MOTE MARINE LABORATORY

VENTING
To Vent or Not To Vent ?

Vent vs. Non-Vent

  Many fish have swimbladders or gas bladders. As a general rule, fish with spiny fin rays have closed swimbladders and fish with soft fin rays have open swimbladders. The closed bladder acts like a balloon that has been tied off with the gas trapped inside. The open bladder acts like an untied balloon and gas is free to move in and out.
   You may have noticed that some fish caught at depths have their intestines coming out of their anus, eyes bulging out of their heads and stomachs protruding out of their mouths. This is because these fish have closed gas bladders that have expanded and ruptured and the gasses from the bladder have displaced the internal organs as they were brought rapidly to the surface.
   The pressure exerted on their bodies decreases during ascent, allowing the gases to expand in their bodies. If the fish is kept, this is not a problem, but if it is thrown back, it helplessly floats on the surface, exposed to the elements and is easy prey for sea birds and predatory fish.
   An alternative to throwing the bloated fish back is to purge the air from it's body before release. This can be accomplished by using a hollow puncturing device like a syringe or a sharpened basketball needle. Proper purging of the air will allow the fish to swim back to the bottom and find a suitable hiding place. It is assumed that this purging increases it's chances of survival.
   This study is being conducted to investigate that assumption. Mote Marine Lab and RSMAS are conducting this study with funding by FL Sea Grant.

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Objectives

The objectives of the study were as follows:

1. To determine; if the act of abdomen deflation itself is harmful to 5 reef fish species. (red grouper, gag, red snapper, vermilion snapper, mutton snapper and mangrove snapper.
2. To scientifically determine the potential for using abdomen deflation as a means of enhancing the survival of released undersized grouper and snapper.
3. To test the hypothesis that abdomen deflation enhances survival in some reef fish species but not in others, and that critical water depth is also species specific.
4. To provide hard scientific data to evaluate Sea Grant's educational material on abdomen deflation.
5. To provide guidelines on species treatment; and deflation methodology.
6. To provide data useful for reef fish stock assessment to assist in management for the 6 species.
   
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Species Involved

Species Involved: Red Grouper, Epinephelus morio, Gag, Mycteroperca microlepis, Red Snapper, Lutjanus campechanus, Vermilion Snapper, Rhomboplites aurorubens, Mutton Snapper, Lutjanus analis and Mangrove Snapper, Lutjanus griseus.

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Methodology

1. All released fish were tagged and the tag data sheet completed
2. Half of the tagged were vented and half were not  vented.

To differentiate possible effects of deflation (venting) from those due to depth of capture, fish were tagged and released in equal numbers with deflated versus non-deflated abdomens in discrete depth categories: 0-4, 41-70, 71-100, 101-200 and +201.

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Statistics

The total number as of October 16, 2002,  of tagged, recaptured and vented fish vs. the number non-vented fish for each species is shown in the provided table. The goal in providing this table is to show which categories need to be filled. For example, there are smaller numbers in all the +201 depth category. Also, there are more vented fish than non-vented fish at this depth.

The purpose of this study is to determine the potential survival of vented versus non-vented fish, so equal numbers of these fish must be treated to determine if a difference in survival rates exists. Obviously, it is necessary to vent fish  that are caught in the +201 ft. depth category but for the purpose of this study, a number of fish need to be returned unvented and recorded for the control group.

In the 0-4-ft. depth category, there are few vented fish. Although it may not be necessary to vent fish at this depth, these vented fish will be used to determine if venting causes infections and/or physical harm to the fish. Please vent more fish in this depth category.

As shown in the following table, more gag, red grouper, red snapper, vermilion snapper, mangrove snapper and mutton snapper need to be tagged for this project. Please use these tables as a guide in determining whether or not to vent your catch. For our statistics to be valid, we must have equal numbers (as close as possible) of treated fish vs. non-treated fish to compare.

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Comparison of Fish Vented Vs. Non-Vented at Various Depths

From October 1990 through October 16, 2002

Last Updated: June 15, 2004

Web Site Design: Don Marshall