How to sample very large fish

I spent most of the month of May traveling around Florida training  fishing charter captains how to collect biological samples of goliath grouper (formerly known as jewfish). These trips were phase one of a 3-year grant that our lab was awarded to collect age structure information about the goliath grouper population in Florida. Since 1990 when a moratorium was placed on all goliath grouper landings, the population has since increased and goliath grouper are now a common sight at reefs and wrecks along the Florida coastline. Just how much the population has rebounded remains in question and information about the age-structure of the population (How many young fish are there? How many adults?) is critical to determining just how much the population has increased and what kinds of management decisions must be made to prevent another near extinction.

The typical way to age a fish is to use the otolith, or ear bone, that most fish grow inside their heads. Used for maintaining balance when the fish is alive, once removed the otolith can be used to determine how old the fish was. Otoliths grow in spurts, just like fish do, that roughly correspond to the seasons, and so the otolith grows ring by ring, much like a tree. Obviously, counting otolith rings requires killing the animal, something we would like to avoid doing to an endangered species. Instead we use the rays of the dorsal fin which also grow rings like a tree. The rays are embedded in resin, cut into thin sections, and then the rings are counted under a microscope.

In the past few years our lab has caught and sampled about 250 fish, but at least 1000 are needed to reliably estimate the population. This is where the charter captains come in. Once trained by us, they will be able to land and sample goliath grouper if they happen to catch one during a charter. We get the samples and the customers get a rare, up-close experience with a huge fish!

Check out the lab’s website for more info. And here is a video detailing the complete sampling protocol. Spoiler alert: big fish and biologists below!

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One thought on “How to sample very large fish

  1. fishinnpost

    I literally had no idea you could measure something from the location of a fish like you did. I would love to have a cool job like you do. I imagine its tough to man handle these fish when they are moving and freshly caught on fishing line or nets.. Wow just thinking about the chaos. Appreciate your cool job because mine sucks! Haha

    Reply

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