Texas Tech University
TTU Biology Dr. Wilde

Does Venting Promote Survival of Released Fish

Coral Trout
Photograph courtesy of Bill Sawynok.

I recently completed a meta-analysis of the effects of venting fish with over-inflated swim bladders. I was able to locate 17 studies that provided paired estimates of survival of vented and unvented fish. Among these studies, there was a total of 39 paired estimates, from 22 species or species groups, based on thousands of fish.

Eighteen estimates were from experimental studies, in which fish were captured, vented (or not), and then held in cages, aquaria, etc. for observation and survival rates were recorded. Twenty-one estimates were from capture-recapture studies, in which fish were captured, tagged, vented (or not), released, and then recaptured. In these studies, recapture rates were considered to be surrogate measures of survival.

I combined these estimates using Relative Risk. Essentially, this is the survival (recapture) rate of vented fish divided by the survival (recapture) rate of unvented fish. If the relative risk = 1, there is no affect attributable to venting. If relative risk is greater than 1.0, then venting promotes survival.

Venting had no significant effect (relative risk was not different from 1.0) in 32 of 39 estimates. It was less than 1.0 in two samples (venting decreased survival) and was greater than 1.0 in five samples (venting increased survival). Among 10 species for which multiple estimates were available (including three of the five species that appeared to benefit from venting), none showed an overall response to venting. Thus, there is little evidence that venting promotes survival.

Summarizing results from all studies, there was no difference in relative risk between experimental and capture-recapture studies, so the data can be combined. There was no overall response to venting, when results from all species and studies were combined. Venting had no affect on survival.

There was evidence that venting had no effect on, or was slightly beneficial, to fish captured from shallow waters. However, venting was progressively less beneficial, apparently harmful, to fish captured from deep water.

A summary of my paper, published in the January 2009 issue of Fisheries, is available here.