from backcountry rangers, who could fine me if I'm caught without one.
Rangers do patrol the backcountry, and they regularly do canister checks on passing hikers. I'm not aware of any guaranteed technique to avoid a fine, e.g. by raising legal technicalities against warrantless searches or questioning law enforcement jurisdictions, etc. So my recommendation is to absolutely carry a canister if you think you may encounter a ranger, but otherwise consider practicing good bear country techniques instead.
Wild Ideas. Least heaviest, carbon fiber, expensive.
Bear Vault. See-through plastic, second least heaviest, moderately priced.
Backpackers Cache. The original; plastic, heaviest, least expensive.
Bear Keg. Plastic, heaviest, least expensive.
Bare Boxer. Very small volume, thus lightweight; cheap.
Ursack. Their Hybrid model was temporarily approved in 2006, but after extensive field testing that summer the SIABBG determined it unfit for use. This was unfortunate since the Ursack is lighter than all other models (20 oz, which is 12 oz lighter than the next lightest comparable model) and the aluminum liner had some "give" to it, which made it more comfortable inside of a pack.
I do not like canisters and probably never will. But I can think of a few improvements that would make less hostile towards them:
Make them lighter, duh! A canister that weighs about a pound would be awesome.
Give them a flat side. I don't know anyone who has a back that is so concave that a canister nestles into it perfectly. Instead, the cylindrical shape of today's canisters exert pressure along one narrow point, right up against the spine. Canisters are particularly painful with frameless packs.
Introduce adjustable volume. The best feature of Platypus water bottles is their collapsibility: you can adjust their volume (from zippo to 80 oz) to minimize the space it needlessly consumes in your pack or side pockets. A canister manufacturer that can figure out how to do this would own the market.
Make them soft-sided. The canister would be much more comfortable to carry if they had some "give" to them -- they would be easier to pack and more comfortable to carry.
Sequoia-Kings Canyon National Park