coming back from the Canary islands I was struck by their beaches which were nice and sandy but were completely devoid of shells. Living in the UK and NZ, the beaches are full of shells. I presume that it is lack of food for the shellfish, but what and why?
I’m guessing that there are probably plenty of shells, it’s just that the shell harvesters beat you to the beach. They pick them up and resell them to tourists and to serious shell collectors over the Internet and via mail order. The same thing happens on famous shell islands, like Sanibel on the Florida Gulf Coast–you literally have to get up pretty early in the morning to beat the serious shell collectors and the harvesters.
Strombus canarium, 2" long, from the Canary Islands, 75 cents.
http://www.einsteins-emporium.com/life/seashells/ls515.htm
Conus guiniacus, 25-28 mm, ditto, $4.00
http://www.seashells.net.au/CONIDAE.html
Also, the shells on UK beaches are probably just basically clamshells, aren’t they? Cold northern seas don’t tend to produce the fancy collectible shells.
And are the NZ shells like these?
http://www.krampf.com/galleryfolder/g_zealand_folder/g_Zealand1.html
I’m guessing that either these aren’t collectible, either, or that the NZ shell-harvesting industry just hasn’t gotten with the times yet. 
I am not quite convinced. those conch shells you mentioned may well have been gathered by divers. While whole shells may have been gathered by souvineer hunters, there should still have been lots of broken shells or very small shells - which there isn’t. There just seems to be a total lack of shells and shellfish. I presume there must be lots of other places in the world with no shells on snady beaches - so I was wondering in general if some aquatic environments are not condusive to sealive of that kind?
I know along the New Jersey shoreline, some areas are littered with shells, and some are devoid of them (taking into account any clean up, etc. Some remote areas and isolated area aren’t subject to these issues)
From 30+ years of experience, the coastline along the immediate area seems to play some part. In areas with a gradual drop, where you could be 30 yards out and only up to your hips in water, shells were abundant, especially when tides were strong and the high/low difference was strong. Shells were carried in/out easily. Tuff on the feet.
In areas with a shelf like drop, the beaches are cleaner…more sand…just sand…fewer stones…and hardly any shells.
Also, we’ve learned that jumping off a boat that is near a shore line that drops gradually is dangerous on the feet because of the accumaltion of shells - some are sharp. Areas with a steep drop are much cleaner/softer.
This is my observation. I cannot say whether it adresses the area you are talking about, but give it some consideration.
Lots of reasons might affect the quantity you see, as both DDG and Philster have mentioned…
Earlier shell collectors,
Sandy vs hard substrate,
Availability of food for shell producing species,
Storm history,
Inclination of slope,
Pollution,
Water clarity, etc.
If we’re talking about leisure beaches, the sand may not be naturally-occurring; some resorts top up their beaches with thousands of tons of imported beach sand every year; this might not only bury any existing empty shells, but also might provide an environment in which molluscs cannot thrive.