muchalls wrote: Have tried a couple of times to return big fish (seldom catch monsters...) without luck.

Dead right MB - many report that their fish swam away strongly, but that doesn't mean it survived - the embolism damage has already been done.

Even the promotion and popularity of deflation needles is probably based less on science than on marketing and the feel-good factor. 

There's plenty of literature online on this subject - here's a sample, with my emphasis. 

The last sentence is interesting - taking it a step further I wonder how our snapper stocks would respond if it was illegal to release a legal sized fish.

As a captured fish is brought to surface...gases may leave solution and form bubbles (emboli) in the blood, and various tissues and organs, including the eyes, brain, heart, arteries, gills, spleen, fins, musculature, and the dermis beneath the scales.

The Australian National Strategy for the Survival of Released Line Caught Fish recently endorsed venting as has, in effect, the U.S. National Marine Fisheries Service, which now requires offshore anglers in U.S. territorial waters in the Gulf of Mexico to have venting devices in their possession.

The available information provides virtually no support for the practice of venting as a means of increasing survival of captured and released fish. This result is consistent across a variety of experimental and field study protocols, within and among various species of fish, including species captured in freshwater and in saltwater, and from various depths.

Although fish that can swim away or submerge commonly are considered to have survived catch and release, this assumption is largely untested and there is some evidence that the ability to swim away is unrelated to survival. It is, perhaps, the counterintuitive nature of this result, along with some wishful thinking, that has perpetuated the practice of venting.

However, the physiological effects of barotrauma are not remediated simply by returning the fish to its capture depth (Morrissey et al. 2005) and many barotrauma injuries are unaffected by recompression.

The available evidence fails to demonstrate that venting fishes exhibiting symptoms of barotrauma promotes post-release survival. In fact, it is possible that this practice decreases survival of fish captured from deeper waters, presumably because of the greater severity of their barotrauma symptoms. Venting fish should not only be discouraged by fishery management agencies, but given the possibility that venting adversely affects survival of released fish, this practice should be prohibited, rather than mandated.