node.js child_process.spawn ENOENT error - only under supervisord - node.js

Question

I'm running a command with Node.js using child_process.spawn:
#!/usr/bin/js
var spawn = require("child_process").spawn;
var stockfish = spawn("stockfish");
This works fine using $js spawntest.js from the command line; it just hangs like you would expect because the subcommand is waiting for input.
When I set this script up as a supervisord program, however, it fails:
$ sudo supervisorctl start spawntest
spawntest: ERROR (abnormal termination)
Here is the contents of the stderror output log that supervisor keeps in /var/log/supervisor/spawntest-stderr---supervisor-RyULL0.log:
events.js:72
throw er; // Unhandled 'error' event
^
Error: spawn ENOENT
at errnoException (child_process.js:980:11)
at Process.ChildProcess._handle.onexit (child_process.js:771:34)
events.js:72
throw er; // Unhandled 'error' event
^
Error: spawn ENOENT
at errnoException (child_process.js:980:11)
at Process.ChildProcess._handle.onexit (child_process.js:771:34)
events.js:72
throw er; // Unhandled 'error' event
^
Error: spawn ENOENT
at errnoException (child_process.js:980:11)
at Process.ChildProcess._handle.onexit (child_process.js:771:34)
events.js:72
throw er; // Unhandled 'error' event
^
Error: spawn ENOENT
at errnoException (child_process.js:980:11)
at Process.ChildProcess._handle.onexit (child_process.js:771:34)
The supervisorctl program is configured like this:
[program:spawntest]
command=/home/gus/dev/spawntest.js
autostart=false
autorestart=false
user=gus
Why is this failing only under Supervisor? Are there any Node.js modules/global variables etc I can use to see what the differences are between invoking a script directly and through Supervisor?